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Cover photo: 

Green Anaconda photo by Lutz Dirksen. Photo permission granted for use.
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Giant Constrictors: Biological and Management 

Profiles and an Establishment Risk Assessment 

for Nine Large Species of Pythons, Anacondas, 

and the Boa Constrictor

By Robert N. Reed and Gordon H. Rodda 

Open-File Report 2009–1202

U.S. Department of the Interior

U.S. Geological Survey
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U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia 2009

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Suggested citation:

Reed, R.N., and Rodda, G.H., 2009, Giant constrictors: biological and management profiles 

and an establishment risk assessment for nine large species of pythons, anacondas, and the 

boa constrictor: U.S. Geological Survey Open-File Report 2009–1202, 302 p.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.

Frontispiece: Clockwise from upper left: Southern African Python, Python natalensis, in 

South Africa (courtesy of G. Alexander); A clutch of hatchling Burmese Pythons (Python 

molurus bivittatus) in Florida (courtesy of S. Snow); A brooding female Burmese Python lifted 

from her eggs in Florida (courtesy of S. Snow); Green Anaconda, Eunectes murinus, in Brazil 

(courtesy of L. Dirksen); DeSchauensee's Anaconda, Eunectes deschauenseei (courtesy of 

L. Dirksen); Boa Constrictor, Boa constrictor, on Aruba Island, West Indies (courtesy of R. 

Reed); Reticulated Python, Broghammerus reticulatus, on Sulawesi Island, Indonesia 

(courtesy of B. Lardner).
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Table of Contents

Chapter One–Introduction and Synopsis of Conclusions...................................................................... 1

Synopsis of Conclusions .............................................................................................................................. 4

Chapter Two–Materials and Methods....................................................................................................... 9

Introduction .................................................................................................................................................. 9

Length Measurements ............................................................................................................................... 10

Climate Space............................................................................................................................................ 11

International Trade..................................................................................................................................... 14

Climate Matching ....................................................................................................................................... 15

Reasons for Preferring our Method to the use of Environmental Niche Models ......................................... 16

Chapter Three–Synopsis of Eradication Tools...................................................................................... 21

Introduction ................................................................................................................................................ 21

Primary Tools............................................................................................................................................. 23

Primary Tool I–Limit Spread: Barriers..................................................................................................... 23

Primary Tool IIa–Localize Targets: Visual Searching ............................................................................. 25

Primary Tool IIb–Localize Targets: Public Reporting.............................................................................. 29

Primary Tool III–Localize Targets: Trapping........................................................................................... 30

Primary Tool IV–Localize Targets: Detector Dogs.................................................................................. 32

Primary Tool V–Kill Unlocalized Snakes: Toxicants ............................................................................... 33

Summary of Primary Tools ..................................................................................................................... 34

Secondary Tools ........................................................................................................................................ 35

Secondary Tools I–Judas Snakes .......................................................................................................... 35

Secondary Tools II–Biocontrol................................................................................................................ 36

Secondary Tools III–Habitat Manipulation .............................................................................................. 38

Secondary Tools IV–Bounties and Adding Value ................................................................................... 38

Secondary Tools V–Frightening Devices................................................................................................ 38

Secondary Tools VI–Reproductive Inhibition.......................................................................................... 39

Secondary Tools VII–Egg/Nest Destruction ........................................................................................... 40

Secondary Tools VIII–Pheromonal Attractants....................................................................................... 40

Summary of Secondary Tools ................................................................................................................ 42

Overall Summary.................................................................................................................................... 42

Chapter Four–The Indian or Burmese Python, Python molurus.......................................................... 43

1.0 Introduction .......................................................................................................................................... 43

1.1 Species............................................................................................................................................. 43

1.2 Common Names............................................................................................................................... 44

1.3 Evolutionary Context ........................................................................................................................ 44

2.0 Description ........................................................................................................................................... 45

2.1 Unique Morphological Features........................................................................................................ 45

2.2 Size .................................................................................................................................................. 45

2.3 Sexual Size Dimorphism .................................................................................................................. 46

2.4 External Sexual Differentiation ......................................................................................................... 46

3.0 Distribution in Space and Time ............................................................................................................ 47

3.1 Native Range.................................................................................................................................... 47
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3.2 Habitat Range................................................................................................................................... 50

3.2.1 Maximum Elevation.................................................................................................................... 50

3.3 Climate Range.................................................................................................................................. 50

3.4 Microhabitat...................................................................................................................................... 51

3.5 Introduced Ranges ........................................................................................................................... 52

3.6 Seasonal Activity .............................................................................................................................. 52

3.7 Daily Activity ..................................................................................................................................... 52

3.8 Foraging Mode/Sensory Modalities .................................................................................................. 52

4.0 Life History ........................................................................................................................................... 53

4.1 Reproductive Mode .......................................................................................................................... 53

4.2 Reproductive Phenology .................................................................................................................. 53

4.3 Reproductive Effort/Fecundity .......................................................................................................... 54

4.3.1 Clutch or Litter Size/Frequency.................................................................................................. 54

4.3.2 Egg Size .................................................................................................................................... 55

4.4 Growth.............................................................................................................................................. 55

4.5 Maturation ........................................................................................................................................ 56

4.6 Longevity .......................................................................................................................................... 56

5.0 Diet/Trophic Role ................................................................................................................................. 56

5.1 Prey .................................................................................................................................................. 56

5.2 Predators.......................................................................................................................................... 57

5.3 Diseases and Parasites.................................................................................................................... 58

6.0 Demography......................................................................................................................................... 58

6.1 Population Density............................................................................................................................ 58

6.2 Size Distribution................................................................................................................................ 58

7.0 Interactions with Humans..................................................................................................................... 58

7.1 Human Utilization and Persecution................................................................................................... 58

7.2 Human Health Risks......................................................................................................................... 59

7.3 Human Aesthetic Relations .............................................................................................................. 59

7.3.1 Pet Trade ................................................................................................................................... 59

7.3.2 Other Uses................................................................................................................................. 60

8.0 Pathway Factors: Pet Trade................................................................................................................. 61

8.1 Volume of International Trade .......................................................................................................... 61

8.2 Volume of Domestic Trade ............................................................................................................... 61

9.0 Entry Potential (Survival in Transit – Meant Mainly for Unintentional Transport).................................. 61

10.0 Colonization Potential ........................................................................................................................ 62

10.1 Likelihood of Escape/Release ........................................................................................................ 62

10.2 Climate Match: Areas of the United States at Greatest Risk .......................................................... 62

10.3 Prey Availability .............................................................................................................................. 63

10.4 Predation Risk ................................................................................................................................ 64

10.5 Reproduction Requirements........................................................................................................... 64

10.6 Hibernation Requirements.............................................................................................................. 64

10.7 Tools for Eradicating a Colonization ............................................................................................... 64

11.0 Spread Potential................................................................................................................................. 65

11.1 Dispersal Ability .............................................................................................................................. 65

11.2 Ability to Develop Races or Strains ................................................................................................ 65

11.3 Tools for Managing Spread ............................................................................................................ 65

12.0 Economic Impact Potential................................................................................................................. 65
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12.1 Pathogen Vector............................................................................................................................. 66

12.2 Predator on Livestock..................................................................................................................... 66

12.3 Predator on Pets............................................................................................................................. 66

12.4 Electrical Power Systems ............................................................................................................... 66

12.5 Traffic Accidents/Human Attacks.................................................................................................... 67

12.6 Tourism .......................................................................................................................................... 67

12.7 Impacts on Commerically Important Wildlife................................................................................... 67

12.7.1 Species Hunted with Gun or Binoculars................................................................................... 67

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture........................................................ 68

13.0 Environmental Impact Potential.......................................................................................................... 68

13.1 Species of Special Concern as Prey or Competitors ...................................................................... 68

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) .................. 73

13.3 Vector for Disease Spread to or Within Native Faunas................................................................... 73

13.4 Species that Might be Adversely Impacted by Control Measures................................................... 73

14.0 Perceived Impact (Social and Political Influences)............................................................................. 73

14.1 Pet Industry .................................................................................................................................... 73

14.1.1 Domestic Breeders Affected? .................................................................................................. 73

14.1.2 International Trade/Alternate Pets ........................................................................................... 74

Chapter Five–The Reticulated Python, Broghammerus reticulatus or Python reticulatus................ 75

1.0 Introduction .......................................................................................................................................... 75

1.1 Species............................................................................................................................................. 75

1.2 Common Names............................................................................................................................... 76

1.3 Evolutionary Context ........................................................................................................................ 76

2.0 Description ........................................................................................................................................... 77

2.1 Unique Morphological Features........................................................................................................ 77

2.2 Size .................................................................................................................................................. 78

2.3 Sexual Size Dimorphism .................................................................................................................. 79

2.4 External Sexual Differentiation ......................................................................................................... 79

3.0 Distribution in Space and Time ............................................................................................................ 79

3.1 Native Range.................................................................................................................................... 79

3.2 Habitat Range................................................................................................................................... 82

3.3 Climate Range.................................................................................................................................. 83

3.4 Microhabitat...................................................................................................................................... 84

3.5 Introduced Ranges ........................................................................................................................... 84

3.6 Seasonal Activity .............................................................................................................................. 85

3.7 Daily Activity ..................................................................................................................................... 86

3.8 Foraging Mode/Sensory Modalities .................................................................................................. 86

4.0 Life History ........................................................................................................................................... 86

4.1 Reproductive Mode .......................................................................................................................... 86

4.2 Reproductive Phenology .................................................................................................................. 87

4.3 Reproductive Effort/Fecundity .......................................................................................................... 89

4.3.1 Clutch or Litter Size/Frequency.................................................................................................. 89

4.3.2 Egg Size .................................................................................................................................... 89

4.4 Growth.............................................................................................................................................. 89

4.5 Maturation ........................................................................................................................................ 90

4.6 Longevity .......................................................................................................................................... 90

5.0 Diet/Trophic Role ................................................................................................................................. 90
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5.1 Prey .................................................................................................................................................. 90

5.2 Predators.......................................................................................................................................... 91

5.3 Diseases and Parasites.................................................................................................................... 91

6.0 Demography......................................................................................................................................... 92

6.1 Population Density............................................................................................................................ 92

6.2 Size Distribution................................................................................................................................ 92

7.0 Interactions with Humans..................................................................................................................... 92

7.1 Human Utilization and Persecution................................................................................................... 92

7.2 Human Health Risks......................................................................................................................... 92

7.3 Human Aesthetic Relations .............................................................................................................. 94

7.3.1 Pet Trade ................................................................................................................................... 94

7.3.2 Other Uses................................................................................................................................. 94

8.0 Pathway Factors: Pet Trade................................................................................................................. 95

8.1 Volume of International Trade .......................................................................................................... 95

8.2 Volume of Domestic Trade ............................................................................................................... 95

9.0 Entry Potential (Survival in Transit - Meant Mainly for Unintentional Transport) .................................. 95

10.0 Colonization Potential ........................................................................................................................ 95

10.1 Likelihood of Escape/Release ........................................................................................................ 95

10.2 Climate Match: Areas of the Country at Greatest Risk ................................................................... 96

10.3 Prey Availability .............................................................................................................................. 98

10.4 Predation Risk ................................................................................................................................ 98

10.5 Reproduction Requirements........................................................................................................... 99

10.6 Hibernation Requirements.............................................................................................................. 99

10.7 Tools for Eradicating a Colonization ............................................................................................... 99

11.0 Spread Potential................................................................................................................................. 99

11.1 Dispersal Ability .............................................................................................................................. 99

11.2 Ability to Develop Races or Strains ................................................................................................ 99

11.3 Tools for Managing Spread ............................................................................................................ 99

12.0 Economic Impact Potential............................................................................................................... 100

12.1 Pathogen Vector........................................................................................................................... 100

12.2 Predator on Livestock................................................................................................................... 100

12.3 Predator on Pets........................................................................................................................... 100

12.4 Electrical Power Systems ............................................................................................................. 100

12.5 Traffic Accidents/Human Attacks.................................................................................................. 100

12.6 Tourism ........................................................................................................................................ 101

12.7 Impacts on Commercially Important Wildlife................................................................................. 101

12.7.1 Species Hunted with Gun or Binoculars................................................................................. 101

12.7.2 Species That Impact Forestry, Agriculture, or Horticulture..................................................... 101

13.0 Environmental Impact Potential........................................................................................................ 102

13.1 Species of Special Concern as Prey or Competitors .................................................................... 102

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) ................ 102

13.3 Vector for Disease Spread to or Within Native Faunas................................................................. 102

13.4 Species That Might be Adversely Impacted by Control Measures................................................ 102

14.0 Perceived Impact (Social and Political Influences)........................................................................... 102

14.1 Pet Industry .................................................................................................................................. 102

14.1.1 Domestic Production Affected?.............................................................................................. 102

14.1.2 International Trade/Alternate Pets ......................................................................................... 103
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Chapter Six–The Northern African Python, Python sebae, and the Southern African Python, 

Python natalensis .................................................................................................................................. 104

1.0 Introduction ........................................................................................................................................ 104

1.1 Species........................................................................................................................................... 104

1.2 Common Names............................................................................................................................. 104

1.3 Evolutionary Context ...................................................................................................................... 105

2.0 Description ......................................................................................................................................... 106

2.1 Unique Morphological Features...................................................................................................... 106

2.2 Size ................................................................................................................................................ 107

2.3 Sexual Size Dimorphism ................................................................................................................ 108

2.4 External Sexual Differentiation ....................................................................................................... 109

3.0 Distribution in Space and Time .......................................................................................................... 109

3.1 Native Range.................................................................................................................................. 109

3.1.1 Native Range of Python sebae ................................................................................................ 111

3.1.2 Native Range of Python natalensis .......................................................................................... 115

3.2 Habitat Range................................................................................................................................. 116

3.3 Climate Range................................................................................................................................ 117

3.3.1 Climate Range of Northern African Pythons ............................................................................ 117

3.3.2 Climate Range of Southern African Pythons............................................................................ 118

3.4 Microhabitat.................................................................................................................................... 119

3.5 Introduced Ranges ......................................................................................................................... 120

3.6 Seasonal Activity ............................................................................................................................ 121

3.7 Daily Activity ................................................................................................................................... 121

3.8 Foraging Mode/Sensory Modalities ................................................................................................ 122

4.0 Life History ......................................................................................................................................... 122

4.1 Reproductive Mode ........................................................................................................................ 122

4.2 Reproductive Phenology ................................................................................................................ 122

4.3 Reproductive Effort/Fecundity ........................................................................................................ 123

4.3.1 Clutch or Litter Size/Frequency................................................................................................ 123

4.4 Growth............................................................................................................................................ 124

4.5 Maturation ...................................................................................................................................... 126

4.6 Longevity ........................................................................................................................................ 127

5.0 Diet/Trophic Role ............................................................................................................................... 127

5.1 Prey ................................................................................................................................................ 127

5.2 Predators........................................................................................................................................ 128

5.3 Diseases and Parasites.................................................................................................................. 128

6.0 Demography....................................................................................................................................... 129

6.1 Population Density.......................................................................................................................... 129

6.2 Size Distribution.............................................................................................................................. 129

7.0 Interactions with Humans................................................................................................................... 130

7.1 Human Utilization and Persecution................................................................................................. 130

7.2 Human Health Risks....................................................................................................................... 131

7.3 Human Aesthetic Relations ............................................................................................................ 131

7.3.1 Pet Trade ................................................................................................................................. 131

7.3.2 Other Uses............................................................................................................................... 131

8.0 Pathway Factors: Pet Trade............................................................................................................... 132
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8.1 Volume of International Trade ........................................................................................................ 132

8.2 Volume of Domestic Trade ............................................................................................................. 133

9.0 Entry Potential (Survival in Transit – Meant Mainly for Unintentional Transport)................................ 133

10.0 Colonization Potential ...................................................................................................................... 133

10.1 Likelihood of Escape/Release ...................................................................................................... 133

10.2 Climate Match: Areas of the Country at Greatest Risk ................................................................. 133

10.3 Prey Availability ............................................................................................................................ 135

10.4 Predation Risk .............................................................................................................................. 136

10.5 Reproduction Requirements......................................................................................................... 136

10.6 Hibernation Requirements............................................................................................................ 136

10.7 Tools for Eradicating a Colonization ............................................................................................. 136

11.0 Spread Potential............................................................................................................................... 137

11.1 Dispersal Ability ............................................................................................................................ 137

11.2 Ability to Develop Races or Strains .............................................................................................. 137

11.3 Tools for Managing Spread .......................................................................................................... 137

12.0 Economic Impact Potential............................................................................................................... 137

12.1 Pathogen Vector........................................................................................................................... 138

12.2 Predator on Livestock................................................................................................................... 138

12.3 Predator on Pets........................................................................................................................... 138

12.4 Electrical Power Systems ............................................................................................................. 138

12.5 Traffic Accidents/Human Attacks.................................................................................................. 138

12.6 Tourism ........................................................................................................................................ 139

12.7 Impacts on Commercially Important Wildlife................................................................................. 139

12.7.1 Species Hunted with Gun or Binoculars................................................................................. 139

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture...................................................... 139

13.0 Environmental Impact Potential........................................................................................................ 140

13.1 Species of Special Concern as Prey or Competitors .................................................................... 140

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) ................ 140

13.3 Vector for Disease Spread to or Within Native Faunas................................................................. 140

13.4 Species that Might be Adversely Affected by Control Measures .................................................. 140

14.0 Perceived Impact (Social and Political Influences)........................................................................... 141

14.1 Pet Industry .................................................................................................................................. 141

14.1.1 Domestic Production Affected?.............................................................................................. 141

14.1.2 International Trade/Alternate Pets ......................................................................................... 141

Chapter Seven–The Boa Constrictor, Boa constrictor ....................................................................... 142

1.0 Introduction ........................................................................................................................................ 142

1.1 Species........................................................................................................................................... 142

1.2 Common Names............................................................................................................................. 145

1.3 Evolutionary Context ...................................................................................................................... 145

2.0 Description ......................................................................................................................................... 146

2.1 Unique Morphological Features...................................................................................................... 146

2.1.1 Color and Pattern..................................................................................................................... 146

2.1.2 Scalation and Meristics ............................................................................................................ 148

2.2 Size ................................................................................................................................................ 148

2.3 Sexual Size Dimorphism ................................................................................................................ 150

2.4 External Sexual Differentiation ....................................................................................................... 150

3.0 Distribution in Space and Time .......................................................................................................... 151
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3.1 Native Range.................................................................................................................................. 151

3.2 Habitat Range................................................................................................................................. 155

3.3 Climate Range................................................................................................................................ 156

3.4 Microhabitat.................................................................................................................................... 157

3.5 Introduced Ranges ......................................................................................................................... 158

3.6 Seasonal Activity ............................................................................................................................ 159

3.7 Daily Activity ................................................................................................................................... 161

3.8 Foraging Mode/Sensory Modalities ................................................................................................ 161

4.0 Life History ......................................................................................................................................... 162

4.1 Reproductive Mode ........................................................................................................................ 162

4.2 Reproductive Phenology ................................................................................................................ 162

4.2.1 Seasonality .............................................................................................................................. 162

4.2.2 Reproductive Behaviors........................................................................................................... 165

4.2.3 Vitellogenesis and Ovulation.................................................................................................... 165

4.3 Reproductive Effort/Fecundity ........................................................................................................ 166

4.3.1 Clutch or Litter Size/Frequency................................................................................................ 166

4.4 Growth............................................................................................................................................ 167

4.5 Maturation ...................................................................................................................................... 167

4.6 Longevity ........................................................................................................................................ 167

5.0 Diet/Trophic Role ............................................................................................................................... 168

5.1 Prey ................................................................................................................................................ 168

5.1.1 Mammals ................................................................................................................................. 168

5.1.2 Birds......................................................................................................................................... 170

5.1.3 Reptiles.................................................................................................................................... 170

5.2 Predators........................................................................................................................................ 170

5.3 Diseases and Parasites.................................................................................................................. 171

6.0 Demography....................................................................................................................................... 171

6.1 Population Density.......................................................................................................................... 171

6.1.1 Spacing and Aggregation......................................................................................................... 172

6.2 Size Distribution.............................................................................................................................. 172

7.0 Interactions with Humans................................................................................................................... 173

7.1 Human Utilization and Persecution................................................................................................. 173

7.2 Human Health Risks....................................................................................................................... 173

7.3 Human Aesthetic Relations ............................................................................................................ 174

7.3.1 Pet Trade ................................................................................................................................. 174

7.3.2 Other Uses............................................................................................................................... 175

8.0 Pathway Factors: Pet Trade............................................................................................................... 176

8.1 Volume of International Trade ........................................................................................................ 176

8.2 Volume of Domestic Trade ............................................................................................................. 176

9.0 Entry Potential (Survival in Transit – Meant Mainly for Unintentional Transport)................................ 176

10.0 Colonization Potential ...................................................................................................................... 176

10.1 Likelihood of Escape/Release ...................................................................................................... 176

10.2 Climate Match: Areas of the United States at Greatest Risk ........................................................ 177

10.3 Prey Availability ............................................................................................................................ 179

10.4 Predation Risk .............................................................................................................................. 180

10.5 Reproduction Requirements......................................................................................................... 180

10.6 Hibernation Requirements............................................................................................................ 180
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10.7 Tools for Eradicating a Colonization ............................................................................................. 180

11.0 Spread Potential............................................................................................................................... 181

11.1 Dispersal Ability ............................................................................................................................ 181

11.2 Ability to Develop Races or Strains .............................................................................................. 181

11.3 Tools for Managing Spread .......................................................................................................... 181

12.0 Economic Impact Potential............................................................................................................... 182

12.1 Pathogen Vector........................................................................................................................... 182

12.2 Predator on Livestock................................................................................................................... 182

12.3 Predator on Pets........................................................................................................................... 183

12.4 Electrical Power Systems ............................................................................................................. 183

12.5 Traffic Accidents/Human Attacks.................................................................................................. 183

12.6 Tourism ........................................................................................................................................ 183

12.7 Impacts on Commercially Important Wildlife................................................................................. 184

12.7.1 Species Hunted with Gun or Binoculars................................................................................. 184

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture...................................................... 184

13.0 Environmental Impact Potential........................................................................................................ 184

13.1 Species of Special Concern as Prey or Competitors .................................................................... 184

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) ................ 185

13.3 Vector for Disease Spread to or Within Native Faunas................................................................. 185

13.4 Species that Might be Adversely Affected by Control Measures .................................................. 185

14.0 Perceived Impact (Social and Political Influences)........................................................................... 186

14.1 Pet Industry .................................................................................................................................. 186

14.1.1 Domestic Production Affected?.............................................................................................. 186

14.1.2 International Trade/Alternate Pets ......................................................................................... 186

Chapter Eight–The Yellow Anaconda, Eunectes notaeus, and DeSchauensee’s Anaconda, 

Eunectes deschauenseei ...................................................................................................................... 187

1.0 Introduction ........................................................................................................................................ 187

1.1 Species........................................................................................................................................... 187

1.2 Common Names............................................................................................................................. 188

1.3 Evolutionary Context ...................................................................................................................... 188

2.0 Description ......................................................................................................................................... 189

2.1 Unique Morphological Features...................................................................................................... 189

2.2 Size ................................................................................................................................................ 190

2.3 Sexual Size Dimorphism ................................................................................................................ 191

2.4 External Sexual Differentiation ....................................................................................................... 191

3.0 Distribution in Space and Time .......................................................................................................... 192

3.1 Native Range.................................................................................................................................. 192

3.1.1 Yellow Anaconda, Eunectes notaeus....................................................................................... 192

3.1.2 DeSchauensee’s Anaconda, Eunectes deschauenseei........................................................... 194

3.2 Habitat Range................................................................................................................................. 194

3.3 Climate Range................................................................................................................................ 195

3.3.1 Climate Range of Yellow Anacondas....................................................................................... 195

3.3.2 Climate Range of DeSchauensee’s Anaconda ........................................................................ 195

3.4 Microhabitat.................................................................................................................................... 196

3.5 Introduced Ranges ......................................................................................................................... 197

3.6 Seasonal Activity ............................................................................................................................ 197

3.7 Daily Activity ................................................................................................................................... 197
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3.8 Foraging Mode/Sensory Modalities ................................................................................................ 198

4.0 Life History ......................................................................................................................................... 198

4.1 Reproductive Mode ........................................................................................................................ 198

4.2 Reproductive Phenology ................................................................................................................ 198

4.3 Reproductive Effort/Fecundity ........................................................................................................ 199

4.3.1 Clutch or Litter Size/Frequency................................................................................................ 199

4.4 Growth............................................................................................................................................ 200

4.5 Maturation ...................................................................................................................................... 200

4.6 Longevity ........................................................................................................................................ 201

5.0 Diet/Trophic Role ............................................................................................................................... 201

5.1 Prey ................................................................................................................................................ 201

5.2 Predators........................................................................................................................................ 202

5.3 Diseases and Parasites.................................................................................................................. 202

6.0 Demography....................................................................................................................................... 202

6.1 Population Density.......................................................................................................................... 202

6.2 Size Distribution.............................................................................................................................. 203

7.0 Interactions with Humans................................................................................................................... 203

7.1 Human Utilization and Persecution................................................................................................. 203

7.2 Human Health Risks....................................................................................................................... 204

7.3 Human Aesthetic Relations ............................................................................................................ 204

7.3.1 Pet Trade ................................................................................................................................. 204

7.3.2 Other Uses............................................................................................................................... 205

8.0 Pathway Factors: Pet Trade............................................................................................................... 205

8.1 Volume of International Trade ........................................................................................................ 205

8.2 Volume of Domestic Trade ............................................................................................................. 206

9.0 Entry Potential (Survival in Transit—Meant Mainly for Unintentional Transport) ................................ 206

10.0 Colonization Potential ...................................................................................................................... 206

10.1 Likelihood of Escape/Release ...................................................................................................... 206

10.2 Climate Match: Areas of the Country at Greatest Risk ................................................................. 207

10.3 Prey Availability ............................................................................................................................ 209

10.4 Predation Risk .............................................................................................................................. 210

10.5 Reproduction Requirements......................................................................................................... 210

10.6 Hibernation Requirements............................................................................................................ 210

10.7 Tools for Eradicating a Colonization ............................................................................................. 210

11.0 Spread Potential............................................................................................................................... 211

11.1 Dispersal Ability ............................................................................................................................ 211

11.2 Ability to Develop Races or Strains .............................................................................................. 211

11.3 Tools for Managing Spread .......................................................................................................... 211

12.0 Economic Impact Potential............................................................................................................... 211

12.1 Pathogen Vector........................................................................................................................... 212

12.2 Predator on Livestock................................................................................................................... 212

12.3 Predator on Pets........................................................................................................................... 212

12.4 Electrical Power Systems ............................................................................................................. 212

12.5 Traffic Accidents/Human Attacks.................................................................................................. 212

12.6 Tourism ........................................................................................................................................ 213

12.7 Impacts on Commercially Important Wildlife................................................................................. 213

12.7.1 Species Hunted with Gun or Binoculars................................................................................. 213
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12.7.2 Species that Impact Forestry, Agriculture, or Horticulture...................................................... 213

13.0 Environmental Impact Potential........................................................................................................ 213

13.1 Species of Special Concern as Prey or Competitors .................................................................... 213

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) ................ 214

13.3 Vector for Disease Spread to or Within Native Faunas................................................................. 214

13.4 Species that Might be Adversely Affected by Control Measures .................................................. 214

14.0 Perceived Impact (Social and Political Influences)........................................................................... 215

14.1 Pet Industry .................................................................................................................................. 215

14.1.1 Domestic Production Affected?.............................................................................................. 215

14.1.2 International Trade/Alternate Pets ......................................................................................... 215

Chapter Nine–The Green Anaconda, Eunectes murinus, and Beni Anaconda, Eunectes

beniensis ................................................................................................................................................ 216

1.0 Introduction ........................................................................................................................................ 216

1.1 Species........................................................................................................................................... 216

1.2 Common Names............................................................................................................................. 217

1.3 Evolutionary Context ...................................................................................................................... 217

2.0 Description ......................................................................................................................................... 218

2.1 Unique Morphological Features...................................................................................................... 218

2.2 Size ................................................................................................................................................ 219

2.3 Sexual Size Dimorphism ................................................................................................................ 220

2.4 External Sexual Differentiation ....................................................................................................... 221

3.0 Distribution in Space and Time .......................................................................................................... 221

3.1 Native Range.................................................................................................................................. 221

3.2 Habitat Range................................................................................................................................. 224

3.3 Climate Range................................................................................................................................ 224

3.4 Microhabitat.................................................................................................................................... 225

3.5 Introduced Ranges ......................................................................................................................... 225

3.6 Seasonal Activity ............................................................................................................................ 225

3.7 Daily Activity ................................................................................................................................... 226

3.8 Foraging Mode/Sensory Modalities ................................................................................................ 226

4.0 Life History ......................................................................................................................................... 226

4.1 Reproductive Mode ........................................................................................................................ 226

4.2 Reproductive Phenology ................................................................................................................ 227

4.2.1 Seasonality .............................................................................................................................. 227

4.2.2 Reproductive Behaviors........................................................................................................... 227

4.2.3 Vitellogenesis and Ovulation.................................................................................................... 228

4.2.4 Parturition ................................................................................................................................ 228

4.3 Reproductive Effort/Fecundity ........................................................................................................ 228

4.3.1 Clutch or Litter Size/Frequency................................................................................................ 228

4.4 Growth............................................................................................................................................ 229

4.5 Maturation ...................................................................................................................................... 230

4.6 Longevity ........................................................................................................................................ 230

5.0 Diet/Trophic Role ............................................................................................................................... 230

5.1 Prey ................................................................................................................................................ 230

5.2 Predators........................................................................................................................................ 231

5.3 Diseases and Parasites.................................................................................................................. 232

6.0 Demography....................................................................................................................................... 232
xv

6.1 Population Density.......................................................................................................................... 232

6.2 Size Distribution.............................................................................................................................. 233

7.0 Interactions with Humans................................................................................................................... 233

7.1 Human Utilization and Persecution................................................................................................. 233

7.2 Human Health Risks....................................................................................................................... 234

7.3 Human Aesthetic Relations ............................................................................................................ 234

7.3.1 Pet Trade ................................................................................................................................. 234

7.3.2 Other Uses............................................................................................................................... 235

8.0 Pathway Factors: Pet Trade............................................................................................................... 236

8.1 Volume of International Trade ........................................................................................................ 236

8.2 Volume of Domestic Trade ............................................................................................................. 236

9.0 Entry Potential (Survival in Transit—Meant Mainly for Unintentional Transport) ................................ 236

10.0 Colonization Potential ...................................................................................................................... 237

10.1 Likelihood of Escape/Release ...................................................................................................... 237

10.2 Climate Match: Areas of the Country at Greatest Risk ................................................................. 237

10.3 Prey Availability ............................................................................................................................ 238

10.4 Predation Risk .............................................................................................................................. 239

10.5 Reproduction Requirements......................................................................................................... 239

10.6 Hibernation Requirements............................................................................................................ 239

10.7 Tools for Eradicating a Colonization ............................................................................................. 239

11.0 Spread Potential............................................................................................................................... 240

11.1 Dispersal Ability ............................................................................................................................ 240

11.2 Ability to Develop Races or Strains .............................................................................................. 240

11.3 Tools for Managing Spread .......................................................................................................... 240

12.0 Economic Impact Potential............................................................................................................... 240

12.1 Pathogen Vector........................................................................................................................... 241

12.2 Predator on Livestock................................................................................................................... 241

12.3 Predator on Pets........................................................................................................................... 241

12.4 Electrical Power Systems ............................................................................................................. 241

12.5 Traffic Accidents/Human Attacks.................................................................................................. 241

12.6 Tourism ........................................................................................................................................ 242

12.7 Impacts on Commercially Important Wildlife................................................................................. 242

12.7.1 Species Hunted with Gun or Binoculars................................................................................. 242

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture...................................................... 242

13.0 Environmental Impact Potential........................................................................................................ 243

13.1 Species of Special Concern as Prey or Competitors .................................................................... 243

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food Supply) ................ 243

13.3 Vector for Disease Spread to or Within Native Faunas................................................................. 243

13.4 Species that Might be Adversely Affected by Control Measures .................................................. 243

14.0 Perceived Impact (Social and Political Influences)........................................................................... 244

14.1 Pet Industry .................................................................................................................................. 244

14.1.1 Domestic Production Affected?.............................................................................................. 244

14.1.2 International Trade/Alternate Pets ......................................................................................... 244

Chapter Ten–The Risk Assessment ..................................................................................................... 245

Introduction .............................................................................................................................................. 245

Risk of Establishment............................................................................................................................... 245

1.Organism with Pathway..................................................................................................................... 245
xvi

2.Entry Potential ................................................................................................................................... 246

3.Colonization Potential........................................................................................................................ 246

4.Spread Potential................................................................................................................................ 250

Probability of Organism Establishment .................................................................................................... 253

Consequence of Establishment................................................................................................................ 254

5.Economic Impact Potential ................................................................................................................ 254

6.Environmental Impact Potential......................................................................................................... 255

7.Perceived Impact (Social and Political Influences) ............................................................................ 257

Organism Risk Potential........................................................................................................................... 259

Acknowledgments ................................................................................................................................. 261

References Cited.................................................................................................................................... 262

Appendix. Trade Volume of Giant Constrictor Snakes Imported to the United States ................... 300

Figures

Figure 2.1. Schematic of climate space diagrams, showing representative American cities

near the reference line intersections .......................................................................................................... 13

Figure 4.1. Schematic representation of the head of P. molurus............................................................... 45

Figure 4.2. Native range of P. molurus, as deduced from the sources cited in text................................... 48

Figure 4.3. Climate space for P. molurus, with reference lines and localities as introduced 

in Fig. 2.1 ................................................................................................................................................... 51

Figure 4.4. CITES records for importation to the United States of live P. molurus .................................... 60

Figure 4.5. Areas of the United States matching the climate envelope expressed by 

P. molurus in its native range (Fig. 4.3) under two hypotheses of hibernation duration ............................. 63

Figure 5.1. Schematic illustration of the head of a Reticulated Python...................................................... 77

Figure 5.2. Native range of B. reticulatus .................................................................................................. 80

Figure 5.3. Climate space for B. reticulatus, with reference lines and localities as introduced 

in Fig. 2.1 ................................................................................................................................................... 84

Figure 5.4. CITES records for importation to the United States of live B. reticulatus................................. 94

Figure 5.5. Areas of the mainland United States matching the climate envelope expressed 

by B. reticulatus using only those portions of the range that are deemed occupied by all 

observers ................................................................................................................................................... 97

Figure 5.6. Areas of the mainland United States matching the climate envelope expressed 

by B. reticulatus using all portions of the native range deemed occupied by any of the 

observers cited in section 3.1..................................................................................................................... 98

Figure 6.1. Schematic representation of the head of P. sebae................................................................ 106

Figure 6.2. Native range of the Northern African Python, Python sebae ................................................. 110

Figure 6.3. Native range of the Southern African Python, Python natalensis, estimated using 

Broadley (1984) and additional sources................................................................................................... 111

Figure 6.4. Climate space for P. sebae, with reference lines and localities as introduced 

in Fig. 2.1 ................................................................................................................................................. 118

Figure 6.5. Climate space for P. natalensis, with reference lines and localities as introduced 

in Fig. 2.1 ................................................................................................................................................. 119

Figure 6.6. Relationship between dam body size and clutch size in 8 captive Northern 

African Pythons in Cameroon................................................................................................................... 124

Figure 6.7. Growth of a captive juvenile P. sebae measured at monthly intervals................................... 125
xvii

Figure 6.8. Relationship between snout-vent length and mass for a sample of P. natalensis

from South Africa near the southern limits to the species’ range ............................................................. 126

Figure 6.9. Annual imports of giant African pythons, Python sebae/P. natalensis (not 

distinguished in trade) to the United States during the period 1977-2007................................................ 132

Figure 6.10. Areas of the United States matching the climate envelope expressed by P. sebae

in its native range (Fig. 6.5)...................................................................................................................... 134

Figure 6.11. Areas of the United States matching the climate envelope expressed by 

P. natalensis in its native range................................................................................................................ 135

Figure 7.1. Schematic representation of the head of B. constrictor......................................................... 146

Figure 7.2. Estimated native range of the Boa Constrictor (Boa constrictor), with approximate 

Mexican range in inset box ...................................................................................................................... 152

Figure 7.4. Annual imports of Boa Constrictors, Boa constrictor, to the United States during 

the period 1977-2007 ............................................................................................................................... 174

Figure 7.5. Areas of the United States matching the climate envelope expressed by 

B. constrictor in its native range ............................................................................................................... 178

Figure 7.6. Areas of the United States matching the climate envelope expressed by 

B. constrictor in its native range excluding records for the Argentine Boa (B. c. occidentalis) ................. 179

Figure 8.1. Schematic representation of the head of E. notaeus............................................................. 189

Figure 8.2. Estimated native range of the Yellow Anaconda (Eunectes notaeus, shown in 

blue) and DeSchauensee’s Anaconda (Eunectes deschauenseei, shown in yellow)............................... 193

Figure 8.3. Climate space for E. notaeus, with reference lines and localities as introduced in 

Fig. 2.1 ..................................................................................................................................................... 195

Figure 8.4. Climate space for E. deschaeunseei, with reference lines and localities as 

introduced in Fig. 2.1................................................................................................................................ 197

Figure 8.5. Annual imports of Yellow Anacondas, Eunectes notaeus, to the United States 

during the period 1977-2007 .................................................................................................................... 205

Figure 8.6. Areas of the mainland United States matching the climate envelope expressed 

by E. notaeus in its native range .............................................................................................................. 208

Figure 8.7. Areas of the mainland United States matching the climate envelope expressed 

by E. deschauenseei in its native range................................................................................................... 209

Figure. 9.1. Schematic representation of the head of E. murinus............................................................ 218

Figure 9.2. Estimated native distribution of the Green Anaconda (Eunectes murinus, shown 

in blue) and the Beni Anaconda (Eunectes beniensis, cross-hatched oval)............................................. 222

Figure 9.3. Climate space for E. murinus based on 77 known localities from the native range, 

and for E. beniensis based on three known localities............................................................................... 224

Figure 9.4. Annual imports of Green Anacondas, Eunectes murinus, to the United States 

during the period 1977-2007 .................................................................................................................... 235

Figure 9.5. Areas of the United States matching the climate envelope expressed by 

E. murinus in its native range................................................................................................................... 238

Figure. A.1. Number of Indian Pythons (P. molurus), Reticulated Pythons (B. reticulatus), and 

African Pythons (Northern (P. sebae) and Southern (P. natalensis) not distinguished in trade) 

imported to the United States during the period 1977-2007..................................................................... 300

Figure A.2. Number of Green Anacondas (E. murinus) and Yellow Anacondas (E. notaeus) 

imported to the United States during the period 1977-2007..................................................................... 301

Figure A.3. Number of Boa Constrictor (B. constrictor) imported to the United States during the 

period 1977-2007..................................................................................................................................... 301
xviii

Tables

Table 2.1. Estimated body proportions for the nine giant constrictors considered in this work .................. 11

Table 4.1. Total lengths and masses of typical P. molurus........................................................................ 46

Table 4.2. Imperiled wildlife potentially vulnerable to some ontogenetic stage of P. molurus 

or other giant constrictors in Florida ........................................................................................................... 69

Table 5.1. Total lengths and masses of typical B. reticulatus .................................................................... 78

Table 5.2. Peak oviposition dates of B. reticulatus by latitude ................................................................... 88

Table 6.1. Total lengths and masses of typical P. sebae and P. natalensis............................................. 108

Table 7.1. Subspecies of Boa constrictor as recognized by Peters and Orejas-Miranda (1986), 

with the addition of the disputed B. c. longicauda as proposed by Price and Russo (1991)..................... 144

Table 7.2. Total lengths and masses of typical B. constrictor .................................................................. 149

Table 7.3. Recorded and inferred seasonality of reproductive activity in Boa constrictor from 

various parts of the range ........................................................................................................................ 164

Table 8.1. Total lengths and masses of typical E. notaeus...................................................................... 190

Table 9.1. Total lengths and masses of typical E. murinus...................................................................... 220

Table 10.1. Hypothesized attributes affecting survival of potential invaders during 

establishment (from Rodda and Tyrrell, 2008), as applied to giant constrictors ....................................... 248

Table 10.2. Hypothesized attributes affecting reproduction of potential invaders during 

establishment (from Rodda and Tyrrell, 2008), as applied to giant constrictors ....................................... 250

Table 10.3. Eleven traits not previously mentioned that have been hypothesized to influence 

spread success in reptiles and amphibians.............................................................................................. 252

Table 10.4. Probability of organism establishment for nine giant constrictor species, and 

constituent values .................................................................................................................................... 253

Table 10.5. Consequences of establishment for nine giant constrictor species, with their 

associated confidence ranges.................................................................................................................. 259

Table 10.6. Organism Risk Potential (ORP) algorithm adopted by the Aquatic Nuisance 

Species Task Force (ANSTF, 1996) ........................................................................................................ 260

Table 10.7. Organism Risk Potential (ORP) for nine giant constrictor species ........................................ 260

Table A.1. Numbers of individuals of six species of giant constrictor snakes imported to the 

United States during the period 1977-2007, as reported by CITES ........................................................ 302
1

Chapter One–Introduction and Synopsis of 

Conclusions

In this report, we compile summaries of the biology of nine very large constrictor 

species and consider what effects these species might have on the ecology, economy, and 

domestic tranquility of the United States were such snakes to become established. 

Representatives of each group have been discovered in the wild in Florida, though 

evidence of reproduction is presently available for only three species, the Burmese 

Python (Python molurus bivittatus), the Northern African Python (Python sebae), and the 

Boa Constrictor (Boa constrictor). The occurrence of these three large constrictors in the 

wild in the same area of Florida may be a coincidence, but southern Florida has a climate 

that may be suitable for all of the giant constrictors and much of the commercial trade in 

giant constrictors passes through southern Florida. Thus the probability of exotic species 

establishment is greater in south Florida, though not limited to it. For each of the nine 

giant constrictor species under consideration, we review their climate tolerances and the 

areas of the country at risk (see section 10.2 in each of the species accounts: chapters 

Four–Nine). However, because we have established populations in south Florida, our 

geographic focus will always include Florida.

The selection of giant constrictor species was based on concern over the size of 

the potential invaders combined with their prevalence in international trade. We include 

the four largest species of snake, as well as similar and closely related species, and the 

Boa Constrictor. The four largest snakes are the true “giants,” with maximum lengths 

well exceeding six meters: Northern African Python (Python sebae), Indian Python 

(Python molurus1

In addition to the four true giants and the Boa Constrictor, we included four 

species that are so similar in appearance to one of the giants that they might be confused 

with them, and which are in some cases not distinguished in the international live animal 

trade. For example, the Beni Anaconda (Eunectes beniensis) is a recently described and 

poorly known anaconda closely related to the giant Green Anaconda. Because these 

species are visually similar and the range of the Beni Anaconda is within the range of the 

Green Anaconda, we review the former species in the same chapter as the latter. 

Similarly, there are now two species in the yellow anaconda group: the Yellow Anaconda 

), Reticulated Python (Broghammerus reticulatus), and Green 

Anaconda (Eunectes murinus). Although Morelia amethestina, the Scrub Python, and 

some Austropapuan relatives, which we did not include, often exceed the length of our 

shortest species, the Boa Constrictor, we selected the species not only for their size, but 

also for the likelihood of establishment. The boa is very large, much more commonly 

owned, and is already established in Florida; thus it is a species of special concern. The 

Scrub Python is less frequently encountered in the trade, and is not yet known to be a 

problem in the United States.

 1 We treat the Burmese Python as a subspecies (Python molurus bivittatus) of the Indian Python (Python 

molurus). See chapter Four for taxonomic details.
2

(Eunectes notaeus), which has appeared in the wilds of Florida, and a rare and similar 

closely related species, DeSchauensee’s Anaconda (Eunectes deschauenseei). Finally, 

the Southern African Python, Python natalensis, is the sister species of the Northern 

African Python, but the two are not distinguished in international trade. These two 

species are easily distinguished by technical characters such as head scalation, but many 

people do not recognize the distinction. Thus we treat all nine of these large constrictors 

in this risk assessment; the Boa Constrictor, the four true giants, and their four lookalikes.

Although the size threshold on our characterization of a snake as a “giant” was 

arbitrary, the heightened focus on large constrictors has two firm foundations. First, only 

the largest nonvenomous snakes constitute a safety hazard for humans. Citizens have a 

legitimate expectation that their government will be protective of their personal safety. 

Second, there are no native American snakes of comparable size; thus establishment of a 

giant exotic snake would force native wildlife to survive in the presence of a novel-size 

snake predator. The history of extinctions is rife with examples of the ecological damage 

caused by the introduction of novel predators. Blackburn and others (2004) found that 

the only strong explicator of extinctions of island birds was the number of introduced 

predators. Novelty is less readily quantified, but most ecologists are convinced that 

introduced predators do significantly more damage if their predatory method includes a 

novel tactic for which the prey species has not evolved a defense (Rodda and others, 

1997). A classic example is the doomed birds of Guam, which were unfamiliar with 

snakes and had not evolved any defense against a nocturnal arboreal snake predator such 

as the Brown Treesnake (Boiga irregularis, Savidge, 1987). The issue is not limited to 

islands, however; Allen (2006) found that novelty of body size was the best statistical 

predictor of which bird species have successfully invaded southern Florida.

Although giant constrictors constitute an exceptional threat to the integrity of 

native ecosystems, they also constitute an important element of the large and growing 

trade in living reptiles. We can testify to these snakes’ attraction personally, as we both 

have kept pet giant constrictors. We can attest to these snakes’ beauty, companionability, 

and educational value. The love of nature is often originally fostered in one’s own arms, 

where close contact with living things engenders a connection not otherwise possible. 

And size does impress. Thus the social value of protecting native ecosystems must be 

weighed against the social value of fostering positive attitudes about the protection of 

nature through giant constrictor ownership. Economic benefits are also associated with 

importation and trade in giant constrictors; we tabulate importation, but do not quantify 

the economic benefits of the trade. A risk assessment quantifies as much as possible the 

ecological factors associated with the introduction of a species; decision-makers must 

account for the societal values from all viewpoints of any potential regulatory action. We 

will not do so in this risk assessment.

This document addresses primarily the biological impacts associated with 

potential colonization of the United States by any of the nine giant constrictors, and it 

tabulates the biological information germane to potential economic and social impacts. 

What it does not do is consider or assess the diverse regulatory actions that might be 

taken to mitigate or prevent colonization by these animals. Regulatory action could occur 

at the Federal level (for example, prohibit importation or interstate transfer of reptiles), or 

at the State level (for example, statutory or regulated restrictions on the behavior of 
3

private owners). Each of these steps would have specific effects on the potential for 

establishment in the wild and on the operation of the pet trade, and the merits of each 

possibility could be weighed in a regulatory evaluation. However, such an evaluation is 

beyond the scope of this risk assessment. We contrast only the projected biological (and 

related) impacts of establishment against the conditions that would prevail in the absence 

of colonization by these species.

Risk assessment, by its very nature, entails uncertainty. “A risk assessment model 

cannot absolutely determine whether or not an introduced exotic species will establish 

and if it does what impact it will have” (Aquatic Nuisance Species Task Force, 1996; 

Bomford, 2008). We have tried to draw attention to the greatest sources of uncertainty, 

but all elements of a risk assessment embody some uncertainty. The ANSTF (Aquatic 

Nuisance Species Task Force, 1996) protocol that we follow in this document highlighted 

three primary sources of uncertainty:

1. uncertainty of the process–(methodology)

2. uncertainty of the assessor–(human error)

3. uncertainty about the organism–(biological and environmental unknowns).

As is explained more fully in the following chapter on methods, the biological and 

environmental unknowns associated with giant constrictors are numerous and profound. 

However, the problem is not unique to giant constrictors. The ANSTF (1996) pointed 

out, “...it is the biological uncertainty more than anything else that initiated the need for 

developing a nonindigenous risk process. Common sense dictates that the caliber of a 

risk assessment is related to the quality of data available about the organism and the 

ecosystem that will be invaded. Those organisms for which copious amounts of high 

quality research have been conducted are the most easily assessed.” The basic natural 

history of the giant constrictors is largely unknown; our risk assessment reflects this 

uncertainty.

In the next chapter, we explain the methods used for the risk assessment. The 

subsequent chapter (Three) summarizes what is known about snake eradication. Snake 

eradication has not been attempted with giant constrictors to the best of our knowledge. 

Thus our review considers the available data for other species, based primarily on 

experience with the Brown Treesnake in Guam and the Habu in Japan (Rodda and others, 

1999d). Eradication methods are an important consideration for a risk assessment, as the 

risks of establishment are less worrisome if impacts can be easily eliminated or prevented 

by eradicating the invader. The tool-by-tool review in chapter Three provides the 

background material needed to put the risks in a realistic management context.

The species accounts (chapters Four–Nine) include both a summary of each 

species’ relevant biology and a summary of the relevant risks and management factors 

that apply to each species uniquely. Management attributes that are the same for all 

species of giant constrictor are described in chapter Three.

The formal risk assessment is the last chapter (Ten), in which we follow the 

format originated by the Aquatic Nuisance Species Task Force (ANSTF, 1996). 

Reflecting a consensus of the field, the task force concluded that the overall probability

of establishment is a product of the risk of the organism being in a certain pathway (such 

as the pet trade) multiplied by the risk of the organism surviving in that pathway 

multiplied by the risk of that organism escaping from the pathway and colonizing the 
4

wild multiplied by the risk of the colony spreading. The task force also concluded that 

the consequences of establishment include the sum of economic, ecological, and 

social/political costs. The species-specific cost and risk components are tallied within 

each species account and brought together in chapter Ten.

A word on terminology—a variety of terms has been used to describe an 

organism that is not native to the place in which it is found: exotic, introduced, invasive, 

nonindigenous, non-native, colonists. In this report we make no distinction among these 

terms. A colonization is the original population of a non-native species, perhaps a single 

female and her offspring. A colonization is usually considered “established” when there 

is physical evidence that the species’ entire life cycle can be completed in its new 

environment. In practical terms this means population recruitment, usually the discovery 

of wild-born hatchlings or neonates. However, most individual snakes go undetected by 

humans, and hatchlings are especially hard to detect. Thus many populations recruit 

successfully prior to the time when establishment has been seen by scientists and 

therefore documented. Though often unobserved until much later in time, 

“establishment” is a milestone of primary importance to the assessment of risk. 

At some point, an expanding population exceeds the size at which it can still be 

considered a colony. A colony generally refers to a population that is compact enough 

that it could be eradicated with decisive and effective control measures. However, the 

areal capabilities of eradication tools are poorly known and improving daily; thus there is 

no preordained acreage beyond which a colonization is too widespread to be eradicated. 

Nonetheless, eradication is the final defense that could completely reverse an invasion, 

and thus expansion of an invading population beyond the eradication size limit is the final 

milestone for invasion risk.

Synopsis of Conclusions

In this risk assessment, we estimate the ecological risks associated with 

colonization of the United States by the world’s four largest snake species: Green 

Anaconda, Indian or Burmese Python, Northern African Python, and Reticulated Python 

(Eunectes murinus, Python molurus, Python sebae, and Broghammerus reticulatus,

respectively), the Boa Constrictor (Boa constrictor), and four look-alikes (Southern 

African Python, Python natalensis; Yellow Anaconda, Eunectes notaeus; 

DeSchauensee’s Anaconda, Eunectes deschauenseei; and Beni Anaconda, Eunectes 

beniensis). At present, the only probable pathway by which these species would become 

established in the United States is the pet trade. Importation for the pet trade entails a 

risk of establishment of these animals as exotic or invasive species, but it hardly 

guarantees that establishment. Federal regulators have the task of appraising the 

importation risks and balancing those risks against economic, social, and ecological 

benefits associated with the importation. This risk assessment quantifies only the 

ecological risks, recognizing that ecosystem processes are complex and only poorly 

understood. We outline the types of economic impacts that may be experienced, but we 

leave quantification of economic costs to economists. There is great uncertainty about all 

aspects of this risk assessment, but we have tried very hard to bring the best available 

information to bear. Scientific sources are referenced in chapters Two–Ten, and omitted 

from this synopsis.
5

One element of uncertainty is the degree to which a colonization of one of these 

snakes could be eliminated if appropriate financial resources were available to support an 

eradication attempt. We have some indication from the management response to the 

establishment in south Florida of the Burmese Python, but we can also draw on additional 

insight from eradication attempts against the Brown Treesnake in Guam, the Habu in 

Japan, the Boa Constrictor on Cozumel Island in Mexico, and other snake management 

efforts worldwide. None of these management programs have progressed to the point of 

attempting eradication on any area nearly the size of the Burmese Python population in 

Florida, and no eradication effort has been successful on a scale larger than a few 

hectares. Chapter Three reviews the management toolbox in some detail—covering 

especially traps, active search, and toxicants—and concludes that future advances in 

technology may greatly expand the scope of snake eradication, but extant technology 

would not appear to be capable of eradicating any of the giant constrictors from south 

Florida unless the colonization were caught very early, perhaps around the present range 

of the Boa Constrictor population now in Miami (several hundred hectares or less). 

There is great uncertainty about this management prognosis because: (1) the availability 

of financial resources is unknown, (2) future improvements in eradication technology are 

uncertain, and (3) the known tools have not been adequately tested against any species of 

giant constrictor.

The giant constrictors differ in a number of ways important to management or 

control efforts. For example, the anacondas are found almost exclusively in water, 

complicating their control in wetlands such as the Everglades, but limiting their spread in 

dry upland areas. At the other extreme is the Boa Constrictor, a species that may use 

water but is found primarily in terrestrial environments, including arboreal habitat. The 

pythons appear to be completely at home in aquatic and terrestrial environments, 

rendering it problematic to curtail their spread.

Another important difference among the giant constrictors is their size and 

temperament. Although the largest individuals of all of the species covered in this work 

are probably capable of killing an adult human, most seem disinclined to do so. 

Unprovoked attacks on humans are largely limited to Reticulated Pythons, and only the 

larger Burmese/Indian and Northern/Southern African pythons have killed adult humans.

One very important difference among the species is the degree to which they 

naturally inhabit areas that freeze. The Indian or Burmese Python is exceptional in its 

ability to tolerate cold weather through hibernation; most of the other species are likely 

limited to areas where below-freezing weather is short-term and can be avoided by 

submergence in shallow burrows or water.

The giant constrictors differ in the degree to which they are presently in the pet 

trade. In the last thirty years, over 1 million of these snakes have been imported into the 

United States, of which about 60 percent were Boa Constrictors (based on CITES 

records: Appendix). Other important imports are Indian Pythons (~300,000), Reticulated 

Pythons (~150,000), Northern/Southern African pythons (~33,000), and Green 

Anacondas (~13,000). The least traded species among those documented is the Yellow 

Anaconda, which has been imported in small numbers (<2000), but this species has still 

turned up at several localities in or near Big Cypress National Preserve, Florida. These 

totals are documented imports, to which must be added an unknown number of imports 

not ascribed to any particular python species, and sales of domestically produced giant 
6

constrictors. Domestic production is undocumented, but believed to be greatest for 

Reticulated and Burmese Pythons. For Burmese Pythons in particular, the domestic 

production is judged to be as large as or larger than importation.

This list of differences among the species is less striking than are the ecological 

commonalities among them. All of the species we consider are evolutionarily closely 

related, in two recognizable clades: boids (the anacondas and boa: live-bearing) and 

pythonids (the pythons: egg laying). They are of similar size, the larger species reaching 

a maximum of nearly 10 m in total length, though few individuals exceed 6 m. Most 

species occur in a very wide range of habitats, though the Reticulated Python is largely 

limited to tropical forest, and two species of anacondas are found in a restricted range of 

tropical wetland habitats. All are capable of climbing trees, especially when young, and 

all will seek out water to avoid harassment or predators. All can be active at night or 

during the day, though they seem to prefer warm weather. All well-studied species are 

known to live within urban areas, at least under some conditions. Most significantly, 

they all hunt by a similar strategy of restful alertness concealed near localities 

(waterholes, rodent burrows) where a prey animal is likely to transit. This sit and wait

predatory style is probably responsible for their ecological success, and the extreme 

difficulty humans have in finding them. Detection probabilities for the Burmese Python 

in the Everglades are on the order of 1 in 1,000 per day, meaning that only a tiny fraction 

of the population can be found on demand. Sit and wait predators may also be very 

difficult to trap, as they encounter traps only during their infrequent moves between 

hiding places.

All of the species under consideration are fecund, with the Reticulated Python 

having the second largest number of young of any snake (maximum 124), and the other 

pythons are close behind. All of the species under consideration can probably move large 

distances over short time periods when so inclined. These two factors combine to make it 

hard to limit the spread of their colonies. All of the species under consideration grow 

relatively quickly; the fastest snake-growth records are all held by one or another of the 

giant pythons. The giant pythons mature at a relatively young age, with some able to 

breed in their second year and most capable of breeding by age 3–5, at least in captivity 

(maturation ages are generally unknown in the wild). Despite their quick maturation and 

large clutch sizes, the giant constrictors are capable of a long life, with captive longevities 

reported in excess of 30 years.

Introduced predators are considered to be a greater threat as invaders if they are 

generalist feeders, because no matter where they escape or are released they may find 

suitable food. The giant constrictors are generalist predators on terrestrial vertebrates, 

although the anacondas also regularly consume aquatic vertebrates. Generalist predators 

are more likely to cause the extinction of vulnerable native prey because they continue to 

thrive on alternate prey as they drive the vulnerable prey species to extinction. Specialist 

predators rarely thrive when their preferred prey become rare.

All of the giant constrictors would have few predators on themselves in the 

United States because they are so large. Large alligators and panthers would be capable 

of eating the occasional giant constrictor, but large constrictors will likely eat alligators 

and panthers. It is unclear whether any native predator would enjoy a net benefit.

There are a variety of reptile ticks that appear capable of transferring among any 

of the giant constrictors; as examples of many potential pathogens of medical or 
7

veterinary importance, snake ticks originating in Africa may carry heartwater disease 

(potentially fatal to cattle) and ticks from Asia can convey capillariasis, a disease of 

human health significance.

There are few data on which to estimate the likely population density of giant 

constrictor populations, though the densities of Burmese Pythons near Everglades 

National Park suggest that they can reach very high densities where rodents are abundant.

The list of traits shared by the giant constrictors includes many of the traits that 

either increase the severity of their probable ecological impacts or exacerbate the 

challenge of controlling or eradicating them:

• large size

• habitat generalist

• arboreal when young

• tolerant of urbanization

• sit and wait predator

• low detectability

• high fecundity

• long-distance disperser

• rapid growth

• early maturation

• generalist predator

• host for diseases of economic and human health significance

• high-population density

Thus in comparison to potential invaders lacking these traits, this group of snakes 

constitutes a particularly high risk. 

Using the rating system proposed by the Aquatic Nuisance Species Task Force 

(1996), the components of risk include four elements of probability of establishment and 

three elements of consequences of establishment. For the giant constrictors under 

consideration, we judge the risk of establishment to range from medium (Reticulated 

Python, DeSchauensee’s Anaconda, Green Anaconda, and Beni Anaconda) to high 

(Indian or Burmese Python, Northern African Python, Southern African Python, Boa 

Constrictor, and Yellow Anaconda), with the understanding that three of the species 

(Southern African Python, DeSchauensee’s Anaconda, and Beni Anaconda) warrant their 

rank primarily because they are actually or potentially sold in commerce labeled as if 

they were a similar and closely related species (Northern African Python, Yellow 

Anaconda, and Green Anaconda, respectively). If they were to be unequivocally 

distinguished from their look-alikes, and if the current low volume of trade in these 

species were to continue, they might constitute an absolutely smaller risk.

For the giant constrictors under consideration, we judge the consequences of 

establishment to range from low (DeSchauensee’s Anaconda and Beni Anaconda) to 

medium (Reticulated Python, Yellow Anaconda, and Green Anaconda) to high (Indian or 

Burmese Python, Northern African Python, Southern African Python, and Boa 

Constrictor). The overall organism risk potential, which is the sum of both probability of 

establishment and consequences of establishment, was found to range from medium 

(Reticulated Python, Green Anaconda, DeSchauensee’s Anaconda, and Beni Anaconda) 

to high (Indian or Burmese Python, Northern African Python, Southern African Python, 

Boa Constrictor, Yellow Anaconda).
8

Certainties were highly variable within each of the seven elements of the risk 

assessment, varying from very uncertain to very certain; see the tables in chapter Ten for 

certainty ratings associated with each element. In general, the highest certainties were 

associated with those species unequivocally established in Florida (Burmese Python and 

Boa Constrictor) because of enhanced ecological information on these species from 

studies in both their native range and in Florida.
9

Chapter Two–Materials and Methods

Introduction

The core of this work–the biological profiles–are a work of traditional library 

scholarship, gleaning relevant data from the peer-reviewed literature and talking to 

experts active in each taxon (augmented by unpublished data from relevant sources and 

by our own experiences with these species in the field and in captivity). We started with 

keyword searches of relevant bibliographic databases, added relevant references from our 

personal bibliographic records, and traced all relevant citation threads from primary and 

secondary sources to the original publications. The latter approach was the most time 

consuming, and it provided the most useful sources. We translated key original sources if 

published in a European language, and contracted translation of a small number of 

essential references in Asian languages. We relied on secondary sources only to the 

extent that no better record was available, or if the secondary source credibly paraphrased 

or translated a difficult-to-obtain primary reference. We generally did not attempt to 

verify distributional records, and in most cases relied on recent comprehensive 

distributional publications. In a few cases, we challenged certain key localities by 

pursuing the original records or asking curators to verify specimen identifications, but for 

most distributional details we relied on credible literature compendia.

In addition to secondary sources, we compiled the following number of primary 

sources: Indian Python, 210; Reticulated Python, 205; Northern African Python, 71; 

Southern African Python, 76; Boa Constrictor, 183; Green Anaconda, 67; Beni 

Anaconda, 2; Yellow Anaconda, 30; and DeSchauensee’s Anaconda, 16. As is suggested 

by these counts, DeSchauensee’s and Beni Anacondas are poorly known; it is at least in 

part for this reason that we have combined those accounts with better known related taxa. 

However, “better known” is a relative term, and it is noteworthy that none of the giant 

constrictors are well known. Despite the giant constrictors’ persistent intense public 

interest and renown, only a few of these species have been the subject of modern 

ecological field studies. For example, credible dietary analyses have been published for 

only a few species, and using data from only a few localities. We are unable to 

characterize the body size, breeding season, or diet of any giant constrictor species 

throughout its range. Furthermore, available studies have focused on divergent 

ecological topics, complicating any attempt at direct comparisons of traits across species. 

As examples, the interaction between ecology and reproduction is best known from freeranging Green Anacondas, whereas harvested Yellow Anacondas have yielded excellent 

data on size at maturity and various other reproductive parameters, and radiotelemetry of 

Southern African Pythons has revealed interactions between ecology and thermal 

biology. No single species has received across-the-board ecological study, and the 

ecology of some species is almost completely unknown.

Knowledge of the biology of these giant constrictors may be scanty, but 

knowledge of appropriate management tools for these species is almost nonexistent. 
10

Thus for the management profiles we relied to varying degrees on inference from the 

management of other snake species, primarily the Brown Treesnake in Guam and the 

Habu in the Ryukyu Islands (Rodda and others, 1999d). Management experience with 

these snakes has identified natural history attributes that confine and steer management in 

particular directions (see especially the Epilogue in Rodda and others, 1999d). For 

example, the degree of arboreality of each snake species is a good predictor of the 

challenges associated with snake barrier construction and the extent to which the species’ 

presence will be associated with electrical system power outages. We have used such 

guidance to infer likely management issues that are conveyed in the management profiles 

of each species account. Necessarily such inferences contain an element of uncertainty, 

which is reflected in the species account narratives and explicitly stated in the uncertainty 

ratings contained in the last chapter. Although some elements of management differ by 

species, the giant constrictors share many natural history attributes, and the management 

factors that are shared among all species are conveyed in chapter Three, the synopsis of 

eradication tools.

Length Measurements

The lengths of snakes are often given in terms of the snake’s full length (including 

the tail, as the tail may be hard to distinguish without having the animal in hand). In this 

work, we use the abbreviation TL to designate the full or total length. However, 

scientists often prefer to measure snout-vent length (SVL), which can be measured 

somewhat more accurately and is less vulnerable to variation due to loss of the tail tip 

from predators, desiccation, and so forth. Neither measurement is very precise in snakes, 

because snakes are so elastic, and the giant constrictors are especially prone to 

measurement error if measured alive, as their muscles routinely overpower those of the 

measurer. Freshly killed material has a relatively consistent length, as long as the 

specimen has not been appreciably cut or dislocated during the killing process. Alas, 

many historical specimens of giant constrictors died a violent death and were distorted 

prior to measurement. Moreover, there are almost no precise measurements in the 

literature providing both SVL and TL (or equivalently, providing tail-length 

measurements, which vary slightly with sex); thus converting from one measurement to 

another is somewhat speculative.

In this work we provide TL measurements, estimated from the following table 

(Table 2.1), which was compiled from all of the available literature sources (see 

Bioprofiles). When the source data are measurements of total length in either feet/inches 

or SI (metric) units, we express the lengths in either mm or m without further annotation. 

If the source data are SVL measurements, we give the SVL value in parentheses.
11

Table 2.1. Estimated body proportions for the nine giant constrictors considered in this work, by 

sex (M or F) or of mixed-sex samples. These values used to convert SVL measurements to the TL 

values reported in the text.

Tail (% of TL) SVL/TL TL/SVL

Species M F Mixed M F Mixed M F Mixed

Indian Python 0.12 0.10 0.11 0.88 0.90 0.89 1.14 1.11 1.12

Reticulated Python 0.15 0.12 0.135 0.85 0.88 0.865 1.18 1.14 1.16

Northern African Python 0.12 0.11 0.115 0.88 0.89 0.885 1.14 1.12 1.13

Southern African Python 0.12 0.11 0.115 0.88 0.89 0.885 1.14 1.12 1.13

Boa Constrictor 0.14 0.10 0.12 0.86 0.90 0.88 1.16 1.11 1.14

Yellow Anaconda 0.14 0.13 0.135 0.86 0.87 0.865 1.16 1.15 1.16

DeSchauensee’s Anaconda 0.14 0.13 0.135 0.86 0.87 0.865 1.16 1.15 1.16

Green Anaconda 0.14 0.13 0.135 0.86 0.87 0.865 1.16 1.15 1.16

Beni Anaconda 0.14 0.13 0.135 0.86 0.87 0.865 1.16 1.15 1.16

Climate Space

“Weather” characterizes atmospheric conditions at a single point in time; 

“climate” characterizes seasonally average conditions, which are identified over time, 

typically 30 years (World Climate, 2007). Characteristic climate parameters are average 

rainfall or temperature at a particular time of year; typical weather values are whether it is 

now sunny. Climate space describes the long-term (greater than one generation) climatic 

conditions under which a population is known to persist, and is not equivalent to the 

weather that an individual can tolerate. Individual or physiological tolerance can be both 

greater and less than the climate space that supports a population. Individual tolerance 

depends on acclimatization, which may occur gradually. At the wrong season, or with 

the “wrong” recent experience, a snake may not be able to tolerate weather conditions 

(for example, cold) that it could tolerate at other times of year, or when primed by 

gradually cooling temperatures. Similarly, a snake in a burrow or hibernaculum can 

tolerate atmospheric conditions that include weather that it could never survive if 

subjected to directly. Exposed rattlesnakes cannot survive freezing weather, but 

populations of rattlesnakes often live in climates that freeze! Similarly, desert tortoises 

will die from midday heat if placed above ground on a Mojave Desert summer day, but 

they are protected by their behavior of remaining in burrows during these conditions. 

Conversely, a snake that is placed on the surface of the ground at 5°C will 

probably survive (it is physiologically capable of surviving 5°C), but such a snake is so 

cold that it would probably be incapable of protecting itself from a predator (it is not 
12

ecologically viable); thus physiological tolerance is a misleading guide to ecological 

success. Our “climate space” is a bivariate (rainfall and temperature) characterization of 

the climates of occupied habitats and is not to be confused with individual physiological 

tolerance.

Although it is widely recognized that climates are important and vary among the 

native ranges occupied by giant constrictors, there appears to be no standard method for 

characterizing climate space. Perhaps this is because climate has traditionally been 

viewed as fixed in relation to geographic space. We now know that the climate is 

presently changing at an evolutionarily unprecedented rate, and therefore it seems 

appropriate to explicitly record the climate conditions under which the constrictors now 

occur (and which presumably reflect geologically recent history and the snakes’ recent 

evolution).

We used published sources to infer each species’ native range (these distributions 

are summarized in each Species Account, section 3.1). We used exact specimen 

locations whenever available, and more general regional information when unavoidable, 

paying particular attention to records from high elevations and high latitudes. As we 

were focused on the climatic extremes tolerated by the species, we compiled primarily 

those locality records within 3 lat/long degrees of the periphery of the species’ range 

(spot checking of more interior localities indicated that inclusion of interior localities 

failed to appreciably expand the observed climate envelopes). 

We used two approaches to extract climate profiles from each species’ 

distributional records. Where suitable climate records were obtainable from weather 

stations (Indian Python and Reticulated Python), we used those empirical records (see 

Rodda and others, 2009). On the other hand, there are many places in the world where 

weather station data are sparse or unreliable. For those localities we used modeled 

climate from Hijmans and others (2005). For the two Asian pythons, “presence” 

localities were matched to the geographically closest choice from among the 85,000 

weather stations reported in the World Climate (2007) data set, paying particular 

attention to ensure an elevation match (where known). When possible, we used 

individual weather stations that reported both mean monthly rainfall and mean monthly 

temperature, but in a few cases combined records from nearby stations to obtain both 

climate data types. The World Climate stations are grouped into lat/long cells of 1 

degree; we matched these to locality records in the same cell whenever possible, but for a 

few important localities could find matching weather records only for an adjacent cell 

(only stations with similar elevations were considered). We were able to obtain a few 

useful climate records for locations hosting Indian Pythons in Nepal from Schleich and 

Kästle (2002). To analyze rainfall on a logarithmic scale and include weather stations 

that reported zero rainfall during particular months, we coded zero rainfall means as 0.01 

mm/mo (this value is slightly lower than the minimum possible from a weather station 

recording a typical 30-year climate record). 

To facilitate comparisons between different species, we plotted the occupied 

climate space for each species on graphs with identical scaling and fixed reference lines 

(Fig. 2.1). To make it easier to comprehend the values associated with the reference 

lines, we identified familiar American cities that had seasonal climates that approximated 

the intersections of the reference lines (Fig. 2.1). Thus the warm wet nexus is most 

closely approximated by Hilo, Hawaii in summer; the warm dry nexus by Yuma, Ariz., in 
13

0

5

10

15

20

25

30

35

40

0.01 1 100 10000

Mean monthly precipitation (mm)

Mean monthly temperature (C)

El Paso TX - Feb Sitka AK - Sep

Hilo HI - Aug

Yuma AZ - Oct

cool cool

dry dry

hot hot

wet wet

late summer; the cool dry nexus by El Paso, Tex. in winter; and the cool wet nexus by 

Sitka, Alaska in fall. By comparing each species’ climate space to the positions of the 

reference lines, it should be straightforward to assess their relative association with cool, 

warm, wet, and dry conditions respectively.

Figure 2.1. Schematic of climate space diagrams, showing representative American cities 

near the reference line intersections. Yuma, Arizona is a hot dry city in the desert along the 

California/Arizona border. Hilo, Hawaii is a wet windward city on the Big Island of Hawaii. Sitka, 

Alaska is on the windward coast of a cool rainforest island in Southeastern Alaska. El Paso, Texas 

is a dry cool (in Feb.) city in west Texas.

We plotted each of the climate records as 12-sided polygons, each vertex 

representing the mean conditions for one month of the year. The 12-sided polygons often 

doubled back on themselves to form figure 8s or other complex shapes. Interpretation of 

these 12-sided polygons depended on whether the species in question was known to 

hibernate or aestivate. 

For species that hibernate or aestivate (for example, Indian Python), a minimum 

convex polygon of climate records could overstate the range of conditions suitable for the 

species to be active. For hibernating species, we treated the coldest vertices as 

representing the conditions prevailing during hibernation, and we considered at least two 

plausible hibernation temperature thresholds to quantify the likely duration of hibernation 

from the native range climate records. If these counts of months in hibernation did not 

conform to the hibernation durations in the literature, we inferred that the hibernation 

temperature threshold we hypothesized was in error and chose another temperature 

threshold that would produce a plausible hibernation duration. Once the quantified 

hibernation duration was in line with published values, we tentatively accepted the 
14

hypothesized temperature threshold as plausible. Similarly, for species that are known or 

suspected to aestivate, we evaluated periods of extreme aridity in the same way. 

By progressively flagging the first, second, and third months of greatest aridity or 

cold against the graphical background of the climate polygons, we inspected the 

aggregated 12-sided climate polygons to determine the number of arid or cold months 

that were largely confined to sparsely occupied climate space. In some cases we could 

identify no clear break between sparsely occupied and densely occupied climate space. 

In that case we relied on hibernation or aestivation durations from the literature. For 

example, Minton (1966) reported a four-month hibernation period for Indian Pythons in 

Pakistan; thus we evaluated alternate hypotheses of 3 (Clim3) or 4 (Clim4) months of 

hibernation.

For each hibernation hypothesis, we fit the closest convex polygon that included 

all points believed to represent climatic conditions experienced by active pythons (that is, 

excluding those points deemed hibernation or aestivation), and checked these climate 

hypotheses against field observations reported in the literature or by personal 

communication from appropriate experts. 

For species that do not hibernate, we considered all points of the 12-sided 

polygons to represent climatically suitable conditions, and we fitted the closest convex 

polygon that enclosed all occupied climate space. We then mapped all locations of the 

United States that fell within minimum convex polygon’s climate space and scored 

excluded localities by the feature (cold, hot, too wet, and too dry) that caused their 

exclusion. If there were climatically important disagreements among authorities 

regarding the native range boundaries (for example, Reticulated Python), we derived 

climate envelopes for both minimum and maximum estimates of the native range.

International Trade

To estimate the level of legal international trade in the giant constrictors, we 

queried the CITES (Convention on International Trade on Endangered Species) trade 

database (available at www.cites.org). All boas and pythons are listed under CITES in 

Appendixes I or II, and signatories to CITES are required to annually submit records of 

imports and exports of these species. We obtained CITES records of imports to the 

United States from 1977 through 2007 for the species of interest; results are presented in 

the Appendix and include records of over 1,100,000 individuals of these species imported 

to the United States during this period. Although we present records through 2007, the 

CITES website indicates that only records through 2005 are presently complete; more 

recent records should therefore not be considered definitive. We summed annual trade 

without regard to declarations of subspecies within any given species, as subspecific 

epithets are not required except for taxa listed under CITES Appendix I, and thus the 

subspecific identity of most individuals identified to species cannot be determined from 

the documentation. Many of these species are somewhat similar in appearance, and their 

taxonomy changes at alarmingly frequent intervals; such changes (for example, the 

splitting of the former P. sebae into P. sebae and P. natalensis) may not be reflected in 

import records. There are also LEMIS (Law Enforcement Management Information 

System) records from the U.S. Fish and Wildlife Service that provided some of the trade 

data we cite. However, there are many disagreements between LEMIS and CITES 

records, as has been documented for wildlife trade in general (Blundell and Mascia, 
15

2005). Blundell and Mascia (2005) conclude that the records are “notoriously unreliable” 

for a host of reasons. We can confirm their unreliability, but know of no better source of 

information for quantifying the legal trade. Although the illegal trade in reptiles is 

considerable (Christy, 2008), we know of no means to quantify the level of the illegal 

trade and did not attempt to add the illegal trade volume to the CITES results. We are 

also chary of accepting as fact the declared country of origin for many records due to 

multiple anecdotal reports of cross-border smuggling and re-export of many of these taxa, 

so in this document we did not attempt to identify the main countries of origin or major 

trade routes. Accordingly, we consider the CITES import records to be the best available 

metric of the international trade in these species, but we recognize several potential 

shortcomings of the declared volume of trade. 

Climate Matching

One controversial component of species-specific management is projection of the 

areas of the United States that are climatically suitable for each giant constrictor (section 

10.2 of each species account). There are many proposed methods for “climate 

matching,” and we have chosen one that we believe most accurately portrays the regions 

of the country that may be at risk. However, there are other approaches, which have their 

own devotees. In particular, software designed for creating “Environmental Niche 

Models” are often used for climate matching, as the environmental niche data supplied 

with the software are primarily or exclusively climate measurements. In the following 

section of this chapter, we explain the method we used; in the concluding section of this 

chapter, we offer a brief defense of this selection. Please note that climate matching is a 

rapidly evolving field of endeavor and some methods will no doubt prove unreliable and 

be discarded in favor of methods not yet invented. We believe that current tools should 

be relied upon to give an indication of the relative size and location of the geographic 

area at risk, but should be used only with great circumspection to identify specific 

localities at risk.

We also applied our climate envelope hypotheses to current world climate data 

layers for monthly temperature and precipitation modeled from weather station data from 

around the world to a 1-km resolution (Hijmans and others, 2005) to verify if all occupied 

native range sites were identified as suitable.

Finally, we applied the identified climate envelopes to the current climate 

for the United States. We obtained average monthly precipitation (cm) and average 

monthly temperature (°C) data from the online Daymet database for the United States 

(http://www.daymet.org; Thorton and others, 1997). Thorton and others (1997) used 

daily observations from over 6,000 stations across the United States, collected from 1980 

to 1997, to create the surfaces at a 1-km2 resolution. 

The equations defining the climate space minimum convex polygons were 

implemented using Visual Basic for Applications with ESRI’s ArcGIS 9.0 ArcObjects to 

produce the U.S. map of climate suitability. Where we considered alternative hibernation 

hypotheses, the final maps were compared using the Raster Calculator in ArcGIS to 

determine areas where the hibernation scenarios matched and differed.
16

Reasons for Preferring our Method to the use of Environmental 

Niche Models

Our method uses a priori identification of suitable climate metrics; Environmental 

Niche Models (see next paragraph) generally use automated selection of climate metrics. 

Both approaches incur the risk of selecting the wrong measure. We chose mean monthly 

precipitation and mean monthly temperature as adequately representing the climate 

attributes best associated with giant constrictor range limitations. The choice of monthly 

metrics insures that the climate will be suitable during every month of the active season, 

and that the inactive season length will not exceed that experienced in the native range. 

We do not believe that daily values are appropriate for snakes that have access to natural 

refugia, as the low metabolic demands of reptilian physiology, as well as the huge meals 

eaten by giant constrictors, insures that they do not need to venture out every day or even 

every week in order to maintain a net positive energy balance. We do not believe that 

annual climate averages are meaningful to a giant constrictor, as a low annual 

temperature average, for example, could reflect either continuously low temperatures (not 

acceptable to a giant constrictor) or warm summers and frigid winters (acceptable to 

hibernating species). A giant constrictor does not require that the conditions be 

continuously suitable, but conditions must be suitable for a sufficient number of months 

per year. Most species can probably aestivate for months to minimize exposure to 

seasonally inclement weather or food unavailability, but this is not known with assurance. 

While we chose mean monthly temperature and precipitation to represent the ecological 

conditions pertinent to giant constrictor population viability, there are many other climate 

metrics that could be used and may be plausible. For example, it may be that mean 

minimum January temperature is a better metric, or mean winter minimum, or mean dry 

season minimum or maximum, or extreme June maxima, number of frost-free days, or 

some other characterization. We could simply try every possibility until we discovered 

which one gave the most desired answer, but that would leave us guilty of the statistical 

error known as “fishing,” which often gives spurious correlations. If you try enough 

statistical tests, you will eventually discover one that “works,” but just by chance. 

Fishing is expressly invalid as a statistical procedure whenever the chosen metric was 

adopted because it gave the desired answer. A priori methods, in which the metrics are 

chosen for a biological reason in advance, without knowledge of the statistical outcome, 

are valuable because they offer statistical protection against spurious correlation, but they 

do not ordinarily discover the best possible fit between climate conditions and a species’ 

native range boundary.

A popular alternative to a priori identification of climate metrics is the use of 

automated programs that select the climate metrics on the basis of which metric gives the 

best fit to the native range distribution. Generically such programs are often called 

Environmental Niche Modeling programs, of which the current leader is MAXENT 

(Phillips and others, 2004, 2006; Phillips, 2008; see also BIOMOD: Thuiller, 2003). In 

the past GARP, BIOCLIM, CLIMEX were widely used (Nix, 1986; Sutherst and others, 

1996, 1998; Stockwell and Peters, 1999; Peterson and Vieglais, 2001; Stockwell and 

others, 2005; Beaumont and others, 2005).

Environmental Niche Models were initially used to characterize the habitat 

features associated with plot data, wherein a particular species was encountered 
17

(presence) or not (absence), so that nearby sites that were likely to support “presences” 

could be easily identified (geographic interpolation). For example, if one searched 50 

plots in a nature reserve for the presence of one plant species, and the plant was found 

only on north-facing slopes at elevations between 2,000 and 2,100 m, it would be prudent 

to search for additional nearby populations of this plant on north-facing slopes at 

elevations of 2,000–2,100. This localized use is relatively robust against overfitting and 

lack of statistical independence among points, in that nearby sites are likely to have the 

same suite of statistical confounds and collinearity as did the original plots. Collinearity 

is said to exist when two variables covary; for example, mean January temperature and 

minimum January temperature covary among sites. Elevation and temperature have the 

same sort of collinearity. When interpolating geographically, the collinearity structure 

between various metrics remains largely the same and therefore doesn’t greatly bias the 

identification of suitable localities. However, if one chooses to use MAXENT to deduce 

climate drivers or extrapolate climate space from one part of the globe to another, the 

collinearity among climate attributes is very likely to have a different structure in the 

projection sites, and due to the overfitting that is characteristic of such programs 

(Beaumont and others, 2005), one is likely to get gross underprediction (too few sites 

match the exact combination of characters that was present in the source location) and 

erroneous identification of climate drivers (Randin and others, 2006). Using our plant 

example, it is possible that north-facing slopes at an elevation of 2,000–2,100 m at lat 40° 

N have a climate similar to a different locality (at say lat 50° N), where the same plants 

might be found on south-facing slopes at an elevation of 3,200–3,400 m. However, if the 

MAXENT model was calibrated on the original site only, it would overfit and fail to 

predict the species’ occurrence at the second site. 

A strike against MAXENT (and comparable automated fitting algorithms, 

BIOCLIM, GARP, and others) is that it is methodologically opaque; the computations 

cannot be reproduced by hand, as the procedure is complex and not fully explained in any 

publication to date. Yet, it is so easy to use (software and data are available free on the 

internet, one isn’t responsible for identifying the biases in other peoples’ data, and it takes 

only a few minutes to crank out the maps) that many users accept the results and default 

settings without exploring the sensitivity of the result to the choices made. For example, 

we find MAXENT to be ultrasensitive to the boundary conditions set for selection of 

pseudo-absences. In several papers we have seen authors inappropriately take pseudoabsences from the entire rest of the globe. Others take them from unoccupied parts of 

entire continents. These gross distinctions provide really good apparent-model fits (see 

discussion of Area Under the Curve, or “AUC” below), but poor biological insight. The 

creator of MAXENT is clear, however (Phillips, 2008), that taking pseudo-absences from 

unrealistic localities is inappropriate; one should restrict pseudo-absences to those 

portions of the globe that are potentially occupiable by the species (that is, nearby and not 

separated from occupied range by dispersal barriers such as salt water, alpine zones, and 

so forth). If this is done, one gets much more realistic AUC values.

A brief digression into AUC. AUC is a good metric for evaluating models 

intended for binary judgments based on an uncertain threshold of a continuous variable. 

An example would be a model for estimating presence or absence (a binary trait) from a 

continuous habitat suitability score (for example, one that ranged from 1 to 10). 

Environmental Niche Models assume a single optimum condition for a species presence, 
18

and every site is given a continuous score that reflects how close the site matches the 

deduced optimum. Yet a species can only be either present or absent. Perhaps 

“presences” would be best estimated for sites with habitat scores over 2.7, or maybe 

presences would be better discriminated using a threshold of 3.1. Different thresholds 

yield different binary selections, but which discrimination model works the best overall? 

AUC deals with this uncertainty by plotting the accumulation of true positives against 

false positives (that is, discrimination success) under all possible thresholds. The curve 

itself is called a “Receiver Operating Characteristic” or ROC curve; discrimination 

success is measured by the area under such an ROC curve, as the ROC curve bulges 

upward maximally when true positives most greatly exceed false positives (Hanley and 

McNeil, 1982). Thus the Area Under the Curve (AUC) is highest (on a scale of 0 to 1) 

for the mathematical models that discriminate best.

If the model is totally lacking in discriminatory power it will yield an AUC of 0.5 

(false results accumulate as fast as true results); progressively better discriminators will 

have AUC values progressively closer to 1, and perfect reverse discrimination–for 

example, scoring all black sheep white–will give an AUC of zero. As computation of 

AUC is automated in MAXENT, and AUC often produces very satisfying values, it is 

widely reported. If the true negatives (that is, pseudo absences) are outrageous choices 

(for example, Siberia for tropical species), the model can be a really poor discriminator in 

delineating the native range, but it will encounter so few hard choices that it will yield a 

really good AUC. We’ve recently seen a suite of implausible models, none of which had 

a reported AUC of less than 0.96. AUC may depend more on the choice of pseudoabsences than on the fit of the model.

A very serious problem with MAXENT occurs when most of the native range is 

on islands (for example, Reticulated Python) from which there are no suitable adjoining 

land areas that are unoccupied. If the introduced or native range of a terrestrial species 

extends to saltwater or any other physical barrier to dispersal, there is no climatic

interpretation to the range limit (absences should be given no statistical weight, Arntzen, 

2006). All we can state is that the climate is suitable in the places where the species 

occurs. If the native range limit occurs somewhere without a dispersal barrier, we can 

infer that the species’ absence beyond the range limit is due to something, of which 

climate is a viable hypothesis. 

MAXENT gives equal weight to each point. This is especially problematic if the 

data points were obtained from opportunistic museum collection localities (as is typically 

done for climate matching). Therefore, if museum collectors do not randomize their 

effort across the native range (and they don't), MAXENT is biased by the same factors 

that bias the distribution of collecting locations. This is strikingly evident in many 

studies we have seen (for example, Stockwell and others, 2005) where major portions of 

a species’ range were undercollected and were therefore deemed unsuitable habitat by 

MAXENT, GARP, and so forth. Environmental Niche Models are often primarily 

inferring where collectors go to collect.

One reason why naïve application of MAXENT yields results with poor 

biological insight is that it automatically keeps adding more and more parameters as long 

as some improvement in the model fit can be obtained. Because each climate axis can be 

fit by any function (shape) that best matches the data set (a posteriori fitting), the model 

can include a potentially infinite number of parameters—even if the number of axes is 
19

severely constrained (for example, five). This is a textbook case of statistical fishing, and 

it leads to overparameterization or overfitting. There are statistical tools that can be used 

to constrain the number of parameters (for example, a priori selection of hypotheses, 

parsimonious choice of function complexity, Akaike’s Information Criterion; Burnham 

and Anderson, 2002), but in most geographic applications, these statistical protections are 

not invoked. Overfitting leads to underprediction, as is evident in many MAXENTderived maps showing that parts (often large and important parts) of the occupied native 

range are projected to be unsuitable (for example, Pyron and others, 2008). 

For an introduced population to survive and spread, the climate of the colonized 

site must be within the range of climate conditions under which the introduced species 

can survive and reproduce, a necessary but not sufficient condition (Guisan and Thuiller, 

2005). Because population persistence is a complex interaction between the species and 

its predators, parasites, prey, competitors, dispersal barriers, and transient habitat 

conditions (Brown and others, 1996), the inferred climatic optima are context specific, 

not a single absolute optimum such as is computed by MAXENT. A climate that, on 

average, allowed a population to have a lambda of 0.98 in one place (a lambda slightly 

less than 1 indicates a population that is slowly disappearing) might sustain the same 

species in some other context (lambda = 1.01: a slowly increasing population). Thus we 

fundamentally are in pursuit of the relationship of climate to demographic persistence 

(lambda), a very complex, often indirect, and poorly understood function, which is likely 

to be limited by different conditions in different places. The climate-demography 

relation of an entire species cannot be adequately characterized as a single optimum in 

climate space. At the local scale, Environmental Niche Models work well because the 

demographic context is likely to be similar across the region used for small-scale 

geographic interpolation. Environmental Niche Models can produce wildly inaccurate 

results when applied to intercontinental extrapolation or deduction of climate drivers.

The ecological context prevailing in a species’ native range is likely to differ from 

the context in its introduced range. Aside from habitat, topographic, and assemblage 

differences, an introduced species typically sheds many parasites and predators when it is 

translocated to a new environment. It is also likely to have passed through a genetic 

restriction, meaning the new population is not a carbon copy of the old gene pool and for 

this reason alone the outcome is likely to differ.

Given the contextual paradigm shift that may take place during a translocation 

from native to introduced range, our preference is that climate similarity be treated as the 

necessary but not sufficient condition (a one-sided prediction; on the correct side of a 

given criterion the climate is potentially suitable) rather than being rolled into an 

ecological black box to be inferred by automated data fitting (which assumes one optimal 

condition, with incremental degradation to either side of the optimum). That is, any 

occupied locality (whether native or introduced range), by definition, has a suitable 

climate; the suitable-climate envelope may be larger than the occupied climate space 

(indeed, additional climatically suitable but unoccupied localities are extremely likely for 

such poor dispersers as reptiles and amphibians), but the climate envelope cannot be 

smaller than that demonstrated by occupied areas. Some researchers distinguish a 

fundamental or potential niche (maximum occupiable climate space) from a realized 

niche (the actual climate space in a specific area, as constrained by biological factors 

such as dispersal barriers, predators, competitors, and so forth) (Guisan and Thuiller, 
20

2005). Araujo and Pearson (2005) stated, “By relying on observed distributions (which 

inherently reflect multiple range determinants, both historical and ecological) to 

determine ecological requirements, this methodology is likely to underestimate the true 

range of climate variation that species are able to tolerate.” 

If one accepts the necessary but not sufficient conceptualization of climate 

matching, the analytical goal would be to delineate the relevant climate characteristics 

that encompass all occupied range. The word “relevant” is included so as to omit climate 

characteristics that might apply to a particular locality but are not determining of the 

species’ presence. For example, elevation is sometimes included in climate matching 

algorithms, and it might correlate with a species’ boundary, but not determine it (perhaps 

temperature, which correlates with elevation, is determining). In another place, the 

temperature would still be determining, but the range limit would be at a different 

elevation. Inclusion of elevation in a climate-match model in this case would be an 

example of collinearity (temperature and elevation are correlated) and overfitting

(because the collinearity is site-specific and may not extrapolate). When such an overfit 

model is projected to a different continent/context it gives an underprediction because the 

elevation would most likely be a mismatch.

An alternate to the necessary but not sufficient conceptualization is the black box 

approach, which holds that climate is a convenient proxy for many factors about habitat 

(prey abundance, vegetative structure of habitat, temperature tolerance, and so forth), and 

our analytical goal is to find the climate attributes that most closely correlate with the 

boundary of the occupied native range. Knowing the closest climatic correlate allows us 

to guess the likely spread of an invader without having to figure out the black box of 

habitat-species interactions. Unfortunately, a climate projection based on such a 

conceptualization will work well only if the demographically limiting factors in the 

native range are duplicated exactly in the introduced range.

In this study we do not use climate matching as a tool for determining which 

factors affect a species’ distribution; rather climate matching using plausible a priori 

climate axes provides one measure of the areas of the United States that may be at risk 

from a species invasion. Species with broad climate tolerances (for example, Indian 

Pythons) will exhibit a broader United States climate match than species with a narrow 

climate tolerance (for example, Reticulated Pythons). Such differences are visualized 

geographically by comparing maps in section 10.2 of the different species accounts. 

Demographically limiting factors that apply solely or primarily in the destination site 

alone will constrain actual range limits to less than that shown on the map, an example of 

overprediction. Conversely, demographically limiting factors that apply solely or 

primarily in the native range will induce underprediction in the map; thus the area 

actually at risk may be much larger than that shown. Current knowledge is insufficient to 

assess the relative probabilities of under- and overprediction of these maps. To the extent 

that all species accounts are vulnerable to the same magnitude of under- and 

overprediction, the maps can be profitably compared to infer the relative geographic risks 

associated with establishment in the United States.
21

Chapter Three–Synopsis of Eradication Tools

Introduction

The thrust of this risk assessment is to tabulate and assess the ecological and 

economic costs that could result from establishment in the United States of selected giant 

constrictors, so as to inform efforts to prevent that establishment. Why then is this 

chapter devoted to control of established snake populations rather than prevention of their 

arrival? The biodiversity losses and economic costs attributable to colonization of a 

mainland United States site by an invasive giant snake depend crucially on whether it is 

practical to eliminate (eradicate) the colonization through control measures (Bomford, 

2003). If the cost of eradication is high, the penalty for failure to prevent a colonization 

includes the need to mount an expensive eradication program. If the cost of eradication is 

relatively low and new colonizations can be routinely eliminated, a higher risk of 

colonization may be tolerable. If eradication is not possible (or the cost is exorbitant), the 

risk of colonization may not be acceptable, even if the known impacts of an introduction 

are minor or few. Thus any risk assessment for a potential species introduction must 

consider the costs for eradicating unwanted colonizations, and the prospects that such an 

effort would be successful (Bomford, 2003).

The short answer is that no introduced populations of reptiles have been 

intentionally eradicated anywhere on Earth to the best of our knowledge (F. Kraus, 

personal commun., 2008). Thus, our toolbox of proven techniques is empty. One would 

have little confidence that a snake colonization could be eliminated if one chose to do so. 

However, few wildlife managers have made the decision to attempt eradication, and until 

recently little research effort was devoted to identifying and perfecting reptile eradication 

techniques (Rodda and others, 1999d). Based on the literature and our experience with 

the Brown Treesnake on Guam, local eradication (that is, in small areas) is possible, but 

may be costly. The prospects for success are strongly dependent on the geographic 

extent of the population (Rodda and others, 1999a); most observers feel that eradication 

of a large continental population (for example, Burmese Pythons in Florida) is 

impractical, but opinions on the level of difficulty vary. Certainly we have no examples 

to draw from, but ongoing research is producing a steady stream of innovations that 

might be tapped. Here we review potential techniques for reptile eradication.

Note that this chapter does not attempt to review or even consider management 

actions short of eradication. In the case of an established population, it may be prudent to 

undertake a variety of management actions short of eradication. We know that rats are 

controlled in every city in the world, even though there is no hope of eradicating rats 

from those cities or the world. Pythons may be controllable in the vicinity of bird 

rookeries for example, even if they cannot be eradicated from all of Florida. We may be 

able to protect isolated places like the Lower Florida Keys from giant constrictors. 

However, management recommendations for that sort of local control requires a careful 

study of local conditions and careful application of management expertise as it applies to 
22

the particular constrictor species in question. Such a course of action could be described 

and evaluated in a management plan for a particular situation, but for the purposes of this 

work, we will limit the focus to whether and under what circumstances it is likely to be 

practical to eradicate a giant constrictor population.

Management commonalities among giant constrictors. In this synopsis of 

eradication tools, we do not provide separate analyses for the various giant constrictor 

species, as there is no management literature or experience upon which to base species 

distinctions. Furthermore, the giant constrictors share a suite of key traits that dictate 

similar eradication practices (see Epilogue in Rodda and others, 1999d, for examples of 

the relevance of such traits). For example, giant constrictors are notoriously cryptic and 

often hidden and immobile and are therefore difficult to detect. An invader such as an 

ibis is conspicuously colored and moves often; therefore it is very easy to spot if you are 

near it (that is, detection probability near 1), promoting effective control by shooting. In 

contrast, giant constrictors are routinely hidden from view inside soil, vegetation, or 

murky water (that is, detection probability often <0.001). The giant constrictors all rely 

heavily on ambush predation; thus control tools that rely on the snakes’ movements (for 

example, traps) may be limited in application. The giant constrictors are all strongly 

attracted to endothermic prey and are likely to be vulnerable to similar endothermic foodbased attractants (for traps or toxicants). The giant constrictors all eat infrequently, 

complicating their control by food-based attractants. Thus given the similarities in their 

natural histories, we anticipate that a similar suite of tools would be used for all of the 

giant constrictors, with accommodation for the more aquatic habits of the anacondas 

(though all of the giant constrictors spend time in aquatic environments, and those 

environments must be an important component of any eradication). 

Introduction to primary eradication tools. Because there is no literature on 

eradication techniques or even control techniques for giant constrictors, we must cast a 

taxonomically wider net to find examples of control tools. Witmer and others (2007) 

reviewed all control tools for invasive terrestrial vertebrates in the United States. Veitch 

and Bell (1990) reviewed tools used for invasive endotherms in New Zealand. Both 

reviews note that the available tools are strongly dependent on taxon-specific attributes; 

Witmer and others (2007) list only four tools for invasive reptiles and amphibians: 

trapping, toxicants, dogs, and barriers. Barriers are used primarily to contain dispersal of 

an expanding population or prevent recolonization of areas already cleared of invaders, 

whereas trapping and toxicants are devices to collect targets or kill them. Dogs are not 

usually the direct agents of lethal control, but are used to enhance detection and 

localization of reptiles that can then be killed by some other method. Thus we can group 

tools on the basis of whether they limit spread (barriers), localize the targets (dogs, traps), 

or kill the targets (toxicants). To this list of four tools should be added visual searching 

by humans, which is used extensively for many herpetofauna, including Brown 

Treesnakes (Rodda and others, 1998; Campbell and others, 1999; Vice and Pitzler, 2002). 

Searching for many species, especially snakes, can be done at night with the aid of 

electric lights, and such direct searching is therefore often called spotlighting (Engeman 

and Vice, 2001). While dedicated searches are usually far more effective per person-hour 

than opportunistic detections by the general public, the general public has a vastly greater 

number of eyes with which to make detections. Thus we will discuss enlisting help from 

the general public as a tool for greatly increasing the number of detections.
23

Introduction to secondary eradication tools. In addition to these five primary 

eradication tools are several variations and some concepts that may have value in control 

of invasive snakes. Witmer and others (2007) mention: (1) Judas animals, (2) introduced 

predators, (3) habitat manipulation, (4) bounties, (5) frightening devices, (6) reproduction 

inhibition, and (7) egg/nest destruction. Pheromonal attractants (8) have frequently been 

mentioned as a tool for increasing the species specificity of trapping and toxicants. 

Introduced predators are an example of biocontrol, but diseases and parasites are another 

type of biocontrol that might be used to reduce (but not likely eradicate) an introduced 

snake population (Simberloff, 2001). We will consider both introduced predators 

(macropredators) and micropredators (diseases and parasites) in the section on biocontrol. 

We will review each tool in turn.

Standard of success. In this review, our standard of success is whether the tool 

would enable an incipient colonization to be eradicated. This is not a unitary threshold, 

as populations differ appreciably in the geographic extent that is infested at the time that 

the population is discovered or control is attempted. What works on a 2-ha colonization 

may not succeed with a 20,000-ha colonization. Thus we will also discuss in a relative 

way the geographic scale over which a control tool is likely to be successful (absolute 

geographic determinations must await more information). Keep in mind that the primary 

challenge of an eradication is not achieving a high body count or rapidly killing the bulk 

of the population, but getting to the last individual. Any eradication program that kills 

only the accessible animals, or the feeding ones, or the adults, or all but the largest few 

individuals will be an eradication failure. The individual that is the least accessible and 

the least vulnerable must be the defining focus of an eradication program—a standard 

that is very difficult to meet.

One practical concern in the application of snake control tools is fire. Control 

measures that involve the long-term placement in the field of control devices (especially 

traps, toxicant bait stations, and barriers) can be incinerated by a wild fire. Fire is not 

likely to eradicate any giant constrictor, as many individuals will escape the fire flames 

by refuging underground or in water, but plastic barriers or bait stations and trap 

attractants (such as live rats) may be impractical to relocate in advance of a rapidly 

spreading fire. This factor should be kept in mind when contemplating such devices in 

fire-prone areas.

Primary Tools

Primary Tool I–Limit Spread: Barriers

One can envision barriers as being physical (a fence), chemical (repellents), 

ecological (adverse habitat barrier), or a gauntlet of control tools (for example, five rings 

of toxicants/traps) that minimize the chances of a snake successfully passing through. 

Physical barriers are used extensively in the Ryukyu Islands to aid trap capture of Habu, 

Protobothrops flavoviridis (Shiroma and Akamine, 1999). Physical barriers have been 

used for Brown Treesnakes, primarily to minimize the contamination of cargo or enhance 

detection of snakes in potentially contaminated cargo leaving Guam (Perry and others, 

1998). However, the large size of the giant constrictors suggests that the risk of 

undetected snakes in cargo will be much lower for the giant constrictors than for the 
24

relatively diminutive Brown Treesnake. Numerous proposals have been floated for use 

on Guam of Brown Treesnake barriers to prevent the recolonization of natural areas from 

which Brown Treesnakes have been eliminated for restoration of endangered prey species 

(Rodda and others, 1999a). In this application, barriers would enclose a patch of habitat 

from which the extant population of invasive snakes can be eliminated. However, for 

giant constrictors in the United States mainland, barriers have been suggested for 

minimizing spread of the constrictors to previously uncolonized habitat (for example, 

protect Florida Keys or prevent northward spread of Burmese Pythons). Although the 

current management focus for giant constrictors emphasizes prevention of spread to 

novel areas, management emphasis may shift to local eradication in a fenced reserve if 

nationwide eradication proves impossible.

Are barriers effective, and if so, at what cost? Physical Brown Treesnake barriers 

have been extensively tested and can be effective (Perry and others, 1998, 2001), but 

other types of barriers (chemical, ecological, and control) have not been tested for Brown 

Treesnakes. No barriers of any type are known for giant constrictors, but the Brown 

Treesnake experience may be of value in anticipating the magnitude of costs and the 

circumstances under which physical barriers would be effective. Brown Treesnakes are 

much more slender than the giant constrictors; thus Brown Treesnakes can cantilever a 

higher proportion of their body across gaps (Jayne and Riley, 2007). Nonetheless, the 

reach of an ascending snake is primarily dependent on its length; longer snakes can 

ascend further. In this regard the longer and more slender Reticulated Python is apt to be 

a particular challenge for barrier design, whereas the shorter (Boa Constrictor) or more 

heavily bodied species (anacondas) would be less of a challenge. Species-specific testing 

would be needed, but it might be a reasonable first approximation to assume that a snake 

can extend itself vertically between one-half and three-fourths of its body length (the 

three-fourths value is based on Brown Treesnakes).

Left to their own devices, plants will grow onto a barrier and render it ineffective; 

thus, control of encroaching plants is a significant maintenance cost for all barriers (Perry 

and others, 1998, 2001). Maintenance costs can easily exceed construction costs over the 

life of a barrier; the ratio of these costs will depend on details of the barrier 

implementation.

Four environmental features are notable for sharply raising the construction cost 

of Brown Treesnake barriers: (1) salt content of the air, (2) hurricanes, (3) surface-water 

flow, and (4) steep terrain. Barriers can be established for areas lacking these attributes 

for relatively low cost (exact cost strongly dependent on size of area and other features) 

using metal mesh barriers. However, in areas with salty air, the metal mesh rusts within a 

few years. Hurricane frequency is very high on Guam, but hurricanes are also a 

consideration throughout the southeastern United States. If the purpose of a barrier is to 

keep exotic snakes out of a nature preserve permanently, the barrier must be designed to 

sustain the worst weather possible at the site, a costly consideration. Barriers can be 

rebuilt after a hurricane, but it is highly unlikely that they could be rebuilt fast enough to 

prevent surrounding snakes from penetrating the area and requiring an expensive 

reextermination. Surface-water flow is a dominant environmental characteristic of the 

south Florida areas that are most likely a point of original colonization for all of the giant 

constrictors. Conversely, steep terrain will rarely obstruct barrier construction in Florida. 
25

In fire-prone areas such as the Everglades, it might be necessary to limit barrier designs 

to those using only nonflammable materials.

All of the giant constrictors swim (see Bioprofiles) and likely would be willing to 

enter water to bypass a barrier that terminated at the water’s edge. There are no known 

snake barrier designs that prevent this (this is also a chronic problem for peninsular 

barriers for invasive mammals in New Zealand; see problems reported in Lovegrove and 

others, 2002).

Short lengths of hurricane-resistant physical barriers for Brown Treesnakes have 

been designed and installed in the Mariana Islands, but these routinely incur construction 

costs of >$500 per linear meter (unpub. data). Presumably barriers for giant constrictors 

would be substantially more expensive (due to the greater height requirement), and costs 

could balloon further if the barrier needed to cope with surface-water flow. Barriers will 

interfere with the movement some non-target species. Thus physical barriers for giant 

constrictors are probably limited to protection of relatively small geographic areas. There 

are ample opportunities for research to identify and develop more cost effective physical 

barriers for giant constrictors, as well as to test chemical, ecological, and control tool 

barriers for these species.

Primary Tool IIa–Localize Targets: Visual Searching

Direct searching is the most widely used tool for locating reptiles and amphibians 

(Guyer and others, in press). Effective traps or toxicants have not been developed for 

most herpetofauna and thus visual searching is often the only tool available. For those 

species for which effective traps have been developed, such as for Brown Treesnakes or 

Habu (Rodda and others, 1999d), trapping tends to supersede visual searching as the most 

effective tool. However, visual searching for Brown Treesnakes remains the primary 

detection method under three circumstances: (1) on perimeter fences surrounding high 

value areas such as ports and airports (Vice and Pitzler, 2002), (2) for juvenile Brown 

Treesnakes (which are not easily trapped: Rodda and others, 2007), and (3) in high-prey 

environments, where traps work poorly (for example, on the island of Saipan, where prey 

have not yet been depleted by Brown Treesnakes; Gragg and others, 2007). As the two 

latter circumstances are defined by the capabilities (or lack thereof) of traps, and traps 

have not been developed for giant constrictors, it is unknown whether the same logic 

would apply to control of giant constrictors. Snakes climbing chain-link fences are 

relatively conspicuous, which is the attraction of that particular implementation for 

Brown Treesnakes, but chain-link fences are infrequent in Florida and many other 

mainland United States sites, and we expect relatively little use by the comparatively 

heavy-bodied giant constrictors. 

It is notable that most giant constrictors are not systematically collected in their 

native ranges; most are taken when encountered by chance (Shine and others, 1998a). 

Thus there is not a body of native-range expertise to tap for refining visual search 

techniques.

Visual searching is very labor intensive and therefore costly, but its application to 

giant constrictors may be more constrained by limitations on physical access for visual 

searchers than by labor cost. The three primary challenges in Florida (the most likely site 

of colonization) are inundation, obstructing vegetation, and private property. Whereas 

the site of original colonization of Burmese Pythons was Everglades National Park (thus 
26

private property is not an important factor in visual searching the area); dense low-level 

vegetation and chronic inundation block effective searching of most Everglades habitat. 

This is well illustrated by the low number of visual detections associated with the pythons 

that were known (2005–2008) to be in an area by virtue of their radiotransmitter signal. 

In 14.4 radiotelemetered python-years, we are aware of only four detections unaided by 

use of the radio signal (S. Snow, pers. commun., 2008). During the radiotracking period, 

there were visitors and searchers in a position to see pythons in the area every day. 

Despite this human presence, the average python was detected by searchers or the public 

about once per three years (4/5,270 days or 1/1,318 days). This implies that on any given 

day, the probability of anyone finding an arbitrary python without the assistance of 

radiotelemetry is only about one out of 1,318 days or 0.001 or 0.1 percent. This value 

should be considered trustworthy only to order of magnitude, but the order of magnitude 

has important implications for interpreting sighting rates and projecting control 

effectiveness. Whereas some of the pythons were undetected because no one was 

searching for them, and a few others were near a searcher but undetected due to 

obscuring vegetation, the vast majority of nondetections occurred simply because 

pythons rarely visit levees, roads or road berms, trails, or other locations frequented by 

humans (Mazzotti and others, unpub. data, 2009). In most cases a python went 

undetected simply because it was too far from any dry land or open water to be seen by 

any observer walking, driving, or boating.

Radiotelemetric data are not available to judge this phenomenon in the other giant 

constrictor species, although visual detections of insular Boa Constrictors in Honduras 

are biased towards microhabitats that are seldom used by telemetered individuals (Reed 

and others, 2007). The behavior of anacondas and Reticulated Pythons suggests that they 

would be even less inclined to spend time on land (see Bioprofiles). The other pythons 

and boas are less well known in this regard, though Boa Constrictors in some parts of 

their native range have little association with water (chapter Seven). Thus more 

information is needed to determine detection probabilities across a range of species and 

habitat types.

Extended wavelength searching. A concept that is intriguing to some researchers 

is that visual detection rates have the potential to be greatly enhanced through 

technological means, such as providing different wavelengths of nighttime illumination 

or extending human visual perception with tools such as FLIR (Forward Looking 

InfraRed) designed to detect infrared radiation (heat) or other special value wavelengths 

(R. Engeman, pers. commun., 2008). The prospects for extended wavelengths to enhance 

visual detection are highly sensitive to the reason for lack of visual detection with 

ordinary light. If the targets are within line of sight but are routinely overlooked (as a 

result of the snake’s crypsis), a potentially productive approach might be technological 

aids to visual detection such as FLIR, but if the cause of the snakes’ lack of detection is 

intervening vegetation or water, extended-spectrum aids would be of no use.

The use of infrared (heat) detection has not yet been tested on giant constrictors in 

the field. The fundamental problem with this approach is that nonbrooding giant 

constrictors are ectothermic, meaning that they do not generate appreciable heat. Whereas 

humans and other endothermic creatures are readily detected by the greater heat 

production of their surfaces relative to background, snakes will differ from background 

radiation levels only if they have recently moved from a medium with a different 
27

temperature, or absorb solar radiation at a rate greater than background. Thus if a 

basking snake entered cooler water, it would present a thermal contrast with its 

surroundings (though not detectable with infrared sensing equipment once the snake 

submerged). Conversely, if a snake emerged from cooler water into vegetation or onto 

ground heated by sunshine, it would present a cooler signal than its surroundings, until 

thermal equilibrium was achieved (at which point the snake would disappear from a pure 

thermal image). The attainment of thermal equilibrium is a function of the mass of the 

snake and its thermal flux, which is itself a function of the starting temperature 

differential (Christy and others, 2007). A very large python would take hours to 

approach equilibrium if the starting temperature differential was large (for example, 

emerging from cold late-winter water and basking in strong sunlight), but a small snake 

would be close to equilibrium within a few minutes under the small environmental 

temperature differences that prevail at night and through much of the year. Because giant 

constrictors will generally attempt to achieve higher temperatures in the United States 

mainland in the winter, the best opportunities for thermal contrast are likely to occur at 

mid-morning with basking winter snakes. However, visual detection at mid-morning is 

already near maximal due to excellent visible-wavelength lighting conditions. In the 

evening, giant constrictors are apt to be in the water to take advantage of the daytime heat 

that is retained longer by the higher thermal mass of water. As a submerged snake is not 

detectable with infrared sensing equipment, the opportunities for enhanced detection at 

night seem pretty limited.

There are practical difficulties in using FLIR equipment to spot giant constrictors 

in daylight. Recall that snakes that have reached thermal equilibrium are not visible to 

FLIR and are best seen with unaided eyes. Snakes that have just emerged into sunlight 

from cooler water and those that have absorbed a lot of solar radiation and are basking 

next to damp (evapotranspiring) vegetation are maximally visible to FLIR. One 

presumes that the searcher wishes to detect all classes of snakes (those just arrived, those 

at equilibrium, and any that are hotter than background). This implies that detecting all 

snakes would require scanning each scene once with FLIR and once without. A plausible 

solution involves using two observers (each dedicated to their own wavelength—and a 

driver if conducted from a vehicle), but this increases costs over using a single observer. 

It remains to be seen if such an arrangement would be more cost-effective than using the 

excellent visible-wavelength light available at midday.

Brooding pythons showing shivering thermiogenesis should be thermally 

detectable when they are shivering (among our giant constrictors, only P. molurus

unequivocally exhibits shivering thermiogenesis; Ellis and Chappell, 1987). 

Unfortunately, Burmese Pythons shiver only under certain circumstances. For example, 

the female may shiver if her clutch is somewhat cool, but not excessively so–if 

excessively so, the brooding mother gives up attempting to warm the eggs (Vinegar and 

others, 1970). We have no information on how often those circumstances would apply in 

the United States mainland. The more likely impediment to the use of thermal imaging, 

however, is that brooding pythons are usually hidden under dense and thick vegetative 

cover. Thus, there would most likely be few or no visual angles from which the 

temperature differential could be detected. Overall, the prospects for enhanced detection 

through infrared signals are likely to have strong seasonal and temporal limits when 

applied to giant constrictors in the United States mainland.
28

The relative difficulty of detecting giant pythons has vital implications for 

controlling the population using either trained searchers or volunteers. If the average perday-detectability of a python is 0.001 and there is no heterogeneity among individuals in 

their detectability, only a very small proportion of the population would be removed with 

even a very substantial searching effort. With the number of searchers plausibly 

available for python control, the rate of removal would be so low that natural recruitment 

(roughly 50 eggs per female per year; see individual Bioprofiles) would presumably 

exceed the capture rate.

An additional inference from so low a detection rate is that one should generally 

assume that roughly 1,000 pythons are present for each one detected in habitats similar to 

the Everglades. This should not be taken to imply that any individual sighting represents 

a phalanx of undetected comrades; on average there will be many undetected individuals 

(to the nearest order of magnitude, about 1,000) for each one detected. Naturally, the 

exact detection probability is unknown and would presumably increase in areas that are 

more accessible (for example, the rangelands and agricultural lands of Central Florida), 

but the realized detectability may be lower on private lands that do not have the large 

number of visitors and searchers found in the Everglades region. The point is not to 

focus on the exact value, but to appreciate that all searching activity is seriously limited 

by the low visibility of snakes, even giant constrictors, in low visibility and inaccessible 

environments.

Detecting a giant constrictor is a key step in killing it, but one does need to 

consider the challenges of capturing a snake that has been seen but not yet secured. Our 

experience in Florida is largely limited to Burmese Pythons and Boa Constrictors, both of 

which are remarkably easy to subdue unless they are very large or poised to flee into deep 

murky water. They are “easy” in the sense that when on land they generally do not bolt 

in response to visual contact with a searcher (in contrast to racers and other snakes whose 

predator defense hinges on rapid flight). Though spectacularly muscular, most giant 

constrictors do not move quickly at the sight of a human. In many cases they will hold 

until they or the surrounding vegetation is contacted. 

For safety reasons, no solitary searcher should touch a giant constrictor, but two 

people can safely handle giant constrictors up to about 3–4 m in total length. Larger 

snakes warrant additional handlers, though the basic approach is the same (noose or 

obtain a grip behind head, and use additional handlers to restrain coils). Several 

herpetoculturists have provided advice on the safe number of handlers needed for giant 

constrictors of various sizes (Barker and Barker, 1997; de Vosjoli and Klingenberg, 

2005). The latter paper cites with approval a guideline from the American Federation of 

Herpetoculturists that solitary individuals should attempt handling only giant constrictors 

less than 2.4 m (8 ft) total length, with an additional person present for each additional 

1.2 m (4 ft) in total length. These standards comport well with our personal experience.

Bites from giant constrictors can do considerable tissue damage (see a particularly 

ghastly wound from a Reticulated Python in Barker and Barker, 1997), so care is needed 

to avoid their many sharply recurved teeth. Rapid flight is rarely a problem with solitary 

Burmese Pythons on land. Individuals in shallow water can often be handled in the same 

way, with the additional challenge of determining where the head is located, as it is often 

obscured by murky water or vegetation. Capture of giant constrictors that are swimming 
29

in deep water is probably best handled by noosing with the support of sufficient handlers 

to keep the struggling animal from endangering the stability of the handlers or their boat.

Shooting. Shooting has often been suggested for safety in capturing and 

restraining giant constrictors. Shooting is most attractive when the target is readily 

detectable but will not easily allow a searcher to get close enough to permit hand capture 

or noosing. Thus shooting is often used for ungulates, birds, and carnivores, but is rarely 

used for small mammals (for example, rodents, with the exception of nutria), reptiles, or 

amphibians (Witmer and others, 2007). Shooting is often restricted in National Parks or 

urban settings for reasons of safety or visitor equanimity. Shooting may be warranted for 

reptiles that are particularly dangerous to touch (for example, venomous snakes or 

crocodilians); very large individuals of the giant constrictor species will fall into the 

dangerous-to-touch category. An appreciable drawback of reliance on firearms is the 

restriction to ricochet-safe areas and to use trained shooters. Mortally wounded reptiles 

(even those shot through the head) are notorious for crawling some distance before dying, 

and the head of a giant constrictor is a relatively small target; shooting can be an 

unreliable method of euthanasia. Shooting with a stun gun is used in Australia to 

incapacitate large crocodilians and probably could be used to advantage on giant 

constrictors as well (G. Zug, pers. commun., 2009).

Primary Tool IIb–Localize Targets: Public Reporting 

Trained searchers are the best for visual searching, but there are comparatively 

few of them (Stanford and Rodda, 2007). The general public is, or can be, well informed 

on the significance of sighting giant constrictors, and most people will eagerly submit 

such information to responsible authorities. Opportunistic sightings are presently being 

reported to the Python hotline in southern Florida (S. Snow: 305 815-2080). Hotlines of 

this sort have proven to be of enormous value in locating Brown Treesnakes on islands 

where Brown Treesnakes are not established (Hawley, 2007). In addition, volunteers can 

be induced to provide systematic searches in areas of special interest. Both approaches 

(opportunistic detections and volunteer searches) are powerful tools that not only address 

a crucial information need, but also generate interest and support among the public. 

The pool of volunteers is not unlimited in number or stamina, however. The key 

is to ensure that public reporting is used where it is most effective. Maximal efficacy is 

likely to occur in areas where sightings are so rare that early detection is at a premium, 

and too few professional searchers are available to address the need. Thus opportunistic 

call-ins and volunteer searches are most productively steered towards areas at the fringe 

of the known occupied range of an invasive giant constrictor. 

This focus on peripheral areas may conflict with the desires of volunteers. 

Volunteers are most enthusiastic about searches that result in many captures. Low 

reward rate is well known to diminish the efficacy of searchers, even trained searchers 

(Henke, 1998). The Brown Treesnake Rapid Response Team uses a variety of gambits to 

minimize the inherent problem of low reward rate; for example, the placement of 

physical target models (rubber snakes) in the search area with rewards for those who 

detect them. Volunteers also participate in training workshops conducted in high-targetdensity areas, with concomitant high-reward rates (Stanford and Rodda, 2007). Another 

approach is to ask the searchers to report other species of interest. In Florida, for 

example, searchers could be asked to report all snakes seen, so that the searchers’ minds 
30

stay focused on a vision of a slender cryptic reptile in the vegetation, while providing 

valuable baseline data on encounter rates with competing species at risk. 

One would expect that enthusiasm for volunteer searches would be highest in 

areas where the presence of the snakes is novel. As the population of invaders becomes 

better established, one would expect the pool of volunteers to wane. Unlike bird 

watching, which is an established form of recreation often conducted from the comfort of 

an automobile, searching for dangerous snakes in a swamp at night has fewer steadfast 

devotees. During a high-profile snake hunting contest conducted on Guam at the peak of 

the Brown Treesnake irruption, a fully loaded four-wheel-drive pickup truck was 

awarded to the duo who collected the most snakes in one month (Rodda and others, 1998, 

1999e). Out of a human population of more than 100,000 potential searchers, hundreds 

stepped forward to volunteer for snake searching, but at the end of the month 85 percent 

of the snakes had been brought in by just three pairs. The ecological consequence of this 

contest was that the island’s snake population was reduced by only an estimated 0.2 

percent (Rodda and others, 1998, 1999e).

Another consideration on the use of volunteers is that unlike Brown Treesnakes, 

giant constrictors are potentially dangerous to hunters, and misidentification of snake 

species in the southern United States can lead to fatalities. Legal liabilities need to be 

addressed forthrightly and in advance. Also, if volunteers are to be used in nature 

reserves (such as National Parks), some mechanism needs to be in place to ensure that 

volunteers do not collect native species opportunistically while they are on “patrol.”

In summary, hunting for giant constrictors by sight probably offers the best 

opportunity to involve the public in reporting new locations for a spreading population. 

However, the low detectability of giant constrictor snakes in heavily vegetated 

environments (especially in the southeastern United States) probably precludes use of this 

tool for eradication. Searching may yet prove to be an important component for local 

control, especially in high visibility environments such as mowed grass on public 

property. Research opportunities exist to enhance the effectiveness of visual searching 

by new optical tools, careful selection of searching times and conditions (for example, 

only on days colder than X degree in sites with abundant basking sites, and so forth.), and 

detailed knowledge of the behavior of snakes with regard to their visibility.

Primary Tool III–Localize Targets: Trapping

In combination with toxicants, trapping is the backbone of control for most 

secretive animals (Veitch and Bell, 1990; Witmer and others, 2007). The majority of 

Brown Treesnakes are taken by traps (Vice and Pitzler, 2002). The vast majority of 

invasive rodents are taken with toxicants (Veitch and Clout, 2002). As both tools rely on 

food-based attractants, at least as now operationally deployed for snakes, the primary 

difference is that traps provide greater opportunity for the release of non-target species, 

but traps tend to be more expensive to build, deploy, and maintain.

The primary drawback for trap control of giant constrictor snakes is that there are 

not yet any designs for giant constrictor snake traps that have demonstrated effectiveness. 

A variety of ad hoc designs have been reported in the literature, but their effectiveness 

has neither been demonstrated nor quantified (for example, Auliya, 2006). Thus they 

may or may not be practical. Though traps have proven adequate for local eradication of 

Brown Treesnakes (Campbell, 1996), Brown Treesnake traps suffer from inability to 
31

capture juvenile snakes (Rodda and others, 2007; Tyrrell and others, 2009). Thus 

discovery of an effective giant constrictor trap design does not guarantee that traps can be 

used efficiently for eradication (which requires the removal of all individuals).

The scope for improvement of giant constrictor traps is probably substantial, as 

the capture rate of Brown Treesnake traps was improved by about two orders of 

magnitude during research on that tool (Rodda and others, 1999b). However, there are 

several additional challenges associated with trapping giant constrictors. Most obviously, 

the traps must be much larger; this implies significantly greater challenges in fabricating, 

moving, deploying, and storing giant constrictor traps. Also, Guam is largely upland 

habitat, with no inundation at trap locations. Traps for use in South Florida, or for any 

species that is often found in water (that is, most of the giant constrictors) would need to 

accommodate a wide variety of water depths and periods of inundation. Giant constrictor 

traps used in the mainland United States would need to provide avenues for escape or 

release of the many species of non-targets present in the focal area. Among the key nontargets are rats (which are sometimes endangered and in all cases prone to damaging traps 

through gnawing), alligators (somewhat dangerous to remove from traps, and inclined to 

damage traps when trying to escape), raccoons (notorious for dexterous destructiveness), 

large species of protected snakes such as Indigo Snakes (cannot be automatically released 

without allowing small giant constrictors to leave), venomous snakes (dangerous), and 

birds (cannot be left in traps for extended periods). However, if traps are checked 

regularly (for example, daily), it should be relatively straightforward to release the nontargets safely.

A key consideration for traps is that giant constrictors appear to be primarily sitand-wait or ambush foragers (Shiroma, 1999, discusses trapping implications). A 

fundamental premise of detection is that moving targets are best detected with stationary 

devices and stationary targets are best detected with moving devices (Koopman, 1946). 

To the extent that giant constrictors sit and wait for long periods of time, they may be illsuited to control with stationary traps. However, some traps are designed to force the 

occasional movements of a target into the vicinity of a trap. When applied to reptiles, the 

forcing device is called a drift fence, and it works for many species because the moving 

animal tends to follow the line of the fence rather than change to an entirely new 

trajectory. Basically the traps are placed alongside long stretches of barrier fencing; a 

snake that runs into the barrier slithers along the barrier until it runs into a food-attraction 

or barrier trap, which effectively collects the snakes from an area defined by the length of 

the fence. As fences can be made very long, drift-fence traps can be highly effective, as 

they are for the sit-and-wait Habu (for example, Hayashi and others, 1999). Whether this 

concept will work for giant constrictors remains to be seen (anecdotal evidence suggests 

that this concept does not work for Brown Treesnakes; our unpub. data).

The primary limitation of traps as a large-scale management tool is that they 

provide no economies of scale. Two hundred traps cost about twice as much to run as 

one hundred traps, largely because the cost is mostly the labor of baiting, checking, and 

maintaining the traps. This limits the size of the area that can be controlled with trapping. 

We are aware of no snake trapping operations that exceed a few thousand traps. The size 

of the area covered by a few thousand traps depends on trap spacing. The successful use 

of traps for sit-and-wait snakes has required very short intertrap spacings, such as 10 m 

(presumably because sit-and-wait snakes move only short distances per day; see Shiroma 
32

and Akamine, 1999; Katsuren and others, 1999). At 10-m spacing, each hectare requires 

100 traps (Shiroma, 1999, suggests 200 traps per hectare), and thus 1,000 traps would 

service about 10 ha (about one-twenty-fifth of a square mile). In contrast, Brown 

Treesnake traps are fully effective when deployed at about 16/ha or one-eighth the trap 

density of Habu traps. In summary, traps may be a cost effective tool for local control of 

invasive giant constrictors, but they need R&D and probably cannot be used for 

eradication of extensive colonizations.

Primary Tool IV–Localize Targets: Detector Dogs

Detector dogs are widely used for transportation system interdiction of biological 

agents, including invasive snakes (Shiroma and Ukuta, 1999; Engeman and others, 2002). 

We can call such dogs cargo-sniffing dogs. Dogs are often used as an adjunct to hunting 

ungulates such as pigs or goats (hunting dogs). However, dogs are rarely used for 

landscape control of invasive reptiles, with the exception of some experimental use for 

Brown Treesnakes (USGS unpub. data) and Habu (Shiroma and Ukuta, 1999). Our 

experience with them is that they are reasonably good at finding Brown Treesnakes even 

when the snake is too high in the trees for the dog to physically contact, but there are 

challenges in searching dense vegetation with detector dogs (we have not attempted to 

locate snakes in association with water, but doubt its practicality for that application). 

Overall, USGS detector dogs on Guam have detected a known radiotelemetered snake in 

a defined search plot in about 35 percent of our trials. This indicates that a substantial 

fraction of the snakes was not located, but that dog teams are far more effective than 

human searchers (humans detect 7 percent of available snakes per search at 16-m searchtransect spacing on Guam; Christy and others, unpub. data). Thus there may be a potent 

role for detector dogs in locating giant constrictors, at least in appropriately accessible 

habitat. This discussion is focused on dogs, but there is no compelling reason why other 

types of sniffer animals could not be used. However, the limitations on detector dogs 

would apply to a wide variety of other sniffer species as well (see below). 

A likely drawback of sniffer dog detection of giant constrictors is that the dogs, 

which are remarkably expensive to train and maintain, could be killed by their targets. 

All of the giant constrictors have demonstrated an appetite for dogs. Even if an attempted 

ingestion were to be unsuccessful, the dog could become reluctant to hunt for giant 

constrictors if it had a bad experience with one. This was the outcome of a detector dog 

encounter with a Habu (Shiroma and Ukuta, 1999); after getting bitten it was unwilling to 

hunt snakes. Perhaps a large dog could be trained to signal the presence of a giant 

constrictor at a safe distance. Considering the various limitations on use of a sniffer dog 

for detection of giant constrictors in the United States (such as limited searchable habitat, 

time of day, size of snake, and so forth), it seems unlikely that detector dogs would play a 

large role in the landscape-level control of a giant constrictor. Where they might be 

indispensible is in detection of the last few snakes in an incipient colonization that was 

detected early enough to carry out an eradication by other means, or perhaps when 

attempting to define the limits of the expansion of a population. Training of a sniffer dog 

requires a substantial commitment of resources for facilities, and human and dog training. 

If the dog were to be used to determine if an eradication had been completed, the dog 

would have to be able to distinguish the target giant constrictor species from native 

snakes (false positives are unacceptable when the question is whether any of the targets 
33

remain). Experience with detector dogs indicates that this capability is well within a 

good dog’s abilities, but requires lengthy training (probably >1 yr). Training of detector 

dogs is unlikely to be justifiable for a giant constrictor species that is not yet known to be 

present in the United States. If training was delayed until the population was 

unequivocally established, the requisite training and testing time might be too much for 

the dog team to be available for the timely eradication of an incipient population. These 

factors warrant due consideration in any plan to use detector dogs for giant constrictor 

control.

Primary Tool V–Kill Unlocalized Snakes: Toxicants

Toxicants are the premier tool for eradicating introduced rodents (Wodzicki, 

1978; Veitch and Bell, 1990), and they have also been used for invasive birds, mammals, 

and carnivores (Witmer and others, 2007). Rodents are particularly attractive targets for 

toxicants because rodents gnaw impulsively, some species cannot regurgitate, and their 

small size and high metabolism dictate nearly continuous feeding (Wodzicki, 1978). 

Snakes, on the other hand, often volunteer to go without eating for months at a time; this 

is especially true of giant constrictors (see Bioprofiles; Murphy and Henderson, 1997, 

recount multi-year fasts among most of the giant constrictors). Toxicants 

(acetaminophen) have been successfully applied on a local scale for Brown Treesnakes, 

and ongoing work may perfect aerial application for landscape scale use on Guam 

(Savarie and others, 2007). However, there are few or no non-target species on Guam; 

there are no native terrestrial snakes on Guam (Rodda and others, 1999c). The key 

challenge for toxicant application to invasive giant constrictors on the United States 

mainland is finding a way to prevent harm to non-target species, especially non-target 

snakes. In the southeastern United States, a special challenge is associated with 

protecting the Indigo Snake (Drymarchon couperi), both a State and Federally listed 

Threatened Species. Because the Indigo Snake is large (maximum total length 2.8 m) 

and has a diet that overlaps with that of the giant constrictors, it is challenging to identify 

a device that would exclude this particular non-target while poisoning all sizes of giant 

constrictors. The Indigo is of special interest, but many other vertebrates may take 

poison baits intended for giant constrictors, including most carnivorous or omnivorous 

birds, mammals, and reptiles (and larger frogs), and as far as is known all of these native 

species are vulnerable to acetaminophen toxicity. Brown Treesnakes are very slender, 

facilitating their exclusive entry into narrow cylindrical bait stations, but the giant 

constrictors are relatively, well, giant, making it more challenging to configure a bait 

station that would allow their unimpeded entry while defeating such clever and dexterous 

competitors as raccoons, or such numerous and relentless non-targets as crabs and rats. 

Scientific opinion is divided on whether this problem is surmountable, but no definite 

ideas for doing so have emerged. Toxicant developers routinely struggle with the issue 

of preventing non-target take of poison baits, and we see little evidence that any of the 

mechanisms proposed for other species would be applicable to acetaminophen use on 

giant constrictor snakes on the United States mainland.

Although we are not aware of any giant snake toxicants that are chemically 

harmless to the non-targets of concern, it is possible that such a compound may exist. 

The cost of identifying and registering such a product would be sensitive to the 

availability of appropriate pharmacological information. Acetaminophen was chosen for 
34

Brown Treesnake poisoning because it was already registered for other uses and was 

associated with vast quantities of pharmacological data. At the time of acetaminophen’s 

registration, the registrants judged that only known products were within the economic 

range of practicality for the substantial expense of obtaining a registration for use on 

Brown Treesnakes. Registering an entirely new product takes years of expensive testing 

and perhaps a decade of experimental documentation. Presumably the same fiscal 

constraints would apply for obtaining a registration for a giant constrictor toxicant. There 

is presently no toxicant registered for use on any snakes in the United States mainland. 

An additional challenge for giant constrictors is that the dosage would need to be 

sufficient to poison snakes weighing up to about 150 kg (for anacondas; less for smaller 

species). This is crucial, because many of the non-targets have smaller body masses and 

would therefore be vulnerable, even if the toxin were to be relatively less toxic to them. 

For example, acetaminophen is highly toxic to felines including domestic cats, a nontarget on Guam. Cats are protected to a degree on Guam because most cats are an order 

of magnitude or more heavier than a typical Brown Treesnake. Whereas cats may be 

sickened by ingestion of a Brown Treesnake bait on Guam, they would not receive a 

lethal dose, as each poison bait contains only enough to kill a Brown Treesnake. 

However, the non-targets on the United States mainland are uniformly smaller than an 

adult giant constrictor, creating an additional challenge for the identification of a safe 

toxin.

Summary of Primary Tools

Given current knowledge, the primary tools for eradicating invasive reptiles are 

traps and toxicants. Both have been used successfully for control but not eradication of 

invasive snakes. However, there are significant non-target challenges in using toxicants 

for giant constrictors on the United States mainland. It is very difficult to see how one 

would poison invasive snakes without considerable and possibly unacceptable ancillary 

damage to non-target carnivorous or omnivorous birds, mammals, amphibians, and 

reptiles. In the one use of snake toxicants to date (acetaminophen for Brown Treesnakes 

on Guam), the poison baits (dead neonatal mouse with acetaminophen tablet inserted 

inside) appear generally effective for adult snakes, but are not eaten by smaller Brown 

Treesnakes (our unpub. data). There is no reason to think that such a limitation would 

apply with equal force to giant constrictors, but the existence of such a limitation for the 

well-studied Brown Treesnake is a reminder that one cannot assume that any 

hypothesized tool for giant constrictors will be free of such constraints. Given the current 

state of knowledge and funding, it would be presumptuous to assume that any 

colonization of a giant constrictor could be eliminated through the use of toxicants, or 

even that an appropriate toxicant could be discovered and registered in time. 

Traps are also likely to have significant limitations for eradication of an incipient 

colonization. Above all, the practical size of an area that could be trapped is small, and 

thus it would be necessary for an incipient colonization to be discovered and treated 

before the population spread very far. Successful traps for giant constrictors are presently 

unknown, with an unknown time interval needed to develop effective models, if that is 

even possible. Regardless, there may be serious constraints on their use, potentially 

including a high requisite trap density (as the giant constrictors are sit-and-wait predators 

and thus would be aware only of nearby traps), limitation of use to favorable substrates 
35

(may not be practical in areas of inundation or impassible vegetation), vulnerability to 

wildland fires, and high labor and transportation costs needed with frequent monitoring to 

release non-targets unharmed. However, there appears to be no intrinsic obstacle to the 

development of cost-effective traps for effective local control, if timelines are sufficiently 

accommodating. For example, traps could be an essential component of keeping 

Burmese Pythons from spreading to the Lower Florida Keys, or to reduce their impacts 

on colonial bird rookeries.

Other tools that have been used for invasive snakes are visual searching (unlikely 

to result in eradication unless the infested area is very small) and detector dogs (most 

useful for monitoring the completion of an eradication using some other tool).

Secondary Tools

Secondary Tools I–Judas Snakes

Judas animals have been used most often for highly social ungulates, such as 

goats, as part of an eradication program based on shooting. The first several goats taken 

in such an eradication effort are live-trapped and radiocollared, then marked 

conspicuously (should be visible to any hunter) and released. The released goats seek out 

their own kind and accompany them, allowing radiotrackers to locate and decimate each 

Judas goat’s herd at will. When its last compatriot is shot, the Judas goat will most likely 

seek out a new herd to accompany, with the same detrimental effect on its colleagues. 

The Judas animal approach has been invaluable for invasive goat removal, as it 

eliminates the need to find the herd, costs only one radiotransmitter per Judas animal, 

provides locality information for all goats, including both sexes and all sizes, and 

continues working as long as the radio works (and the Judas animal lives).

The application of the Judas animal stratagem to giant constrictors is more 

circumscribed, as giant constrictors seek out their own kind primarily or exclusively for 

mating. Thus juvenile snakes are not likely to be vulnerable to exposure by the Judas. 

However, many of the giant constrictors accompany their prospective mates for long 

periods of time during the mating season. Most notable are anacondas, Burmese Pythons, 

and Southern African Pythons (boas accompany their prospective mates in small parts of 

their range, and the other species may do so under some circumstances, but long-duration 

cohabitation appears limited to a few species). From a demographic perspective the most 

valuable targets are adult females; thus one would radio-mark adult males, which could 

lead one (at the appropriate season) to adult females. On an informal basis this has been 

done successfully with Burmese Pythons in Everglades National Park (S. Snow, pers. 

commun., 2007). A constraint on this approach with giant constrictors is that the Judas 

snake would generally lead one to only a single female at a time. Whereas several males 

might be accompanying this female (so the yield of captured snakes may be more than 

one per tracking event), the yield would rarely include more than one female. The cost of 

the requisite radiotracking is substantial (radios must be surgically implanted; snakes may 

need to be followed for weeks before one finds them with a mate), and the practicality of 

this approach will hinge on the “force multiplier”: how many targets are killed for the 

cost of the radiotracking effort. As with trapping, it is difficult to obtain any economies 
36

of scale when using Judas snakes, but the approach works and may benefit from research 

to make it more cost effective.

Secondary Tools II–Biocontrol

Biocontrol takes two forms: predator-based and pathogen-based. Predator-based 

biocontrol is the introduction of a higher order predator, which it is hoped will depress 

the population of the target. Pathogen-based biocontrol is the introduction of a disease, 

parasite, or fungus that one hopes will be fatal and highly contagious, and thereby depress 

the population. The primary advantage of biocontrol over other tactics is that the agent of 

control is self-sustaining. The primary disadvantage of biocontrol is that the agent of 

control is self-sustaining; if it has undesirable effects, there may be no way to eliminate 

it. If it fails to sustain itself, it would not continue to control the target. An ideal 

biocontrol agent would attack only one species and be just successful enough to sustain 

itself without impacting non-target species, and with enough demographic vulnerability 

itself that it could be destroyed if conditions warranted doing so. Caudell (2001) 

screened 57 potential Brown Treesnake predators, but found that none was socially 

acceptable as a biocontrol agent introduction to Guam. Vertebrate predator-based 

biocontrol has been attempted on numerous occasions, especially on islands (Davis and 

others, 1976; Howarth, 1999). The record of successes is small, and the number of 

catastrophic failures (non-target species decimated or even driven extinct by the predator) 

is large (Davis and others, 1976; Bykovskii and Kandybin, 1988; Howarth, 1991). As a 

result, it is against U.S. Fish and Wildlife Service policy to introduce predator-based 

biocontrol agents for vertebrates (there is also a question as to which higher order 

predator would qualify for preying on giant constrictors!), and we will not entertain that 

notion further.

Pathogen-based biocontrol has a stronger track record, but almost exclusively 

with invertebrate hosts (Davis and others, 1976; Bykovskii and Kandybin, 1988). For 

example, Waage (1997) (and Waage and Mills, 1992) as President of the International 

Association for Biological Control strongly promoted biocontrol agents for invertebrates, 

but opined that no acceptable agents were known for vertebrates, because the putative 

vertebrate agents lacked sufficient species specificity. Furthermore, whereas 

invertebrates have rudimentary immune systems, they lack the ability of vertebrates to 

rapidly mount immune responses triggered by pathogen-specific structures such as tcells, and to produce new types of t-cells in response to a new pathogen (Davis and 

others, 1976). Vertebrate immune systems tend to interact with pathogens to produce 

progressively lower virulence in the biocontrol agent over generations (Dobson and 

others, 1992; Nokes, 1992). This has been a strong pattern, with few vertebrates that 

have been subjected to arguably successful pathogen-based biocontrol (Nokes, 1992). 

The most often cited example is the use of rabbit hemorrhagic fever and myxoma virus 

with rabbits in Australia (Mutze and others, 1998). Both resulted in a significant 

depression in the rabbit population immediately following the release of the disease 

agent, but in both cases the rabbits recovered in a few generations as the pathogen 

evolved lower virulence (Fenner, 1983; Bykovskii and Kandybin, 1988; see also Berthier 

and others, 2000).

Pathogen-based biocontrol has been studied by a number of researchers in Brown 

Treesnakes (primarily native range parasites and paromyxo virus) and Habu (primarily 
37

Entamoeba invadens), with no appropriate agents yet identified. Nichols (1990, 1999; 

Nichols and Lamirande, 2001) studied captive Brown Treesnakes infused with paromyxo. 

Dobson (1988) proposed the use of Entamoeba invadens for use on Brown Treesnakes, 

but provided no data (see Telford, 1999, for support for use of parasites). Others have 

screened a variety of putative control agents from natural populations (classical 

biological control) of Brown Treesnakes (MacKerras, 1961; Schad, 1962; Ewers, 1968; 

Whittier and others, 1997; Caudell and others, 2002), but found no agent that appeared to 

lower populations. Ishii and coworkers have long pursued use of Entamoeba invadens 

for population reduction in Habu (Ishii and others, 1971, 1976, 1979, 1981; Ishii and 

Noboru, 1971; Ishii and Hayashi, 1978; Ishii and Sawai, 1999), without advancing to 

field trials or identifying a species-specific or highly virulent strain.

It is noteworthy that biocontrol has not been used for the eradication of invasive 

species (Simberloff, 2001). This is due to the unlikelihood of a disease agent causing the 

extinction of its own species-specific host (and therefore itself). In general, disease 

transmission slows and stops as the hosts become rare; thus pathogen-based biocontrol 

has not been known to extirpate an introduced host (Holmes, 1982). When a disease 

agent causes the extinction of a host, it is normally because the extirpated host is a 

secondary host (secondary in the sense of lesser importance), and the numbers of the 

pathogen are sustained by a persisting primary host (Holmes, 1982). However, one 

would rarely get permission to release a pathogen in order to kill a secondary host, as that 

implies a greater risk to a non-target species. If the primary host was also an undesirable 

invasive species, one can imagine that such a scenario could be approved by regulatory 

authorities, but we are hard pressed to envision such a scenario involving giant 

constrictors on the United States mainland.

One highly significant impediment to the application of biocontrol for giant 

constrictors or any snakes is that the science of snake epidemiology is primitive in the 

extreme. Virtually nothing is known about the virulence, spread, or pathways of diseases 

of snakes in the wild. Most snake diseases, such as the paromyxo studied in Brown 

Treesnakes in the 1990s (Nichols, 1999), are unknown in the wild and may not propagate 

there, though they do impact giant constrictors in captivity (Manvell and others, 2000). 

Viruses are also notoriously unstable and prone to jump to non-target hosts (Hoddle, 

1998). Presumably one would have to conduct a very substantial body of research to 

obtain the necessary biological facts about a putative control agent. For example, in the 

early 2000s a blue-ribbon panel was convened (under pathologist Thierry Work, USGS, 

Honolulu) to guide research into pathogen-based biocontrol of the Brown Treesnake. 

The panel concluded that such a research program was warranted only if one could 

devote about 10 years and $50M to the program, as lesser amounts were unlikely to 

progress to the point of a suitable agent. Many proponents of pathogen-based biocontrol 

have advocated the use of advanced genomic science to enhance the virulence of a 

pathogen-based agent (Interdisciplinary Center for Biotechnology Research, 1992). 

While we do not doubt that such an enhancement is possible, one has to presume that 

such an enhancement would engender substantial additional research cost, both for 

development and testing of the novel biological product. In most scenarios, the 

genetically modified enhancement would be vulnerable to the evolutionary pressures that 

tend to diminish a pathogen’s virulence over generations. While pathogen-based 
38

biocontrol of giant constrictors in the United States mainland might be possible, the 

requisite costs seem beyond that which would be available for this purpose.

Secondary Tools III–Habitat Manipulation

Habitat manipulation is a tool to manage pests, not eradicate them. Habitat 

manipulation of a gross sort is commonly used for control of Habu (Mishima and others, 

1999) and Brown Treesnakes (Campbell and others, 1999), for example, by removing 

trees from the vicinity of transportation facilities. Brown Treesnakes are less inclined to 

cross mown grass to reach an objective, though they are capable of doing so. We can 

think of no habitat manipulation that would absolutely prevent incursion by a Brown 

Treesnake, and the same limitation is likely to apply to giant constrictors in the United 

States mainland.

Secondary Tools IV–Bounties and Adding Value

Bounties do not have a very favorable reputation for the management of invasive 

species, in that they tend to pay people for activities that the people would do anyway 

(hence there is no gain), and bounties rarely result in eradication of a colonization 

(Singleton and others, 1999). Typically bounties lose their potency as the target species 

becomes rare because the financial reward becomes an insufficient motivator for the 

progressively greater effort needed to capture the remaining individuals. Bounties have 

never been used successfully with invasive reptiles. Given the various restrictions on 

access inherent in the complex wetlands of southern Florida, it is difficult to see how a 

bounty could be raised to a high enough level to stimulate hunting of rare giant 

constrictor snakes in all suitable habitat.

If the bounty was raised to a level that would stimulate such hunting, one risks 

creating a perverse incentive for hunters to both distribute the pest to new areas (spread 

the wealth-yielding opportunities) and lobby for continuation of the lucrative business 

(that is, prevent the pest’s extirpation). One can even imagine hunters holding a few 

individuals back to insure continuation of their business model. In St. Lucia, Lesser 

Antilles, bounty hunters turned to breeding Bothrops snakes (Fer-de-Lance) for the high 

bounty payments they could obtain from the litters of nearly 100 neonates (Devaux, 

1995). The giant constrictors also have very large litters/clutches. This perverse 

potential of bounties can be generalized by noting that any feature that adds value to an 

invasive species (selling the hides for leather, making tourist trinkets of them, and 

promoting safaris to come see the novelty) creates economic pressure to assure the 

population’s continuation, and further spread. If eliminating an invasive is a social good, 

one should not create economic incentives for its persistence.

Secondary Tools V–Frightening Devices

Snakes appear relatively insensitive to sound (they can hear, but do not seem to 

react to the sounds that startle humans, birds, and mammals; doesn’t every visitor bang 

on the glass of snake cages at zoos, to no avail?), and thus audible frightening devices of 

the sort used to scare birds from airports or crops are unlikely to be successful. With 

reference to the Reticulated Python, Wall (1926) stated, “It lies for hours completely 

inert, treating contemptuously all efforts on the part of spectators to provoke a 
39

movement.” Visual cues and chemical means are the primary sensory channels that have 

been suggested for repelling snakes. Both have been the subject of experiments with 

Brown Treesnakes (McCoid and others, 1993; Caprette, 1997), though only chemical 

sprays have been shown to have any effectiveness (see McCoid and others, 1993, for the 

failure of a static chemical barrier). Chemical sprays such as cinnamon oil (including 

cinnamon-scented air-freshening sprays) will induce Brown Treesnakes to flee from close 

confinement (Clark and Shivik, 2002), such as in cargo, but there has been no suggestion 

that this tool would have application on a landscape scale. We expect that the same 

limitation would apply to repellents/ejectants discovered for use on giant constrictors.

Secondary Tools VI–Reproductive Inhibition

The first question to ask of any reproductive inhibitor is, “In what way is 

contraception an improvement over a toxicant?” Bomford and O’Brien (1997) 

formalized the search for circumstances favoring contraception and found few applicable 

to Australia, as few contraceptive systems were effective or cost effective. When dealing 

with charismatic large mammals in urban settings (typically deer), reproductive inhibition 

may be chosen due to animal welfare promoters’ support for nonlethal tools (the public 

would prefer not to kill ‘Bambi,’ but they’re okay with deer being sterile) and concern for 

non-target death of pets (which is a risk with many toxicants in urban areas). There is a 

more subtle advantage of reproductive inhibitors that applies to social species in which 

the dominant male (typically a harem master) behaviorally suppresses the reproduction of 

same-sex social inferiors (Caughley and others, 1992; Kirkpatrick and others, 1997). In 

this case, one gets a multiplier; for every dominant male whose reproduction is 

suppressed, several subordinate males are also kept from reproducing. In contrast, if the 

dominant male in such a species were to be killed, the harem master would quickly be 

replaced by another sexually competent male. Thus there are three situations in which 

reproductive inhibition might be favored over ordinary toxicants (Guynn, 1997): (1) 

environments where non-target victims of a toxicant are highly valued (for example, 

Endangered Species); (2) target species that society values alive, even if they are sterile; 

and (3) behavioral dominance that keeps subordinates from breeding. Giant constrictors 

are not known to be a magnet for animal welfare concerns, nor are they deemed better 

alive and sterile than dead (pt. 2), and giant constrictor societies are not known to have 

harem masters2

Could a reproductive inhibitor be found that would impair the reproduction of 

giant constrictors in the United States mainland but not that of non-targets such as Indigo 

Snakes? If so, reproductive inhibitors would have a clear advantage over use of a 

toxicant. Unfortunately, nothing is known of reproductive physiology of giant 

constrictors that suggests a uniqueness that could be exploited for the purpose of 

contraception. Hormonal and immunological systems are thought to be relatively 

conserved in the evolution of snakes (Hoddle, 1998) and would likely be identical 

between giant constrictors and native American snakes. 

(pt. 3), but many of the non-targets associated with giant constrictor 

toxicant use are highly valued (for example, Indigo Snakes); thus point 1 might apply in 

some contexts.

 2 Subordinate individuals defer to the harem master, a pattern unknown in wild giant constrictors. Malemale scramble competition is routinely seen, but this does not appear to have an element of deferring to the 

dominant male except in captivity (Barker and others, 1979) where escape is impossible.
40

This would appear to leave only two options for reproductive inhibition of giant 

constrictors: (1) embark on a study of the reproductive physiology of giant constrictors in 

the hope of discovering a vulnerability specific to these species (basic research 

approach), and/or (2) prepare a delivery system that would keep native snakes from being 

exposed to a generalized inhibitor of snake reproduction (exclusive delivery system). 

Both approaches have potential, but the basic research approach runs a high risk of being 

time consuming, expensive, and fruitless, and a putative exclusive delivery system could 

just as easily be applied to toxicants (so why not apply it to toxicants?). As we discussed 

in the toxicant section, there is no intrinsic reason that such an approach would fail, but 

no one has yet suggested a plausible means to make it succeed. 

In summary, reproductive inhibition remains a theoretical possibility, but would 

require a substantial investment in research to discover a physiological basis for 

distinguishing native snakes from giant constrictors, or invention of a non-targetexcluding delivery system more likely appropriate for toxicant application.

Secondary Tools VII–Egg/Nest Destruction

Egg/nest destruction is normally used only for birds, usually colonial nesting birds 

whose nests are easy to find and destroy (but the birds themselves flee successfully). 

Python nests (the other giant constrictors are live bearers and do not have nests) are 

exceedingly difficult to find and are almost always guarded by the brooding female, 

which is unlikely to flee. If one can find brooding females, it would be more effective to 

destroy them.

Secondary Tools VIII–Pheromonal Attractants

Pheromonal attractants have been suggested as the solution to the non-target 

problem because they are species-specific. That is, if the attractant in a trap, toxicant, or 

reproductive inhibitor is a chemical messenger (pheromone) recognized only by the 

target species, few non-targets would fall prey to the trap/toxicant/reproductive inhibitor 

(Mason, 1999). Furthermore, if evolved resistance to the delivery system is a concern 

with long-term lethal control, one need not fear evolved resistance of a species to its own 

pheromone(s). Evolved resistance should not be a concern if the introduced population 

were to be eradicated, but could become a concern if eradication failed and chronic 

control of the invasive species were to become necessary. Thus chemical 

characterization and synthesis of a pheromone that draws giant constrictors into a trap or 

to a poison bait would be a scientific accomplishment of the first magnitude; we see no 

downside to this.

However, it could be difficult to do for the following reasons: 

(1) Only one snake pheromone has been synthesized to date (Mason, 1992); 

considerable work on other snake systems including Brown Treesnake pheromones has 

failed to isolate the compound (Waye, 2007, was originally an attempt to isolate the 

Brown Treesnake courtship pheromone). No one is certain how to solve this problem of 

analytical chemistry. If the Brown Treesnake pheromone being pursued turns out to be a 

mixture (“bouquet”) of compounds rather than a single chemical, it could be very 

challenging to identify the appropriate mixture as the number of potential combinations is 

astronomical, and non-targets might respond to the mixture or some of its constituents.
41

(2) If a pheromone were to be successfully synthesized, it might induce a behavior 

that is not very useful. For example, Brown Treesnake mating pheromones clearly 

induce Brown Treesnake males to attempt copulation with a pheromone-containing 

substrate, but they have not been found to draw the snakes into a trap or ingest a bait.

(3) Most snake pheromones are believed to be nonvolatile (Mason, 1992). That 

is, instead of wafting through the air, there must be a trail of the pheromone leading to the 

target. In theory such a trail could lead a snake into a trap or to a bait, but to date no one 

has induced a snake to do that. Substrate-born attractants draw snakes in from only a 

very limited area (in contrast to airborne compounds), and there may be significant 

practical problems associated with laying the trails inexpensively and as often as 

required. Anecdotal suggestions that pheromones may be occasionally volatile in pythons 

(for example, Diamond Pythons, Morelia spilota; Brien and others, 2008) have not been 

confirmed by research. 

(4) The leading model of snake attractant pheromones (Mason, 1999) has the 

pheromone being released by the skin of gravid females, inducing approach by sexually 

competent adult males. Though this has been studied and documented in Brown 

Treesnakes and other colubrid snakes, it very probably occurs in giant constrictors as 

well. A problem is that such a system captures only adult males. Adult males are the 

most demographically expendable stratum of the population. Unless one eliminates all 

adult males repeatedly until all of the juvenile males have matured and been killed, the 

population reduction may be minor.

(5) It makes biological sense that a sexual pheromone should decay very rapidly, 

as no male snake would want to be chasing a scent trail left weeks earlier (whose

perpetrator may be long gone). An artificial scent trail that needed frequent renewal 

could be biologically successful but impractical.

(6) To be useful for managers, a pheromone must be species specific, but the 

discovery of a species-specific pheromone requires considerable work with each species. 

One has difficulty envisioning that the potentially large expense of discovering, isolating, 

characterizing, and synthesizing a novel pheromone would be funded unless a specific 

species were established in the United States to justify such a protracted research project. 

In that case, the pheromonal product would be unlikely to be available in time to play a 

role in eradicating an incipient colonization, and there is no guarantee that an appropriate 

pheromone exists. Whereas the pheromonal “bullet” is potentially silver, it is unlikely to 

be timely for the purpose of preventing establishment. If a synthetic pheromone becomes 

available but attracts only adult males, it may still be useful for early detection of 

individuals expanding the species’ range into new areas (for example, pheromonal 

“sentinel traps” in the Florida Keys).

The leading pheromone attractant model mentioned above (female scent draws 

adult males) is not the only possibility. It is theoretically possible for a pheromone to 

apply to both sexes and all ages. For example, some snakes are attracted to scent left by 

conspecifics of any age or sex (Burghardt, 1980, 1983). Waye (2007) found evidence for 

the existence of a pheromone that promoted aggregation in Brown Treesnakes. Such 

social (as opposed to sexual) pheromones could prove very useful for population control. 

To our knowledge such social pheromones have not been isolated, much less 

characterized or synthesized for any snake species, but they remain a very attractive 
42

scientific goal, especially if they should prove to be species specific, chemically stable, 

and inexpensive to make. 

Summary of Secondary Tools

Of the various secondary tools we have considered, one is likely to be actively 

detrimental (bounties or adding value), and three are unlikely to be useful for landscapescale control (habitat manipulation, frightening devices, and egg/nest destruction). A 

fourth (Judas snakes) has potential for local population control, but seems difficult to 

scale up to the level needed for landscape control. Biocontrol, reproductive inhibition, 

and pheromones are potentially applicable at almost any scale. Biocontrol is ill-suited for 

eradication and success would likely require an unprecedented investment in ophidian 

epidemiological research. Biocontrol has not had long-term success for vertebrate hosts, 

but it has the theoretical potential for reducing snake populations on a large scale. For 

reproductive inhibition to be species specific and an improvement over species-specific 

delivery of toxicants, it would require either an unexpected breakthrough in our 

understanding of basic snake reproductive physiology, or a delivery system that achieved 

species specificity in a way that would not be available for delivery of a toxicant. No 

such system has been suggested. Pheromones are a possible silver bullet, but the leading 

paradigm for their use would deplete only adult males. Research into species-specific 

social aggregation pheromones could solve this problem, but to date no such compound 

has been characterized for any reptile.

Overall Summary

Many tools have the potential to benefit from additional research, but none is 

ready for landscape-level control or eradication of giant constrictor populations. 

Pheromone research might open a new pathway for control or eradication, but would 

most likely require an appreciable increase in the level of research funding. It is highly 

likely that visual searching and traps will continue to be the primary tools for control of 

invasive giant constrictors. Improvement in these primary tools is probable as a result of 

continued research, but current methods exhibit capabilities to eradicate an incipient 

population only if the measures are vigorously invoked at the very beginning of a 

colonization. However, a monitoring and response program for timely detection of 

incipient colonizations of reptile invaders has not yet been established on the United 

States mainland, and thus eradication is unlikely to be successful.
43

Chapter Four–The Indian or Burmese Python, 

Python molurus

Biological Profile

1.0 Introduction

Each species in this risk assessment package is described and evaluated in three 

sections: the biological profile; the management profile; and the formal risk assessment. 

The former two sections are part of the respective species’ chapters (Four–Nine), whereas 

the formal risk assessments for all species are merged in chapter Ten. Each bioprofile 

outlines the biology of the species, largely as it is manifest in the species’ native range, 

though in some cases the biology is better documented where the species is introduced. 

Each management profile addresses the practical extent to which the species can be 

managed in the United States. This covers questions such as: Where might it occur in the 

U.S.? How soon are officials likely to be aware of a new population? Would it be 

difficult or expensive to eradicate a new population? These are questions germane to 

assessing the risk of establishment. However, the formal risk assessment is the third 

section (in chapter Ten), and it considers all nine species simultaneously, ranking each 

species in relation to each identified risk factor, following the format suggested by the 

Aquatic Nuisance Species Task Force (1996; see also Orr, 2003).

1.1 Species

The species Python molurus ranges widely over southern and southeastern Asia. 

At times the species has been divided into subspecies recognizable primarily by color 

pattern (Barten and others, 1985; Ross and Marzec, 1990). For example, the dark form in 

Sri Lanka was historically distinguished as pimbura, though that name is no longer in 

wide use. Today the form in India, Pakistan, Sri Lanka, and much of Bangladesh is 

generally recognized as the nominate subspecies P. m. molurus. It is generally 

distinguished from the form in the remainder of the range (P. m. bivittatus) by having a 

lighter overall coloration, an arrowhead marking on the top of the head that fades 

anteriorly, and the absence of a scale row between the supralabials (upper lip scales) and 

the oculars (single ring of scales at the margins of the eye). However, various researchers 

have found the distinguishing attributes to be incongruent (de Rooij, 1917; Mertens, 

1930), and as the two forms are parapatric (abutting but not overlapping), introgression 

between the two forms is assumed to occur frequently enough for the combined 

population to qualify as a species (Underwood and Stimson, 1990; Kluge, 1993; Welch, 

1994; Walls, 1998b; McDiarmid and others, 1999; the Reptile Database, accessed 2008; 
44

ITIS (Integrated Taxonomic Information System) accessed 2008). However, the species 

concept is in flux and some herpetologists, especially those focused on herpetoculture 

(Daniel, 2002; O’Shea, 2007; Barker and Barker, 2008a), recommend splitting the 

population into two species at the contact line. Evidence that both forms occur in the 

same area without interbreeding would measurably strengthen the argument for splitting; 

in captivity, the taxa produce healthy crosses (Townson, 1980).

1.2 Common Names

The most widely used name for the entire species is Indian Python (Frank and 

Ramus, 1995; Murphy and Henderson, 1997; Reptile Database accessed 2007), with P. 

m. bivittatus routinely distinguished as the Burmese Python. As the pet trade is 

composed almost entirely of P. m. bivittatus, most popular references to the species refer 

to it simply as the Burmese Python. Previously popular names include Indian Rock 

Python (for example, Pope, 1961), Asian Rock Python (Walls, 1998b), or simply Rock 

Python (Ditmars, 1931). None of these names is entirely satisfactory. Indian Python is 

somewhat ambiguous, as it may refer to the nominate subspecies, or the full species, 

which has a distribution that extends well beyond India. Burmese Python suggests an 

origin in Burma (=Myanmar), but the species has become scarce in southern Burma (G. 

Zug, pers. commun., 2008), and the name Burmese Python is often incorrectly used to 

characterize the entire species. Indian Rock Python or Asian Rock Python or Rock 

Python suggests a limitation to rocky habitats, which is incorrect (see section 3.2). It also 

creates a potential confusion with P. sebae, which is also called the Rock Python, or more 

often the African Rock Python, though it too occupies many habitats bereft of rocks 

(Broadley, 1999).

1.3 Evolutionary Context

The sister taxon of the Indian Python is the Northern African Python, P. sebae 

(Underwood and Stimson, 1990; Rawlings and others, 2008) which is taxonomically 

clustered with the South African Python (P. natalensis). P. molurus is difficult to 

distinguish from P. sebae, as the morphology and range of coloration and scale patterns 

overlap extensively, and both African forms are reported to interbreed successfully with 

P. molurus in captivity (Branch and Erasmus, 1984; Coborn, 1991; Walls, 1998b). One 

can easily surmise that in a more mesic (wet) geological period the distributions of these 

sister taxa may have joined in the Middle East. Evidently they have not developed prezygotic reproductive isolating mechanisms since their separation, which is not surprising, 

as reproductive isolating mechanisms are thought to evolve more readily in species that 

overlap and therefore have a need to prevent lower viability hybrids (Mayr, 1974). The 

evolutionary position of the P. molurus/sebae group within the python clade is less 

certain (Kluge, 1993; Rawlings and others, 2008), but most observers agree that it is not 

closely related to the sympatric (geographically overlapping) Reticulated Python 

(Broghammerus reticulatus) and its sister taxon, B. timoriensis (Lesser Sundas Python).
45

Figure 4.1. Schematic representation of the head of P. molurus.

2.0 Description

2.1 Unique Morphological Features 

While details of the scalation differ somewhat among the giant constrictors, the 

functional features of the morphology of the Indian Python share more similarities than 

differences with other snakes of similar size. For example, all the giant snakes are 

nonvenomous constrictors having short prehensile tails (P. molurus tails about 12 percent 

of total length: Kabisch, 2002); heavy bodies; large, angular but supple skulls with high 

counts of small, severely recurved teeth; vertical pupils; temperature-sensing thermal pits 

in their lips (=labial scales; except Eunectes; note that Boa also lacks obvious labial pits, 

but we do not consider this species as a morphologically “giant” constrictor); narrow 

belly scales; bodies circular in cross section; flanks with many rows of tiny scales; and 

the blotchy color pattern characteristic of species that depend more on crypsis than 

mobility for prey capture and predator defense (Fig. 4.1). Vestigial pelvic girdles are 

used by the males for courting females (Gillingham and Chambers, 1982). The blotchy 

color patterns of Indian Pythons are individually unique (Bhupathy, 1990). 

2.2 Size

Humans are fascinated by the large size of the Indian Python and have discussed 

the matter extensively (Oliver, 1959; Pope, 1961; Minton and Minton, 1973; Ernst and 

Zug, 1996; Murphy and Henderson, 1997; Walls, 1998b; Bellosa, 2003, 2004; Zug and 

Ernst, 2004; Bellosa and others, 2007), despite having relatively few data (large 

specimens rarely reach museums, for practical reasons). However the large size of the 

Indian Python may be its primary ecological adaptation. In combination with an 

extraordinary ability to recruit stomach tissue quickly to digest a large meal (Secor, 1995, 

2003; Secor and Diamond, 1995, 1998; Secor and others, 1994, 2000), the large body 
46

size of the Indian Python extends the size range of its predatory niche well beyond those 

of all sympatric snakes except the Reticulated Python (with which its shares some 

habitats in insular and mainland Southeast Asia). Estimates of the maximum size of a 

species depends on the size of the sample of measurements obtained (though rumors of 

rare but gargantuan snakes abound), and credible measurements are sparse when 

museums do not preserve a large series or large individuals (Zug and Ernst, 2004; 

Manthey and Grossman, 1997). Many experts state that Indian Pythons over 5 m in total 

length (all length measurements in this work are total length) are exceptionally rare 

(Ernst and Zug, 1996; Murphy and Henderson, 1997), though there is a recent record of a 

8.22-m total length captive-reared Indian Python (Bellosa, 2003). Some typical 

benchmarks are given in Table 4.1. Useful references on this topic are: Wall, 1912, 

1921; Smith, 1943; Lederer, 1956; Schmidt and Inger, 1957; Pope, 1961; Acharjyo and 

Misra, 1976, 1980; Van Mierop and Barnard, 1976; Ernst and Zug, 1996; Walls, 1998b;

Cox and others, 1998; Zug and Ernst, 2004; Bellosa and others, 2007.

Table 4.1. Total lengths and masses of typical P. molurus (compiled from the literature 

sources listed in the text).

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 480-790 75-165 2.0 ~5 ~4.5 ~50

F: 480-790 75-165 2.6 ~10 8.22 182

2.3 Sexual Size Dimorphism

A diversity of metrics has been suggested for quantifying sexual size dimorphism 

(Lovich and Gibbons, 1992), but none of these has been applied to the Indian Python. 

Although hard data are absent, there appears to be a consensus that most very large 

Indian Pythons are females (for example, Walls, 1998b), which according to Lederer 

(1956), exceed the total length of males by 5 to 6 feet (1.5–1.8 m). A size difference of 

this magnitude probably allows large female Indian Pythons to eat prey items that would 

be unavailable to males.

2.4 External Sexual Differentiation

Both sexes of Indian Python exhibit vestigial remnants of pelvic girdle, visible 

externally as tiny protruding spurs, which are proportionately larger in males (Gillingham 

and Chambers, 1982). Pythons of exceptionally large size are generally females (section 

2.3).
47

3.0 Distribution in Space and Time

3.1 Native Range

The range of the Indian Python is poorly documented, at least in part because 

such large snakes have not fared well amidst the dense human population that now occurs 

in its native range (Groombridge and Luxmoore, 1991). Thus the current distribution is 

likely to have shrunk over that occurring naturally (Minton, 1966; Khan, 2006). As our 

primary interest is in the ecological conditions under which it may occur, we have not 

dwelled on range shifts (presumably anthropogenic) reported within historic time. 

Groombridge and Luxmoore (1991) published the most recent compendium of native 

range distributions, though their interest was in trade regulation rather than biogeography, 

and the quality of their records was highly dependent on the caliber of trade monitoring 

in each country. Murphy and Henderson (1997) reviewed the relevant literature. Rodda 

and others (2009) reported the data also tabulated here (Fig. 4.2). Associated with a call 

for unrestricted importation, Barker and Barker (2008a) argued for a more restricted 

distribution, partially relying on nonpeer-reviewed or unpublished information about 

current distributions. The references cited in this paragraph are not repeated in the 

country by country summary below. Countries are listed in approximate order from west 

to east.

Pakistan–P. molurus has been recorded from mostly riparian zones in the lower and 

upper reaches of the Indus Valley, with only minor penetration into the highly arid lands 

in the west or the coldest portions of the extreme north (Wall, 1912; Smith, 1943; 

Minton, 1962, 1966; Khan, 2002, 2006). No explanation for its absence in the middle 

Indus has been published, though there seems to be agreement that it is not found there.

India–The Indian Python is found virtually throughout the country, though it is probably 

absent from higher elevations in the extreme north (for example, Kashmir, Arunachal 

Pradesh) and the extremely arid parts of the Thar or Great Indian Desert along the 

Pakistan border (Wall, 1912, 1921; Smith, 1943; Deoras ,1965; Whitaker, 1978, 1993; 

Das, 2002b).

Sri Lanka–Found throughout (Wall, 1921; Deraniyagala, 1955; de Silva, 1990).

Nepal–Found throughout the lowlands (Swan and Leviton, 1962; Malla, 1968; Zug and 

Mitchell, 1995; Kabisch, 2002); the maximum elevation is a matter of some uncertainty 

(see discussion in 3.2.1).

Bhutan–No unique literature, but presumed to have a distribution ecologically similar to 

that in neighboring Nepal.
48

Figure 4.2. Native range of P. molurus, as deduced from the sources cited in text. The dividing 

line between the subspecies has not been subject of a recent review, but is believed to lie in the 

narrow neck of the range separating South Asia from Southeast Asia. Many specimens in the 

foothills of the Himalayas appear to be P. m. bivittatus, as are scattered specimens in lowland 

eastern India.
49

Bangladesh–Found throughout the country (Wall, 1912, 1921; Smith, 1943; Kock and 

Schröder, 1981; Das, 1996).

Burma (Myanmar)–Range poorly understood; may not reach the northernmost border 

(Wall and Evans, 1900; Wall, 1912; Smith, 1943; Dowling and Jenner, 1988; 

Auffenberg, 1994; Zug and others, 1998; Manthey and Grossman, 1997; Das, 2002a; 

Pauwels and others, 2003).

Thailand–Similar to Burma/Myanmar (Taylor and Elbel, 1958; Cox, 1991; Manthey and 

Grossman, 1997; Cox and others, 1998; Chan-ard and others, 1999) and does not extend 

south of the Isthmus of Kra (Pauwels and others, 2003).

Laos–Found throughout, with the possible exception of higher elevations at the northern 

border (Deuve,1970; Manthey and Grossman, 1997).

Cambodia–Found throughout (Manthey and Grossman, 1997).

Vietnam–Found throughout, with the possible exception of high elevations at the 

northern border (Campden-Main, 1970; Manthey and Grossman, 1997; Ziegler, 2002).

China–Very poorly documented in publications of the West. The most authoritative 

recent source (Ji and Wen, 2001) does not explain the detailed range map presented (see 

also Pope, 1935; Vinegar and others, 1970; Lin, 1983; Liu, 1986; Zhao and Adler, 1993; 

Zhong, 1993).

Malaysia–The snake is believed absent from Borneo (see note under Indonesia) and if 

present in peninsular Malaysia, it would be so only at the most northern extreme 

(Boulenger, 1912; Manthey and Grossman, 1997; Cox and others, 1998; Inger and Voris, 

2001).

Indonesia–The distribution in Indonesia is disjunct from the rest of the range, the snake 

being found only on the islands of Java, Sumbawa, and Sulawesi, and in Sulawesi only 

on the western side of the southwestern arm (de Rooij, 1917; Kopstein, 1930; Mertens, 

1930; de Haas, 1950; David and Vogel, 1996; Inger and Tan,1996; Manthey and 

Grossman, 1997; How and Kitchener, 1997; Stuebing and Inger, 1999; Inger and Voris, 

2001; Malkmus and others, 2002; de Lang and Vogel, 2005; McKay, 2006; Das, 2006). 

Auliya (pers. commun., 2008) cited an unconfirmed report of this species on Lombok. 

Although a 19th century specimen attributed to Indonesian Borneo exists (Smith, 1943; 

Kabisch, 2002), the absence of any subsequent documentation casts doubt on the validity 

of the geographic attribution and modern authors do not recognize the presence of P. 

molurus on Borneo (Inger and Tan, 1996; Stuebing and Inger, 1999; Malkmus and others, 

2002; Kabisch, 2002; Das, 2006; Auliya, 2006). 
50

3.2 Habitat Range

A dedicated study of the habitat range of Python molurus has not been published. 

However, it is evident both from the wide range documented and the informal habitat 

descriptors mentioned in the literature that this species occurs in virtually every habitat 

from lowland tropical rainforest (Indonesia, Southeast Asia) to thorn-scrub desert (in 

Pakistan) and grasslands (Sumbawa, India) to montane warm temperate forests (Nepal, 

China). Zug and Ernst (2004) describe the habitat as mangrove to montane. Smith 

(1943) mentions both upland and riparian habitats. Murphy and Henderson (1997) state,

"rain forest, mangrove swamps, savanna, scrub forests and semideserts." The most arid 

occupied habitats are near the species western limit (India, Pakistan), where Khan (2006) 

lists two habitat types: "Sind Kohistan and southern Baluchistan" ("clumps of cactus-like 

Euphorbia dominate the landscape") and "Dry Temperate Semi-evergreen Scrub Forests" 

("Olea cuspidata, Acadia modesta, Artemisia...with occasional trees"). Wall (1921) 

emphasizes the degree to which this species is comfortable in water. Smith (1943) 

stresses this point and declares that it might even be considered semiaquatic in habit; 

however Wall (1921) notes that although it may remain in fresh water for long periods 

(potentially throughout a shedding period), “it shows no special affection for the sea.” 

Whitaker (1978) claimed a preference of this species for relatively undisturbed habitats 

(“pythons require large undisturbed areas to hunt and hide in.”); it is not clear whether 

absence from more developed sites is a product of direct human persecution (snake 

populations killed in developed areas), indirect human persecution (snakes may learn to 

remain away from settlements so as to avoid harassment), or a natural habitat preference 

(for example, snakes prefer mature forest). Whitaker’s (1978) observation is reinforced 

by other observers only to the extent that the python seems to have disappeared from 

densely populated areas in recent history (Groombridge and Luxmoore, 1991). In a later 

paper (Whitaker, 1993), he stressed the ability of pythons to coexist with humans in 

disturbed habitats: “Because P. molurus can conceal itself in a remarkably small patch of 

live or dead vegetation and being nocturnal, it is sometimes able to dwell unnoticed for 

long periods near human habitation. However, as habitat loss increases by expanding 

human development, marginal forest and scrub forest zones favored by P. molurus are 

disappearing."

3.2.1 Maximum Elevation

No studies focused on this question have been published, but various authorities 

have asserted that they occur at elevations up to 2,500 m (Whitaker, 1993; Murphy and 

Henderson, 1997). Other mentioned limits include 1,500+ m (Pope, 1935), 1,650 m 

(Pope, 1961), 2,000 m (Whitaker, 1978; Kabisch, 2002; Whitaker and Captain, 2004; 

Khan, 2006), and 2,400 m (Ernst and Zug, 1996). As some of the highest occupied 

elevations in their native range are located at the highest occupied latitudes of their native 

range, the individuals present in such locations presumably hibernate (see 3.5, below).

3.3 Climate Range

The Indian Python shows an extraordinary range of climates, including both 

temperate and tropical, as well as both very wet and very dry environments (Fig. 4.3). 

The nominal subspecies (P. m. molurus) ranges into some very dry areas around the Thar 
51

Desert of western India and Pakistan; both subspecies occupy cold areas in the 

neighborhood of the Tibetan Plateau and Himalaya Mountains (Fig. 4.3). Notice that the 

assumption of the shorter hibernation period implies a lower thermal tolerance (when 

active), whereas the assumption of a longer hibernation period spares active snakes from 

exposure to such low temperatures.

Figure 4.3. Climate space for P. molurus, with reference lines and localities as introduced in Fig. 

2.1. The Chinese localities (magenta lines) and Southeast Asia (blue lines) are entirely within the 

subspecies P. m. bivittatus. Clim3 (heavy dashed line) presumes a hibernation period of 3 months 

whereas Clim4 (heavy solid black line) posits a hibernation period of 4 months.

3.4 Microhabitat

Very little information is available on microhabitat. Whitaker (1978) comments 

that “They live in rocky clefts and caves, abandoned mammal burrows, hollow trees, 

dense water reed and mangrove thickets, usually near a permanent water source.” Wall 

(1921) noted the prevalence of this species “partially or wholly submerged near the bank 

of a river or jheel. As in captivity, it will lie for hours showing nothing but the tip of its 

snout, which is pushed out to raise the nostrils above the surface, and permit breathing.” 

From the historical context, it is not clear whether such behavior is a preferred 

resting/foraging mode, or simply the best way to avoid excessive harassment by humans.

0

5

10

15

20

25

30

35

40

0.01 0.1 1 10 100 1000 10000

Mean monthly temperature C) 

Mean monthly precipitation (mm)

Islands

South Asia

Southeast Asia

China

Clim4

Clim3

Mean monthly temperature (C)

Mean monthly precipitation (mm)
52

3.5 Introduced Ranges

Outside of Florida there are no known introduced populations of this species. 

Various commentators have speculated that disjunct (Walls, 1998b) or putatively disjunct 

(Barker and Barker, 2008a) populations might be introduced, but no direct evidence has 

been published to support this speculation.

3.6 Seasonal Activity

No studies on the seasonal activity of this species have been published. The 

species is understood to hibernate at the northern edge of its range (Wall, 1912, 1921; 

Shaw and others, 1939; Smith, 1943; Daniel, 2002). Minton (1966) gives approximate 

dates for entering and leaving hibernation, implying a hibernation duration of 3–4 

months. It is not clear if hibernation in this species is equivalent to the deep burial 

associated with high latitudes in North America. Daniel (2002), for example, comments 

that hibernacula include “any convenient retreat such as a tree hole, a hole in a bank, or 

under rocks or grass heaps.” Bhupathy and Haque (1986) and Bhupathy and Vijayen 

(1989) describe intermittent winter activity near hibernacula, though their study site was 

not at the northern limit of the species range.

3.7 Daily Activity

Opinions differ with regard to the daily activity cycle of the Indian Python. This 

could reflect differing activity patterns in different localities or times of year. In areas 

where the python is subject to human persecution they may have learned to avoid 

daytime exposure. Whitaker (1978) stated, “Pythons sleep or bask in the sun during the 

daytime. At night they prowl in search of prey or lie in wait near a waterhole or a regular 

mammal pathway.” Bhatt and Choudhury (1993) studied the python in the cooler 

seasons in central India, where they found them to be largely diurnal in the coolest 

months and crepuscular in the warmer months. A similar pattern has been reported in 

Florida (Snow and others, 2007b). Many observers note that pythons will be inactive for 

a period of days to weeks following a large meal (Wall, 1912, 1921; Whitaker, 1978; 

Daniel, 2002).

3.8 Foraging Mode/Sensory Modalities

Dedicated studies on foraging mode or sensory modalities have not been 

published for this species. There is a consensus that pythons conceal themselves near 

pathways used by their prey (ambush site chosen on the basis of the snake’s detection of 

odor trails left by the prey species), which they strike when prey individuals walk in front 

of the snake’s mouth. Wall (1921) for example, opines that predatory strikes are visually 

guided and may be accompanied by tail movements that could constitute a distraction 

display (Mullin, 1999): “The snake, roused to activity by the sight of food, advances 

towards its prey often with quivering tail...” Given the sophisticated thermally sensitive 

pits in the head of pythons, they could presumably sense the passing prey by some 

combination of visual (Goris and others, 2007; Grace and Matushita, 2007), vibration, 

chemical, and thermal information (Wall, 1912; Bullock and Barrett, 1968; Auliya, 2006; 

Grace and Matushita, 2007). Auffenberg (1994) cites his own unpublished work showing 

an ability of pythons to commute between refugia and ambushing sites by following their 
53

own scent trail from previous passages. Daniel (2002) suggests that pythons may 

actively acquire immobile prey under some circumstances: “Heronries, nesting colonies 

and large bird roosts usually have attendant pythons which take their toll at night.”

4.0 Life History

4.1 Reproductive Mode

The Indian Python shows a fairly standard snake-life history in that it hatches 

from an egg at a relatively small size, but is independent from birth, grows rapidly and 

matures in a few years, searches for mates (males search) or waits for males to find it 

(females) during the mating season, and females lay eggs to repeat the cycle. The 

primary uniqueness of the Indian Python is that females brood their eggs, enclosing the 

eggs inside their coils throughout incubation, raising the clutch’s temperature if needed 

by shivering thermiogenesis (Valenciennes, 1841; Hutchinson and others, 1966; Vinegar 

and others, 1970; Müller, 1970; Vinegar, 1973; Van Mierop and Bernard, 1978). Wall 

(1921) reported “in all cases the dam has shown great solicitude for the fate of her eggs.” 

The female may raise the temperature of her clutch by 6–8°C (Hutchinson and others, 

1966; Vinegar and others, 1970; Ernst and Zug, 1996).

4.2 Reproductive Phenology

Reproductive phenology (the time of year when reproductive events occur) is 

expected to vary with locality (especially. latitude), but for the Indian Python has been 

quantified only in Florida (Snow and others, 2007a, b; Krysko and others, 2008) and 

South Asia (Wall, 1912, 1921; Bhupathy and Vijayan, 1989). The general pattern in 

areas of seasonally reduced activity seems to be for courtship to occur as air temperatures 

begin to warm. Mating aggregations or mating balls (Rivas and others, 2007a) form in 

south Florida in March/April (R.W. Snow, pers. commun., 2008); previous observations 

of this phenomenon were limited to captivity (Acharjyo and Misra, 1976), or inferred 

from aggregations seen in Dec.–Mar. in India (Shaw and others, 1939; Smith, 1943; 

Bhupathy and Vijayan, 1989; Daniel, 2002) or Pakistan (Minton and Minton, 1973).

Copulation in the Indian Python has not been carefully quantified, but appears to 

take typical snake form (Gillingham, 1987): (1) following shedding by the female, (2) 

males are incited to repeatedly and fairly rapidly rub ventrally against the dorsal surface 

of the newly shed female, and if she is receptive, (3) intromission follows the female’s 

gaping of her cloaca, which facilitates her cloacal grasping of the male’s nearest everted 

hemipene (Barker and others, 1979; Nakamoto and Toriba, 1983). If cloacal gaping by 

the female is required, the potential for forced copulation would seem to be quite limited 

in this species; we have found no reports of forced copulation in P. molurus. The one 

unique aspect of python courtship is that the vestigial hind limbs of the male are vibrated 

at a frequency of 30–80 movements per minute to stimulate the corresponding area on the 

female (base of tail; Gillingham and Chambers, 1982). Eighteen timed copulations 

(Pope, 1961; Barker and others, 1979; Nakmoto and Toriba, 1983) lasted 35–1,160 min 

(Nakamoto and Toriba, 1983, reported a median of 360 min for their 11 observations), 

though due to the intertwining of tails (and concomitant obstruction of view), there may 

be some uncertainty about whether intromission was present throughout.
54

Copulation by the male is not essential for development of viable eggs in the 

Indian Python (Durnford, 2003; Groot and others, 2003). Instead, the female apparently 

has the ability to fertilize her eggs with her own genetic material, though it is not known 

how often this occurs in the wild. Several captive studies reported viable eggs from 

females kept for many years in isolation. For example, Pope (1961) cites a 1951 report 

by C. Leigh of a python that laid eggs nine, ten, eleven, and thirteen years after mating.

This was once interpreted as evidence for long-term sperm storage, but may instead be 

evidence for facultative parthenogenesis (self-fertilization).

The length of time between copulation and oviposition (gestation) has not been 

measured in the wild, and may vary with locality. Wall (1921) infers from anecdotes in 

the wild that 3–4 months elapses, but Stemmler-Morath (1956; cited in Pope, 1961) and 

Walls (1998b) reported only 2 months in captivity. Van Mierop and Barnard (1976) 

observed 113–114 d in captivity.

Most captive females stop eating at some point prior to oviposition (Acharjyo and 

Misra, 1980); Barker and others (1979) state that female reproductive aphagia (fasting) 

begins at the time of copulation. Presumably cessation of feeding is a reflection of 

competition from follicles/eggs for space within the narrow confines of a snake’s body 

(Wagner, 1976; Van Mierop and Barnard, 1976, 1978; Barker and others, 1979). All 

observers agree that females also do not generally eat while they are brooding their eggs; 

thus the female may lose considerable weight during the lengthy period of reproductive 

aphagia (38 percent of pre-breeding mass lost (that is, includes mass lost through egg 

formation); Van Mierop and Barnard, 1976).

The mean date or degree of synchrony within a population of oviposition is not 

known with certainty for any site, but Wall (1921) reports anecdotes of Mar–Jun for India 

and as late as Aug for Sri Lanka. Harvey and others (2008) cites May–Jun for Florida. 

Wagner (1976) noted that females shed about 14 d prior to oviposition. Harvey and 

others (2008) give Jul–Aug as the dates for hatching in Florida, but comparable values 

are not apparently known for the native range.

4.3 Reproductive Effort/Fecundity

Reproductive effort is classically computed as the percentage of the female’s 

body mass represented in the clutch (RCM or relative clutch mass). To our knowledge, 

this has not been computed for Python molurus (though see mass loss reported by Van 

Mierop and Barnard, 1976, in section 4.1).

4.3.1 Clutch or Litter Size/Frequency

Clutches of 8–107 have been reported (Wall, 1921; Smith, 1943; StemmlerMorath, 1956, cited in Pope, 1961; Ernst and Zug, 1996; Cox and others, 1998), although 

the mean clutch size is towards the low end of that range (Pope, 1961, recognized only 

two clutches of more than 55). Karsen and others (1986) claimed a maximum number of 

30, but that is inconsistent with recent records from Florida (mean 36, Brien and others, 

2007), and does not approach two recent clutches of 79 and 85 from the Florida 

Everglades (Krysko and others, 2008). Ernst and Zug (1996) reiterate Wall’s (1921) 

claim that the Indian Python’s maximum of 107 is the maximum for all snake species, but 

Cox and others (1998) have a record of 124 for B. reticulatus and Zug (pers. commun., 

2009) notes a Puff Adder (Bitis arietans) with 157. 
55

Most observers assume that Indian Pythons produce only one clutch a year. 

Given the extreme physiological (clutch production, brooding, and reproductive aphagia) 

and temporal (months of brooding) cost of producing a clutch, and the absence of more 

than one reported breeding season per year for this species, the assumption of no more 

than one clutch per year seems sound. Indeed, it is possible that energetically challenged 

females may be unable to reproduce every year in some localities; this possibility has not 

been examined in the wild. It is probable that males attempt to reproduce annually in all 

localities.

4.3.2 Egg Size

Eggs are symmetrical, leathery, roughly 60 x 120 mm (Smith, 1943), and adherent 

when freshly laid (Pope, 1961; c.f. Walls, 1998b). Oviposition sites include open ground, 

tree holes, termite mounds, and under fallen logs (Wall, 1921). Eggs of minimum size

(diameter as low as 47 mm in captivity, Acharjyo and Misra, 1980; length as low as 75 

mm, Lederer, 1956) tend to have low viability and may be influenced by conditions of 

captivity. Mass of snake eggs is especially vulnerable to abnormal hydric conditions of 

captivity: reported values for P. molurus are 140–303 g (Lederer, 1956; StemmlerMorath, 1956, cited in Pope, 1961; Walls, 1998b). Lederer (1956) pointed out that eggs 

shrink a few days before the eggs hatch.

4.4 Growth

Embryos are relatively advanced when oviposited, with total lengths of 25–83 

mm (Pope, 1961; Van Mierop and Barnard, 1976). From that point on, the rate of growth 

will be highly dependent on ambient temperature, and captive observations may not 

reflect wild conditions. Unfortunately, all available data are from captivity (Wunder, 

1934, cited in Pope, 1961; Lederer, 1956; Schmidt and Inger, 1957; Pope, 1961; Vinegar, 

1973; Van Mierop and Barnard ,1976; Black and others, 1984). Pope (1961) noted a 

length of 275 mm after 43 d; Van Mierop and Barnard (1976) noted 289 mm (18 g) at 25 

d, 580 mm at 47 d.

Hatchling lengths and masses are given in Table 1 (Section 2.2); key references 

are Wunder (1934) cited in Pope (1961), Smith (1943), Schmidt and Inger (1957), Pope 

(1961), Vinegar (1973), Van Mierop and Barnard (1976), Ernst and Zug (1996), and Cox 

and others (1998). Incubation time is highly sensitive to temperature, which is not 

always known or natural in captivity. Reported incubation times are in the range 55–80 d 

(Schmidt and Inger, 1957; Pope, 1961; Karsen and others, 1986; Clercq, 1988; Van 

Mierop and Barnard, 1976; Ernst and Zug, 1996). Hatchlings use an egg tooth to slit the 

shell one or more times (Lederer, 1956). Hatchlings shed 10–21 d after emergence, and 

probably rely on yolk until this first shed (Valenciennes, 1841; Schlott, 1935, cited in 

Pope, 1961; Lederer, 1956; Van Mierop and Barnard, 1976; Clercq, 1988).

Postemergence growth is among the fastest recorded for snakes, with captive 

individuals approaching 200 mm/mo, though typical rates are 70 mm/mo (Wall, 1921), 

75 mm/mo (Acharjyo and Misra, 1980), 89 mm/mo (Pope, 1961), or 98–103 mm/mo 

(Van Mierop and Barnard, 1976). One extraordinary individual was recorded as growing 

245 mm/mo (Frye and Mader, 1985). Growth rate declines with age; Parker (1963) 

expressed the trajectory as an increase in total length of 170 percent in the first year and 

70 percent in the second. Acharjyo and Misra (1980) gave the mean growth rate as 
56

declining from 76 mm/mo (1st year) to 45 mm/mo (2nd year) to 52 mm/mo (3rd year) to 

15.5 mm/mo (4th year). Wall (1921) gave the mean monthly growth of a 3.6-m TL 

captive as 40 mm/mo.

Few data exist on the frequency of shedding in the Indian Python, which may 

scale with growth and would therefore decrease with increasing size. Wall (1921) states 

that a captive (size not given) shed 5–6 times per year (see also Daniel, 2002).

4.5 Maturation

Maturation in captivity is likely to be accelerated when captives are fed 

generously. Unfortunately, data are not available from the wild. Frye and Mader (1985) 

report on an extraordinary male captive that matured in five months, at a total length of 

1.7 m. This is consistent with Wall’s (1921) assessment of male maturation occurring at 

1.7 m (all unqualified length measurements in this work are total length). However, 

females are reported (Wall, 1921; Lederer, 1956) to mature at around 2.6 m, which 

implies a greater age at female maturation or faster female growth. Pope (1961) argues 

that males may mature in 2 y and females in 3 y; Walls (1998b) asserts 3 y. Ross and 

Marzec (1990) state, “They can reach ten feet [3 m] in length by one year of age if 

overfed, and can reach sexual maturity at 18 months to two years. With more appropriate 

growth, sexual maturity is reached at 2 1⁄2 to 3 years.” These latter rates are a reasonable 

expectation for normally fed animals (Walls, 1998b). Acharjyo and Misra (1980) found 

two females kept out of doors in Orissa, India, to be mature at an age of 3 y 7.5 mo.

4.6 Longevity

Few records exist for long-term captives, and no data are available from the wild. 

Bowler (1977) gives two records for captives >28 y in captivity, the longest of 34 years, 2

months for an Indian Python acquired as an adult.

5.0 Diet/Trophic Role

5.1 Prey

With only a few reported exceptions, Indian Pythons eat terrestrial vertebrates, 

though they eat a very wide range of terrestrial vertebrates (lizards, frogs, crocodilians, 

snakes, birds, and mammals). Special attention has been paid to the large maximum size 

of prey taken from python stomachs; the most famous include a leopard (Begbie, 1907), 

alligators (Ditmars, 1912; Snow and others, 2007a), antelope (Wall, 1921; Raja of Jasden 

1953, cited in Pope, 1961), dogs (Ernst and Zug, 1996), deer (Wall, 1921; Snow and

others, 2007a), jackals (Wall, 1921), goat (Wall, 1921), porcupine (Wall, 1921), wild 

boar (Whitaker, 1978), pangolin (Pope, 1961), bobcat (Snow and others, 2007a), pea fowl 

(Wall, 1921), frigate birds (Snow, pers. commun., 2008), Great Blue Herons (Snow, pers. 

commun., 2008), Wood Stork (Snow, pers. commun., 2009), langur (Wall, 1921), and 

flying fox (Foster and Price, 1997). At the small end of the size spectrum are items such 

as wrens (Snow and others, 2007a), toads and frogs (Wall, 1921), and mice (Whitaker, 
57

1978; Clercq, 1988). Within this prey size range are species of special concern3

While this statement may be an accurate characterization of the population of 

Indian Pythons, Wall (1921) presented evidence that individual pythons may be more 

idiosyncratic in dietary preferences. He noted a captive individual that consistently 

refused house rats (Rattus), while readily consuming jerboas (Dipodidae). Although 

lizards (Wall, 1921; Auffenberg, 1994), snakes (Wall, 1921), frogs (Wall, 1921) and 

toads (Wall, 1921) have been found in python stomachs, they do not appear to be 

common prey. Too few have been documented to assess the snake’s prey range in those 

taxa, though it would appear that many North American reptiles and amphibians are too 

small to be of much interest to pythons. An exception is alligators (Snow and others, 

2006, 2007a), which frequently interact with pythons in the Everglades, both as predator 

and prey.

. In 

Florida, the species of special concern known to have been eaten include Endangered 

Key Largo Woodrats (Neotoma floridana small, Greene and others, 2007), Round-tail 

Muskrats (Neofiber alleni, Snow and others, 2007a), Limpkin (Snow and others, 2007a), 

and White Ibis (Snow and others, 2007a). It appears that Indian Pythons will eat any bird 

or mammal of an appropriate size. 

Fish eating does not appear to be a common trait in Indian Pythons. Although 

Keays (1929) claimed it was common, at least at certain venues (see also Ernst and Zug, 

1996), there are few other records documenting fish eating, despite the frequent close 

proximity of fish and pythons.

A number of vegetable items have been found in python stomachs (Wall, 1912, 

1921); these fruits are presumed to be either ingestion errors (the rotting mango smelled 

of carrion) or secondary ingestion (the python swallowed a frugivore whose stomach 

contained some berries). The python’s willingness to eat carrion or carrion analogues 

might facilitate the use of artificial attractants, as suggested by the observation of Pope 

(1961): “S. Mookerjee reported the discovery of four mangoes in the esophagus of an 

Indian python. Tooth marks were visible on the mangoes, which were infested with 

insect larvae, an indication that they were not fresh.”

5.2 Predators

Predation on Indian Pythons has not been systematically documented. 

Presumably, eggs and hatchling pythons experience the greatest vulnerability due to their 

small size. Alligators (Snow and others, 2006), monitor lizards (Auffenberg, 1994), 

jackals (Bhupathy and Vijayan, 1989), and humans (Murphy and Henderson, 1997) are 

documented predators or scavengers of pythons. Adult pythons are relatively 

invulnerable to predators, except when they are digesting a large meal (Wall, 1921; 

Murphy and Henderson, 1997).

 3 Our use of lower case letters in the phrase “species of special concern” indicates that we are using those 

words in their common English usage. Formal designation using the same words would be capitalized. For 

example, the State of Florida has a special legal category for species the Florida Wildlife Conservation 

Commission designates as Species of Special Concern (these are so noted under the Florida column in 

Table 4.2).
58

5.3 Diseases and Parasites

Little is known about the diseases or parasites of pythons in the wild. Morris 

(1932), Pope (1961), Frank and Haefner (1981), Riley and Self (1981), Ismail (1984), and 

Bhupathy and Vijayan (1989) mention the occurrence of internal parasites of the Indian 

Python, but none of these sources presents evidence that the parasites affect python 

populations. 

6.0 Demography

6.1 Population Density

To our knowledge, there are no credible published estimates of population density 

of the Indian Python. Bhupathy and Vijayan (1989) summed the maximum number seen 

at different dens (over the course of a winter) from a 29-km2 park in India, but they 

recognized that not all dens were known, and additional individuals were present away 

from dens. They also acknowledge that den switching may occur in midwinter, 

potentially leading to double counting. Accordingly, the bias of their ~5/km2 estimate is 

difficult to assess. The density of the population studied by Bhupathy and Vijayan 

(1989) may have been depressed by losses to humans (well documented by Bhupathy and 

Vijayan).

6.2 Size Distribution

A satisfactory assessment of the size distribution of a population requires an 

understanding of the size dependence of detectability. Pythons donated to authorities by 

interested citizens are likely to have caught their attention because of the snake’s large 

size, for example; conversely, preserved museum specimens are routinely selected for 

their small size (to save space and preservative). Although lengths are given for some 

individuals taken from the python’s native range, these have not been aggregated into a 

usable size distribution; this has been done for the Florida population (Snow and others, 

2007b). The Florida record indicates that the modal size class is 3.0–3.25 m in total 

length, with the majority of individuals below this mode (mean = 2.4 m).

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

In South Asia, humans kill pythons in substantial numbers for their meat and 

skins, and for perceived improvements in human safety (Groombridge and Luxmoore, 

1991). Due to legal protection associated with low python population density in South 

Asia, the documented hide trade is limited to Southeast Asia. At the time of 

Groombridge and Luxmoore’s (1991) study, exports had peaked in 1985 in Thailand (at 

188,000 skins plus an unquantified number of exports tabulated in terms of skin length, 

area, or weight). Following that peak year, trade in skins originating in Thailand was 

suspended, coincident with a surge in exports from Malaysia (where the species is not 

known to occur).
59

It is not possible to assess the impact of this trade on the health of wild stocks 

using available information; however, Groombridge and Luxmoore (1991) believed that 

the trend was not favorable. They cited evidence for population declines in Pakistan, 

India, Bangladesh, Sri Lanka, Nepal, Burma, Thailand, and Laos.

7.2 Human Health Risks

A full-grown female Indian Python constitutes a safety risk to humans, but 

virtually all known human fatalities are associated with pet manipulation (Minton and 

Minton, 1973; Chiszar and others, 1993; Murphy and Henderson, 1997; Flank, 1997; 

Associated Press, 2006), not unprovoked attacks. A remarkable exception occurred 

during production of this report (1 July 2009), in which an unattended 2-year-old in 

Florida was sought out and fatally constricted by the family’s 2.5 m (5.5 kg) pet 

amelanistic Burmese Python (Miller, 2009). Events of this sort may have happened 

before, but gone unreported or underreported, as families are understandably reluctant to 

share details of so personal a tragedy with the media. Most putatively unprovoked 

attacks occur from an undocumented large constrictor in parts of Asia where the more 

likely perpetrator is a Reticulated Python (Minton and Minton, 1973), though the death of 

a Chinese baby near Hong Kong in 1900 (Wall, 1921) would be outside of the known 

range of B. reticulatus and was therefore most likely an Indian Python. Given the large 

number of humans living in the native range of the Indian Python, it is remarkable that 

more human fatalities have not been reported, as herpetoculturists believe that any python 

over 8 feet (2.4 m) in length is capable of killing an adult human (Flank, 1997). “Large 

constrictors have the ability to kill humans, and in areas of southern Florida where these 

snakes occur, children should be kept away from water edges and dense vegetation, 

which is a common precaution anyway because of the much higher likelihood of alligator 

attacks” (Snow and others, 2007b). Snow and others (2007b) also note that large 

constrictors crossing roads could cause traffic accidents.

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

Indian Pythons are desirable because of their beauty and ease of handling. These 

attributes, combined with a very low cost for wild juveniles coming out of Southeast 

Asia, has long allowed the Burmese Python to be a mainstay of the exotic reptile trade. 

Groombridge and Luxmoore (1991) reported that export of live Burmese Pythons from 

Thailand peaked in 1985 at 25,000, after which Thailand curtailed the trade. The 

following year, Malaysian exports of the nonresident species jumped from 94 to 15,006, 

suggesting that much of the trade was rerouted in response to Thailand’s export ban. 

Reed (2005) tabulated U.S. importation records for 24 species of boas and pythons during 

the period 1989–2000, which indicated that Burmese Pythons4

 4 There is no legal trade in Indian Pythons that are not of the Burmese subspecies; thus all importation 

records for the species are referred to Python molurus bivittatus.

were the fourth most 

popular snake (behind P. regius, B. constrictor, and B. reticulatus), at 12,466 recorded 

importations. The number of snakes imported increased sharply over the last decade, 

with 99,000 individuals imported from 1996 through 2006 (Harvey and others, 2008, 
60

based on LEMIS import records). It is notable that not all import records were 

identifiable to species (thus the cited figure is a minimum), and domestic sources 

(undocumented) are believed to constitute a significant additional source (unpub. records 

of Florida Wildlife Comm., Law Enforcement). CITES records (Fig. 4.4) show wide 

swings in annual imports, with a sharp decline in recent years that CITES notes may be at 

least partially due to incomplete records tabulated since 2005.

Figure 4.4. CITES records for importation to the United States of live P. molurus.

7.3.2 Other Uses

Pythons are valued in their native range for rodent control, religious symbols, 

entertainment, and medicinal value (Wall, 1921; Pope, 1961; Minton and Minton, 1973; 

Murphy and Henderson, 1997; Walls, 1998b). In the U.S., Burmese Pythons play a 

traditional role in zoos, circuses, carnivals, and roadside attractions; occasionally they are 

used as an accoutrement for exotic dancers (Pope, 1961; Coborn, 1991; Walls, 1998b).

Management Profile

This section follows the format suggested by the Aquatic Nuisance Species Task 

Force (1996) in which the first four elements review the sequential likelihood of species 

establishment (Pathway × Entry potential × Colonization potential × Spread potential), 

and the concluding three elements consider the sum of social, biological, economic, and 

political consequences that might occur if the species becomes established or legally 

regulated (Economic impact + Environmental impact + Social and political influences). 

These seven factors are rated and compiled for all giant constrictor species in the final 

0

5000

10000

15000

20000

25000

30000

19771979198119831985198719891991199319951997

19992001

200320052007

Year imported

Number imported

Year imported

Number imported
61

chapter (Ten) of this risk assessment. This section discusses the seven risk assessment 

areas as they apply to P. molurus.

8.0 Pathway Factors: Pet Trade

Although transport to the United States by another pathways is imaginable (for 

example, circuses, zoos, scientific study, food), the overwhelming preponderance of live 

Indian Pythons in the United States today arrive via the pet trade.

8.1 Volume of International Trade 

As indicated in section 7.3.1 of this chapter, Indian Pythons are one of the most 

often imported snakes, routinely appearing in the top five species, with 99,000 imported 

from 1996 through 2006. Snakes in the international trade pathway constitute a 

somewhat higher risk than domestically bred animals, in that wild snakes often carry 

exotic parasites or pathogens that may transfer to other captive snakes during transport, 

sales, and pet ownership, or to native snakes or livestock once released. In addition, 

wild-caught snakes consist of genotypes that have proven ability to survive in the wild. 

Potentially, the survival experience or learning of the previously free-ranging animal is 

also of some value in improving the individual’s prospects for subsequent survival in the 

wild.

8.2 Volume of Domestic Trade 

The volume of domestic trade in Python molurus is unknown, but many experts 

believe it to be comparable or to exceed the volume of international trade (Florida 

Wildlife Commission law enforcement specialists, pers. commun., 2007). The Pet 

Industry Joint Advisory Council submitted testimony in 2008 to the Fish and Wildlife 

Service that domestic production of P. molurus greatly exceeded imports. The Burmese 

Python breeds readily in captivity and has been selectively bred for numerous “designer 

morphs” (Walls, 1998b; de Vosjoli and Klingenberg, 2005), which attract buyers with 

their beauty and novelty. Herpetoculture experts (for example, de Vosjoli and 

Klingenberg, 2005) routinely advise prospective buyers to purchase only captive-bred 

individuals as they are likely to be healthier. Accordingly, the captive-bred hatchlings 

generally command a higher price. Except for novel genotypes sought for breeding 

purposes, adult pythons do not command a high price, as supply exceeds demand for 

large individuals.

9.0 Entry Potential (Survival in Transit – Meant Mainly for 

Unintentional Transport)

This heading addresses the prospect of a species surviving in the pathway by 

which it might enter the United States. For species that arrive unintentionally as 

stowaways there is considerable uncertainty. But pythons arrive intentionally, in 

packages made up especially for their survival. As the pet trade makes no profit from the 

transport of any that die, survival of Indian Pythons in commerce is very high, 

approaching 100 percent for these sturdy animals.
62

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

Pythons may be released either intentionally or unintentionally (see Reed and 

Rodda, 2008). Unintentional release is promoted by the impressive muscular strength 

and persistence of a confined python. Some indication of the frequency of python escape 

can be gleaned from the numerous newspaper accounts of escaped pythons reportedly 

terrorizing suburban neighborhoods. We are aware of no systematic compilation of such 

events, but they are familiar to any herpetoculturist.

Intentional releases appear to be motivated primarily by an owner’s inability to 

properly care for such a large and voracious pet. Such owners are evidently unable or 

unwilling to find another owner for the undesirable giant constrictor (zoos are inundated 

with requests for taking over care of adult giant constrictors, requests that they are unable 

to satisfy), and may choose to release their pet in an area of biologically suitable habitat 

rather than kill the animal and protect the environment from potential invasion. For 

example, the initial colonization of the Burmese Python in Florida appears to have 

derived from animal(s) clandestinely released at the far end of the main entrance road in 

Everglades National Park (Snow and others, 2007b). Presumably the owner(s) of the 

pet(s) drove the >40 miles to the end of the dead-end road to ensure that their pet would 

be maximally surrounded by biologically suitable habitat.

10.2 Climate Match: Areas of the United States at Greatest Risk

Due to the wide rainfall tolerance and extensive semitemperate range of P. 

molurus, large areas of the southern United States mainland appear to have a climate 

suitable for survival of this species (Fig. 4.5). United States areas climatically matched at 

present ranged up the coasts and across the south from Delaware to Oregon, and included 

most of California, Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Alabama, 

Florida, Georgia, and South and North Carolina. By the year 2100, projected areas of 

potential suitable climate extend northward beyond the current limit to include parts of 

the States of Washington, Colorado, Illinois, Indiana, Ohio, West Virginia, Pennsylvania, 

New Jersey, and New York. Thus a substantial portion of the mainland United States is 

potentially vulnerable to this ostensibly tropical invader. In addition to the mapped areas 

of the U.S. mainland, the territories of Guam, Northern Mariana Islands, American

Samoa, Virgin Islands, and Puerto Rico would appear to have suitable climate. Areas of 

the State of Hawaii with elevations under about 2,500 m would also appear to be 

climatically suitable. Climate suitability is just one factor in the establishment of an 

invasive species, a necessary but not sufficient condition.
63

Figure 4.5. Areas of the United States matching the climate envelope expressed by P. molurus

in its native range (Fig. 4.3) under two hypotheses of hibernation duration: clim3 (assumed duration 

of hibernation 3 months) and clim4 (assumed duration of hibernation 4 months). See chapter Two 

for details of analytical method.

10.3 Prey Availability

Dietary studies (see Biological profile 5.1) indicate that P. molurus will readily 

consume any terrestrial vertebrate of a size suitable for ingestion by the individual snake. 

In Florida, Burmese Pythons have eaten prey as small as wrens and as large as deer, with 

diet records that encompass multiple orders of mammals, birds, and reptiles excluding 

snakes and turtles. All indicated portions of the United States have prey of a suitable 

size, but some insular areas may lack prey of sufficient abundance in certain size ranges. 

For example, some islands in the Northern Mariana Islands have few terrestrial

endotherms larger than rats. Rats are a key prey for juvenile and small adult pythons, but 

may not be sufficient for healthy growth of large adult females. Thus an invasive 

population of pythons in the Northern Mariana Islands might be subject to stunting, due 

to limited availability of large prey. On islands with feral cats, pigs, goats, and so forth, 

there would appear to be no such prey size limitation. A complete prey size distribution 

appears to be present in most of the U.S. mainland.
64

10.4 Predation Risk

Few predators on the U.S. mainland are of a size sufficient to safely engage an 

adult Burmese Python. Because of their size and predatory capabilities, panthers and 

crocodilians are likely to be especially effective. However, we know of no direct 

evidence on which to judge the probability of either species preying on pythons with 

sufficient frequency to depress the population of pythons or limit its spread. The 

circumstantial evidence from southern Florida is that the combination of panthers and

alligators has not noticeably constrained establishment or spread. It is possible, however, 

that spread will accelerate if and when Burmese Pythons spread beyond the area densely 

inhabited by panthers and alligators.

10.5 Reproduction Requirements

As Burmese Pythons do not deposit their eggs in the ground, but hold and warm 

their eggs to protect them from cooling and perhaps from mold and predators, they 

appear capable of reproducing without further specialized features in any locality of 

suitably warm climate (Fig. 4.3), though it is possible that additional requirements exist 

but have not yet been identified.

10.6 Hibernation Requirements

Indian Pythons hibernate underground, but do not appear to dig their own 

hibernacula. In India they are most often associated with porcupine burrows (for 

example, see Bhupathy and Vijayan, 1989). Thus the existence of suitable animal-made 

or natural burrows would seem to be required for the persistence of P. molurus

populations. We are aware of no systematic data collection on the availability of burrows 

in the United States, though natural crevices service the needs of hibernating snakes in 

localities such as Manitoba, well north of the area climatically suitable for P. molurus

(Fig. 4.5). Much of the southeastern United States has Gopher Tortoise burrows that are 

probably suitable. Much of the southern United States has medium-size mammals 

(skunks, raccoons, foxes) that would provide suitable burrows. However, there well may 

be places where small burrows (for example, under wind-thrown root balls) are available, 

but climatically buffered large burrows (dug burrows or large natural crevices) are rare. 

For example, sandy areas in coastal Virginia and elsewhere (B. Savitsky, pers.commun., 

2007) may have soils too weak to support large underground cavities and/or a paucity of 

large burrowing vertebrates. In such environments, pythons might not be able to 

overwinter, or they might be limited to riparian zones where beaver or other digging 

mammals create burrows.

10.7 Tools for Eradicating a Colonization

The prospects for preventing python colonization were outlined in the generic 

management chapter (Three). That chapter drew most heavily from the experience with 

the current P. molurus infestation in Florida and thus is most directly applicable to this 

species. To date there has been no evidence that a colonization could be eradicated by 

management activities.
65

11.0 Spread Potential

The localities into which pythons might spread are very similar to the localities in 

which pythons might survive (see section 4 above). However, the rate of spread 

(dispersal ability) is limited by python movement rate (vagility) and fecundity. 

11.1 Dispersal Ability

Burmese Python vagility and fecundity are among the highest of any snake, with 

short-term (ca. 1 month) straight-line movements of 40 km recorded in Florida (S. Snow, 

pers. commun., 2008) with radiotelemetry, and the reported clutch sizes (max. 107) being 

third-ranked among snakes (second is the Reticulated Python). Thus the spread of 

Burmese Pythons is unlikely to be limited by dispersal ability. Indeed, the rate of spread 

is likely to be near maximal compared to other snake species.

11.2 Ability to Develop Races or Strains

Nothing is known about the viability of pythons of various strains or color 

morphs. At least three albino Burmese Pythons have been encountered in the wild in 

southern Florida, including those with wild prey in their stomachs, and an albino 

individual was found in an ambush posture under a bird feeder within a few days of 

escaping its cage (S. Snow, pers. commun., 2008). Thus albinism per se does not appear 

to be a lethal trait for wild individuals. Given the great diversity of native range localities 

that have contributed stock to the pet trade, it is likely that novel genetic combinations 

(that is, unrepresented in nature) could be created by existing or subsequent releases of 

captive stock (as was observed for introduced Anolis sagrei in Florida; Kolbe and others, 

2004). However, no data are available on this topic.

11.3 Tools for Managing Spread

The chapter (Three) on generic management of giant constrictors specifically 

addressed the eradication of incipient colonizations. That chapter concluded that 

eradication has not been achieved for any giant constrictor and is unlikely on the U.S. 

mainland with the technology and early detection infrastructure presently implemented. 

However, spread and local population density may be somewhat more readily managed, 

especially if the area to be protected from spread is geographically isolated. Thus there 

are improved management opportunities associated with interdiction to islands such as 

Hawaii or the Florida Keys. Such interdiction would draw heavily from the vigorous 

interdiction program presently in place for Brown Treesnakes on Pacific Islands (Vice 

and Pitzler, 2002). Interdiction opportunities between geographically connected sites in 

the U.S. mainland would appear much less favorable, especially given the State-by-State 

variability in regulations on the ownership and transport of giant constrictors.

12.0 Economic Impact Potential

To the best of our knowledge, no one has undertaken a study of the economic 

impacts of P. molurus in either its native or introduced range. We are thus forced to 

speculate in an unquantified way on the basis of the known biological facts; many issues 

of great potential importance may be overlooked or mischaracterized by this approach.
66

12.1 Pathogen Vector

Very little is known about the disease vectors associated with Indian Pythons in 

the native range, and nothing is known regarding potential new interactions where 

introduced. It seems probable that ticks and other fairly general ectoparasites could be 

transmitted to native reptiles in the United States. Diseases borne by such parasites could 

potentially impact U.S. industry. The most famous possibility is heartwater disease, a 

potentially catastrophic disease of hoofed animals including cattle that is vectored by 

ticks found on African pythons (for example, P. sebae), but the ticks are capable of 

transferring to other species of the genus Python in captivity (see analysis of this 

phenomenon in P. sebae management chapter). If a tick bearing heartwater was to attach 

itself to a P. molurus that was subsequently released, there would be at least the 

possibility of introducing heartwater disease to North America. Alternately, if heartwater 

was to become established in North America by some other mechanism, newly resident 

pythons could support ticks that would harbor the disease (complicating management of 

the epidemic) or transmit the disease to previously unaffected areas by dispersing with 

infected ticks.

12.2 Predator on Livestock

Although cattle and adult sheep are unlikely to be taken often by P. molurus, all 

smaller livestock would be at risk if unconfined or confined in porous structures (for 

example, many barns have holes or crevices large enough for entry of pythons). Freerange poultry such as chickens and geese have already become victims (S. Snow, pers. 

commun., 2007). Newborn mammals are likely to be especially vulnerable, as they are 

with the much smaller Brown Treesnakes (Chiszar, 1990; Fritts and McCoid, 1991). 

Accounts of wild prey of P. molurus often mention wild pigs, so we expect that domestic 

pigs would be favored prey.

12.3 Predator on Pets

Loss of pet animals may be of greater psychological importance than what would 

be suggested by the raw number of individuals lost. Puppies and kittens are the pets most 

often taken by Brown Treesnakes (Rodda and others, 1999c), probably because they are 

the least able to defend themselves. Losses of caged birds are generally a one-off affair, 

as the snake is often unable to depart between the cage wires due to its excessively 

bulging stomach. This state of affairs may be distressing to the bird owner discovering 

the replete but only partially confined python.

12.4 Electrical Power Systems

Brown Treesnake economic damages are primarily a result of damage to electrical 

power circuits and supporting hardware such as transformers (Fritts and others, 1987; 

Burnett and others, 2006). Burnett and others (2006) projected potential Brown 

Treesnake annual power system losses of $4.5B in Hawaii; thus the magnitude of 

potential losses is appreciable. However, juvenile Burmese Pythons are less capable 

climbers than are Brown Treesnakes (adult pythons are too stout and heavy for climbing 

on wires), and thus the economic loss per unit of land area is likely to be appreciably less 

from pythons than from Brown Treesnakes. We are not aware of any documented power 
67

line problems from the large population of Burmese Pythons in south Florida, and thus 

this problem may be no more severe than that already associated with power line 

movements by rat snakes. 

12.5 Traffic Accidents/Human Attacks

Although Burmese Pythons are the nonvenomous snake species most often 

associated with human fatalities, these generally involve interactions initiated by the pet 

owner (“illegitimate bites”); however, legitimate attacks have been documented and some 

visitors will find this disconcerting. In python-infested areas where American Alligators 

are also found, we expect that human health risks from reptiles would continue to be 

dominated by the risks from alligators. We do not anticipate P. molurus becoming a 

significant source of human mortality in the United States, though the loss of anxiety-free 

recreation space (terrestrial areas where alligators do not occur) may be important to 

some users.

As mentioned by Snow and others (2007b), pythons have a small potential for 

creating or contributing to traffic accidents by blocking traffic lanes or contributing to 

bystander accumulation. The economic cost associated with such activity would 

presumably be immeasurably small.

12.6 Tourism

The potential economic impacts on tourism can be exceedingly difficult to 

measure (Shwiff, 2008), as impacts that are large in aggregate magnitude may 

accumulate when immeasurably small antipython preferences are held by a large number 

of potential tourists. Furthermore, there may be some small offsetting benefits to tourism 

from the opportunity to witness novel predators. However, the potential exists for costly 

losses in tourist revenues if a high-profile encounter or fatality occurs and generates 

adverse publicity. The effect would likely be stronger in localities that do not presently 

have dangerous animals such as alligators. For example, several economic analyses of 

tourism in Hawaii have projected that colonization by the Brown Treesnake could cost 

Hawaiian businesses hundreds of millions of dollars. Presumably a colonization of 

Hawaii by the much larger and more dangerous Indian Python would engender a 

proportionately larger loss of tourism revenues.

12.7 Impacts on Commerically Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

South Florida ecotourism is heavily dependent on bird watching. Birds that are 

imperiled or visually striking are especially desired. Imperiled species are the ones most 

likely to be impacted by python predation (see section 5.1), as they are demographically 

least able to sustain a modest increase in predation pressure. Thus to the extent that 

visitation is tied to success at bagging a sighting of a rare bird, the presence of a novel 

predator on rare birds is likely to be detrimental to tourism.

South Florida is not heavily dependent on visitation by gun hunters, but hunting is 

an economically important activity further north, one that is potentially adversely 
68

impacted by a stealthy predator that competes with hunters by eating desired species such 

as quail, turkey, feral hogs, and deer.

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture

Burmese Pythons are unlikely to have a dramatic impact on forestry, agriculture, 

or horticulture. To the extent that managed lands are now impacted by rat, nutria, or 

beaver numbers, there might be a small positive effect related to rodent population 

reduction.

13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

A very large number of imperiled species are at risk from giant constrictors in the 

State of Florida. Table 4.2 lists all species tracked by the Florida Natural Areas 

Inventory (FNAI) that are of a size suitable for consumption by at least some ontogenetic 

stage of Burmese Pythons (10g–100kg) and that occur in suitable habitat within the State 

of Florida (139 taxa: species, subspecies, or populations judged by the Inventory to have 

taxonomic uniqueness). The FNAI tracks taxa that are either biologically imperiled or 

listed by a State or Federal agency as being of special conservation concern. The degree 

of imperilment is quantified by the number and size of discrete populations and is 

summarized on a scale of 1 to 5, where 1 is critically endangered (only a few individuals 

remain) and 5 is secure from a conservation perspective (many populations of many 

individuals). The number is preceded by various letters indicating the reference group: G 

stands for global and indicates that the reference group consists of all members of the 

same species found anywhere in the world; T indicates that the reference group is a 

special taxon (usually subspecies). A “Q” or “?” following the number indicates that 

there is some taxonomic or informational uncertainty afflicting the rating (see 

www.fnai.org for additional details on the assignment of rarity ratings). There are 75 

biologically rare taxa within the State of Florida (Table 4.2) that are vulnerable to python 

predation and meet the Inventory’s rareness criteria 1–3, where 3 is either very rare and 

local throughout its range (21–100 occurrences or less than 10,000 individuals) or found 

locally in a restricted range or vulnerable to extinction from other factors. Using Federal 

or State listing as a metric of rarity, we find that 71 “listed” taxa from Florida are at risk 

of python predation (Table 4.2). We have not mapped out the global ranges of these 71, 

75, or 139 imperiled taxa, but the majority appear to have distributions that would put 

them at risk of global extinction should the python spread throughout the area mapped to 

be climatically suitable in Figure 4.5.
69

Table 4.2. Imperiled wildlife potentially vulnerable to some ontogenetic stage of 

P. molurus or other giant constrictors in Florida, as tracked by the Florida Natural Areas Inventory 

(FNAI). Species with special legal status in either Florida or the United States are given in the last 

two columns respectively. See text for interpretation of the FNAI risk categories.

Class Common Name Scientific Name FNAI Florida U.S.

Amphibians Gopher Frog Rana capito G3

Spec. 

Conc. -

Birds Cooper's Hawk Accipiter cooperii G5 - -

Birds Bachman's Sparrow Aimophila aestivalis G3 - -

Birds

Louisiana Seaside 

Sparrow

Ammodramus maritimus 

fisheri T4 - -

Birds

Macgillivray's Seaside 

Sparrow

Ammodramus maritimus 

macgillivraii T2 - -

Birds

Cape Sable Seaside 

Sparrow

Ammodramus maritimus 

mirabilis T1 Endangered Endangered

Birds Scott's Seaside Sparrow

Ammodramus maritimus 

peninsulae T3Q

Spec. 

Conc. -

Birds

Florida Grasshopper 

Sparrow

Ammodramus savannarum 

floridanus T1 Endangered Endangered

Birds Brown Noddy Anous solidus G5 - -

Birds Florida Scrub Jay Aphelocoma coerulescens G2 Threatened Threatened

Birds Limpkin Aramus guarauna G5

Spec. 

Conc. -

Birds Great Egret Ardea alba G5 - -

Birds Great White Heron Ardea herodias occidentalis T2 - -

Birds Florida Burrowing Owl

Athene cunicularia 

floridana T3

Spec. 

Conc. -

Birds Short-tailed Hawk Buteo brachyurus G4 - -

Birds

Ivory-billed 

Woodpecker Campephilus principalis GH Endangered Endangered

Birds Crested Caracara Caracara cheriway G5 Threatened Threatened

Birds Snowy Plover Charadrius alexandrinus G5 Threatened -

Birds Piping Plover Charadrius melodus G3 Threatened Threatened

Birds Wilson's Plover Charadrius wilsonia G5 - -

Birds Antillean Nighthawk Chordeiles gundlachii G4 - -

Birds

Worthington's Marsh 

Wren

Cistothorus palustris 

griseus T3

Spec. 

Conc. -

Birds Marian's Marsh Wren

Cistothorus palustris 

marianae T3

Spec. 

Conc. -

Birds Mangrove Cuckoo Cossyzus minor G5 - -

Birds Florida Prairie Warbler

Dendroica discolor 

paludicola T3 - -

Birds Kirtland's Warbler Dendroica kirtlandii G1 Endangered Endangered

Birds Cuban Yellow Warbler

Dendroica petechia 

gundlachi T4 - -

Birds Little Blue Heron Egretta caerulea G5

Spec. 

Conc. -

Birds Reddish Egret Egretta rufescens G4

Spec. 

Conc. -

Birds Snowy Egret Egretta thula G5

Spec. 

Conc. -

Birds Tricolored Heron Egretta tricolor G5

Spec. 

Conc. -

Birds Swallow-tailed Kite Elanoides forficatus G5 - -
70

Birds White-tailed Kite Elanus leucurus G5 - -

Birds White Ibis Eudocimus albus G5

Spec. 

Conc. -

Birds Merlin Falco columbarius G5 - -

Birds Peregrine Falcon Falco peregrinus G4 Endangered -

Birds

Southeastern American 

Kestrel Falco sparverius paulus T4 Threatened -

Birds Magnificent Frigatebird Fregata magnificens G5 - -

Birds Key West Quail-dove Geotrygon chrysia G3 Threatened Threatened

Birds Whooping Crane Grus americana G1

Spec. 

Conc. Endangered

Birds Florida Sandhill Crane Grus canadensis pratensis T2 Threatened -

Birds American Oystercatcher Haematopus palliatus G5

Spec. 

Conc. -

Birds Bald Eagle Haliaeetus leucocephalus G5 - -

Birds Worm-eating Warbler Helmitheros vermivorum G5 - -

Birds Least Bittern Ixobrychus exilis G5 - -

Birds Black Rail Laterallus jamaicensis G4 - -

Birds Wood Stork Mycteria americana G4 Endangered Endangered

Birds

Black-crowned Nightheron Nyctanassa nycticorax G5 - -

Birds

Yellow-crowned Nightheron Nyctanassa violacea G5 - -

Birds Osprey Pandion haliaetus G5

Spec. 

Conc. -

Birds Painted Bunting Passerina ciris G5 - -

Birds White-crowned Pigeon Patagioenas leucocephala G3 Threatened -

Birds Brown Pelican Pelecanus occidentalis G4

Spec. 

Conc. -

Birds

Red-cockaded 

Woodpecker Picoides borealis G3

Spec. 

Conc. Endangered

Birds Hairy Woodpecker Picoides villosus G5 - -

Birds Roseate Spoonbill Platalea ajaja G5

Spec. 

Conc. -

Birds Glossy Ibis Plegadis falcinellus G5 - -

Birds Mangrove Clapper Rail

Rallus longirostris 

insularum T3 - -

Birds Florida Clapper Rail Rallus longirotsris scottii T3? - -

Birds American Avocet Recurvirostra americana G5 - -

Birds Snail Kite

Rostrhamus sociabilis 

plumbeus T3Q - -

Birds Black Skimmer Rynchops niger G5

Spec. 

Conc. -

Birds Louisiana Waterthrush Seiurus motacilla G5 - -

Birds American Redstart Setophaga ruticilla G5 - -

Birds

White-breasted 

Nuthatch Sitta carolinensis G5 - -

Birds Least Tern Sterna antillarum G4 Threatened -

Birds Caspian Tern Sterna caspia G5 - -

Birds Roseate Tern Sterna dougallii G4 Threatened Threatened

Birds Sooty Tern Sterna fuscata G5 - -

Birds Royal Tern Sterna maxima G5 - -

Birds Gull-billed Tern Sterna nilotica G5 - -

Birds Sandwich Tern Sterna sandvicensis G5 - -

Birds Bachman's Warbler Vermivora bachmanii GH Endangered Endangered

Birds Black-whiskered Vireo Vireo altiloquus G5 - -
71

Birds Zenaida Dove Zenaida aurita G5 - -

Mammals

Sherman's Short-tailed 

Shrew

Blarina carolinensis 

shermani T1

Spec. 

Conc. -

Mammals Red Wolf Canis rufus G1Q - Endangered

Mammals

Rafinesque's Big-eared 

Bat Corynorhinus rafinesquii G3 - -

Mammals Big Brown Bat Eptesicus fuscus G5 - -

Mammals Florida Bonneted Bat Eumops floridanus G1 Endangered -

Mammals Hoary Bat Lasiurus cinereus G5 - -

Mammals Salt Marsh Vole

Microtus pennsylvanicus 

dukecampbelli T1 Endangered Endangered

Mammals Southeastern Weasel Mustela frenata olivacea T4 - -

Mammals

Florida Long-tailed 

Weasel Mustela frenata peninsulae T3 - -

Mammals Southeastern Bat Myotis austroriparius G3 - -

Mammals Gray Bat Myotis grisescens G3 Endangered Endangered

Mammals

Northern Long-eared 

Myotis Myotis septentrionalis G4 - -

Mammals Indiana Bat Myotis sodalis G2 Endangered Endangered

Mammals Round-tailed Muskrat Neofiber alleni G3 - -

Mammals Key Largo Woodrat Neotoma floridana smalli T1 Endangered Endangered

Mammals Gulf Salt Marsh Mink Neovison vison halilmnetes T3 - -

Mammals

Atlantic Salt Marsh 

Mink Neovison vison lutensis T3 - -

Mammals

Southern Mink, So. FL 

pop Neovison vison pop 1 T2Q Threatened -

Mammals Key Deer

Odocoileus virginianus 

clavium T1 Endangered Endangered

Mammals Pine Island Rice Rat Oryzomys palustris pop 1 T1Q - -

Mammals Sanibel Island Rice Rat Oryzomys palustris pop 2 T1Q

Spec. 

Conc. -

Mammals Key Rice Rat Oryzomys palustris pop 3 T2Q Endangered Endangered

Mammals

Key Largo Cotton 

Mouse

Peromyscus gossypinus pop 

1 T1Q Endangered Endangered

Mammals

Choctawhatchee Beach 

Mouse

Peromyscus polionotus 

allophrys T1 Endangered Endangered

Mammals

Santa Rosa Beach 

Mouse

Peromyscus polionotus 

leucocephalus T1 - -

Mammals

Southeastern Beach 

Mouse

Peromyscus polionotus 

niveiventris T1 Threatened Threatened

Mammals

St. Andrews Beach 

Mouse

Peromyscus polionotus 

peninsularis T1 Endangered Endangered

Mammals Anastasia Beach Mouse

Peromyscus polionotus 

phasma T1 Endangered Endangered

Mammals

Pedido Key Beach 

Mouse

Peromyscus polionotus 

trissyllepsis T1 Endangered Endangered

Mammals Florida Mouse Podomys floridanus G3

Spec. 

Conc. -

Mammals Key Vaca Raccoon Procyon lotor auspicatus T1? - -

Mammals Key West Raccoon Procyon lotor incautus T2 - -

Mammals Florida Panther Puma concolor coryi T1 Endangered Endangered

Mammals Mangrove Fox Squirrel Sciurus niger avicennia T2 Threatened -

Mammals

Southeastern Fox 

Squirrel Sciurus niger niger T5 - -

Mammals Sherman's Fox Squirrel Sciurus niger shermani T3

Spec. 

Conc. -
72

Mammals Lower Keys Cotton Rat Sigmodon hispidus exsputus T2 - -

Mammals Insular Cotton Rat

Sigmodon hispidus 

insulicola T1 - -

Mammals Lower Keys Rabbit Sylvilagus palustris hefneri T1 Endangered Endangered

Mammals Eastern Chipmunk Tamias striatus G5

Spec. 

Conc. -

Mammals Florida Black Bear

Ursus americanus 

floridanus T2 Threatened -

Reptiles American Alligator Alligator mississippiensis G5

Spec. 

Conc. -

Reptiles

Gulf Coast Smooth 

Softshell Apalone mutica T5 - -

Reptiles Spotted Turtle Clemmys guttata G5 - -

Reptiles American Crocodile Crocodylus acutus G2 Endangered Threatened

Reptiles Eastern Indigo Snake Drymarchon couperi G3 Threatened Threatened

Reptiles

Red Rat Snake, Lower 

Keys pop Elaphe guttata, pop 1 T2

Spec. 

Conc. -

Reptiles

South Florida Rainbow

Snake

Farancia erytrogramma 

seminola T1 - -

Reptiles Gopher Tortoise Gopherus polyphemus G3 Threatened -

Reptiles Barbour's Map Turtle Graptemys barbouri G2

Spec. 

Conc. -

The species most immediately in danger of extinction by the introduced P. 

molurus include the Key Largo Woodrat (Neotoma floridana smalli), whose global range 

is restricted to northern Key Largo in the Upper Florida Keys; the Key Largo Cotton 

Mouse (FNAI refers to this as Peromyscus gossypinus pop 1), similarly restricted in 

distribution; the Cape Sable Seaside Sparrow (Ammodramus maritimus mirabilis), whose 

global range is within Everglades National Park; and the Round-tailed Muskrat (Neofiber 

alleni), whose global distribution is limited to Florida and parts of southern Georgia. The 

seaside sparrow nests in low shrubs highly accessible to pythons. The muskrat has habits 

and habitats that put it maximally in harm’s way, and though its resting platforms are 

conspicuous where present, the platforms have already become noticeably rarer in the 

areas occupied by the python (S. Snow pers. commun., 2008).

Should the python spread further into the Keys (presently it is known in Key 

Largo but does not clearly have a population established there), it would put several 

additional endemic populations or species at risk: Key Deer (Odocoileus virginianus 

clavium), Lower Keys Rabbit (Sylvilagus palustris hefneri), Lower Keys Cotton Rat 

(Sigmodon hispidus exsputus), Key Rice Rat (Oryzomys palustris pop 3), and Key Vaca 

Raccoon (Procyon lotor auspicatus). Should the python spread northward to Georgia, it 

would increase the risk on virtually all of the taxa listed in Table 4.2. Based on the 

experience of introduced prey sustaining artificially elevated introduced predator 

populations at the expense of native prey species (see references in Fritts and Rodda, 

1998), the greatest biological impact of an introduced predator such as P. molurus is 

likely to be loss of imperiled native prey species. Species that are not presently listed as 

imperiled may become so or be extirpated. Species presently listed as at risk are in the 

greatest danger.

Although most endangerment from giant constrictors is likely to be associated 

with overt predation, it is possible that disturbance of birds at colonial roosts through 
73

nighttime visitation by snakes could cause abandonment of dependent young and loss of 

recruitment, even without predation (P. Frederick, pers. commun., 2009).

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

It is possible for an imperiled predator to benefit from the new prey species. For 

example, the Florida Panther (Puma concolor coryi) might obtain food by preying on 

Burmese Pythons. We have seen python carcasses in Everglades National Park that 

appeared to have been killed by predators, most likely panthers. However, this food 

supplementation effect is likely to be relatively minor and may be more than offset by 

competition between the two predators for food that is now diminished in abundance by 

the presence of the python, and by direct predation on panthers (especially panther cubs) 

by the python. Alligators and crocodiles may be in a similar situation of “intraguild 

predation”: individual predation events may favor one or the other predator, but the 

overall demographic effect is likely to be negative. We see no strong evidence for any 

predators that would consistently benefit from the presence of the python.

13.3 Vector for Disease Spread to or Within Native Faunas

Too few data exist to evaluate this consideration, though the risk of importation of 

parasites or pathogens cannot be excluded.

13.4 Species that Might be Adversely Impacted by Control Measures

If an indiscriminate control technique such as acetaminophen toxicants were to be 

applied to areas infested with Burmese Pythons, native species such as the Indigo Snake 

(Drymarchon couperi) or Florida panther (Puma concolor coryi) could be adversely 

impacted. It is difficult to project such ecological costs without a more distinctly 

enumerated set of management tools.

14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Breeders Affected?

Domestic producers of Burmese Pythons for the pet industry and pet owners have 

raised objections to potential restrictions on the sale of this giant constrictor. Objections 

include concern for lost sales, concern that such a proposal would lead to other 

restrictions on pet ownership, and ideological objection to the concept of government 

regulation of pet ownership. If only importation of P. molurus were to be proscribed, 

domestic producers would presumably benefit via reduced competition from cheap 

imports.

There are a very large number (possibly exceeding 100,000) of Burmese Pythons 

in private ownership in the United States, though no one has data that would quantify the 

number or characterize the owners. Many are traded or exchanged in situations outside 

of conventional retail channels. Florida and Texas have begun to regulate ownership of 
74

these animals, but it is too early to judge compliance with these measures (Environmental 

Law Institute, 2008). 

Regulatory measures to restrict trade or ownership could have negative ecological 

effects in terms of dealers freeing stock (the release of multiple animals at the same time 

and same place enormously increases the risk over single releases such as those typically 

done by pet owners), owners releasing animals for which they don’t have or cannot 

acquire appropriate licenses, and ecovandals determined to release animals as a 

imprecisely directed assault on the government.

14.1.2 International Trade/Alternate Pets

Companies that import Burmese Pythons would lose sales of these reptiles if 

importation restrictions were to be applied. As reptile importation has historically 

promoted both certain staple species (for example, Green Iguanas) and novelties (for 

example, Solomon Islands Monkey-tailed Skinks, amelanistic snakes), there is reason to 

believe that much of the importation effort associated with a particular species could be 

transferred to another species. In addition to likely economic impacts associated with 

such a shift, a large increase in importations of other species could significantly alter the 

risk environment associated with other potential invasive species. However, 

quantification of the economic impacts of trade regulations is a subject better handled by 

the economic impact report.
75

Chapter Five–The Reticulated Python, 

Broghammerus reticulatus or Python reticulatus

Biological Profile

1.0 Introduction

None of the giant constrictors is well known from field studies (though there is a 

substantial secondary literature), but the Reticulated Python stands out as poorly known 

even among these mysterious snakes. The few data come primarily from two sources: 

anecdotes from the colonial era (for example, Wall, 1926) and size, diet, and sex 

tabulations accumulated during the preparation of snakeskin leather (Shine and others, 

1998a, b, 1999; Auliya, 2006). To date there is no body of literature based on field 

studies, with the conspicuous exception of one-third of one dissertation (Auliya, 2006). 

Though admirable in its scope and execution, Auliya was hampered by lack of 

preexisting information about field techniques and therefore was able to capture only 22 

snakes, of which only one was a definite female. A recent compendium on the boas and 

pythons (Henderson and Powell, 2007) has no information from wild Broghammerus 

reticulatus and provides laboratory data only on the snake’s digestive physiology. 

Therefore, though this report borrows heavily from the superb compendium provided by 

Auliya (2006), who synthesized all extant knowledge, we know very little about this 

species in the wild.

1.1 Species

Described by Schneider (1801) in the genus Boa, Python reticulatus or 

Broghammerus reticulatus, the Reticulated Python, is probably the world’s longest snake 

(Bellosa and others, 2007). Although native range boundaries are disputed, the species 

conservatively ranges across much of mainland Southeast Asia, from Bangladesh to 

Vietnam, and virtually all of the islands between Asia and New Guinea (not found on 

New Guinea or the Aru Islands; it is found on Borneo, Sumatra, Java, Sulawesi, and the 

Lesser Sunda, Molucca, and Philippine Islands). The species is a giant constrictor 

characterized by a beautiful network (reticulations) of earth tones: white, tan, brown, 

yellow, and red.

There are three named subspecies of the Reticulated Python: saputrai

(southwestern arm of Sulawesi and the island of Selayar), jampeanus (the island of 

Tanahjampea, south of Sulawesi), and reticulatus (the remainder of the range). Auliya 

(2006) opined that additional subspecies are likely to occur in the Sulawesi area.
76

1.2 Common Names

P. reticulatus or B. reticulatus is now almost always called the Reticulated Python 

in English (occasionally Reticulate Python, Wall, 1926; Pope, 1961; Frank and Ramus, 

1995), though a century ago the name Regal Python was sometimes used for this species 

in honor of its grand size (Ditmars, 1931). However, the scientific name of the 

diminutive Ball Python (Python regius) carries a similar implication, and “Regal” or 

“Royal” descriptors are now used for the Ball Python or avoided entirely to preclude 

confusion. 

The smaller subspecies are sometimes given distinctive common names for 

marketing purposes. In particular the jampeanus subspecies is often called a Dwarf 

Reticulated Python (commonly “Dwarf Retic”) to highlight its more manageable adult 

size (usually ~ 2-m total length).

1.3 Evolutionary Context

The Reticulated Python is very closely related to a smaller python, the Lesser 

Sundas Python (Python timoriensis or Broghammerus timoriensis), found on only a few 

of Indonesia’s Lesser Sunda Islands (but not Timor, despite the scientific name). The 

Reticulated Python and Lesser Sundas Python are clearly two twigs on the same 

evolutionary branch (“sister” taxa). However, the position of this branch in the 

evolutionary tree of pythons is less certain. The taxonomy of the Reticulated Python has 

been relatively stable (especially compared to Australasian Pythons), being located in the 

genus Python according to most 20th century observers (Underwood and Stimson, 1990; 

Kluge, 1993; Reptile Database, 2008). However, both evolutionary biologists and 

herpetoculturists (McDowell, 1975; Underwood and Stimson, 1990; Kluge, 1993; Walls, 

1998b) pointed out that P. reticulatus was not only similar in several characters to P. 

timoriensis but it was also intermediate in many characters between the Afro-Asian 

pythons (for example, P. molurus, P. sebae, P. curtus) and the Australasian pythons 

(Antaresia, Apodora, Aspidites, Bothrochilus, Leiopython, Liasis, Morelia). Hoser 

(2004) recognized the unique intermediateness of the Reticulated Python by transferring 

it to the genus Broghammerus, in honor of one of his mentors. However, the justification 

for a monotypic genus of pythons was not widely accepted. Recently, Rawlings and 

others (2008) published a comprehensive molecular cladogram, which held that the 

phylogenetic grouping that included both the Reticulated Python and the other species in 

the genus Python also included all of the Australasian pythons, dictating that either the 

Reticulated Python + Lesser Sunda Python clade needed to be cleaved from the genus 

Python, or all of the Australasian pythons needed to be merged into Python. Rawlings 

and others (2008) opted for splitting, especially as a suitable name was already in use in 

the literature for the Reticulated Python. Adherents of this point of view will wish to use 

the name Broghammerus reticulatus. This name is not accepted by a number of 

prominent herpetologists (G. Zug, pers. commun., 2009), and conservative practice 

suggests retention of the older name Python reticulatus.
77

Figure 5.1. Schematic illustration of the head of a Reticulated Python.

2.0 Description

2.1 Unique Morphological Features

Whereas details of the scalation differ somewhat among the giant constrictors, the 

functional features of the morphology of the Reticulated Python share more similarities 

than differences with other snakes of similar size. For example, all the giant snakes are 

nonvenomous constrictors having short prehensile tails (B. reticulatus tails are 12–15 

percent of total length); heavy bodies; large, angular but supple skulls with high counts of 

small, severely recurved teeth; vertical pupils; temperature-sensing thermal pits in their 

labial scales (except Eunectes); narrow belly scales; bodies circular in cross section; 

flanks with many rows of tiny scales; and the blotchy color pattern characteristic of 

species than depend more on crypsis than mobility for prey capture and predator defense. 

Vestigial pelvic girdles are present, as are spurs (pointed scales capping the pelvic 

girdles), which are proportionately longer in males than females (Shine and others, 

1998a).

Reticulated Pythons differ from other giant constrictors and Python in having 

relatively lighter body mass (shared with P. timoriensis), lacking a wedge-shaped color 

marking (light or dark) on the top of the head (shared with Eunectes and some P. 

brongersmai); possessing a mid-sagittal dark line on the top of the head (Fig. 5.1; shared 

with P. timoriensis, and the blood python group P. breitensteini, P. brongersmai, and P. 

curtus); having a red/orange iris, especially when adult (Fig. 5.1; shared with blood 

python group); possessing a parallel-sided (as opposed to wedge-shaped) post-orbital 

dark mark (Fig. 5.1; shared with P. anchietae and P. regius); lacking a pre-orbital dark 

mark extending towards the nostril (shared with Eunectes and P. timoriensis); lacking 

posterior sutures in the rostral scale (shared with Eunectes and P. timoriensis); having a 

large number of subcaudal scales (55–102; no other Python species has more than 83, 
78

though P. timoriensis is poorly known); and large size (0.6–9.7 m; shared with Eunectes). 

The genus Broghammerus (shared with B. timoriensis) is defined (Rawlings and others, 

2008) externally “by having the supralabial thermoreceptive pits less well defined than 

the infralabial pits (converse arrangement in Python); by infralabial pits set in a 

longitudinal groove defined ventrally by a longitudinal fold [and] colour pattern of the 

suborbital supralabial region similar to the rest of the supralabials, compared with 

Python, in which there is a dark suborbital patch.” Mark Auliya (in litt., 2009) suggested 

that Rawlings and others (2008) accidentally reversed the polarity of the character 

comparing the depth of the supralabial and infralabial pits; our impression agrees with 

Auliya. The Reticulated Python is easily distinguished from B. timoriensis in possessing 

reticulated body coloration, lacking head shields darkened at the edges compared to the 

centers, and usually possessing a red iris.

Individual Reticulated Pythons can be distinguished on the basis of their blotch 

pattern (Vijayakumar, 1999) and the serial arrangement of paired and unpaired 

subcaudals (Auliya, 2006).

2.2 Size

As with all giant constrictors, the maximum length of the Reticulated Python is 

discussed far more than it is actually measured. In a rare exception, Cox (1991) 

measured a dead one, “a few centimeters less than ten meters.” Zug and Ernst (2004) 

acknowledged a 10-m Reticulated Python (presumably based on Raven, 1946), and most 

other authorities credit this value or list slightly lower figures (for example, Oliver, 1959; 

Pope, 1961; Murphy and Henderson, 1997; Tweedie, 1953; Walls, 1998b; O’Shea, 2007; 

Bellosa and others, 2007). However, specimens over 4 m in total length are rare in the 

wild (Shine and others, 1998a, b, 1999; Auliya, 2006), and even rarer in museum 

collections. Some benchmarks for wild populations are given in Table 5.1 (captives are 

not reported here, as they tend to be better fed and attain larger sizes, though Barker and 

Barker, 1997, give exceptionally small total lengths (457 mm) for some hatchlings). 

Useful references on size are: Wall, 1926; Smith, 1943; Ross and Marzec, 1990; Cox, 

1991; Shine and others, 1998a, b, 1999; Walls, 1998b; Cox and others, 1998; Ziegler, 

2002; Zug and Ernst, 2004; Auliya, 2006; and Bellosa and others, 2007. Note that 

different measurement samples (most were based on commercial collecting for skins) 

diverge appreciably in benchmarks such as size at maturation, and too few samples exist 

to generalize about the range of variation possible throughout the species. 

Table 5.1. Total lengths and masses of typical B. reticulatus (compiled from the literature 

sources listed in the text).

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 550-850 95-210 1.6-3.5 1-10 ~7 ~90

F: 550-850 95-210 2.4-4.5 2.5-15 ~10 ~150
79

2.3 Sexual Size Dimorphism

Females mature at a larger size and attain a larger maximum size (see Table 5.1) 

than males, but the ratio of mean male and female size depends on the sexes’ relative 

mortality schedules, which are not known for even one locality. Also, any size bias in 

sampling of the two sexes can skew estimates of sexual size dimorphism. Females are 

slightly longer in slaughterhouse samples, but they tend to be slightly lighter in weight 

for matched length samples (Shine and others, 1998a, b, 1999; Auliya, 2006). The 

degree to which female Reticulated Pythons exceed the size of their male counterparts is 

variable among samples; Shine and others (1998a) asserted that Reticulated Pythons 

show more extreme sexual size dimorphism than do most other snakes, with adult 

females 42 percent longer than adult males on average. However, Shine and others 

(1999) noted that the body size distributions of male and females were relatively similar 

(because more of the females in their sample were immature), and the sexual size 

dimorphism was not as extreme in the later sample from a different site on Sumatra (SSD 

not reported in the later study). Too few data exist to quantify the degree of SSD 

rangewide.

2.4 External Sexual Differentiation

Males have slightly longer tails and their spurs tend to be longer, but no definite 

method for external sexing has been derived, as the observed values have overlapping 

ranges between the sexes (Shine and others, 1998a, b, 1999; Auliya, 2006). Other body 

proportions (for example, head length) are relatively uniform between the sexes (Shine 

and others, 1998a, b; Auliya, 2006).

3.0 Distribution in Space and Time

3.1 Native Range

The southern limits of the native range of the Reticulated Python are relatively 

well known, but authorities disagree on the species’ northern limits. Four published 

range maps (Groombridge and Luxmoore, 1991; Murphy and Henderson, 1997; Walls, 

1998b; Auliya, 2006) differ in details of the northwest (India) and northeast (China) 

corners of the range, but the texts associated with these sources often suggest that the 

species may not be present in some northern areas. It is probable that the range of the 

python has contracted in response to human population expansion, but too few data are 

available to track range changes through time. We reprint the distribution of Auliya 

(2006; Fig. 5.2), as that is the authoritative rendition.
80

Figure 5.2. Native range of B. reticulatus as estimated by Auliya (2006), Auliya (in litt. 2009) and 

Bellosa and others (2007). Islands within the heavy outline are believed occupied by the 

Reticulated Python. Primary uncertainties include the entire northern periphery of the range, as 

well as the northwest and northeast corners of the range on the Asian mainland.

Bangladesh–Groombridge and Luxmoore (1991) accepted Khan’s (1982, cited in 

Groombridge and Luxmoore, 1991) assessment that Reticulated Pythons were found only 

in the southeastern “panhandle” of Bangladesh, or perhaps just inside the main unit east 

of the Ganges delta (see also Kock and Schröder, 1981). Noting the records of Daniel 

(2002), Khan (1988, cited in Groombridge and Luxmoore, 1991, but reference not given); 

Das (1994, 1996), and Islam and Islam (1997, cited in Auliya, 2006), Auliya (2006) maps 

out a more expansive distribution, including roughly the southern half of the country.

Brunei–Believed to be found throughout (Groombridge and Luxmoore ,1991).

Cambodia–Consensus is that it occurs throughout (all maps agree; Groombridge and 

Luxmoore, 1991; Murphy and Henderson, 1997; Walls, 1998b; Auliya, 2006), but Auliya 

(2006) notes that the only published locality is that of Trapeang Chan, north of 12° N. 

(Saint-Girons, 1972).
81

China–Records are few but most authors who doubt the presence of this snake in China 

rely on the undocumented assumption that its presence there is due to human transport. 

A specimen in the Leiden Natural History Museum from 25° N. is probably the most 

northern record for this species (tied with the record(s) from Arunachal Pradesh; below), 

but was found in a port city and may have been unintentionally from a more southerly 

locale. Wall (1903, cited in Auliya, 2006) and Stanley (1914, cited in Auliya, 2006) give 

records from China, but Auliya (2006) states that he agrees with Pope’s (1935) 

conclusion that all records in mainland China are the result of introductions by humans; it 

is unclear whether any of these introductions have resulted in feral populations. In 

contrast, Walls (1998b) states that it “occurs through southern China (including Hainan 

[Island]).” Auliya (in litt. 2009) concurred with occupation of Hainan Island. We 

consider all Chinese localities to be contested among authorities.

India–All sources agree that the Reticulated Python is resident in India’s Nicobar Islands 

(off the northern tip of Sumatra) but not in the more northern extension known as the 

Andaman Islands (Deoras, 1965; Groombridge and Luxmoore, 1991; Das, 1994, 1996, 

2002b; Murphy and Henderson, 1997; Walls, 1998b; Auliya and others, 2002; Auliya, 

2006). However there is little consensus regarding the mainland distribution. Das (1994, 

2002b) cites a specimen from Arunachal Pradesh, which is well north (26–27° N.) of any 

locality accepted (ca. 24° N.) by Auliya (2006), but consonant with the Assam limit 

stated by O’Shea (2007). Auliya (2006) accepts Deoras’ (1965) locality in Madhya 

Pradesh (82° E; eastern peninsular India), which is nearly 1000 km west of most other 

records (for example, Groombridge and Luxmoore, 1991, recognize no mainland Indian 

localities). Whitaker and Captain (2004) considered its presence likely but unconfirmed 

in northeast India. We considered mainland Indian localities to be contested among 

authorities.

Indonesia–No serious dispute among authorities, except perhaps for a very small number 

of small Maluku islands at the eastern limit of the range. Auliya (2006) accepts as the 

limit Lydekker’s line, which excludes New Guinea and several island groups near its 

western shore: Waigeo, Misool, and Aru Islands. The major islands included in the 

Indonesian native range of the Reticulated Python are Bali, Borneo, Buru, Flores, 

Halmahera, Java, Sumatra, Sumba, Sulawesi, Seram, Sumbawa, and Timor. All minor 

islands between these major islands are believed to be populated. Key references are: de 

Rooij, 1917; de Haas, 1950; Bosch, 1985; Underwood and Stimson, 1990; Groombridge 

and Luxmoore, 1991; Kluge, 1993; David and Vogel, 1996; Auliya and Abel, 2000a; 

Malkmus and others, 2002; Auliya and others, 2002; de Lang and Vogel, 2005; and 

Auliya, 2006. Koch (1926, cited in Auliya, 2006) asserted that the Reticulated Python 

was introduced to Ambon (small island south of Seram), though he did not cite evidence 

for this claim.

Laos–Although there is general agreement that it is found throughout (Deuve, 1970; 

Groombridge and Luxmoore, 1991; Auliya, 2006), specific localities are reported only by 

Stuart (1999, cited in Auliya, 2006), with most records occurring in central and southern 

parts of the country.
82

Malaysia–Observers agree that it is found throughout at suitable elevations. Key 

references are: Boulenger, 1912; Bourret, 1936 cited in Auliya 2006; Groombridge and 

Luxmoore, 1991; Stuebing, 1991; Malkmus, 1992; Inger and Tan, 1996; Barker and 

Barker, 1997; Stuebing and Inger, 1999; and Chan-ard and others, 1999.

Burma (Myanmar)–Authorities are uniform in accepting Myanmar as part of the native 

range, but they disagree somewhat as to the northernmost location in Myanmar. Without 

distinguishing between P. molurus and B. reticulatus, Salter (1983: cited in Auliya, 2006) 

claimed it was common throughout the country. Most other observers believe it present 

only in the southern half, though Salter (1983) notes a specific record for the Kyatthin (= 

Chatthin) Wildlife Sanctuary (23.6°N. 95.5°E) and this locality was confirmed by other 

sources (Zug et al. 1998, G. Zug, pers. comm., 2009). Localities to the north of this 

location are disputed among authorities.

Philippines–Found throughout, including the Batanes Islands (O’Shea, 2007), well north 

of the main archipelago: Taylor, 1922, cited in Auliya 2006; Bourret, 1936; de Haas, 

1950; Leviton, 1963; Brown and Alcala, 1964; Groombridge and Luxmoore, 1991; 

Barker and Barker, 1997; Auliya, 2006; O’Shea, 2007.

Singapore–Resident (de Haas, 1950; Groombridge and Luxmoore, 1991; Lim and Lim, 

(1992, cited by Auliya, 2006; Paperna and others, 2004).

Thailand–Auliya’s (2006) map shows the Reticulated Python to be present throughout. 

This agrees with the other published maps (Groombridge and Luxmoore, 1991; Murphy 

and Henderson 1997; Walls 1998b), though Auliya seems to confirm de Haas’s (1950) 

assertion that it does not occur north of about 18°N, and Barker and Barker’s (1997) 

claim that it is limited to lower (unspecified threshold) elevations.

Vietnam–The northern limit of distribution in Vietnam is uncertain, though it appears to 

be common and widespread at least throughout the south (Campden-Main, 1970). A 

locality between 21° and 22° N. appears in Smith (1943), and other localities around 

Hanoi and further south are undisputed by most authors (Bourret, 1936; Darevsky, 1990; 

Ziegler and Hermann, 2000; Ziegler, 2002; Auliya, 2006). As this report went to press, a 

new volume on the herpetofauna of Vietnam was published (Nguyen and others, 2009); 

these authors do not recognize localities in the northern half of the country. If this 

assessment is accurate, then some of the climate loops currently considered 

“uncontested” in Fig. 5.3, should now be considered “contested.”

3.2 Habitat Range

Almost no information is available on habitat choices by Reticulated Pythons. 

Auliya (2006) made a valiant effort to compile anecdotal reports into a coherent 

description of the habitat and microhabitat (see below) preferences of this species, but the 

only strong generalizations to emerge were a penchant for inhabiting riparian zones 

(Smith, 1943; Campden-Main, 1970; Tweedie, 1953; David and Vogel, 1996; Auliya, 

2006; and many others), and a willingness to utilize subterranean features in karst 
83

landscapes (Hoogerwerf, 1970; Auliya, 2006). It is possible, however, that both of these 

attributes are induced or amplified by human persecution in more accessible places. 

David and Vogel (1996) summarize the habitat breadth of the species, “It is found from 

sea level up to more than 1300 m and it inhabits lowland primary and secondary tropical 

wet forests, tropical open dry forests, tropical wet montane forests, rocky scrublands, 

swamps, marshes, plantations and cultivated areas, and suburban and urban areas.” 

Several authors (for example, Manthey and Grossman, 1997; Malkmus and others, 2002) 

cite the 1,300-m maximum elevation figure reported by David and Vogel (1996), but 

Pope (1961) cited 1,200 m, Stuebing and Inger (1999) cited 1,000 m, and Cox and others 

(1998), Ziegler (2002), and O’Shea (2007) believe it reaches 1,500 m. McKay (2006) 

states that it reaches to 1,200 m on Bali but 1,300 m elsewhere.

The literature contains emphatic and yet contradictory views on the tolerance of 

B. reticulatus for populated places (Groombridge and Luxmoore, 1991). Wall (1926), for 

example, asserted that the “python is only met with in the densest jungles, places 

unknown to Europeans with the exception of a few forest officers, and an occasional 

sportsman.” In contrast, Wall (1926) cites Captain Stanley Flower (1899) who writes of 

a plethora of Reticulated Pythons living in the city of Bangkok, “Strange to say, it is not 

in the quiet jungle-forest that the python seems to prefer to live, but in the busiest spots 

along the Manam [River], where steamers and junks are loading and unloading, steamlaunches whistling, steam-saws buzzing, rice-mill chimneys filling the air with smoke, 

and hundreds of noisy coolies passing to and fro...” Auliya (2006) attempts to reconcile 

these descriptions by suggesting that perhaps pythons in the 1–3 m size range are drawn 

to the high rat densities near human habitation, but that larger pythons find safety in more 

vegetated places. One must also be careful interpreting Captain Flower’s description of 

Bangkok of the 19th century in terms familiar to urban dwellers today. Flower noted that 

as Bangkok became a city the pythons grew scarce. Nonetheless, Cox (1991) asserts that 

occasional individuals continue to be caught within the city limits of Bangkok. Paperna 

and others (2004) found epidemiological data to suggest that Reticulated Pythons avoided 

densely populated areas of Singapore, even though rats were present.

From collections of pythons sold to tanners (Shine and others, 1998a, b, 1999; 

Auliya, 2006), it is evident that small (2–3 m) pythons remain numerous in oil palm 

plantations, rice fields, and other heavily developed agricultural landscapes (Inger and 

Colwell, 1977; Groombridge and Luxmoore, 1991; R. Rozar, pers. commun., 2009). 

However, they are also found in comparatively natural forests (Lim, 1981, cited in 

Groombridge and Luxmoore, 1991; Fredriksson, 2005; Auliya, 2006). The only 

consistent generality is that most individuals are found in association with water bodies.

3.3 Climate Range

Reticulated Pythons occur primarily in areas with a wet tropical climate (Fig. 5.3). 

Though they also occur in areas that are seasonally dry (Fig. 5.3), they do not occur in 

areas that are continuously dry, or very cold at any time.
84

Figure 5.3. Climate space for B. reticulatus, with reference lines and localities as introduced in 

Fig. 2.1. The uncontested sites (orange lines) are those included in the native range maps of all 

peer-reviewed publications; inclusion of the contested sites (blue lines) expands the climate space 

(heavy black lines) from the dashed to the solid perimeter in a coldward direction.

3.4 Microhabitat

Pythons have been located in a wide range of microenvironments (especially near 

water) ranging from subterranean refuges, including the dark zone of caves, to moderate 

heights (6 m) in trees (van Rooijen and van Rooijen, 2002; Auliya, 2006). Auliya (2006) 

made the intuitively reasonable suggestion that arboreality might be more often expressed 

in the lighter weight juveniles (arboreality elevated in juveniles up to 2.0 m [presumably 

total length] according to O’Shea, 2007). Most pythons are found on the ground surface 

or at low perch heights, concealed in vegetation, but that is where most objects are found 

by human searchers. Without more information on the detection probabilities in different 

microenvironments, it is difficult to estimate the degree to which the range of 

observations is due to python behavior or human perception.

3.5 Introduced Ranges

We are aware of no strong evidence for any introduced population of the 

Reticulated Python. However, it is curious how often early observers concluded that 

Reticulated Python sightings at the edge of the known range must represent introductions. 

Such statements have been made for the northeastern (China), southeastern (Molucca or 

Maluku Islands), and northwestern (India) edges of the range. Part of the willingness of 

0

5

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20

25

30

35

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0.01 1 100 10000

Mean monthly precipitation (mm)

Mean monthly temperature (C)

El Paso - Feb Sitka - Sep

Yuma - Oct

Hilo - Aug
85

these observers to ascribe the outlying records to human introduction may be due to 

observations of pythons appearing as stowaways in shipments (potentially attracted to the 

rats accompanying agricultural products). Several famous examples are reported by Wall 

(1926): four Reticulated Pythons were found in a shipment of coconuts from Rangoon; 

one in a ship arriving in Mumbai, India from Moulmein, Myanmar; another arriving alive 

on a ship docking in London (source locality unspecified). The paucity of recent records 

of this sort (in contrast to the persistence, until the last few decades, of Boa constrictor

arriving in shipments of bananas) suggests that modern shipping methods have much 

reduced the likelihood of such accidental transport.

3.6 Seasonal Activity

Too few data exist to make any generalizations about seasonal locomotor activity. 

Given the aseasonal climate associated with most of the python’s range, we would not 

expect there to be any seasonal limitations on activity in many areas. Nonetheless, 

Groombridge and Luxmoore (1991) cite statements from hide buyers that the majority in 

Sumatra are caught in the wet season (up to twice as many a month as in the dry season).

Several authors have commented on the propensity of Reticulated Pythons to be 

found in loose aggregations. Murphy and Henderson (1997) commented on the 

surprising absence of such observations in the literature, though as we noted in the 

introduction to this species, the relevant literature is quite sparse. David and Vogel 

(1996, probably based on Hagen, 1890) noted that aggregations may be found inside tree 

trunks or under plants during the day or in cool weather. A similar phenomenon was 

noted by van Rooijen and van Rooijen (2002) who found three aggregations (one of four 

individuals and two of two) among the 13 individuals they discovered during a short 

(Mar./Apr.) trip to a Malaysian island, Pulau Tioman. It would be helpful to know if 

their visit coincided with the local mating activity, as most aggregations of large 

constrictors have been associated with intermale competition for mates.

Several authors (Lederer, 1944; Barker and Barker, 1997; Walls, 1998b) have 

pointed out that unlike most snakes, Reticulated Pythons bite rival males, often inflicting 

very serious wounds. Nearly all authors comment on the willingness of some Reticulated 

Pythons to bite their human handlers. The word “vicious” is most commonly heard, 

though it is not clear that malice is motivating their behavior. Barker and Barker (1997) 

go to some length to argue that they have periodically encountered Reticulated Pythons 

with a disposition suitable for handling. However, these “pet-quality” individuals are 

evidently relatively rare; Walls (1998b) characterizes the species as “...one of the most 

feared. The Retic is notorious as an aggressive biter that often never tames. Though 

hatchlings and young specimens may have decent personalities, adulthood almost always 

brings with it an awful personality.”

Several authors report evidence that Reticulated Pythons are either very sedentary 

in their habits or inclined to return to sites visited previously—philopatry (Kopstein, 

1926, cited in Groombridge and Luxmoore, 1991; van Rooijen and van Rooijen, 2002; 

Auliya, 2006). As most pythons seem to be killed on first contact with humans, it is 

unlikely that many examples of philopatry would be documentable. In the absence of 

radiotelemetric studies, it is difficult to quantify or assess this propensity.
86

3.7 Daily Activity

No systematic data have been collected on daily patterns of activity or 

locomotion, though van Rooijen and van Rooijen (2002) found that five of seven became 

active after dusk (the remaining two were quiescent after large meals). Distinguishing 

resting behavior for alert but immobile ambush foragers is not easy to do in an animal 

lacking eyelids. Most anecdotes imply nocturnal activity (David and Vogel, 1996; Walls, 

1998b; Auliya, 2006), but Ernst and Zug (1996) and Zug and Ernst (2004) suggest it may 

be more nocturnal where doing so would avoid conflicts with humans.

3.8 Foraging Mode/Sensory Modalities

Given the paucity of observations of Reticulated Pythons capturing prey (Auliya, 

2006, reports one observation), it is plausible to assume that the pythons are primarily sitand-wait or ambush foragers, as is believed characteristic of most large constrictors. 

What senses are used to identify prey are unknown; de Cock Buning and others (1978) 

found that it was easiest to arouse pythons with visual stimuli (movement brought a 

response in 87 percent of trials), though jets of prey-scented air were also effective in 

about 65 percent of trials. Thermal and vibratory cues were of ancillary importance; 

strikes were elicited only in the presence of visual cues. Newman and Hartline (1982) 

showed neurological overlap in processing of visual and thermal information. De Cock 

Buning and others (1978, 1981) and de Cock Buning (1979, 1983) explored the 

physiology of thermal reception in Reticulated Pythons.

4.0 Life History

4.1 Reproductive Mode

Like all pythons, the Reticulated Python lays eggs. The mother surrounds her 

eggs for most of their incubation period of 55–105 days (Wall, 1926; Smith, 1943; 

Branch and Patterson, 1975; Ross and Marzec, 1990; Cox, 1991; Manthey and Grossman, 

1997; Cox and others, 1998; Walls, 1998b; Malkmus and others, 2002; Ziegler, 2002; 

Zug and Ernst, 2004; Auliya, 2006), presumably protecting them from light, desiccation, 

and predators (and potentially having some ability to limit mold or the spread of mold to 

healthy eggs). One would expect higher temperatures to expedite incubation, but hard 

data on incubation temperature in the wild are lacking for B. reticulatus (Shine, 1988; 

Ross and Marzec, 1990; Murphy and Henderson, 1997). As the reported incubation 

times usually lack information on incubation temperature, the incubation times are very 

difficult to interpret. Unlike some other pythons, however, the brooding mother python 

apparently does not warm the eggs with shivering thermiogenesis (Wall, 1926; Vinegar 

and others, 1970; Honegger, 1970; La Panouse and Pellier, 1973; Cox, 1991; Walls, 

1998b; Malkmus and others, 2002), though some observers have claimed that she does 

(Lederer, 1944 (equivocal); Hediger, 1968; Shine, 1985, 1988; Kluge, 1993). Apparently 

heat production, if it occurs, is minimal or contingent on specific conditions.
87

4.2 Reproductive Phenology

Given the paucity of data on pythons from the wild, it is difficult to make 

generalizations about the seasonal course of reproductive activities. Mating is rarely 

observed in the wild, and therefore little can be said about male reproductive phenology 

(though Shine and others, 1998a, found males to have larger testes in October 1994 than 

in April 1995); most inferences are made about female phenology, from the appearance 

of eggs or hatchlings. Within a species, the females can be synchronized throughout the 

range, synchronized within a locality but asynchronous across the native range (often 

associated with latitude as a proxy for onset of rains), or asynchronous within each 

locality. There are, of course, degrees of asynchrony; to our knowledge, uniform 

breeding throughout the year every year has not been documented in any reptile species. 

However, the degree of synchrony within a site can vary from strict seasonality (all local 

individuals breed simultaneously) to many months of some reproductive activity (some 

individuals breed much earlier than their peers), and the time of year for the breeding 

pulse can vary somewhat from year to year (perhaps in relation to local weather patterns). 

A definitive statement for even a single locality would require multiple years of data 

collection to ensure that the pattern observed in one year was representative of all years, 

and no research has been conducted continuously over the course of even one year. For 

example, Shine and others (1998a, b, 1999) made only short visits (about two weeks) 

three to four times a year at two sites in Sumatra. Nonetheless, they declared that 

reproduction is strongly seasonal in both sexes.

From the length of the gestation period (from mating to oviposition) and the 

length of the incubation period (from oviposition to hatching), we can roughly infer a 

locality’s reproductive phenology from the knowledge of a single feature, such as peak 

oviposition date. Oviposition dates for wild pythons are given by five sources (Ross and 

Marzec, 1990; Shine and others, 1998a, 1999; Malkmus and others, 2002; Auliya, 2006). 

Auliya (2006) is the only source that gives dates for the four benchmarks: mating, 

oviposition, incubation, and hatching, though Malkmus and others (2002) also cites 

hatching dates. Groombridge and Luxmoore (1991) estimate gestation at two months; 

Ross and Marzec (1990) state 65 to 105 days, or two to three months. A large number of 

authors (see 4.1) estimate the incubation period at durations ranging from 55 to 105 d, or 

two to three months. 

Reproduction in the tropics often tracks the onset of rainfall, which correlates 

with the movement of the sun (Intertropical Convergence), which is a function of 

latitude. From south to north the available oviposition dates are shown in Table 5.2.

The only source that is clear on the year-round pattern is Auliya (2006) in a 

relatively aseasonal environment with highly subjective and unquantified dates taken 

from local informants. In that environment, oviposition was not recorded only in May 

(the driest month) and July. The peak of oviposition at Shine and others' (1999) nearby 

and equivalently aseasonal site was in Apr.–May. Furthermore, the April peak appears in 

all study localities, suggesting that local variation in reproductive phenology may be 

extensive and may not be closely associated with latitude or rainfall.
88

Table 5.2. Peak oviposition dates of B. reticulatus by latitude (0 = no oviposition known; 

1 = some oviposition known, but believed to be nonpeak; 2 = peak oviposition).

Authority Lat Seasonality J F M A M J J A S O N D

Shine and others, 1998a S 2 Low 1 2 2 2

Auliya, 2006 N 1 Very Low 1 2 2 1 0 1 0 1 1 2 2 2

Shine and others, 1999 N 2 Very Low 1 2 2 1 0 1

Malkmus and others, 2002 N 6 Low 2 2

Ross and Marzec, 1990 N 10-18 Medium 2 2 2

Taken in combination with the assumption that mating precedes oviposition by 2–

3 months, these oviposition dates imply that we would expect mating activity to take 

place in midyear at the lat 2° S. site, the second half of the year at the lat 1° N. site, and 

near the beginning of year at the other sites. The only direct evidence is Auliya’s (2006) 

reasonably congruent estimate that mating occurred Aug.–Feb., with a peak in Nov.–Jan.

Taken with the assumption that oviposition precedes hatching by 2–3 months, 

these oviposition dates imply that hatching should peak at around Oct.–Dec. at the lat 2° 

S. site, Dec.–May at the lat 1° N site, and June–July at the other sites. The only direct 

evidence comes from Auliya’s (2006) figure showing peak hatching from Sept.–Nov. 

(which comports poorly with our estimate from oviposition dates and incubation 

duration), and Malkmus and others’ (2002) statement of a May–June hatching peak 

(which agrees reasonably with our June–July extrapolation). Considerable additional 

data collection will be needed to adequately describe the reproductive phenology of the 

Reticulated Python.

It is not unusual for reptiles to temporarily cease feeding during reproduction. 

The start and end triggers for this phenomenon have not been quantified in the 

Reticulated Python. A typical comment is that of Hagenbeck (letter cited by Taylor, 

1965), “During the period of incubation I offered ducks, fowls, and geese to the python, 

but she refused all nourishment.” Honegger (1970) recorded that a captive male did not 

fast during his mate’s incubation (implying that she did so). In contrast, Barker and 

Barker (1997) stated that males “...cease feeding at the beginning of the breeding 

season...,” though no statement was made about the time of resumption of feeding or 

whether females also undergo reproductive aphagia. Lederer (1944) states that males fast 

during the period of mating and females fast during the period of brooding.

Williamson (1967) noted that a well-fed captive juvenile (of unspecified sex) 

fasted voluntarily at ages of about 12 months (8-week fast) and 24 months (17-week 

fast). Ditmars (1912) claimed that this was the pattern in tropical snakes, “This annual 

fasting period particularly appeals to the boas and pythons, which are of peculiarly 

delicate organization that responds quickly to changes of temperature and undoubtedly 

affects the feeding habits of these reptiles during the well-defined seasons in the tropics.” 

If this hypothesis is true, it may be difficult to infer the causes of anecdotes of aphagia 

observed in captivity.
89

4.3 Reproductive Effort/Fecundity

4.3.1 Clutch or Litter Size/Frequency

Individual females may not lay every year; Shine and others (1998a, b, 1999) 

judged Reticulated Pythons to be highly seasonal reproducers and thereby estimated from 

the peak observed fraction of reproductive females that females in Sumatra laid once 

every two or three years. Fitch (1970) comments that, “Unlike Indian pythons, females 

of this species that bred in captivity, seemed incapable of producing a clutch annually.”

The number of eggs produced is strongly dependent on the size of the female. 

Shine and others (1999) illustrated the relationship (their Fig. 3) and specified its strength 

(r2 = 0.88), but did not provide the regression equation. As only one female in Shine and 

others (1999) exceeded 4 m in snout-vent length, it is difficult to judge the size–fecundity 

relationship for large females. Relative clutch mass (ratio of clutch mass to maternal 

carcass mass) of the one measured female was 0.39.

The clutch “sizes” (number of eggs laid at a time by one female) reported in the 

literature range from 8–124, with typical clutches of 20–40 (Hagenbeck, 1909; Wall, 

1926; Kopstein, 1938; Smith, 1943; Lederer, 1944; Pope, 1961; Hoogerwerf, 1970; 

Groombridge and Luxmoore, 1991; Barker and Barker, 1997; Manthey and Grossman, 

1997; Murphy and Henderson, 1997; Cox and others, 1998; Walls, 1998b; Shine and 

others, 1998a, 1999; Malkmus and others, 2002; Zug and Ernst, 2004; Auliya, 2006). We 

are not aware of any oviparous snake having a larger maximum clutch size.

4.3.2 Egg Size

The eggs are surprisingly large, sometimes described as grapefruit sized. Actual 

measurements of the smaller dimension range from 58–77 mm, the larger dimension 

from 89–115 mm, and the mass from 182–245 g (Wall, 1926; Kopstein, 1938; Lederer, 

1944; Honegger, 1970; Groombridge and Luxmoore, 1991; Manthey and Grossman,

1997).

4.4 Growth

It is difficult to characterize growth of reptiles, as growth is generally size-, diet-, 

and season-specific. All things being equal, linear growth is maximal after hatching and 

declines thereafter. Therefore, although there are three data points on the growth of 

Reticulated Pythons in the wild (Auliya, 2006), these three points would be difficult to 

interpret without knowing the underlying size, diet, and seasonal patterns. Furthermore, 

none of the three is a strong point, as the three recaptures occurred over excessively short 

intervals (7–43 d) and two of the three report a reduction in total length (negative 

growth), probably due to reduced stretching on second capture.

Growth in captivity has a wide range (Lederer, 1944; Williamson, 1967; 

Honegger, 1970; Minton and Minton, 1973; Trutnau, 1980; Kingley, 1987), presumably 

as a result of differing thermoregulatory and caloric conditions. Some captive pythons 

grow only a tiny amount, but Shine and others (1998a) report that captive hatchlings have 

grown to over 4 m in total length in less than 12 months (~275 mm/mo); Barker and 

Barker (1997) imply a similar figure. Such growth may be the fastest that has been 

recorded for any snake (though Frye and Mader, 1985, recorded a similar growth rate of 
90

245 mm/mo in a captive P. molurus), and far exceeds reasonable expectations for growth 

in the wild. Williamson (1967) recorded an initial (30 month) captive growth rate of 141 

mm/mo, which itself is much faster than any growth recorded for a snake in the wild (for 

example, the rapidly growing Brown Treesnake, Boiga irregularis, has an initial average 

growth rate of 38 mm/mo; our unpub. data).

Shedding frequency in the wild is unknown; Honegger (1970) reported that three 

captive hatchlings shed about two weeks after birth (Wall, 1926, stated “about a month,” 

Williamson’s, 1967, python at 8 d) and at intervals averaging 54 days for their next four 

sheds. Williamson’s (1967) well-fed captive averaged 40 days between sheds over the 

first two years, slowing to a rate of once per 61 days in the third year. Pope (1961) cites a 

python that shed an average of every 55 days when it was in the 4–5 m size range. 

Lederer (1944) documented the decline in shedding frequency with age, stating young 

shed 5–9 times per year (40–73 d apart), subadults, 4–7 times (52–91 d apart), and adults, 

3–7 times (52–121 d apart).

Length-weight relations in the wild are known for two samples of pythons with a 

snout-vent length up to about 5 m; southern Sumatra (Shine and others, 1998a) and 

western Borneo (Auliya, 2006).

4.5 Maturation

Shine and others (1998a) infer from captive growth that maturation is reached at 

age 2 years in males and 3 years in females. They explicitly based this on extraordinarily 

rapid captive growth. For example, Cox (1991) records maturation at an age of 5 years 

(unspecified whether this value is for captivity or in the wild). Barker and Barker (1997) 

state that captive males have been observed to court females as early as one year of age, 

but that successful captive breeding is attained only at 18 to 24 months for males and 30 

to 36 months for females. Shine and others (1998a) also emphasized that Reticulated 

Pythons mature at a conspicuously small size relative to the size they might ultimately 

achieve (Table 5.1).

4.6 Longevity

There are no data on life expectancy in the wild; several captive specimens lived 

for nearly 30 years (maximum, 29 years 5 months; Schmidt and Inger, 1957; Pope, 1961; 

Minton and Minton, 1973; Groombridge and Luxmoore, 1991; Snider and Bowler, 1992; 

Walls, 1998b; Slavens and Slavens, web page, accessed 25 Nov. 08, available at 

http://www.pondturtle.com/lsnakea.html#Python). Barker and Barker (1997) refer to 

wild Reticulated Pythons “...with their 40- to 50-year life spans...” but do not cite a basis 

for this inference.

5.0 Diet/Trophic Role

5.1 Prey

Most of the available data are anecdotal, with emphasis on finding the largest item 

recovered from a stomach. Such records seem to emphasize wild boars (Ditmars, 1912; 

Shelford, 1917, cited in Murphy and Henderson, 1997; Jacobsen, 1936, cited in Auliya, 

2006; Groombridge and Luxmoore, 1991; Shine and others, 1998b; Auliya, 2006), 
91

though bears have also been documented (Domalain, 1991, cited in Auliya, 2006; 

Fredriksson, 2005). Ditmars (1912) is unusual in going beyond anecdote to report that 60 

percent of a series of 40 large (criterion for large unspecified) wild-caught Reticulated 

Pythons had boar bristles or hooves in their excreta. He goes on to assert that “Many 

captive specimens of this great snake refuse all other food but swine, and examples 

twenty feet [6 m] in length, swallow a fifty-pound [22.5 kg] pig without difficulty.” 

More recent systematic data collection (Shine and others, 1998a, b, 1999; and Auliya, 

2006) shows a heavy preponderance of chickens and rats, though perhaps the snakes 

turned in for the hide trade are more easily collected from agrarian landscapes where rats 

are exceptionally abundant. Other notable items include monitor lizards (Mertens, 1942; 

Auffenberg, 1994; Shine and others, 1998b), civet cats (Groombridge and Luxmoore, 

1991; Shine and others, 1998b; Auliya, 2006), bats (Auliya, 2006), an immature cow 

(Barker and Barker, 1997; Auliya, 2006), various primates (Groombridge and Luxmoore, 

1991; MacDonald, 1992, cited in Auliya, 2006; Shine and others, 1998b; Wiens and 

Zitzmann, 1999; Gursky, 2002), deer (Jacobsen, 1936; Ussher, 1979, both cited in 

Auliya, 2006; Wall, 1926; Hendrickson, 1966; Groombridge and Luxmoore, 1991; Shine 

and others, 1998b), goat (Kopstein, 1926; Wall, 1926), pangolins (Groombridge and 

Luxmoore, 1991; Shine and others, 1998b; Auliya, 2006), cats (Wall, 1926), dogs (Wall, 

1926; Auliya, 2006), ducks (Wall, 1926), rabbits (Malkmus and others, 2002), tree 

shrews (Auliya, 2006), porcupines (Shine and others, 1998b), and many birds (Shine and 

others, 1998b). Frank Buck (1935, 1939, both cited in Murphy and Henderson, 1997) 

claims to have witnessed the capture of a leopard by a Reticulated Python, but unlike the 

other large animal accounts, this one does not appear to have gained credibility through 

frequent recitation. We also question its veracity, based on the unrealistic details 

provided. Walls (1998b) summarized the python’s diet by concluding, “most mammals 

and birds in its range.” However, it will also eat “large lizards [see also David and 

Vogel, 1996], frogs and fish” (Auliya and Abel, 2000b). Like P. molurus, the Reticulated 

Python recruits digestive tissue at the time of each large meal (Luz, 2002, cited by M. 

Auliya, in litt.; Secor and Ott, 2007; Ott and Secor, 2007a, b).

5.2 Predators

Many authors note that Reticulated Pythons habitually sleep over water, into 

which they plunge when disturbed (van Rooijen and van Rooijen, 2002; O’Shea and 

others, 2004). Though there are a number of accounts of dangerous prey animals that 

inflicted wounds on attacking pythons, there are few accounts that document Reticulated 

Pythons in the stomachs of higher order predators. Indeed we found no accounts 

specifically citing predation on Reticulated Pythons, though small individuals must be 

ready fare for large predatory crocodilians, birds, felids, pigs, and canids. In most areas, 

humans are presumably the most dangerous predator to pythons; this has been frequently 

documented (for example, Ditmars, 1912; Wall, 1926; Raven, 1946; Pope, 1961; Auliya, 

2006).

5.3 Diseases and Parasites

A host of internal and external parasites plague wild Reticulated Pythons, as 

documented by Wall (1926), Shine and others, (1998b), and Auliya (2006). The pythons 

are especially hosts to various protozoans, nematodes, pentastomids, ticks, and lung 
92

arthropods. There are no data to judge whether these infestations are of demographic 

significance. Pythons are known to carry ticks of agricultural significance while captive 

in Florida (Burridge and others, 2000, 2006). Kan and Hii (1979) found that Reticulated 

Pythons can transmit Capillaria (a cestode) that may result in the human disease 

capillariasis.

6.0 Demography

6.1 Population Density

There are no data on the density of wild Reticulated Python populations. Auliya 

(2006) noted that he caught 19 in a 4.4 km2 area over six months, indicating that at least 

that many passed through the study area during the course of the study.

6.2 Size Distribution

The study of size distributions is severely plagued by sampling problems. For 

example, Shine and others, (1998a, b, 1999) and Auliya (2006) were able to document 

the size of pythons turned into skin dealers, but the dealers were uninterested in small 

snakes (and hatchlings are likely to be diverted into the more profitable live-animal 

trade). Furthermore, the skin buyers were located in agrarian areas where rat populations 

were likely to be subsidized by the availability of high-sugar-content crops (Shine and 

others, 1999); thus the size distributions submitted might reflect human disturbance of 

natural vegetation as much as snake availability. Nonetheless, the relative paucity of 

small snakes may suggest rapid initial growth, such as is seen in captive animals. The 

relative paucity of very large snakes may reflect natural and unnatural mortality of these 

long-lived animals.

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

Reticulated Pythons are the premier source of leather from snakes. The most 

complete analysis of the trade (Groombridge and Luxmoore, 1991) covered only the 

years 1986–1989, when export records showed the international trade ranging from 

200,000 skins per year to around 700,000. Internal organs from the snakes killed for their 

skins are used for medicinal purposes, especially the gall bladders (Shine and others, 

1999; G. Zug, pers. commun., 2009). The number of snakes exported live is less well 

documented, but about an order of magnitude smaller (see 7.3.1). No doubt additional 

individual pythons are killed in areas too remote for transport to a buyer. Shine and 

others (1999) note that their informants reported that virtually all pythons encountered 

were killed, “because of the value of the meat and the advantages of removing nuisance 

animals that might otherwise devour chickens, dogs or children.”

7.2 Human Health Risks

The Reticulated Python is a particularly dangerous species of giant constrictor. 

Murphy and others (1994) is the only reputable source we found that argued that
93

Reticulated Pythons “...should not be considered a serious threat to human life.” We can 

divide human fatalities into two categories on the basis of whether the snake was being 

potentially provoked by a human, usually the owner of a pet snake (“illegitimate bites”), 

as opposed to attacks on humans that were not interacting with the snake prior to the bite 

(“legitimate bites”). To our knowledge, illegitimate bites have never resulted in ingestion 

of the human, probably because the bites were defensive in nature, intended merely to 

cause the human to stop bothering the snake (lethal constriction is effective for this). 

Most of the giant constrictor bites clearly documented in the literature refer to illegitimate 

bites, but the Reticulated Python stands out as being the source of many legitimate bites, 

in which the snake often ingests the killed human (unless other humans intervene). 

Among the authorities who have weighed in on this distribution of deaths, Walls (1998b) 

is a minority of one for his assertion that Reticulated Pythons do not exhibit ingestion of 

adults: “...[although] it has been repeatedly charged with killing and eating children...[it] 

has been seen to kill, but not eat, adult humans.” The most widely cited example of child 

eating is of a 14-year-old boy killed on the island of Salibabu, in the Talaud Islands north 

of Sulawesi (Kopstein, 1927). Incidents from this part of the world tend to be poorly 

documented, but this one was reported by a credible scientist and therefore has been very 

extensively cited (Smith, 1943; Tweedie, 1953; Schmidt and Inger, 1957; Pope, 1961; 

Campden-Main, 1970; Caras, 1975; Barker and Barker, 1997; Murphy and Henderson, 

1997; Zug and Ernst, 2004; Auliya, 2006, among others). 

Legitimate bites have not been limited to children; for example, Auliya (2006) 

cites a 60-kg adult removed from the stomach of a Reticulated Python in 1998 from the 

Philippines. Cox (1991) states that Reticulated Pythons are capable of killing adults. 

Muthusamy and Gopalakrishnakone (1990, cited in Murphy and Henderson, 1997) report 

two incidents with adults, at least one of which was unquestionably “legitimate.” Barker 

and Barker (1997) cite a 29-year-old rubber tapper killed in peninsular Malaysia in 1995, 

as well as others earlier near Rangoon, and on Borneo, Sumatra, Sulawesi, and 

Halmahera. Murphy and Henderson (1997) recount numerous attacks and ingestions, 

both illegitimate and legitimate. Barker and Barker (1997) go so far as to aver that, “Of 

the species of giant snakes, there are more verified reports of man-eating reticulated 

pythons than of all the other species combined.” They also assert that most go 

unreported, “with many cases classified simply as ‘missing persons’.” Headland and 

Greene (2007) presented data to show that in a preliterate hunter-gatherer society (Agta 

Negritos) in the Philippines in the 1970s, “...26% of adult males had survived predatory 

attacks by reticulated pythons and fatal attacks still occurred in the 1970s.” A substantial 

number of the tribe’s adults knew personally of a person who had been ambushed and 

killed. It is notable that claims to this effect are lacking for most other portions of the 

Reticulated Python’s range; a possible exception is the undocumented assertion by the 

19th century zoo collector Carl Hagenbeck (1909) that “It is said that in Borneo the 

natives are frequently attacked and devoured by giant serpents, and from what I have 

seen of their capacity for dealing with large prey I have no doubt that a full-grown 

Borneo python could easily swallow a man.”

Davis (1980), Flank (1997), Murphy and Henderson (1997), and Day (2008) 

report some of the illegitimate fatalities.
94

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

There are major discrepancies between reporting agencies on the number of 

Reticulated Pythons imported into the United States, though all sources acknowledge that 

undercounting occurs due to incomplete declarations, smuggling, and contradictory 

records. CITES records (Fig. 5.4) are acknowledged to be incomplete after 2005, but 

show fairly steady imports at around 4,000 per year. LEMIS records of the Fish and 

Wildlife Service for the period 1998–2005 averaged about 12,375 per year, roughly on 

parity with the number of P. molurus importations (C. Romagosa, pers. commun., 2008, 

from LEMIS records), and a dramatic increase from the 2,333 yearly average for the 

period 1989–2000 (Reed, 2005). We are unable to resolve these discrepancies, but all 

sources indicate that thousands of Reticulated Pythons are imported to the United States 

annually.

In addition to these sales of wild-caught or farmed/ranched individuals from the 

python’s native range are added pythons that are captive bred in the United States, a 

substantial but unquantified number. As the captive-reared animals are often bred for 

unique colors, they are likely to be more expensive than the roughly $100 retail price of 

an imported python (C. Romagosa, pers. commun., 2008, from LEMIS records).

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1977197919811983198519871989199119931995199719992001200320052007

Year imported

Number imported

Figure 5.4. CITES records for importation to the United States of live B. reticulatus.

7.3.2 Other Uses

Major human uses of Reticulated Pythons are limited to leather, medicinals, meat 

(all section 7.1), and pets (7.3.1). Number imported

Year imported
95

Management Profile

8.0 Pathway Factors: Pet Trade

As with the other giant constrictors, the overwhelming majority of live 

Reticulated Pythons reach the United States through the pet trade.

8.1 Volume of International Trade

As indicated in section 7.3.1, above, Reticulated Pythons are among the most 

often imported snakes, with thousands imported per year. Section 8.1 of chapter Four 

outlines reasons why imported snakes might constitute a slightly greater risk of 

establishment than captive-reared individuals, though hard data on this phenomenon are 

lacking.

8.2 Volume of Domestic Trade

We are aware of no estimates on the volume of domestic sales or breeding of 

Reticulated Pythons, but it seems to be increasing, especially for designer morphs or 

dwarf lineages. Our impression is that the domestic production is somewhat smaller than 

international trade for the United States, but we cannot verify this impression.

9.0 Entry Potential (Survival in Transit - Meant Mainly for 

Unintentional Transport)

As with all of the giant constrictors, survival in the pet-trade pathway is 

intentional and approaches 100 percent, as giant constrictors can routinely fast for the 

period of time between capture and sale, and the substantial expense associated with the 

purchase of a python generally induces care by owners that is sufficient for survival of 

these relatively robust snakes.

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

The same set of factors that promote release of unwanted pet Burmese Pythons 

(see section 10.1 of Chapter Four) applies with equal or greater force to Reticulated 

Pythons. The only discernible difference is that Reticulated Pythons have a more 

fearsome reputation (which inhibits demand for adults), and they show slightly greater 

growth rate and maximum size than do Burmese Pythons, also limiting demand for 

adults. Reticulated Python individuals that are prone to attack their owners (behavior 

which appears to be more frequent in Reticulated Pythons) will likely induce owners to 

discard problematic individuals.
96

10.2 Climate Match: Areas of the Country at Greatest Risk

Reticulated Pythons have a more tropical distribution than Burmese Pythons. 

Accordingly, the area of the mainland United States showing a climate match of 

uncontested sites is smaller, exclusively tropical, and limited to southern Florida (Fig. 

5.5; note that newly-available information for Vietnam, discussed in Section 3.1, could 

conceivably further reduce the area considered suitable in Florida, but we were unable to 

incorporate this information before going to press). However, if one includes those 

portions of the mainland Asia range that are judged occupied by some but not all 

observers, one obtains the climate match shown in Fig. 5.6, which also includes much of 

central Florida and the lower Rio Grande section of Texas. If the range limit of the 

Reticulated Python at the northern limit of the range reflects competition with Burmese 

Pythons, it is conceivable that additional portions of the United States would be at risk if 

they were not already occupied by Burmese Pythons. Given the current distribution of 

Burmese Pythons in Florida, and the ongoing spread of that species, such a scenario 

seems unlikely to become manifest in Florida, though there is a small chance that it could 

become so in extreme southern Texas. Low and mid-elevation sites in America’s tropical 

territories (Guam, Northern Mariana Islands, American Samoa, Virgin Islands, Puerto 

Rico) and Hawaii would also appear to be climate matched to the requirements of the 

Reticulated Python, whether using all localities or the subset considered uncontested.
97

Figure 5.5. Areas of the United States matching the climate envelope expressed by B. 

reticulatus using only those portions of the range that are deemed occupied by all observers (see 

section 3.1).
98

Figure 5.6. Areas of the United States matching the climate envelope expressed by B. 

reticulatus using all portions of the native range deemed occupied by any of the observers cited in 

section 3.1.

10.3 Prey Availability

The prey range of Reticulated Pythons is essentially the same as that of the Indian 

Python, as far as is known, including all birds, reptiles, and mammals of a size 

appropriate for the individual predator. Large frogs and fish are eaten less regularly, if at 

all, but might sustain an introduced population that had no alternative prey. Given the 

extreme generality of the Reticulated Python’s diet, the mainland sites indicated in Figs. 

5.5 and 5.6 would not appear to have any shortage of appropriately sized endotherms. 

Prey availability for large Reticulated Pythons could be a limiting factor in some insular 

situations, as noted in chapter Four, section 10.3.

10.4 Predation Risk

The Reticulated Python is the longest known snake, and the United States has no 

specialized predators that would take a large adult python. However, smaller generalized 

predators such as crocodilians or panthers would probably take small individuals, as 

noted for Indian Pythons in chapter Four, section 10.4. That narrative suggested that 
99

such “intra-guild” predation would have an equivocal demographic outcome, with each 

predator prevailing where it was larger than an individual prey item, but with little 

demographic significance overall. In comparison to adult Burmese Pythons, Reticulated 

Pythons seem to be more aquatic, and thus adults might have fewer encounters with 

panthers. Hatchling Reticulated Pythons appear to be more arboreal than similar-sized 

Burmese Pythons (Retics are more slender, as befits a climber), and thus juveniles may 

have fewer encounters with crocodilians.

10.5 Reproduction Requirements

We are aware of no specialized reproduction requirements of this species, which 

broods its eggs but does not show shivering thermiogenesis as far as we know. Thus it 

would probably have difficulty incubating eggs in colder climates, which should be 

reflected in the climate envelope and maps above.

10.6 Hibernation Requirements

As far as it known, Reticulated Pythons do not hibernate.

10.7 Tools for Eradicating a Colonization

The tools for eradication of this species are not known to differ from those 

specified in chapter Three, and section 10.7 of chapter Four. Because Reticulated 

Pythons do not hibernate, they might be especially vulnerable to capture while basking on 

cold days at the northern limit of their range.

11.0 Spread Potential

11.1 Dispersal Ability

To the best of our knowledge, there are no relevant data on dispersal ability. We 

have no reason to think that Reticulated Pythons would be less vagile than Indian 

Pythons. The record clutch size for Reticulated Pythons is 124, the largest known clutch 

of any oviparous snake, so they are unlikely to have their population-dispersal ability 

limited by fecundity.

11.2 Ability to Develop Races or Strains

There are no data to evaluate this issue in Reticulated Pythons. As the rare 

subspecies are highly sought after in the pet trade and are therefore disproportionately 

numerous in the trade, release of differing strains is a distinct possibility, though the rare 

subspecies presently command a premium price and attain smaller body sizes, which 

would presumably limit human motivation for release in the wild.

11.3 Tools for Managing Spread

There are no species-specific data germane to managing spread in this species. 

The highly aquatic adults may be difficult to constrain by barriers that end at the water’s 

edge. The arboreal habitats of juveniles suggest that barriers would also be more difficult 

to design and to maintain in forested habitats.
100

12.0 Economic Impact Potential

A formal economic analysis of the potential impacts of introduction of the 

Reticulated Python and possible regulation of the trade may be conducted pursuant to a 

variety of statutes and Exec. Order 13272, Proper Consideration of impact on small 

business. We defer to the economists for hard data; our narrative is provided only to cite 

relevant biological literature and identify germane sections of the bioprofiles.

12.1 Pathogen Vector

Reticulated Pythons are known to harbor a variety of pathogens or vectors that 

could potentially be of human health or agricultural significance; see discussion in 

section 5.3 regarding papers on Reticulated Python ticks (Burridge and others, 2000, 

2006) and cestodes (Kan and Hii, 1979). The epidemiological significance of these 

carriers is unknown.

12.2 Predator on Livestock

Reticulated Pythons in their native range are reported to prey on all sizes of 

livestock. However, because the average Reticulated Python is relatively small, we infer 

that the greater predatory challenge would be on poultry (see chapter Four section 12.2).

12.3 Predator on Pets

Predation on all types of pets (except horses) is within the size range of known 

Reticulated Python predation, but the frequency of occurrence is probably sensitive to the 

snakes’ willingness to reside in suburban situations. In its native range, the snake is 

heavily persecuted by a very high density of humans in mostly agrarian societies where 

few snakes of such a size would go undetected. It is less clear if that dynamic would 

have a similar outcome in American suburbs, which are often depopulated during 

business hours.

12.4 Electrical Power Systems

Juvenile Reticulated Pythons would presumably be capable of climbing overhead 

power lines, though their propensity for doing so is completely undocumented.

12.5 Traffic Accidents/Human Attacks

Human predation by Reticulated Pythons is sufficiently rare that it is not judged 

as a major public health problem in the snake’s native range. Nonetheless, of all the giant 

constrictors, this species has the most abundant evidence for “legitimate” attacks (section 

7.2). One imagines that such an attack in the United States would garner extensive press 

coverage due to its novelty and the iconic reputation of the Reticulated Python. This 

could have an appreciable negative influence on the serenity of residents and tourists. 

That this species does coexist with humans in urban or suburban settings would probably 

heighten anxieties in comparison to fear of species believed to stay in wild places.
101

12.6 Tourism

Of the two regions of the U.S. mainland showing potential evidence of a climate 

match with the native range of Reticulated Pythons (south Texas and southern Florida), 

southern Florida is probably the region most dependent on tourism and therefore most 

vulnerable to disruptions of tourism. However, southern Florida already has an 

acknowledged reputation for unsavory characters, both reptilian and otherwise. Thus the 

potential impact on tourism is probably relatively minor, given that southern Florida is 

already colonized by the Burmese Python. However, a widely publicized humanReticulated Python encounter in an urban area of southern Florida could adversely impact 

tourism there.

Insular areas of the United States might be far more sensitive. For example, 

Hawaii is well known to be a tropical paradise, in part for the absence of snakes. Given 

the range of estimates associated with potential Brown Treesnake impacts on Hawaii 

tourism (up to $3B/year; Shwiff, 2008), a legitimate Reticulated Python attack in Hawaii 

could be very costly. Whereas the dollar cost of an incident would be less in other U.S. 

insular areas (Guam, Northern Mariana Islands, Puerto Rico, Virgin Islands, American 

Samoa), the proportionate impact to those fragile island economies could be highly 

disruptive. The potential economic cost makes this improbable event worthy of serious 

consideration. 

12.7 Impacts on Commercially Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

South Florida and south Texas ecotourism are associated with bird watching. 

Birds that are imperiled or visually striking are especially desired. Imperiled species are 

the ones most likely to be impacted by python predation (see section 7.1). Thus to the 

extent that visitation is tied to success at bagging a sighting of a rare bird, the presence of 

a novel predator on rare birds is likely to be detrimental to tourism. Gun hunting is 

relatively unimportant in southern Florida, and species that are most sought after in 

extreme southern Texas (for example, quail) are apt to be a bit small to be a primary prey 

of Reticulated Pythons. If southern Texas was to become colonized, there would be some 

impact to larger species (for example, deer).

12.7.2 Species That Impact Forestry, Agriculture, or Horticulture

Reticulated Pythons are unlikely to have a direct impact on forestry, agriculture, 

or horticulture, though one can imagine a scenario where agricultural workers might 

become disenthralled with field labor involving the chance of unexpectedly encountering 

a Reticulated Python (for example, sugar cane workers in south Florida; orchard workers 

in south Texas).
102

13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

See section 13.1 of the Burmese Python chapter (Four) for a listing of prey 

species that might be impacted by the Reticulated Python. The Reticulated Python would 

be expected to have a lower impact because (1) its anticipated range potential is 

substantially less (primarily southern Florida, but potentially also southern Texas and 

central Florida), and (2) the Burmese Python will likely have spread throughout the area 

of potential Reticulated Python colonization by the time the Reticulated Python would 

become established. Thus the two pythons are likely to be each other’s most important 

competitor, and it is less likely that the second-arriving species would have as large an 

impact as the first (and therefore novel) giant constrictor.

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

As indicated in section 13.2 of chapter Four, food supplementation by Reticulated 

Pythons is likely to be offset, or possibly more than offset, by intra-guild predation. For 

example, some alligators would no doubt get a meal from the presence of Reticulated 

Pythons in southern Florida, but it is equally likely that some Reticulated Pythons would 

make a meal of juvenile American Alligators, with no clear net result.

13.3 Vector for Disease Spread to or Within Native Faunas

Too few data exist to evaluate this consideration, though the risk of importation of 

parasites or pathogens cannot be excluded.

13.4 Species That Might be Adversely Impacted by Control Measures

If an indiscriminate control technique such as acetaminophen toxicants were to be 

applied to areas infested with Reticulated Pythons, native species such as the Indigo 

Snake (Drymarchon couperi) or Florida panther (Puma concolor coryi) could be 

adversely impacted (due to the lack of specificity of acetaminophen as a toxin). It is 

difficult to project such ecological costs without a more distinctly enumerated set of 

management tools.

14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Production Affected?

Domestic producers of Reticulated Pythons for the pet industry and pet owners 

are likely to raise objections to potential restrictions on the sale of this giant constrictor. 

Objections include concern for lost sales, concern that such a proposal would lead to 

other restrictions on pet ownership, and ideological objection to the concept of 

government regulation of pet ownership. If only importation of B. reticulatus was to be 
103

proscribed, domestic producers would presumably benefit via reduced competition from 

cheap imports.

There are a very large number of Reticulated Pythons in private ownership in the 

United States, though no one has data that would quantify the number or characterize the 

owners. Many retics are traded or exchanged in situations outside of conventional retail 

channels. Florida and Texas have begun to regulate ownership of these animals, but it is 

too early to judge compliance with these measures (Environmental Law Institute, 2008). 

Regulatory measures to restrict trade or ownership could have negative effects in 

terms of dealers freeing stock (the release of multiple animals at the same time and same 

place enormously increases the risk over single releases such as those typically done by 

pet owners), owners releasing animals for which they don’t have or cannot acquire 

appropriate licenses, and ecovandals determined to release animals as an imprecisely 

directed assault on the government.

14.1.2 International Trade/Alternate Pets

Companies that import Reticulated Pythons would lose sales of these reptiles if 

importation restrictions were to be applied. As reptile importation has historically 

promoted both certain staple species (for example, Green Iguanas) and novelties (for 

example, Solomon Islands Monkey-tailed Skinks, Corucia zebrata, amelanistic snakes), 

there is reason to believe that much of the importation effort associated with a particular 

species could be transferred to another species. Thus the industry would be less affected 

overall than specific breeders that have developed stock or expertise in this particular 

species. Quantification of the economic impacts of trade regulations would be the subject 

of an economic impact report (Exec. Order 13272), if needed. We defer to that economic 

analysis for assessment of this issue.
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Chapter Six–The Northern African Python, Python 

sebae, and the Southern African Python, Python 

natalensis

Biological Profile

1.0 Introduction

See section 1.0 of the biological profile of the Indian Python (chapter Four) for a 

description of species-specific and overall components of the risk assessment package.

1.1 Species

Python sebae and Python natalensis are closely related, large-bodied pythons of 

similar appearance found in sub-Saharan Africa. Python sebae was described in 1789, 

while Python natalensis was described as a full species as 1840; within 25 years the latter 

taxon was considered a variant of the former, rather than a distinct species. This 

taxonomy held fairly steady for a century, until Broadley (1984) re-examined museum 

material and concluded that P.s. sebae and P.s. natalensis were valid subspecies. 

Broadley’s (1984) work was partially stimulated by the description of Python saxuloides

(Miller and Smith, 1979), which was described as a sister taxon to P. sebae, but which 

was later deemed a slightly aberrant population of P. s. natalensis by Broadley (1984, 

1999).

Based on morphology, the presence of a single known hybrid individual from 

Tanzania, and an apparent zone of sympatry spanning 900 km from Kenya southwest into 

the Democratic Republic of the Congo, Broadley (1999) later raised P.s. natalensis to a 

full species. This conclusion has been widely accepted in the African herpetological 

community, as reflected in the names used in field guides and journal articles. 

Unfortunately, however, the geographical extent of overlap between these species is still 

only partially known, and many questions remain about how to refer older literature 

records to the correct species. This means that some aspects of African python ecology 

are difficult to tease apart at the species level, especially for observations from areas 

where both species may occur. 

1.2 Common Names

The most common English name for this species complex has been African Rock 

Python. After P. sebae was nominally split from P. natalensis, some authors added 

‘Northern’ and ‘Southern’ as a prefix to this common name (for example, Spawls and 
105

others, 2002). Broadley (1999) opined, “I am unhappy about the use of ‘African Rock 

Python’ as these snakes show no particular association with rocky habitats, so I propose 

the use of ‘Northern African Python’ for P. sebae and ‘Southern African Python’ for P. 

natalensis.” In this document we have adopted Broadley’s recommendations, and refer 

to these snakes as the Northern and Southern African Pythons. When considered 

together, we will refer to both species as the giant African pythons.

As would be expected for species with such large distributions, local common 

names are legion. Auerbach (1987) lists 60 common names in 44 different languages in 

Botswana and its immediate surrounds alone. Of these, commonly encountered names 

include Afrikaanse reusslang, Luislang, or Gewone luislang (Afrikaans), Joboia africana 

(Portuguese), !na//ke: (!Kung), Ngoi (Kikomi), Chatu or Satu (Swahili), and Inhlwathi, 

uMonya, or imFundamo (Zulu). Farther north in Sudan, the Northern African Python is 

called Assalaa (El-Kamali, 2000); a wide range of common names can be expected in the 

northern and western parts of the range of the Northern African Python but are not 

enumerated here.

1.3 Evolutionary Context

The giant African pythons have long been considered members of an AfroAsian 

group, usually including P. sebae, P. molurus, and P. regius, but occasionally also 

including B. reticulatus, P. anchietae, the P. curtus complex (including P. brongersmai

and P. breitensteini), and/or B. timoriensis (McDowell, 1975; Underwood and Stimson, 

1990; Kluge, 1993; Lawson and others, 2004). None of these phylogenetic analyses have 

distinguished between the two giant African pythons, and some analyses (for example, 

Kluge, 1993) have used specimens of both species, while most GenBank records fail to 

give localities for the specimens from which tissues were collected.

More recently, Rawlings and others (2008) examined the phylogeny of pythons 

using nuclear and mitochondrial genes. They concluded that the Northern African 

Python (and by extension its putative sister taxon) is most closely related to P. molurus, 

the Asian python commonly referred to as the Indian or Burmese Python (depending on 

subspecies). Indeed, while the Northern and Southern African Pythons can be readily 

distinguished by head scalation, it is more difficult to distinguish the former from P. 

molurus using any surefire quantitative measure. Rawlings and others (2008) also 

concluded that the AfroAsian pythons (P. sebae, P. molurus, P. brongersmai and 

relatives, and P. regius) represent a basal clade within the python radiation, and that some 

species formerly included within the genus (B. reticulatus and B. timoriensis) should be 

assigned to a new genus (they proposed using the available genus Broghammerus).
106

Figure 6.1. Schematic representation of the head of P. sebae.

2.0 Description

2.1 Unique Morphological Features

The following general description of body color pattern in the giant African 

pythons is based on descriptions in Branch (1988), Spawls and others (2002), Marais 

(2004), Lanza and Nistri (2005), Alexander and Marais (2007), and our personal 

observations. See below for more specific information on head pattern and means of 

discriminating between the two species. The overall impression when viewing these 

pythons is of a brown or grey python with numerous dark dorsal blotches and dark lateral 

spots. Dark brown to brownish-black saddles or bars are present dorsally, usually edged 

in black. These markings vary in size and shape and may be connected by complete or 

nearly complete dark paravertebral stripes on each side. When connected by 

paravertebral stripes, animals appear very dark on the dorsum, with regular “islands” of 

lighter ground color present mid-dorsally – these islands are often constricted in the 

middle, giving them the general shape of a number “8” oriented sideways to the long axis 

of the body. Lateral dark markings in the shape of a “C” are typically present on the 

anterior half of the body, with the open side of the C facing anteriad. The center of the C 

may be filled with a darker color than the surrounding ground color, but the C itself is 

darker yet and typically edged in white or yellow. Posteriad, these markings become 

more sharply defined and take the form of vertical or sinuous bars. Dark spots or dots are 

scattered across the lateral surfaces, often becoming larger on the lowermost scale rows 

adjacent to ventral scutes. The ventral surface is white to grey, with numerous dark grey 

to black blotches of varying size.

Schmidt (1923) stated that color pattern on the head was invariant among a series 

of 18 specimens of the Northern African Python from the Belgian Congo (Democratic 
107

Republic of the Congo), and his description is applicable to most members of the species: 

“The top of the head is dark, with a straight light stripe from the supranasal over the 

nostril and eye on each side to the temporal region. The labial border, except posteriorly, 

is light, connected with the supraocular stripe on the neck, and anteriorly by a broad light 

area below the nostril and on the anterior labials and rostral. This area encloses a dark 

spot on the first and second labials. There are two distinct subocular light lines, one from 

the posterior lower corner to the ninth to twelfth labial, one from the lower anterior 

corner across the sixth labial. There is a median light mark behind the parietals.” In 

contrast to head pattern, however, Schmidt (1923) observed moderate variation in 

scalation, with individuals displaying 11-15 supralabials, 19-24 infralabials, 270-284 

ventrals, 65-69 subcaudals, and dorsal scale formulae from 66-89-43 to 77-95-54. In 

both species, the first two supralabials have obvious thermoreceptive pits, with smaller 

pits visible on the first 4-6 infralabials. The rostral scale also has two lateral pits. In 

Southern African Pythons, there are 78-95 dorsal scale rows at midbody, 261-291 

ventrals, 63-84 subcaudals, 12-15 supralabials, none contacting the eye, and 17-20 

infralabials (Auerbach, 1987; Marais, 2004). Lanza and Nistri (2005) provide a fine 

summary of morphology and pattern for Northern African Pythons from Somalia, as well 

as some of the variation observed elsewhere in the range of the species.

Head scalation and lateral head pattern are used to distinguish Southern and 

Northern African Pythons, as described by Broadley (1984: see his Figure 1, p. 361). 

The frontal scales of the Southern African Python are fragmented, while those of the 

Northern African Python are entire, albeit sometimes fused; the latter situation is also true 

of P. molurus. The head markings of the Northern African Python are much better 

defined and strikingly contrasted than those of its sister taxon; the Northern African 

Python has a pale subnasal stripe, dark subocular and preocular patches, and a broad 

postocular stripe, whereas all of these are greatly reduced or fragmented in the Southern 

African Python. The body markings of the Southern African Python are less defined 

overall (and the Northern African Python in turn exhibits less definition than does P. 

molurus). Otherwise, most characters are similar among the African species, although 

old or incubating individuals of the Southern African Python can be almost solid black 

(Alexander and Marais, 2007). Both African species tend to have higher midbody, 

ventral, caudal, supralabial, and infralabial scale counts than does P. molurus, but the 

fields tend to overlap at least somewhat (Broadley, 1984). 

2.2 Size

As is typical of giant snakes, estimates of the maximum body size attained by the 

giant African pythons are muddied by exaggeration, fable, and error. Some authors state 

that the Northern African Python reaches larger body sizes than does the Southern 

African Python, but we are unaware of any quantitative analysis of this supposition. 

Pope (1961) reported a maximum size of 9.8 m for a P. sebae killed in Ivory Coast in 

1932 – this is perhaps the largest body size that has been given serious consideration by 

those summarizing the biology of the species, but this report lacks verification. To our 

knowledge, no individual snake of any species has ever been found to exceed 30 feet 

(9.14 m), either in the wild or in captivity. More realistic opinions on body sizes of the 

giant African pythons are provided by Ditmars (1931), who considered the maximum 

size of to be about 20 feet (6.1 m), and by Alexander and Marais (2007), who state that, 
108

“It is doubtful that any specimens (of Southern African Python) of more than 6 m have 

existed, even in historical times.” Alexander (2007) stated that this species can exceed 55 

kg body weight. Without distinguishing between the two species, Murphy and 

Henderson (1997) cited multiple accounts of African pythons over 6 m; while these 

claims generally lack verification, there seems to be a moderate amount of evidence to 

suggest the Northern African Python can reach 7 m and that maximal size of the Southern 

African Python is somewhat less.

The largest specimen in a series of 18 individual Northern African Pythons from 

Congo was 4.88 m (Schmidt, 1923), with no apparent size differences between rainforest 

and jungle specimens (in Schmidt’s era, the term “jungle” was applied to scrubby dry 

forest with high stem density, whereas “rainforest” was wet forest with a tall canopy and 

low stem density). In the Gambia, Starin and Burghardt (1992) stated that observed 

pythons ranged in size from 0.5 to about 7.5 m, but these were visual estimates of 

unknown accuracy. Two specimens of Northern African Pythons from southern Somalia 

were purportedly 5.72 m and 4.45 m total length, with girths of 0.41 and 0.46 m, 

respectively (Lanza and Nistri, 2005). In southern Nigeria this species averaged 3.4 m 

total length in natural habitats (n = 96, range 1.0-5.5 m) and 2.9 m in suburban habitats (n 

= 61, range 1.0-4.5 m: Luiselli and others, 2001), while three pythons captured on study 

plots in the same region averaged 2.39 m total length (Luiselli and others, 1998).

Auerbach (1987) said that specimens referable to the Southern African Python 

rarely reach 6.5 m, that specimens average 2-4 m, and that hatchlings average 600 mm. 

Alexander and Marais (2007) state that hatchlings of this species average 500-700 mm 

total length and weigh about 100 g.

Table 6.1. Total lengths and masses of typical P. sebae and P. natalensis (compiled/estimated 

from the literature sources listed in the text). Some values estimated from length/mass 

relationships (for example, Figure 6.8, below).

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 500-700 100 1.7-2.5 2.5-8 4.0? 30?

F: 500-700 100 1.7-3.0 2.5-10 7+ (P. sebae)

6 (P. natalensis)

65?

55

2.3 Sexual Size Dimorphism

Females of both species attain larger body sizes than do males. However, this 

size dimorphism may be somewhat less than what is seen in some of the other giant 

snakes. For example, females were larger in both SVL and mass among 24 Southern 

African Pythons used in a radiotelemetry study in South Africa (mass: one-tailed t = 1.94, 

p = 0.03; SVL: t = 1.84, p = 0.04; our calculations), but the largest female was less than 8 

percent larger than the largest male: 4.6 vs. 4.2 m (4.07 m SVL vs. 3.78 m SVL: 
109

Alexander, 2007). Females may be heavier than males at similar lengths (Lanza and 

Nistri, 2005).

2.4 External Sexual Differentiation

Pelvic spurs of males are larger than those of females, and the proximal portion of 

the tail is often visibly wider in males. While relative tail length (tail length/total length) 

among giant snakes tends to be significantly higher in males than in females, this trend 

may not be consistent for Southern African Pythons, at least using data provided by 

Alexander (2007) for 24 pythons greater than 1.53 m (1.35 m SVL). Among these 

snakes, relative tail length ranged from 0.07 to 0.15 for females and 0.11 to 0.13 for 

males (our calculations).

3.0 Distribution in Space and Time

3.1 Native Range

As with most other giant snakes, the giant African pythons are relatively underrepresented in museum collections; this is due to the great size of adults and the 

associated labor and expense of preserving and maintaining large specimens. For 

example, Broadley (1984) stated that, “Because of the large size attained by this species, 

most museum specimens are juveniles or consist of the skins or severed heads of adults.” 

Distributional surveys based on museum surveys are thus likely to underestimate the 

overall geographic range of these snakes.

Without distinguishing between Northern and Southern African Pythons, Spawls 

and Branch (1995) stated that pythons are “found throughout sub-Saharan Africa, except 

for montane uplands above 2300 m, temperate regions in the south, and extreme desert in 

Namibia and adjacent Botswana, and Somalia and Ethiopia in the Horn.” Within this 

vast distribution, Northern African Pythons (Fig. 6.2) range from the coasts of Kenya and 

Tanzania across much of central Africa to Mali and Mauritania, as well as north to 

Ethiopia and Eritrea; in arid zones, their range is apparently limited to the vicinity of 

permanent water. The Southern African Python (Fig. 6.3) is found from Kenya 

southwest to Angola and south through parts of Namibia and much of eastern South 

Africa. Distributions of the species overlap somewhat, although the southern species 

tends to inhabit higher areas in regions where both species occur. The distributions of 

each species are treated separately by country, below.
110

Figure 6.2. Native range of the Northern African Python, Python sebae, estimated using 

Broadley (1984) and additional sources cited in section 3.1.1. Primary uncertainties include the 

areas denoted with question marks, such as degree of python intrusion into arid zones in the 

northeastern part of the range (especially in Sudan), extent of the Angolan distribution, and areas 

of sympatry or parapatry with the Southern African Python, Python natalensis.

?

?
111

Figure 6.3. Native range of the Southern African Python, Python natalensis, estimated using 

Broadley (1984) and additional sources cited in section 3.1.2. Primary uncertainties include the 

area denoted with a question mark (primarily Angola) and areas of sympatry or parapatry with the 

Northern African Python, Python sebae.

3.1.1 Native Range of Python sebae

Angola–Present in the northernmost third of the country, although most records are from 

localities bordering the Democratic Republic of the Congo and Republic of the Congo 

(Broadley, 1984). Records from the south of the country are referable to P. natalensis.

?
112

Benin–Only one record in Broadley (1984) and three in Chippaux (1999), but apparently 

suitable habitat exists through most of the country. Benin has offered postal stamps 

featuring P. sebae and P. regius (http://www.reptile-stamps.de/html/benin.html; accessed 

22 April 2009).

Burkina Faso (formerly Upper Volta)–Few records available (multiple observation 

localities reported by Chippaux (1999), and apparently suitable habitat exists through 

most of the country). The “python” is listed as Threatened in Burkina Faso, (http://bchcbd.naturalsciences.be/burkina/bf-eng/implementation/doc/monography/chap3-2-3-

2.htm; accessed 20 April 2009), but this could refer to P. sebae and/or P. regius. 

Cameroon–Probably historically present through much of the country in suitable habitat, 

and known from multiple localities (Chippaux, 1999; LeBreton, 1999). Although 

considered rare by Gonwouo and others (2007) in the vicinity of Mt. Cameroon, it was 

found in lowland forest below 1500 m, as well as submontane forest from 1500 to 

1800 m.

Central African Republic–Chirio and Ineich (2006) provided information on 23 

specimens from 10 localities in the CAR and Chippaux (1999) reported 10 observations 

from across the country, while Joger (1989) reported specimens from Parc National Saint 

Floris and the vicinity of Bangoran, as well as several locations just across the border in 

Zaire. Probably historically present through much of the country in suitable habitat.

Chad–Known from Sar (Sahr), in southern Chad along the Chari River (Broadley, 1984). 

Probably historically present around aquatic habitats in southern Chad, especially the 

Logone and Chari Rivers and their tributaries.

Democratic Republic of the Congo (formerly Zaire and Belgian Congo, aka CongoKinshasa)–Apparently present or historically present through much of the country in 

suitable habitat. Largely replaced by P. natalensis in extreme southern DRC, but with 

possible areas of sympatry in parts of Shaba Province (Broadley, 1984).

Equatorial Guinea–Distribution poorly known and represented by only two localities in 

Broadley (1984), but likely was historically present in most suitable habitats. Individuals 

are regularly offered for sale in bushmeat markets on Bioko Island (Fa and others, 2000).

Eritrea–The few records from Eritrea are primarily from the Mareb, Gash, Barca, and 

Anseba River drainages, from the vicinity of Annesley Bay west to the border with Sudan 

(Lanza and Nistri, 2005; M. Largen, pers. commun., 2009). The species is probably more 

common than is reflected by museum records (M. Largen, pers. commun., 2009).

Ethiopia–Known from multiple localities in the southwest and extreme south near 

Mandera, as well as the Rift Valley, Lake Tana, Baro River, and near the northern border 

at Amba Aradam (Broadley, 1984; M. Largen, pers. commun., 2009). Most records are 

associated with watercourses.
113

Gabon–Represented by a small number of records scattered throughout the country (for 

example, Chippaux, 1999). Probably present, at least historically, in suitable habitat 

through much of this low-lying country.

The Gambia–Probably historically present in suitable habitat through much of this small 

country, which is a thin strip of land on each side of the lower Gambia River. Northern 

African Pythons were an obvious component of the vertebrate fauna during one 

ecological study in The Gambia (Starin and Burghardt, 1992), and were observed in 

savannah, marshland, and gallery forest by Hakansson (1981).

Ghana–Probably historically present through much of this low-lying West African 

country in suitable habitat, and represented by nearly 20 localities on the maps of 

Broadley (1984).

Guinea–Poorly known, but probably historically present through much of the country in 

suitable habitat.

Guinea-Bissau–Localities include the coastal city of Bissau and the offshore Bijagos 

Islands (Broadley, 1984), but suitable habitat appears to exist through much of the 

country.

Ivory Coast–Four localities mapped by Broadley (1984). Probably historically present 

through much of the country in suitable habitat

Kenya–Present in the Lake Victoria basin and along the eastern coast, but largely 

replaced by P. natalensis in the southern highlands (Broadley 1984, 1999; Spawls and 

others, 2002; Malonza and others, 2006).

Liberia–Multiple localities recorded (Broadley, 1984), probably historically present 

through much of the country in suitable habitat.

Mali–The only locality mapped by Broadley (1984) was from the vicinity of Bamako. 

Joger and Lambert (1996) added four more localities, all in the southwestern third of the 

country. Chippaux (1999) also provides four localities, including two in the southeast. 

Additional habitat may exist to the east along the Niger River corridor and its many

drainages.

Mauritania–Recently confirmed to be present at multiple sites in the southern half of the 

country; appears to be confined to isolated wetlands in the Sahel (Padial, 2003, 2006).

Nigeria–Although represented by only a few localities in Broadley (1984) and Chippaux 

(1999), likely present through much of the country, including areas such as those in the 

state of Akwa-Ibom that experience extremely high annual rainfall during a wellspecified wet season (May to October, 3146 mm/year: Luiselli and others, 1998). 

Considered vulnerable in all natural habitats in southern Nigeria, but persists better in 

suburbia (Akani and others, 2002).
114

Republic of the Congo (aka Congo-Brazzaville)–Recorded as “present or likely to 

occur” through all of five geographic sectors of the country (Broadley, 1998). 

Considered a common species along the Sangha River (Jackson and Blackburn, 2007) 

and reported by Chippaux (1999) from a few localities along the Congo River.

Rwanda–Poorly known, but present at elevations greater than 1300 m in the vicinity of 

Lake Ihema and Akagera National Park (Broadley, 1984).

Senegal–Represented by multiple specimens, largely around the edges of the country, 

including close to the northern border with Mauritania (Broadley, 1984; Chippaux, 1999) 

and in Niokola-Koba National Park in the southeast region of the country (Joger and 

Lambert, 2002). 

Sierra Leone–Few records, but probably historically present through much of the 

country in suitable habitat.

Somalia–Present through a surprisingly large portion of the country, primarily associated 

with rivercourses and wadis that retain perennial pools; may be present as far north as the 

Wadi Nogal in the Horn of Africa (8.4 ̊ N latitude, 48 ̊ E longitude: Lanza and Nistri, 

2005).

Sudan–Broadley (1984) listed four localities in Sudan, but mapped only two in the 

extreme southern part of the country. However, he listed Khartoum (over 1000 km to the 

north of these records) and White Nile (around 500 km north) as localities; both of these 

are along the Nile River corridor. Pythons are also known from the Mareb River drainage 

in Eritrea near the Sudanese border, as well as the headwaters of the Blue Nile; these 

populations may extend into Sudan. Existing populations appear to be limited to the 

vicinity of watercourses.

Tanzania–Both giant African Pythons are present in Tanzania. Northern African 

Pythons are known from the northwest of the country in the vicinity of Lake Victoria, as 

well as along the northern coastline and in a couple of locations between these regions. 

They are apparently sympatric with Southern African Pythons in part of this distribution, 

but are replaced by the latter in the southern half of the country.

Togo–Few records, but probably historically present through much of the country in 

suitable habitat.

Uganda–Occurs throughout Uganda, except for the northeast (Spawls and others, 2002), 

at elevations up to 2250 m (Spawls and Branch, 1995). Citing Charles R.S. Pitman, Pope 

(1961) stated that the species, “...can be found as high as 7,500 feet in Uganda. 

Extensive studies, chiefly by French workers, in the high country of the great lakes, 

extending from the borders of Uganda southward, indicate ascent to at least 5,900; as the 

4,000-foot level is approached, the records grow more numerous and no doubt the 
115

pythons themselves do. We may conclude that this reptile is certainly found at points 

lying between 6,000 and 7,000 feet, and possibly a little higher.”

Zambia–Probably not present, replaced by P. natalensis. However, Broadley (1984) 

lists a locality just across the border in the Democratic Republic of the Congo.

3.1.2 Native Range of Python natalensis

Angola–listed from several localities by Broadley (1984); most of these are in the 

southern third of the country except for a northern record from the vicinity of Luanda.

Botswana–Recorded from scattered locations in the country, with a concentration of 

records in the southeast corner (Auerbach, 1987). Also a reliable sight record from Twee 

Rivieren, South Africa, in an extremely arid region just across the Botswanan border 

(Haacke, 1984, cited in Auerbach, 1987). Most records from Auerbach (1987) are from 

elevations above 900 m.

Burundi–The only record in Broadley (1984) was from the vicinity of Bujumbura, which 

is on the shores of Lake Tanganyika at elevations below 800 m. Present across most of 

western Burundi according to Spawls and others (2002). Presence in the rest of the 

country (most of which is higher than 1500 m elevation) appears to be poorly known, 

although the species achieves similarly high elevations to the east in Tanzania and Kenya.

Democratic Republic of the Congo (formerly Zaire and Belgian Congo, aka CongoKinshasa)–Present through much of the southern third of the country, including areas of 

possible sympatry with Northern African Pythons (Broadley, 1984). Possibly present 

further north based on records of P. natalensis in Burundi close to the DRC border. 

Replaced in the northern portions of the country by P. sebae.

Kenya–Found in highlands of south-central Kenya, including elevations over 1800 m in 

the Lake Naivasha area (Spawls and others, 2002: this book includes much more detail 

on the species’ distribution in Kenya). Largely replaced by P. sebae in lower elevations 

in Kenya, including the Lake Victoria basin. 

Lesotho–Probably not present, although specimens are known just to the east in South 

Africa (Marais, 2004; G. Alexander pers. commun., 2008). 

Malawi–Known from Rumphi (or Rumpi) at fairly high elevations (higher than 1500 m) 

in the Northern Province (Nyasaland: Stewart, 1968), but likely present in historical times 

through much of the country in suitable habitats.

Mozambique–Known from scattered locations, including the northern coast, the 

southern tip, and along the Save (Sabi) River corridor (Broadley, 1984). However, some 

field guides consider it potentially present through all of southern Mozambique (for 

example, Alexander and Marais, 2007).
116

Namibia–Known to occur in the northern half of the country (Broadley, 1984), 

apparently associated with rivercourses and seasonal/permanent wetlands. Absent from 

extremely arid regions in southern Namibia (Auerbach, 1987).

South Africa–Now largely restricted to the northeast portion of the country, including 

coastal regions starting north of East London and northward, expanding inland to include 

Swaziland, the Northern Province and parts of Gauteng and Northwestern Provinces. 

Small disjunct populations were found east of Port Elizabeth and south of Kalahari 

Gemsbok National Park (Marais, 2004), but these may now be extirpated (G. Alexander 

pers. commun., 2008). A reliable sight record was obtained near Twee Rivieren on the 

Botswanan border (Auerbach, 1987). The species was historically found more widely in 

the Cape region, from which it is now largely absent (Auerbach, 1987). The species may 

have recently extended its range southwards in the area of Gauteng and the Northern 

Cape; Alexander and Marais (2007) speculate that this could be due to global warming. 

The southern range limits in South Africa may be determined by temperatures available 

to females when incubating eggs (Alexander, 2007).

Swaziland–Known from several specimens, mostly in the NE (Broadley, 1984). Occurs 

over the eastern half of Swaziland (Monadjem and others, 2003) in the middleveld, 

lowveld and Lubombos, reaching its western limits in the larger river valleys (Boycott, 

1992).

Tanzania–Present across much of the country, but not in the Lake Victoria basin or 

along most of the coast (Spawls and others, 2002), where it is replaced by P. sebae (see 

above). Fairly common in Arusha National Park, especially in wet areas (Razzetti and

Msuya, 2002). However, pythons were not observed in herpetofaunal surveys of the 

Southern Udzungwa Scarp Forest Reserve in southeast Tanzania at elevations between 

900 and 1900 m (Menegon and Salvidio, 2005).

Zambia–Multiple specimens are known, and Broadley and others (2003) state that it is, 

“Found throughout the country in a variety of habitats, but usually close to water.”

Zimbabwe–Known from many specimens from throughout the country (Broadley, 

1984). 

3.2 Habitat Range

Various authors have opined on the habitat preferences of the giant African 

pythons, but these conclusions are often at odds with one another. For example, Spawls 

and Branch (1995) reported that the giant African pythons (which were not distinguished 

by species in this book) favor evergreen forest or moist savannah, often in rocky areas 

close to water, and Broadley (1990) stated, “Pythons favour moist, rocky, well-wooded 

valley, plantations or bush country, but seldom if ever far from permanent water, for 

which they have a great love...” The Northern African Python was also described as, 

“...widespread in a great variety of sub-Saharan habitats but seldom if ever occurring far 

from permanent water” (Lanza and Nistri, 2005). In arid regions such as the Kalahari, 
117

pythons may have benefitted from the installation of watering holes for livestock and 

game (T. Hibbitts, pers. commun., 2008).

In Nigeria, Northern African Pythons are reported from suburban, forest, 

pond/stream, and swamp habitats (Butler and Reid, 1986), including extensive use of 

Nigerian mangrove habitats (Luiselli and Akani, 2002). In West Africa (The Gambia), 

pythons used savannah, marsh, and gallery forest habitats, usually close to water 

(Hakansson, 1981); pythons were also considered linked with aquatic habitats in the 

southwest portion of the Democratic Republic of the Congo (Trape, 1985). 

In the arid northern parts of its range, Northern African Pythons appear to be 

limited to wetlands, including the headwaters of the Nile, isolated wetlands in the Sahel 

of Mauritania and Senegal, and various watercourses in Somalia, Ethiopia, and Eritrea 

(Broadley, 1984; Padial, 2003; Lanza and Nistri, 2005; M. Largen, pers. commun., 2009). 

3.3 Climate Range

As would be expected by their relative latitudinal positions, the Southern African 

Python tends to inhabit slightly cooler regions than does its sister taxon. However, 

Northern African Pythons in some localities experience climates nearly as cool, 

indicating that they cannot be considered merely a tropical species. Drastic seasonal 

swings in precipitation are a striking feature of many localities for both species, although 

such seasonality does not necessarily appear to be a requirement, at least for Northern 

African Pythons which also inhabit less seasonal locales.

3.3.1 Climate Range of Northern African Pythons

The Northern African Python inhabits regions with some of the highest mean 

monthly temperatures that we identified for any of the giant constrictors’ habitats, with 

means over 35 C in arid northern localities (Figure 6.4). However, many areas inhabited 

by this species are much cooler, especially in the higher elevations of Kenya, Tanzania, 

and Ethiopia. Seasonality in precipitation is a hallmark of many localities inhabited by 

Northern African Pythons; these localities often experience multiple months with little or 

no rainfall. This is true even in parts of western Africa, from which many of the animals 

in the international live animal trade originate. It is uncertain whether this seasonality is 

a requirement of the species, but this seems unlikely given the relatively aseasonal 

regimes experienced in other localities (including other parts of western Africa). Pythons 

also persist in extremely arid regions in the Sahel, but appear to be limited to the vicinity 

of watercourses in most arid climates.
118

0

5

10

15

20

25

30

35

40

0.01 0.10 1.00 10.00 100.00 1000.0010000.00

Mean monthly temperature (C)

Mean monthly precipitation (mm)

Figure 6.4. Climate space for P. sebae, with reference lines and localities as introduced in Fig. 

2.1. Based on 109 known localities from the native range. Climate loops color-coded as follows: 

Red = Southern Africa (Angola, Zambia, Malawi, Mozambique and points south); Turquoise = 

Eastern Africa (Tanzania, Uganda, Kenya, southern Rift Valley countries); Yellow = Western Africa 

(Nigeria and countries to the west that are south of Mauritania, Mali, and Niger); Blue = Northern 

Africa (Mauritania, Mali, Niger, Chad, Sudan, Ethiopia, Eritrea, Somalia); Green = Central Africa 

(Cameroon, Central African Republic, Gabon, Rep. Congo, Dem. Rep. Congo). 

3.3.2 Climate Range of Southern African Pythons

As would be expected for a species that attains significantly higher latitudes, 

Southern African Pythons generally inhabit cooler areas than do Northern African 

Pythons. In some localities, the mean monthly temperatures of the coolest months are 

among the lowest we observed for any of the giant constrictors’ habitats. Other than 

persisting in cooler climates, the climate envelope of Southern African Pythons is similar 

to that of the northern species, including many localities with highly variable rainfall. 

0.01 0.1 1 10 100 1000 10000
119

Figure 6.5. Climate space for P. natalensis, with reference lines and localities as introduced in 

Fig. 2.1. Based on 87 known localities from the native range. Climate loops color-coded as for P. 

sebae.

3.4 Microhabitat

See the biological profile for Boa constrictor for caveats about observed 

microhabitat use based on anecdotal observations. Radiotelemetric locations of freeranging pythons would provide less biased information on microhabitat use, but we are 

aware of only two published radiotelemetric studies of the giant African pythons 

(Lawson, 2004; Alexander, 2007). The former had a sample size of one python, and the 

latter focused on thermoregulation rather than habitat use. Accordingly, few data are 

available to accurately assess fine-scale habitat use in the giant African pythons.

Southern African Pythons are reported to prefer rocky outcrops in both arid and 

moist savanna, as well as using lowland forest (Marais, 2004); we note that “preference” 

for rocky outcrops may simply reflect higher visibility to human observers in these 

habitats, and Broadley (1999) stated that these pythons exhibit no preference for rocky 

habitats. In Arusha National Park, Tanzania, the species is most often observed in or near 

aquatic habitats (Razzetti and Msuya, 2002). Burrows of porcupine, aardvark, and 

warthog are important refugia in northern South Africa (Alexander, 2007). African 

pythons readily climb trees (Broadley, 1990), and may use trees more often than is 

0

5

10

15

20

25

30

35

40

0.01 0.1 1 10 100 1000 10000

Mean monthly precipitation (mm)

Mean monthly temperature (C)
120

commonly assumed (Lawson, 2004); juveniles may be more likely to use arboreal 

habitats (Luiselli and others, 2007).

In contrast to a number of general statements in the literature to the effect that the 

giant African pythons are usually associated with water, pythons in South Africa only 

immersed themselves in water on 3 percent of the days on which they were located via 

radiotelemetry (Alexander, 2007). Conclusions that pythons are reliant on aquatic 

habitats may therefore be biased by anecdotal data and detectability; for example, large 

pythons may be more prone to ambushing large-bodied mammalian prey from the water, 

and these large animals may be more obvious to human observers. It is also possible that 

the Northern African Python may be associated with water to a greater degree than is the 

southern species; 81 percent of locations of a radiotelemetered female P. sebae in 

Cameroon were within 10 m of a stream or river (Lawson, 2004), although we note that 

this area included many agricultural areas and thus the watercourse may have provided 

some of the only remaining forested landscape.

Observations of pythons in multiple habitats of southern Nigeria led Luiselli and 

others (2007) to conclude that they are most abundant in mangrove habitats, but that the 

species overall is a habitat generalist that is especially associated with riverine or other 

aquatic habitats. The apparent abundance of pythons in mangrove habitats in Nigeria 

may well represent higher abundances, but this perceived abundance could also be due to 

higher detectability of pythons in this habitat. A single snake in the adjacent country of 

Cameroon was located via radiotelemetry on 201 occasions over the course of a year, but 

was visually observed only once during this period (Lawson, 2004); on all other 

occasions, the python was concealed underground, in a log/debris, or in a tree. Such a 

difference between perceived and actual habitat use is a stark example of the dangers of 

assuming habitat use based solely on opportunistic observations.

3.5 Introduced Ranges

Several Northern African Pythons have been found in Florida and elsewhere in 

the United States, including observations going back over 100 years (for example, 

Moseley, 1901)—most of these have been assumed to be escaped or released pets. 

African pythons occasionally appear in other extralimital areas, almost certainly as a 

result of the international live animal trade; as an example, two Northern African Pythons 

were found in São Paulo, Brazil during the period 1995-2000 (Eterovic and Duarte, 

2002). An attempt at re-establishing an extirpated population of Southern African 

Pythons in South Africa involved the release of 34 individuals, and discovery of a 

hatchling several years later was an indication of successful reproduction (Branch, 1988); 

we presume that releasing similar numbers of individuals in suitable extralimital habitats 

might result in extralimital populations.

Recently (August 2009), evidence has surfaced pointing to the presence of a 

population of Northern African Pythons along the western border of Miami adjacent to 

the Everglades. Four specimens, including hatchlings and adult females, have been 

confirmed from an area of about two kilometers in diameter, and three of these were 

found in 2009. Another large individual escaped from a credible observer in 2008, while 

a sixth record of a very large female had not been conclusively confirmed at the time of 

this document’s production. A spatially concentrated cluster of sightings of pythons of 

various size classes is fairly strong evidence of a reproductive population, and planning is 
121

underway to delineate and attempt control of this incipient population. This area is 

within the known distribution of Burmese Pythons in Florida, and hybridization between 

these species is known in captivity. The likelihood of hybridization among introduced 

Florida populations is unknown, as are the implications of genetic admixture for risk 

assessment and control purposes.

3.6 Seasonal Activity

Southern African Pythons north of Pretoria, South Africa, exhibited annual 

activity ranges that occasionally exceeded 500 ha. Even in winter, pythons rarely stayed 

below ground for more than a week, emerging to bask at regular intervals (Alexander, 

2007). This study indicated that pythons had activity peaks in March (autumn) and 

August-September (late winter); the former may be due to hyperphagia before the winter 

fast, while the latter may be associated with reproductive behaviors such as matesearching. In Nigeria, pythons exhibited weak activity peaks in January during the dry 

season and in August-September during the latter part of the wet season (Luiselli and 

others, 2007). These authors posited that the January activity peak was related to 

reproductive activity, as previously noted by Starin and Burghardt (1992). In Cameroon, 

a single radiotelemetered female confined her activities to about 7 ha during the rainy 

season but greatly expanded her activity range into forested habitats during the dry 

season (approx. 32 ha: Lawson, 2004). At a higher elevation, Southern African Pythons 

were rarely observed during the cool rainy season in Arusha National Park, Tanzania 

(Razzetti and Msuya, 2002). Pythons in equatorial zones are likely active to some degree 

throughout the year, while those at high elevations and in the south temperate zone 

appear to be relatively inactive during the winter.

3.7 Daily Activity

The giant African pythons may exhibit activity at any hour of the day. However, 

virtually all conclusions on diel activity cycles are based on opportunistic observations 

rather than on unbiased sampling methods. An exception to these biased conclusions was 

provided by Alexander (2007), who showed that most radiotelemetered pythons in South 

Africa sheltered in burrows at night and emerged to bask in the morning – basking 

frequency, duration, and target temperature varied significantly depending on size, sex, 

and reproductive status. In the Abuku Nature Reserve, The Gambia, pythons were 

commonly encountered during daylight hours (Hakansson, 1981), while juveniles (1.5-

2.5 m) in Kenya became active near dusk (Spawls and others, 2002). Adults may be 

more nocturnal than juveniles (Spawls and Branch, 1995), and radiotelemetry of an adult 

female led Lawson (2004) to conclude that diurnal forays from secure retreats were 

uncommon.

Sightings of pythons in natural habitats (n = 96) of southern Nigeria were 

concentrated in the middle of the day (1000-1900), while suburban pythons (n = 61) were 

primarily sighted after dusk, especially after 2200 (Luiselli and others, 2001). However, 

these results are likely to have been somewhat biased by unequal sampling effort across 

years, seasons, and times of day. Of six predatory events recorded by Luiselli and others 

(2001), five occurred during the hours of 2115 - 2350, with the sixth at 1845; these 

results reflect the likely importance of nocturnal foraging, although it was unknown 

whether these occurrences reflect ambush predation or some level of active searching.
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3.8 Foraging Mode/Sensory Modalities

The giant African pythons are primarily ambush foragers, lying in wait for prey in 

burrows, along animal trails, and in water (Branch, 1988; Spawls and others, 2002, 

Alexander and Marais, 2007). Pythons may also engage in active searching for prey to 

some extent (Alexander and Marais, 2007); this strategy appears to be more common 

among juveniles (Spawls and others, 2002). Adults are particularly prone to ambushing 

prey from the water. The African pythons have multiple thermoreceptive pits in the 

rostral and labial scales (see 2.1 Unique Features, above), allowing them to detect and 

strike endothermic prey even in darkness. 

4.0 Life History

4.1 Reproductive Mode

Both the Southern and Northern African Pythons are oviparous.

4.2 Reproductive Phenology

Most available reproductive information is for the Southern African Python. In 

this species, males may trail females during the breeding season (Branch, 1988), and up 

to 13 males have been observed courting a single female (Alexander and Marais, 2007). 

Similar aggregations and trailing behavior were reported for Northern African Pythons in 

The Gambia (Starin and Burghardt, 1992), although these were smaller groups (six or 

fewer individuals). Captive males may engage in male-male combat (Barker and Barker, 

1994), but this behavior has not been reported in the field.

In southern Africa, mating occurs during the winter [between June and September 

(Alexander, 2007; Alexander and Marais, 2007), July and August in Botswana 

(Auerbach, 1987)]. In contrast, Northern African Pythons are reported to mate and 

oviposit year-round around Lake Victoria (Spawls and others, 2002). Large diurnal or 

seasonal temperature variation may not be necessary to evoke reproductive behavior in 

captive Northern African Pythons (Ross and Marzec, 1990). Hybridization between P. 

natalensis and P. molurus has been observed in captivity (Branch and Erasmus, 1984).

Eggs may be laid in hollow trees, termite nests, or mammal holes, and the female 

coils around eggs during incubation (Branch, 1988; Stanford, 1994; Spawls and Branch, 

1995). A female in northern South Africa oviposited in a burrow, approximately 7 m 

from the entrance and 1 m below ground (Alexander, 2007). Elsewhere in South Africa, 

however, a female was found coiled around eggs in open grassland (Porter, 1987). In 

South Africa, eggs are laid between September and December (Alexander, 2007; 

Alexander and Marais, 2007), although Pope (1961) reported a female captured in Natal 

that contained shelled eggs in June. Farther north in Tanzania, a female was observed 

coiled around eggs in early September (Stanford, 1994); based on the maps of Broadley 

(1984), this was most likely the Southern African Python. 

Ditmars (1931) and Auerbach (1987) state that females are capable of shivering

thermiogenesis while brooding; the latter stated that this behavior can result in 

temperature increases of up to 6.5°C. However, most recent authors discount shivering

thermiogenesis in the giant African pythons (Pope, 1961; Vinegar and others, 1970; 

Spawls and Branch, 1995; Alexander and Marais, 2007). Instead, females may leave 
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their eggs to bask (sometimes attaining body temperatures over 40°C), returning to 

transfer body heat to the eggs (Marais, 2004; Alexander, 2007; Alexander and Marais, 

2007). Alexander (2007) provided a summary of the arguments for and against 

thermiogenesis, including the intriguing possibility that Southern African Pythons are 

incapable of thermiogenesis but that Northern African Pythons may be capable. Body 

temperature data of radiotelemetered brooding females in South Africa provided no 

evidence of thermiogenesis (Alexander, 2007). The apparent lack of thermiogenesis in 

these species may mean that these species are less likely to succeed as invasive species in 

cool regions, as compared to thermiogenic pythons.

Incubation is typically 65 days (Spawls and Branch, 1995), similar to many other 

large pythons; although Branch (1988) stated that incubation of Southern African 

Pythons can vary between 60 (at 30-32°C) and 100 (at 25-27°C) days. In captivity, 

incubation is reported at 72-106 days, but typically 72-76 days, with longer incubation 

periods being associated with lower temperatures and resulting in the reduced fitness of 

hatchlings (Ross and Marzec, 1990). Broadley and others (2003) stated that eggs usually 

hatch in December in Zambia. A female in Zimbabwe was found with eggs and 

hatchlings on 24 December (Broadley, 1959, cited in Fitch, 1970), while a female in 

Angola was found with freshly-laid eggs at the end of January, and a female in Natal 

contained shelled eggs on June 2; this last observation appears to be an early outlier. A 

female Southern African Python (see above) in Tanzania incubated eggs for at least 78 

days before they hatched around 20 November (Stanford, 1994). Alexander and Marais 

(2007) state that the hatching success of Southern African Python eggs is greatly 

decreased if incubation temperatures are maintained below 28°C, and only 29 percent of 

eggs hatched that were brooded by a female that was prohibited from basking normally 

for 10 days during incubation, as opposed to 85 percent hatching success for a female 

brooding normally (Alexander, 2007).

Juvenile Southern African Pythons in Tanzania stayed at their nest site for about 

10 days, until shedding, after which they dispersed (Stanford, 1994); in this case the dam 

did not stay after her offspring hatched. Farther south, however, hatchlings may closely 

associate with their dam during the night in order to maintain higher body temperatures 

(Alexander and Marais, 2007). 

4.3 Reproductive Effort/Fecundity

Reproductive effort of females is considerable, as clutch sizes can be large and 

females fast during incubation. Females may lose more than half of their body mass 

during this period, and spent females may die after incubation (Alexander, 2007; 

Alexander and Marais, 2007).

4.3.1 Clutch or Litter Size/Frequency

Branch (1988) stated that an “average” female of 3-4 m total length would be 

expected to lay 30-40 eggs, while Pope (1961) reported an average clutch of 46 eggs, 

individual clutches from 20 to “about 100,” and clutch size increasing with body length 

of the dam (Fig. 6.6). Other authors have reported clutch size as 30-50, occasionally up 

to 100 (Marais, 2004; Spawls and Branch, 1995; Alexander and Marais, 2007). Among 

specific observations of free-ranging females, a Southern African Python from Zimbabwe 
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was found with 39 eggs and hatchlings, and a female in Tanzania of the same species 

(approximately 2.75 m total length) incubated roughly 20 eggs. 

Eggs of Southern African Pythons weigh 130-160 g and are about 10 cm in 

diameter (Marais, 2004). Females likely reproduce only every two to three years 

(Branch, 1988; Alexander and Marais, 2007). 

Figure 6.6. Relationship between dam body size and clutch size in 8 captive Northern African 

Pythons in Cameroon; adapted from data in Stucki-Stirn (1979).

4.4 Growth

We are unaware of any mark-recapture study of the giant African pythons that 

would allow estimation of growth rates among free-ranging pythons, and thus we are 

unfortunately reliant on growth records from individuals in captivity (for example, Fig. 

6.7). Ross and Marzec (1990) stated that Northern African Pythons grow rapidly, but 

provided no accompanying data. A captive individual that grew from 0.53 m to 5.18 m 

in 5.5 years (approx. 70.5 mm/month: Whitworth, 1974) represents the fastest long-term 

growth rate of which we are aware. Lanza and Nistri (2005) provided a literature 

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summary of growth records from captive individuals, including some captives in the 

native range held under semi-natural environmental conditions. 

Growth rates can be rapid among juveniles: Growth of three juveniles in one year 

averaged about 1.20 m (100 mm/month) and 3000 g (250 g/month: starting total lengths 

0.60, 0.60, 0.69 m; starting mass 100, 100, 160; ending lengths 1.75, 1.80, 2.0 m; ending 

mass 3968, 3000, 3200 g), while another juvenile grew from 0.53 to 0.86 m in 14 weeks 

(94 mm/month). Slower growth was observed in 40 juveniles held in Cameroon, with 

length at one year ranging from 0.92 to 1.09 m, and Spawls and others (2002) observed 

growth from 0.50-0.55 m to 0.9-1.37 m in one year (n = 9). Juvenile growth rates are 

likely to be highly dependent on feeding frequency and food quality, as well as suitable 

conditions in captivity. Growth in body length decelerates as pythons get older, although 

body mass tends to continue to increase at a high rate. Lanza and Nistri (2005) provided 

the following observations on captives, culled from the literature: a 3.5 m female grew 1 

m in 3 years, and captives in Cameroon had lengths of 1.80 m (2 years old), 2.50 m (3 

years), 3.20 m (4 years), and a 6 year old specimen of 3.62 m.

In a field study of Southern African Pythons in South Africa, Alexander (2007) 

observed no pythons between 0.76 and 1.53 m (0.67 m and 1.36 m SVL; Fig. 6.9). This 

could indicate that hatchlings grow very quickly, but alternatively this observation could 

be a result of size-based heterogeneity in detectability.

Figure 6.7. Growth of a captive juvenile P. sebae measured at monthly intervals, adapted from 

data tabulated by Raynis (1965). Measurements started when the individual was approximately 

two months old, and the animal was kept at constant temperatures of around 28 ̊C.

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Figure 6.8. Relationship between snout-vent length and mass for a sample of P. natalensis from 

South Africa near the southern limits to the species’ range. Graph courtesy G. Alexander, 

reproduced with permission.

4.5 Maturation

Females may mature at 2.5 m (this presumably refers to total length), which can 

occur in 3-4 years in captivity (Branch, 1988). Males likely mature at about 2 m total 

length. Age at maturity has been estimated at 10-15 years in free-ranging snakes 

(Broadley, 1990; G. Alexander, pers. commun., 2008), but captives in Cameroon matured 

at 5-6 years and 3-4 m (Stucki-Stirn, 1979). Luiselli and others (2007) estimated 

maturation of both sexes to occur at about 1.92 m (1.7 m SVL). Herpetoculturalists are 

regularly able to get male Northern African Pythons to breed at 2 years of age and 

females at 3 years, but this requires feeding large amounts of food to juveniles; similar 

growth rates are likely to be uncommon among free-ranging individuals.

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4.6 Longevity

We are aware of no records for longevity among free-ranging individuals. In 

captivity, Northern African Pythons have lived for 27 yrs 4 mo (Snider and Bowler, 

1992).

5.0 Diet/Trophic Role

Along with rare Nile crocodiles (Crocodylus niloticus), Northern African Pythons 

are considered a top predator in Nigerian mangrove habitats, especially since leopards 

(Panthera pardus) have been eliminated from this region (Luiselli and Akani, 2002).

5.1 Prey

As with most of the giant constrictors, adult giant African pythons primarily eat 

endothermic prey from a wide variety of taxa; Luiselli and others (2001) concluded that, 

“Pythons are opportunistic predators that take whatever suitably sized endothermic prey 

are available.” To this general statement on the palatability of endotherms can be added 

the observation that these pythons may also consume lizards (including monitor lizards, 

Varanus spp.), fish, crocodiles, and frogs (Spawls and Branch, 1995; Branch, 1998; 

Spawls and others, 2002; Marais, 2004). Endothermic prey has included various antelope 

(for example, kob, gazelle, and duiker), Warthog (Phacochoerus africanus), African 

Porcupine (Hystrix cristata), Caracal (Caracal caracal), a variety of rodents, various 

birds including pelicans and chickens, and even a Leopard (Panthera pardus: Wright, 

1960; Branch, 1988 and references therein). Most authors state that juveniles consume 

small rodents and ground-dwelling birds, while adults largely subsist on medium-sized 

mammals.

Among notable diet records, a 47-kg Northern African Python consumed a 26-kg 

Uganda Kob (Adenota kob), and a 4.6-m individual ate a Thomson’s Gazelle (Gazella 

thomsoni), despite a horn briefly erupting from the snake’s body (Hay and Martin, 1966; 

Greene, 1997). A 4.8-m python ate an Impala (Aepyceros melampus) weighing 59 kg 

(Spawls and others, 2002), while a 3.79-m Southern African Python weighing 18 kg 

contained a 13.5-kg Bushbuck in Zimbabwe (Tragelaphus scriptus: D. Broadley, pers. 

commun., 2008). Another Southern African Python in Zimbabwe consumed a White 

Stork (Ciconia ciconia: D. Broadley, pers. commun., 2008). In The Gambia, known prey 

of Northern African Python included Bushbuck and possibly other harnessed antelope 

(Tragelaphus spp.), Red Colubus Monkeys (Procolobus badius), and several captive 

antelopes and monkeys (Starin and Burghardt, 1992). In southern Nigeria, pythons in 

suburban and natural areas consumed dogs, goats, rats, and poultry; rats were not found 

in snakes over 1.55 m, while the other prey classes were consumed by larger snakes 

(2.35-3.51 m: Luiselli and others, 2001). Prey items from three pythons captured in a 

matrix of rainforest/cultivated lands in Nigeria included a Gambian Giant Pouched Rat 

(Cricetomys gambianus), 3 macrochiropteran bats, and 2 guenon monkeys 

(Cercopithecus spp.: Luiselli and others, 1998), while pythons from the Congo consumed 

a rat and a female antelope measuring 1040 mm long and 480 mm high at the shoulder 

(Schmidt 1923). Pythons are considered an effective means of controlling small 

mammals such as Dassies (Procavia capensis) and Cane Rats (Thryonomys 

swinderaianus: Marais, 2004). Domestic animals consumed by giant African pythons 
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include goats, dogs, attempted predation on a calf, and a domestic turkey consumed by an 

extralimital individual in suburban south Florida (Branch, 1988; Murphy and Henderson, 

1997; S. Snow, pers. commun. 2008).

5.2 Predators

Marais (2004) lists mongooses, meerkats, crocodiles, wild dogs, hyenas, Ratels 

(honey badgers) and other snakes as potential enemies, and Pitman (1938: cited in Pope, 

1961) reported that a warthog killed a Northern African Python 12 feet (3.66 m) in 

length. Crocodiles (Crocodylus niloticus) killed and consumed at least six pythons over 

several years at a reserve in The Gambia (Starin and Burghardt, 1992). Senter (2000) 

speculated that the slightly variant color pattern on the tails of hatchling pythons may 

function as a diversionary tail display to keep a predator’s attention away from the 

snake’s head. Branch (1988) stated that, “Man is now the python’s main predator,” and 

Alexander (2007) observed only one of 24 radiotelemetered Southern African Pythons 

lost to predators during over 2500 python-days of observation.

5.3 Diseases and Parasites

Broadley (1990) lists a number of ailments in African pythons, at least some of 

which are likely more prevalent in captivity than in the wild (for example, “mouthrot”, 

fungal growth, etc.). Peirce (1984) observed haemogregarine and helminth parasites in a 

python (named as P. sebae, but probably referable to P. natalensis) in Zambia. The 

former was Haemogregarina sebai, which is apparently known only from African 

pythons. Peirce noted, "...both the nematode and cestode parasites in P. sebae occurred 

in large numbers and appeared to be causing some impaction of the intestine." A 

pentastomid has been identified from Northern African Pythons; this has been referred to 

as Armillifer moniliformis but may represent an undescribed species (Riley and Self,

1981). Armillifer moniliformis has also been isolated from P. molurus, B. reticulatus, and 

Morelia amethistina; effects of this parasite on python health are largely unknown. 

Infestations of Armillifer pentastomids are prevalent in pythons from South Africa and 

can interfere with respiration (Broadley, 1990). As boid snakes, the giant African 

pythons are probably susceptible to retrovirus-associated Inclusion Body Disease 

(Schumacher and others, 1994), but we are unaware of any known cases in these two 

species. In captivity, they are subject to a range of diseases and conditions, including 

paramyxovirus, cryptospiridiosis, etc., (Mader, 1996), but the extent to which such 

problems plague wild pythons is unknown. 

Ticks capable of transmitting diseases of economic and veterinary importance are 

often associated with reptiles imported to the United States in association with the live 

animal trade (Burridge, 2001; Burridge and others, 2000, 2006). Northern and/or 

Southern African Pythons are known hosts of some of these ticks, including Amblyomma 

nuttalli, Amblyomma marmoreum, Amblyomma sparsum, Aponomma exornatum, 

Aponomma flavomaculatum, and Aponomma latum (Burridge, 2001). Once entering the 

country, ticks have proven capable of spreading to new host species (for example, 

African ticks found on Boa Constrictors from the Neotropics, Asian ticks on an African 

Ball Python, an African tick found on a domestic dog, etc.), establishing populations in 

Florida (Allan and others, 1998), and switching to feeding on native reptile species 

(Oliver and others, 1988). 
129

At least eight exotic reptile-borne tick species are deemed capable of becoming 

established in Florida alone, based on climatic suitability and the presence of suitable 

native reptile hosts (Burridge, 2001), as has already happened with at least two species in 

the state. Four of the abovementioned eight species are known or suspected to be 

associated with mammalian diseases, including heartwater and Q fever, and some are 

vectors of reptile haemogregarine parasites and perhaps other pathogens. Heartwater is a 

devastating disease of wild and domestic ungulates, and estimates of ungulate mortality 

in the United States are 40-100 percent of all ungulates should the disease become 

established (Burridge, 2001). Unfortunately, tropical tick systematics, surveys of 

potential hosts, and the prevalence of tick-borne disease have received only modest 

attention in recent years; most of our knowledge of a species group of great 

medical/veterinary interest, for example, is based on a survey that is now 50 years old 

(Theiler and Salisbury, 1959). 

6.0 Demography

6.1 Population Density

To our knowledge, there are no credible published estimates of population density 

of the giant African pythons. The only estimate of which we are aware resulted from 

long-term observational studies at a small study site in The Gambia, which produced 

density estimates of 0.6-0.75 pythons/ha (Starin and Burghardt, 1992). However, these 

numbers were based on tenuous visual recognition of individuals rather than markrecapture or similar quantitative means of estimating population density, and the probable 

absence of top predators in this small and highly isolated nature reserve may have 

elevated prey abundances to an abnormal degree. Nonetheless, this is a surprisingly high 

density of large snakes.

There is little doubt that the giant African pythons can be obvious components of 

the vertebrate fauna of Africa, at least in some areas. In a study of snake community 

ecology in mangrove habitats of Nigeria, the Northern African Python ranked third in 

terms of number of individuals captured over four years (21 pythons/183 snakes total: 

Luiselli and Akani, 2002), and it would likely rank even higher in terms of relative 

biomass. The same species was considered common along the Sangha River in the 

Republic of the Congo by Jackson and Blackburn (2007: note, however, that these 

authors speculated that pythons may be present but difficult to detect in undisturbed 

forest), and as “very common” in forests along the lower Shebelli (Shabeele) River of 

Somalia, despite the overall aridity of this latter region near the northeastern limits of the 

species (Lanza and Nistri, 2005). However, the species was represented by only 3 

individuals from a sample of 237 snakes captured in an area of southeast Nigeria 

characterized by rainforest patches and cultivated areas (Luiselli and others, 1998).

6.2 Size Distribution

See the equivalent section in the profile for Python molurus for problems 

associated with quantifying size distribution without accounting for detectability of 

various size classes. We are aware of no defensible size distribution for any population 

of either of the giant African pythons. Reports of average body sizes include 3-4 m 
130

(Marais, 2004) and 2.8-4 m (max 5.5 m, hatchlings 0.45-0.60 m) in Southern African 

Pythons (Spawls and others, 2002), with similar estimates for the northern species. A 

large (n = 197) sample of Northern African Pythons from Nigeria averaged 3.4 ± 1.2 m 

total length and included snakes from 0.8 m to 5.0 m (Luiselli and others, 2007), while a

sample of approximately 75 Southern African Pythons from north of Pretoria, South 

Africa, included lengths exceeding 4.5 m (4 m SVL) and 55 kg, but the majority were in 

the 2-3.4 m range (1.8-3 m SVL: Alexander, 2007). 

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

African pythons are collected for the skin trade. However, these pythons appear 

to represent a small portion of the overall international trade in python skins. African 

pythons do not represent a significant portion of the reptile skin trade involving tanneries 

in Mexico (Arroyo-Quiroz and others, 2007), and only 25 P. sebae skins entered the 

United States during 1996-2000 according to the USFWS LEMIS import database, while 

over 38,000 skins and skin products of B. reticulatus entered the United States during 

these years. Most African python skins are likely used within Africa.

Northern African Pythons are reported as declining in Nigeria due to intense 

hunting for food and skins (Akani and others, 1999). Pythons do not represent a large 

proportion of snakes observed to have been killed by people in southern Nigeria, but this 

may be because pythons are eaten after being killed and thus were not available for 

observation (Akani and others, 2002). In the Congo, Schmidt (1923) reported that 

pythons are often eaten by humans, and are taken by spearing or by placing nooses at the 

entrance of holes occupied by pythons. Southern African Pythons are protected from 

take in Botswana (Auerbach, 1987) and South Africa, where they are said to be declining 

in some areas, especially around Durban and the Eastern Cape where they may have been 

extirpated (Alexander and Marais, 2007). Causes of mortality for the giant African 

pythons include harvest for food, skins or fat (for medicinal purposes), killing pythons to 

protect livestock, habitat destruction, vehicular traffic when snakes are crossing roads, 

and possibly the rise in electrified fencing around game farms (Branch, 1988; Fa and 

others, 2000; El-Kamali, 2000; Alexander and Marais, 2007). Five of the six pythons 

encountered by Padial (2003) in Mauritania had been killed by local people, and he 

considered these isolated populations to be in urgent need of protection. Juvenile pythons 

were often accidentally captured in fishing nets in the Republic of the Congo (Jackson 

and Blackburn, 2007). A survey of 17 traditional healers in a Johannesburg marketplace 

revealed products from 22 individual pythons, for which 14 different uses were described 

(T. Hibbitts, pers. commun., 2008).

A population of Southern African Pythons in the Eastern Cape of South Africa 

was reportedly extirpated by 1927, prompting a reintroduction project in the 1980s 

(Alexander and Marais, 2007). At a local scale, Northern African Pythons were 

reportedly translocated from a nature reserve to rice fields in The Gambia, presumably 

for rat control (Starin and Burghardt, 1992).
131

7.2 Human Health Risks

As with any large constricting snake, individuals over 3.5 m are potentially 

dangerous to children and/or adults, and some herpetoculturists consider any snake that 

can exceed 8 ft (2.43 m) as being potentially dangerous (Flank, 1997). However, most 

accounts of unprovoked attacks on humans by the giant African pythons are poorly 

substantiated, except for the well-documented deaths of a 13 year old boy in South Africa 

and an adult male who died of internal injuries a day after being constricted (Branch and 

Haacke, 1980; Spawls and Branch, 1995; Spawls and others, 2002). Among notable nonfatal injuries, a python bite to a herpetologist required 57 sutures, and a veterinarian lost 

an eye to a python bite (Spawls and others, 2002).

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

According to USFWS import records, 8,245 live P. “sebae” (not distinguished 

from P. natalensis in import records) entered the United States between 1989 and 2000 

(Reed, 2005). Most of these were likely to have been true P. sebae, based on examination 

of the declared countries of origin. Over a longer period of time (1977-2007), CITES 

records document 32,738 individuals imported, an average of 1,056 per year (range 5-

3488: Fig 6.9). This rate has held fairly steady in recent years and is roughly 10 percent 

of the annual rate of imports of Indian Pythons during the same period. 

The giant African pythons are not particularly popular as pets in the United States, 

especially as compared to Indian Pythons and Reticulated Pythons. This is likely due to 

three observations: 1) the African pythons are commonly perceived as a fairly “vicious” 

snake that is prone to striking with little warning, although this trait has not been 

quantified; 2) whereas P. molurus and B. reticulatus exhibit a bewildering range of 

color/pattern/size “morphs” (named with terms such as albino, granite, green, labyrinth, 

tiger, sunfire, dwarf, etc.), only three such morphs (hypomelanistic, patternless and 

striped) are known for Northern African Pythons (de Vosjoli and Klingenberg, 2005), and 

none are known for Southern African Pythons to our knowledge; and 3) relative to 

molurus and reticulatus, Northern African Pythons have less contrast between ground 

colors and overlying pattern, resulting in a “duller” appearance. The species can be 

easily bred in captivity, but is bred infrequently as compared to other giant pythons, 

likely implying that a greater proportion of African pythons sold in the United States are 

wild-caught.

7.3.2 Other Uses

Major human uses of giant African pythons include meat, skins, medicinals, and 

the live animal trade (see above).
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Figure 6.9. Annual imports of giant African pythons, Python sebae/P. natalensis (not 

distinguished in trade) to the United States during the period 1977-2007. Data from CITES.

Management Profile

See the introduction to the management profile for the Indian Python (chapter 

Four) for an explanation of the format of this section of the risk assessment. 

8.0 Pathway Factors: Pet Trade

8.1 Volume of International Trade

As indicated in section 7.3.1, above, and in the Appendix, the giant African 

pythons represent a fairly small proportion of the overall trade in live giant constrictors as 

compared to perennial favorites such as Boa Constrictor. Although the trade in giant 

African pythons appears low compared to the more popular species, the long-term 

average of about 1,000 imports per year ranks them well above the average for all 

boid/pythonid species in the live animal trade. The annual volume of imported giant 

African pythons has been fairly constant over the past three decades, generally lacking 

the drastic year-to-year changes evident for some other species. Section 8.1 of chapter 

Four outlines broad reasons why imports may constitute higher risk as invasive species

than captive-reared individuals.

We are uncertain as to the specific identity of some individuals in the trade, as P. 

natalensis was split from P. sebae fairly recently and pythons have been imported with 

declared origins in countries with both species. This means that older CITES records and 

those involved in the export/import business may not distinguish between these species in 

international trade. Further examination of CITES export records could be helpful in 

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elucidating the species identity of giant African pythons entering the United States, 

assuming that the declared country of origin is accurate.

8.2 Volume of Domestic Trade

We are aware of no credible estimates of the volume of domestic sales or captive 

propagation of the giant African pythons. Surveys of storefront and online reptile dealers 

appear to indicate a relatively low rate of captive propagation; we strongly suspect that 

this rate is lower than the declared import rate, but have no means to quantify this 

suspicion.

9.0 Entry Potential (Survival in Transit – Meant Mainly for 

Unintentional Transport)

As with all of the giant constrictors, survival in the pet trade pathway is likely to 

be high, as animals are packed and shipped using methods designed to maximize the rate 

of survival in transit. While mortality rates of some reptiles have historically been very 

high during international transport (Franke and Telecky, 2001; Christy, 2008), large 

hardy species of snakes would appear to be among the taxa most likely to survive the 

travails of international transport. Once in the United States and purchased by an 

endpoint consumer, the python is likely to be housed in conditions that are designed to 

meet its needs. Survival in captivity is likely to be high except for snakes possessed by 

extremely negligent owners. 

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

Those factors promoting release of pet Indian Pythons (see section 10.1 of chapter 

Four) are likely to apply to the giant African pythons. Because the majority of giant 

African pythons in the United States live animal trade appear to be imports, they may be 

more subject to ailments caused by parasites, other pathogens, or captivity stress. These 

species also have a reputation of being prone to biting their owners, even as compared to 

other giants such as Reticulated Pythons, and wild-caught imports may be especially 

likely to bite. Both poor health and a temperament poorly suited to captivity may 

increase the odds that problematic individuals are discarded. Observations of multiple 

individuals in a small area of south Florida indicate that release(s) (whether unintentional 

or intentional) have certainly occurred.

10.2 Climate Match: Areas of the Country at Greatest Risk

We extrapolated climate from the native range and mapped it to the United States 

for Northern (P. sebae: Fig. 6.10) and Southern (P. natalensis: Fig. 6.11) African Pythons 

separately. When considered separately, Northern and Southern African Pythons both 

exhibit a climate match to the United States that includes a large portion of peninsular 
134

Florida, extreme south Texas, and parts of Hawaii and Puerto Rico. To this can be added 

apparently suitable conditions for the Southern African Python extending slightly further 

to the north in Florida and also including portions of Texas from about the Big Bend 

region to the southeasternmost extent of the state.

Figure 6.10. Areas of the United States matching the climate envelope expressed by P. sebae in 

its native range (Fig. 6.5), based on 109 known localities. See chapter Two for details of analytical 

method.
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Figure 6.11. Areas of the United States matching the climate envelope expressed by P. 

natalensis in its native range (Fig. 6.5), based on 87 known localities. See chapter Two for details 

of analytical method.

10.3 Prey Availability

As with Reticulated and Indian Pythons, the prey of the giant African pythons 

includes all endotherms of a size appropriate for consumption by an individual snake, as 

well as many reptiles (primarily lizards and crocodiles) and the rare inclusion of 

amphibians (frogs). Fish appear to be an inconsequential part of the diet. The mainland 

areas indicated in Figures 6.10 and 6.11 do not appear to be lacking in prey species of 

suitable sizes and taxonomic affiliations. In fact, if successfully established in south 

Texas, giant African pythons might come into contact with prey species from their native 

distribution (including many species of small to medium-sized antelope), given the 

prevalence of exotic game ranches in this region. Because islands tend to exhibit a 

relative lack of large-bodied mammals, large pythons might be food-limited in insular 

situations such as Hawaii, Puerto Rico, Guam, Northern Mariana Islands, etc.
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10.4 Predation Risk

The United States has no specialized predators capable of keying in on large adult 

giant African pythons (for that matter, no such predator exists anywhere in the world, to 

our knowledge). More generalized predators, including panthers and alligators, would 

certainly take some large individuals, but would likely be repaid in kind by pythons 

consuming juveniles or adults. Smaller pythons are likely to fall prey to a range of 

potential predators, including birds of prey, mammalian mesocarnivores, a few 

ophiophagous snakes, feral hogs, etc. Not enough is known about microhabitat use 

across the range of the giant African pythons for inferences on whether they are more or 

less likely than other giant constrictors to make use of aquatic, arboreal or terrestrial 

microhabitats. Energetic losses associated with moving up through trophic levels means 

that adding a new predator (python) to an ecosystem is unlikely to represent a net benefit 

to native predators, as a python would represent a lower energetic benefit as a prey item 

than the native animals they had consumed to reach any particular body size. Introduced 

pythons would thus be likely to reduce the overall prey biomass available to a predator.

10.5 Reproduction Requirements

Giant African pythons do not appear to exhibit any specialized reproductive 

requirements; they commonly brood their eggs, but the consensus is that neither species 

employs shivering thermiogenesis to warm eggs. The inability to warm eggs by 

shivering would appear to limit their ability to successfully reproduce in cool climates, 

but for the observation that the Southern African Python leaves its eggs to bask, after 

which it returns to the eggs and warms them using heat acquired passively. Most, but not 

all, observations of nesting appear to be associated with underground microhabitats 

(burrows created by other species); nesting underground may further buffer the eggs from 

cool ambient temperatures. 

10.6 Hibernation Requirements

Whereas the giant African pythons do not appear to undergo true hibernation, 

they do appear to be less active during the cool season. At moderate elevations in 

Tanzania, fewer pythons were observed surface-active during the cool season, and 

pythons spent most of their time underground during the South African winter (Razzetti 

and Msuya, 2002; Alexander, 2007). However, pythons were generally surface-active at 

least once per week in the South African study, and late winter saw an increase in python 

activity associated with the onset of the breeding season. The preceding observations 

apply to the Southern African Python; we are unaware of evidence to support hibernation 

or extended aestivation in the Northern African Python, although it would be 

unsurprising if individuals of this species aestivate during the dry season in arid habitats 

bordering the Sahel.

10.7 Tools for Eradicating a Colonization

Available tools for eradication of either species of giant African python are likely 

to be similar to those described in chapter Three and section 10.7 of chapter Four. Many 

observers claim that the Northern African Python is often associated with aquatic 

habitats, implying that development of control tools might be complicated by the 
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necessity of deploying them in aquatic or seasonally flooded habitats. Observations of 

African pythons basking on cool days might make them vulnerable to capture resulting 

from visual surveys during the cool season.

11.0 Spread Potential

11.1 Dispersal Ability

We are unaware of relevant data on dispersal capacity in these species. A single 

individual of the Northern African Python exhibited an activity range of about 30 ha over 

a year of observations, while activity ranges of Southern African Pythons can approach 

500 ha. Given the relatively close relationship among the giant African Pythons and the 

Indian Python, there appears to be no convincing reason why the former would not be 

capable of movements of similar magnitude to those observed for the latter in Florida. 

Clutch sizes of the giant African pythons average 30-50 eggs, but often exceed this 

number and clutch sizes can approach 100 eggs. The upper end of this clutch size range 

puts the species very close to the maximal observed for giant constrictors (107 in P. 

molurus, 124 in B. reticulatus), and the rate of population dispersal is thus unlikely to be 

limited by fecundity. 

11.2 Ability to Develop Races or Strains

There are no native species in the United States with which either giant African 

python would be capable of interbreeding, negating the risk of developing new races or 

strains by that route. However, the African species are capable of hybridizing with the 

Indian Python, which is already established in south Florida. The fertility and long-term 

viability of such hybrids is unexplored. It is conceivable that introduction of African 

genes to the Indian Python population could result in increased genetic variability that 

could allow exploitation of new ecological or physiological niches and/or result in some 

other type of hybrid vigor. Such a scenario has become more likely in the face of recent 

evidence for a population of Northern African Pythons along the western edge of Miami, 

an area within the introduced range of Indian (Burmese) Pythons.

11.3 Tools for Managing Spread

No species-specific tools are available to combat spread. The apparent propensity 

to use aquatic habitats, especially in the Northern African Python, may complicate 

deployment of barriers to dispersal.

12.0 Economic Impact Potential

Regulatory agencies have not decided whether a formal economic analysis of the 

potential impacts of introduction of, and regulation of trade in, the giant African pythons 

will be separately conducted pursuant to a variety of statutes and Executive Order 13272 

(Proper consideration of impact on small business). Our narrative below is provided only 

to cite relevant biological literature and identify germane sections of the biological 

profiles.
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12.1 Pathogen Vector

There is more information on giant African pythons as potential pathogen vectors 

than for any of the other giant constrictors (see section 5.3 of this chapter), but this 

information is still woefully inadequate. A range of pathogens of medical, veterinary, or 

agricultural importance are known or suspected to be vectored by tick species that use 

giant African pythons as hosts, and these pythons may harbor parasites or other 

pathogens capable of being transmitted to vertebrates in the United States; the level of 

threat is difficult to predict given existing knowledge gaps.

12.2 Predator on Livestock

In their native range, giant African pythons have been reported to kill livestock up 

to the size of a 43-kg calf, and one python contained six goats. Such events are unlikely 

to be common, as very large individuals would be expected to represent a small 

proportion of any given python population. Smaller domestic animals (poultry, etc.) are 

more likely to experience predation by the average-sized python, and an escaped/released 

P. sebae consumed a domestic turkey in south Florida (S. Snow, pers. commun., 2007). 

If Northern African Pythons are closely associated with aquatic habitats as suggested by 

some observers, then domestic waterfowl production may suffer in areas with introduced 

python populations.

12.3 Predator on Pets

Dogs have been recorded in the diet of giant African pythons, and it is likely that 

virtually any endothermic household pet, from gerbils to pot-bellied pigs, could be 

consumed by a python of sufficient size. Akani and others (2002) suggested that in 

recent years pythons actually persist better in suburban situations than in natural areas, 

and the average suburban python is about 3.0 m long. A snake of this size would be 

capable of consuming domestic cats, small dogs, etc., but whether a Nigerian suburb 

bears any resemblance to the typical American suburb is another question, which renders 

direct comparisons problematic.

12.4 Electrical Power Systems

The frequency with which giant African pythons would be expected to reach 

elevated power lines is unknown, although radiotelemetry of a single Northern African 

Python revealed extensive use of arboreal habitats. Pythons are unlikely to be nearly as 

adept at climbing utility poles as are Brown Treesnakes or other snake species with 

specialized adaptations for climbing. We expect python-associated damages to electrical 

grids to be minimal.

12.5 Traffic Accidents/Human Attacks

Legitimate attacks on humans that result in fatalities are exceedingly rare in the 

native range of the giant African pythons, but such attacks are not unknown. If an attack 

were to occur in the United States, the negligible public health risk would likely be far 

outstripped by media-induced panic among the populace. 
139

Both African species reach body sizes large enough to cause automobile 

accidents, should a vehicle traveling at a high rate of speed impact a large python; this 

type of accident occurs with unfortunate regularity in the southeast United States when 

vehicles hit alligators, but its potential frequency is difficult to predict.

12.6 Tourism

Potential impacts to tourism are likely to be similar to those associated with 

introduction of Reticulated Pythons (see section 12.6 in chapter Five), although the 

higher propensity for Reticulated Pythons to attack humans in their native range means 

that they might have a greater impact on tourism after such an attack takes place. While 

there are some tourists who might travel to an area for the express purpose of viewing an 

introduced giant African python, experience with Indian Pythons in south Florida 

suggests that these will represent a tiny fraction of the overall visitation rate.

12.7 Impacts on Commercially Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

Bird watching in south Florida and south Texas is a major contributor to local 

economies. Many of the most sought-after birds are endemic to these areas and/or are of 

conservation concern, and pythons are likely to have the greatest impacts on imperiled 

species (see section 5.1). The presence of a novel predator on rare birds is likely to be 

detrimental to bird watching tourism if pythons reduce populations and thus reduce 

sighting rates. Birds are a predictable component of the diet of African pythons in their 

native range. Numerous ungulates considered trophy game species have been recorded in 

the diet of both giant African pythons. Potential impacts to similar species in the United 

States would likely be limited to white-tailed deer and introduced game ungulates (many 

of which are from Africa themselves), although important non-ungulate game species (for 

example, turkey) could be impacted as well.

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture

We do not anticipate that giant African pythons would have a direct impact on 

these industries. However, the presence of giant constricting snakes in forested or 

agricultural habitats could certainly reduce the available pool of laborers willing to work 

in such habitats. Presence of such species in natural landscapes might also induce 

employers to institute measures such as are used in bear country, including special 

training, requirements for safety equipment, and/or requirements to travel in pairs in 

predator-occupied habitat. Such requirements could increase costs associated with some 

industries. It is possible that pythons would suppress populations of rats or other crop 

pests in agricultural settings.
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13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

See section 13.1 of chapter Four for a list of prey species that might be impacted 

by the Indian Python, and the same section of chapter Five for a discussion of why 

another invasive giant constrictor might be expected to have lower impacts. The giant 

African pythons could potentially occupy a larger area than the Reticulated Python based 

solely on climatic variables, but their potential range is smaller than that of the Indian 

Python. Overall impacts of African pythons to species of special concern are therefore 

likely to fall between the other two species, assuming similar densities and dietary 

preferences.

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

As indicated in section 13.2 of chapters Four and Five, food supplementation 

provided by giant African pythons is likely to be offset or upset by intra-guild predation. 

Furthermore, native predators are likely to experience a net loss of prey biomass after the 

introduction of a novel predatory snake, as discussed above in section 10.4.

13.3 Vector for Disease Spread to or Within Native Faunas

Too few data exist to evaluate this consideration, but the risk of importation of 

parasites and pathogens cannot be discounted. Tick species that use giant African 

pythons as hosts have been documented to survive the pet trade pathway to the United 

States, move to new hosts once transported, and establish populations in Florida. Some 

of these tick species are known or suspected to be vectors for diseases that could be 

devastating to native vertebrates.

13.4 Species that Might be Adversely Affected by Control Measures

Control measures for giant African pythons would likely be similar to those 

envisioned for giant constrictors in general (see chapter Three). Some possible avenues 

of control, such as acetaminophen toxicants, are likely to result in non-target mortality to 

native species, especially large snakes capable of accessing toxicant delivery devices 

designed for giant constrictors. Traps, drift fences, and other potential tools may also 

impede movements of non-target native taxa or result in their inadvertent capture. As no 

management tools have been adequately field-tested, however, it is not possible to 

estimate potential ecological costs.
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14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Production Affected?

Domestic production of the giant African pythons in herpetoculture is relatively 

low, almost certainly less than 500 offspring produced per year in the United States, 

although we have no means of verifying this estimate. Restrictions on sales of these 

species are thus likely to impact a relatively small number of producers, at least in 

economic terms, although those who specialize on breeding these species would be 

disproportionately impacted. Such considerations would be better addressed by a formal 

economic analysis. As with Reticulated Pythons (section 14.1.1 of chapter Five), 

however, imposing regulations on trade in giant African pythons would likely result in 

much wider concern over whether such steps are precursors to more general restrictions 

for other species.

As with Reticulated Pythons, regulations on trade or ownership of the giant 

African pythons might result in importers or dealers releasing unsalable stock, owners 

releasing animals for which they cannot acquire permits, etc. Releases by importers, 

breeders, and dealers are of greatest concern, as they are likely to involve multiple 

individuals at the same time and place (= high propagule pressure).

14.1.2 International Trade/Alternate Pets

While the long-term average of 1,000 giant African pythons per year is dwarfed 

by average imports of Boa Constrictors, Indian Pythons, Ball Pythons, etc, the average 

retail price of $100-250 per individual giant African python adds up to a moderate 

income stream for importers even considering retail mark-ups. Restrictions on trade in 

these species would thus produce some economic hardship, but would likely be offset to 

some degree by importers switching to different species that lack such restrictions.

Restrictions on certain taxa are also likely to result in taxonomic loopholes that 

could be exploited by importers, breeders, and retailers. For example, the state of Florida 

recently imposed regulations related to permitting processes for certain large constricting 

snakes, including the Northern African Python (P. sebae). However, the regulations do 

not cover the Southern African Python (P. natalensis), nor several other species of 

slightly smaller body size but which present potentially serious threats as invasive species 

(for example, Boa Constrictor, Yellow Anaconda). It is possible that trade in nonregulated species could increase as a means of circumventing species-specific attempts to 

regulate trade. If the numbers of certain species in trade increase greatly as a means of 

getting around regulations, then the relative risk associated with propagule pressure for 

those species increases as well.
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Chapter Seven–The Boa Constrictor, Boa 

constrictor

Biological Profile

1.0 Introduction

See section 1.0 of the biological profile of the Indian Python for a description of 

species-specific and overall components of the risk assessment package.

1.1 Species

The species Boa constrictor ranges widely over North, Central, and South 

America and dozens of marine and lacustrine islands, and has one of the widest 

latitudinal distributions of any snake in the world. The species displays extensive 

variation in size, color, pattern, scalation, and other traits, and some populations are also 

noticeably divergent in terms of diet, ecology, and/or behavior. Various authors have 

recommended splitting the taxon into several species, but the lack of a thorough revision 

of the species (see below) has kept such recommendations from being widely adopted.

Peters and Orejas-Miranda (1986) listed eight subspecies (Table 7.1), which have 

been largely adopted by some in the academic community and most in the herpetocultural 

community (for example, Fogel, 1997; Russo, 2007; but see Ross and Marzec, 1990 for a 

more conservative viewpoint). Both Walls (1998a) and Peters and Orejas-Miranda 

(1986) gave a synonymy for the species, and the latter included a key to subspecies. 

Smith (1999) provided a rough review of the taxonomy of B. constrictor, including 

references to much of the important literature. Price and Russo (1991) reviewed 

subspecies of B. constrictor, described a new subspecies Boa constrictor longicauda, 

suggested that B. c. nebulosa and B. c. orophias warrant specific status, and downgraded 

B. c. melanogaster (Langhammer, 1983) to a nomen dubium. Savage (2002) briefly 

commented on some potential problems with the approach of Price and Russo (1991) in 

describing B. c. longicauda, but concurred with their castigation of Langhammer (1983). 

Lazell (1964) gave full descriptions and ecological information for B. c. orophias from 

Saint Lucia and B. c. nebulosus from Dominica, comparing both to B. c. constrictor from 

the mainland. More recently, some authors (Walls, 1998a; Bonny, 2007) have considered 

the Saint Lucia and Dominica boas as full species, based on their allopatry and apparently 

independent evolutionary lineages, while others have suggested that the Argentine 

subspecies (B. c. occidentalis) should be considered a full species as well (M. 

Chiaraviglio, pers. commun., 2007). 
143

We suspect that raising B. c. orophias and B. c. nebulosus to full species could 

render some other subspecies paraphyletic with respect to these island populations, 

necessitating the erection of additional species; the consequences of raising B. c.

occidentalis would likely have similar repercussions. Pending a phylogeographical 

review of the genus, we consider the lineage to represent a single wide-ranging species 

for the purposes of this document. 

Despite a fairly large body of literature basing intraspecific taxonomic 

differentiation on the subspecies described in Table 7.1, it is important to note that a more 

nuanced view of geographic variation in Boa Constrictors is likely to emerge with further 

study, and that the current named subspecies may be insufficient to describing this 

variation. Wilson and Meyer (1985) stated that “Boa constrictor is a widespread, 

common and variable species with a taxonomy existing in an abysmal state...it is obvious 

from (a perusal of the literature) that numerous potentially interesting patterns of 

character variation are disarticulated by the application of the subspecies category...if we 

are to understand the systematics of this logistically-difficult-to-study snake, we will have 

to jettison the archaic system of subspecies categories now masquerading as a systematic 

framework.” Similarly, Savage (2002) concluded, “No adequate study of individual and 

geographic variation in this wide-ranging and common species has ever been published,” 

and instead of recognizing traditional subspecies, Savage hesitantly offered only four 

“poorly diagnosable geographic variants...often regarded as races of this boa that 

intergrade in geographically intermediate areas.”
144

Table 7.1. Subspecies of Boa constrictor as recognized by Peters and Orejas-Miranda (1986), with the addition of the disputed B. c. longicauda

as proposed by Price and Russo (1991). Data from sources listed in text. See text for comments on legitimacy of subspecific epithets. For dorsal 

blotch and scale row counts, we give ranges as the recorded minima to recorded maxima; numbers that are contradictory among sources are 

indicated with *.

Subspecies Common name Geographic distribution Dorsal blotches Dorsal scale rows Ventral scale count

B. c. constrictor Red-tailed Boa S. America, inc. Amazon Basin 14*-22* 77*-95 231*-250*

B. c. imperator Common Boa Mexico, C. America, NW S. America 22-30 55-79 225-260*

B. c. orophias St. Lucia Boa St. Lucia, Lesser Antilles 25-31 65-75 258*-288

B. c. occidentalis Argentine Boa Portions of Bolivia, Paraguay, Argentina 22-30 64*-87 242-251

B. c. sabogae Pearl Islands Boa Islands of Gulf of Panamá ? 65-67 241-247

B. c. nebulosus Clouded Boa Dominica, Lesser Antilles 31-35 59-69 258-273

B. c. amarali Bolivian Boa Portions of S. Brazil, Paraguay, Bolivia 22-28 71-79 226-237

B. c. ortonii Peruvian Boa NW Peru 15*-28* 57*-80* 246-252

B. c. longicauda Long-tailed Boa Peru, vicinity of Tumbes 19-21 60-76 223-247
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1.2 Common Names

Common names for the Boa Constrictor are legion, as would be expected of a 

species with such an enormous geographic distribution. In English, this species is one of 

the few with a common name identical to its scientific name. A few examples of regional 

common names include Béquer (Costa Rica: Savage, 2002), Mazacuate, Waula, and 

Ugrwe (Honduras: McCranie and others, 2006), Lampalagua (Argentina: M. 

Chiaraviglio, pers. commun., 2008), Mboi-roy (Paraguay: Waller and others, 1995), 

Macajuel (Trinidad: Boos, 2001), Jumbo Jocko (Tobago: Boos, 2001), and Wowla 

(Belize: Stafford and Meyer, 1999). 

1.3 Evolutionary Context

Boa Constrictors have long been placed within the family Boidae, which 

traditionally included boine, erycine, and pythonine snakes. In recent years, however, 

pythons have been split off to form the Pythonidae, leaving erycines and boines in the 

Boidae. The boine snakes are currently composed of at least 32 species of snakes, 

primarily found in the Neotropics but with two genera in Madagascar and one genus in 

southeast Asia (Kluge, 1991; McDiarmid and others, 1999; Burbrink, 2005). Numerous 

phylogenetic hypotheses of relationships within and among these higher taxa have been 

erected over the decades. 

Based on examination of morphological characters, Kluge (1991) hypothesized 

that the two Madagascan taxa are more closely related to Boa Constrictors than any of the 

three are to other New World boines. This hypothesis was accepted by many 

herpetologists and led to much bewilderment among those attempting to decipher the 

historical biogeography of the subfamily. More recently, Burbrink (2005) explicitly 

examined the phylogenetic position of B. constrictor among other boine snakes, 

incorporating molecular analyses, re-examination of Kluge’s morphological data, and a 

combined analysis. He rejected the hypothesis that Madagascan taxa are the closest 

relatives of Boa Constrictors, and instead proposed a clade that includes all the New 

World boines as a natural grouping, with Boa as the sister taxon to a group that includes 

Corallus, Epicrates, and Eunectes. The informal name “boa” is often applied to the 

boines in general and it may be a component of the common names of many individual 

species (for example, Pacific Island Boa), but in this work we are referring specifically to 

Boa constrictor whenever we use the unmodified term “boa.”
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Figure 7.1. Schematic representation of the head of B. constrictor

2.0 Description

2.1 Unique Morphological Features

See the biological profile for P. molurus for characters that tend to be shared 

among giant constrictor snakes, including B. constrictor. Boa constrictors are easily 

distinguished from pythons because boas have no enlarged head shields or internasals. 

Unlike the pythons, boas also lack thermoreceptive labial pits. In contrast to the four 

species of anacondas (genus Eunectes) boas have laterally-directed eyes, as is 

characteristic of most terrestrial snakes.

2.1.1 Color and Pattern

This species is extremely variable in color and pattern, but distinguishing Boa 

Constrictors from other large-bodied constrictors is straightforward. Boas have no visible 

thermoreceptive pits that open to the environment and no enlarged scales on the top or 

lateral head surfaces, other than slightly enlarged nasals near the tip of the snout. Boas 

also lack the four or five longitudinal head stripes that are present in the anacondas (see 

section 2.1 of chapters Eight and Nine); instead, boas tend to have a single narrow dark 

stripe on the top of the head, which may or may not widen into a narrow arrowhead shape 

towards the back of the head and neck. There is usually a dark band (width varies among 

individuals) from the eye posteriad (we term this the ocular stripe), and this band 

typically extends through the bottom half of the eye and forward to the snout, usually 

becoming wider and angling down to the supralabials—the combination of these bands 

often appears as a single stripe originating on the snout and continuing to the neck. There 

is usually another dark blotch below the eye (often appearing as a triangle with the 

narrower end pointed toward the eye), and two more blotches are typically present on the 
147

infralabials (sometimes lined up with the subocular blotch and the anterior end of the 

ocular stripe where it contacts the supralabials, but offset in many parts of the range). 

In most parts of the range, the dorsal ground color is light tan to brown, and the 

venter is nearly immaculate and white or off-white in color, although B. c. occidentalis

exhibits extensive dark ventral coloration (T. Waller, pers. commun., 2009), as do some 

individuals in other parts of the range (Langhammer, 1983). The dorsal pattern is 

variable, but usually consists of well-defined dark hourglass-shaped dorsal saddles on a 

lighter background. The size, number, and shape of the saddles are extremely variable. 

Most boas also have less distinct lateral markings that are offset from the dark saddles, 

consisting of wide vertical bars or ocelli with lighter centers. In many populations, the 

dorsal saddles become much wider and assume the form of rectangles or oval blotches on 

the tail, with bright chestnut to red colors (hence the common term “red-tailed boas”).

In a description that will distinguish most mainland South American boas from 

other potentially confusing taxa, Lazell (1964) described B.c constrictor as having “15 to 

22 rather neat, geometric, dark dorsal saddles to the level of the anus; the ground color is 

fawn-brown to tan; the venter white to yellow with little dark marking; a dark stripe 

proceeds from the eye directly to the supralabials (subocular stripe), and a second 

proceeds from the eye anteriorly in the loreal region, then curves down onto the 

supralabials (loreal stripe); both stripes correspond to two dark blotches on the 

infralabials and chin.” In El Salvador, B. c. imperator was described as “dorsal ground 

color gray, grayish brown, brown, or yellowish brown, with a series of darker saddle-like 

blotches becoming more reddish brown and contrasting on the tail, where they form dark 

brown rings alternating with cream rings; another series of smaller creamish centered and 

rhomboidal blotches laterally; top of head shares dorsal ground color; a narrow dark 

brown median stripe originating on the snout and extending onto neck” (Köhler and 

others, 2006). Mainland boas from Venezuela have 16-20 dorsal blotches, plus 3-5 on 

the tail (Lancini, 1986), while Dixon and Soini (1986) reported 17 body blotches (range 

14-21) and 4.8 tail blotches (range 4-6) among boas from the Iquitos region of Peru. 

Beebe (1946) described variation in iris color in B. c. constrictor from S. America, and 

stated that in Kartabo, Guyana, there were “two more or less distinct and definite color 

phases, independent of age or sex. One was characterized by rich, warm hues of chestnut 

and browns, and the other was darker and colder in general color, with drab and olive 

greens replacing the warmer tones.” 

Insular B. c. orophias from Saint Lucia have “27 to 31 distinct, subrectangular, 

dark dorsal saddles to the level of the anus; dorsal ground color rich brown; venter white 

with black or grey spotting pronounced; subocular stripe distinct and complete; loreal 

strip largely obsolete; dark pigment on chin and infralabials not closely corresponding to 

the facial stripes” (Lazell, 1964). Insular B. c. nebulosus from Dominica have “32 to 35 

very obscure, irregular transverse markings to the level of the anus; dorsal ground color 

very dark, clouded, grey-brown; venter ash to slate grey, blotched and mottled with 

black; both loreal and subocular stripes absent, or, at most, partially indicated and largely 

obsolete; infralabials and chin merely grey” (Lazell, 1964). Dominican boas occasionally 

exhibit extreme melanism (Malhotra and Thorpe, 1999), and Porras (1999) further 

describes coloration of various insular boas, including their tendency to change color.
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2.1.2 Scalation and Meristics

As would be expected for such a wide-ranging species, B. constrictor exhibits 

considerable geographic variation in scale pattern and number. According to Walls 

(1998a), B. constrictor has 17-25 supralabials and 20-28 infralabials, 55-91 dorsal scale 

rows at midbody, 225-288 ventrals, and 45-69 subcaudals (however, he later listed 

subspecies with counts that do not agree with some of these numbers). This summary 

was from across the entire range of the species and excluded boas from St. Lucia and 

Dominica, which he recognized as full species. Scalation ranges for various subspecies, 

including questionable subspecies, of B. constrictor have been tabulated and/or discussed 

by various authors (Peters and Orejas-Miranda, 1986; en Ben Lotte, 1996, Walls, 1998a; 

Smith, 1999; Monzel, 2005: see also Table 7.1 and discussion of the validity of these 

subspecies in the section 1.1), and Gomes and others (1989) provided line drawings of B. 

constrictor anatomy.

Considerable meristic and morphological divergence can exist between mainland 

and island boas. For example, Lazell (1964) found that B. c. constrictor from 

northeastern South America have a “slightly prominent snout and straight or nearly 

straight canthus; there are 81 to 95 dorsal scale rows at midbody, [and] 234-243 

ventrals.” In contrast, he found that insular B. c. orophias from Saint Lucia have a 

“prominent snout and a convex canthus; 65 to 75 dorsal scale rows at midbody, 270 to 

288 ventrals,” and B. c. nebulosus from Dominica have a “prominent snout and strikingly 

convex canthus; 59 to 69 dorsal scale rows at midbody, 258 to 273 ventrals.” Similarly, 

insular B. constrictor from Belize have a narrow head, longer labial length, longer naresocular distance, shorter internares distance, and larger eyes compared to mainland boas, 

and island snakes were also shorter and lighter at the same length (Boback, 2006).

2.2 Size

As with most giant constrictors, the maximum size of the Boa Constrictor has 

been subject to exaggeration, especially in the older literature. Unfortunately, many of 

these claims of gigantic boas have been perpetuated by more recent authors, and 

ascertaining the actual maximum size is difficult. Part of the confusion stems from 

misapplication of the name Boa Constrictor to other giant snakes, including anacondas 

and even some Old World pythons (Ditmars, 1931). A 5.7 m specimen from Trinidad 

turned out to be a misidentified Green Anaconda (Eunectes murinus: Boos, 1992). 

Lancini (1986) stated that he measured a skin in “an institution in Brazil” that measured 

5.45 m; however, skins can be stretched by over 20 percent as compared to live snake 

length (RNR, pers. obs., 2003). In a review of the species, Greene (1983) stated that B. 

constrictor reaches 5 to 6 m, but Martins and Oliveira (1998) state that the maximum size 

for the species is 4.2 m, and minimum is 0.65 m. Savage (2002) stated that maximum 

size is 4.5 m, but that most adults are 2-3 m. Pope (1961) gave maximum length as 5.5-

5.8 m. Porras (1999) noted that he handled several boas over 4.0 m, including 

individuals of 4.4 m and over 4.88 m, while unpacking a shipment of animals imported 

from Peru. 

The maximum body size based on an available specimen is 4.45 m (WatkinsColwell and Leenders, 2003), but this measurement is from a dried skin from Brazil. 

Although the authors claim that a visual inspection revealed no apparent effects of 

stretching, they provide no quantification of this claim, which we consider dubious based 
149

on our experience with skins removed from boid snakes (RNR, pers. obs., 2008) and the 

published observation that two Boa Constrictors over 3 m total length in life yielded skins 

that were 13.7 percent and 30 percent longer than the live length (Field Naturalists’ Club, 

1893, 1894: cited in Boos, 2001). These findings imply that the maximum size of B. 

constrictor may be in the neighborhood of 4 m. 

Table 7.2. Total lengths and masses of typical B. constrictor (compiled/estimated from the 

literature sources listed in the text). 

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 430-575 34-80 0.9-1.6 0.225-1.75 3.0 ~15

F: 430-575 34-80 1.2-2.3 0.8-5 4.45 ~35

The largest boas tend to be from northern South America, with body size 

generally decreasing to the north and south of the Amazon Basin, as well as numerous 

examples of dwarfed insular forms on Caribbean, Pacific, and Atlantic islands. Mexican 

specimens are generally small, with average lengths generally between 1.0 and 1.75 m 

(Bogert and Oliver, 1945; Davis and Smith, 1953; Myres and Eells, 1968; Gonzalez-Baca 

and Cuaron, 2003), while Central American specimens more commonly reach and 

sometimes exceed 3.0 m (Campbell, 1998; Leenders and Watkins-Colwell, 2004; Guyer 

and Donnelly, 2005; Köhler and others, 2006; Holtzman and others, 2007). Juveniles and 

adults from insular populations near Central America are often smaller than those on 

adjacent mainland areas (Barbour and Loveridge, 1929; Boback, 2005, 2006; Reed and 

others, 2007). Mainland neonates from Belize were 42 ± 4.7 g and 0.48 m (0.42 m SVL: 

Boback, 2005; Boback and Carpenter, 2007). Insular neonates from Belize were 34 ± 3.6

g and 0.44 m (0.39 m SVL: Boback, 2005; Boback and Carpenter, 2007), while 13 

neonates from Utila, in the Bay Islands of Honduras, were 0.49-0.52 m and weighed 

approximately 52 g (Köhler, 2003). There was a captivity effect for island B. constrictor

from Belize, as captive-bred females had longer babies than did wild-bred females 

(Boback, 2005; Boback and Carpenter, 2007). 

Body size in South America is variable, but most reports corroborate the 

observation that northern South American boas reach larger body sizes than do boas to 

the north or south. Beebe (1946) found fairly small boas in Venezuela, but at Kartabo 

(Guyana), “...among the twenty or thirty collected, eight were from eleven feet to twelve 

feet six inches in total length” (3.35-3.81 m). Boas from Tumbes Province, Peru, can 

exceed 2.80 m (Price and Russo, 1991), while boas from Cusco Amazónico, Peru, were 

between 1.2 and about 3 m (Duellman, 2005). On Trinidad, off northeastern South 

America, Mole and Urich (1894) stated that the largest boa with which they were familiar 

was a 3.51 m female, containing 41 “eggs,” and that, “It is probable that Boas (in 

Trinidad, at any rate) never exceed 12 feet” (3.66 m). Mean neonatal size of Brazilian B. 

c. amarali was 0.47 m (0.41 m SVL) and that of B. c. constrictor was 0.52 m (0.45 m 

SVL: Pizzatto and Marques, 2007). Neonatal sizes given by Pizzatto and Marques 

(2007) appear to be somewhat smaller than most values recorded for mainland South 
150

American snakes, probably because they included values from near-term embryos. 

Alternatively, it could be that post-parturient neonates hide in refugia while absorbing 

remaining yolk, emerging only after growing past their lengths at parturition. 

Lazell (1964) collected boas up to 2.36 m on Saint Lucia and 1.85 m on 

Dominica, but saw larger (to about 3 m) boas in the field and in captivity. In Argentina, 

adult male B. c. occidentalis averaged 2.1 m (1.80 m SVL) and 4.1 kg and adult females 

2.28 m (2.05 m SVL) and 6.1 kg (Bertona and Chiaraviglio, 2003). Some authors (Cei 

1993, cited in Chiaraviglio and others, 2003) have stated that the largest B. c. occidentalis

were over 4.0 m in the past, but the largest animals found since 1995 in Córdoba, 

Argentina, were 3.16 m (2.85 m SVL: female) and 2.59 m (2.23 m SVL: male), possibly 

reflecting the intensive harvest pressure on large snakes (or exaggerated claims of body 

size in the past). Tomás Waller (pers. commun., 2009) stated that he is unaware of any 

individuals of this subspecies that exceeded 3.2 m total length and around 18 kg body 

mass, even in areas that have not been harvested. Argentine neonates are 0.47-0.66 m 

and 70-80 g (Monguillot, 1988; Bertona and Chiaraviglio, 2003). 

2.3 Sexual Size Dimorphism

As with the majority of species of large boid snakes, B. constrictor exhibits 

female-biased sexual size dimorphism (SSD). Females tend to reach longer absolute 

lengths, and in many cases are relatively heavier than males of similar body lengths. The 

very largest boas are undoubtedly female. Section 2.2, above, includes some relative 

lengths of male and female boas from various parts of the species’ geographic 

distribution. Unfortunately, few studies have sample sizes large enough for estimation of 

average SSD in a population, and SSD based on small samples and museum specimens 

are likely biased by differences in detection probability among sizes and sexes, and by 

the tendency for only relatively small specimens to be accessioned into museums (thus 

underestimating SSD for a species where females attain appreciably larger sizes). 

Among available records, SSD (calculated as [SVL of larger sex/SVL of smaller 

sex] – 1; Lovich and Gibbons, 1992) was 0.01 and 0.02 among preserved Brazilian B. c.

amarali and B. c. constrictor, respectively, but only 0.01 among neonate B. c. constrictor; 

females of both subspecies had relatively larger heads and shorter tails than did males, 

based on preserved specimens (Pizzatto and Marques, 2007). Other records of SSD in 

length, with varying sample sizes, include 0.142 among Argentine boas (Bertona and 

Chiaraviglio, 2003), -0.03 (males slightly larger: length SSD) and 0.01 (mass SSD) 

among boas on Ometepe Island, Nicaragua (Holtzman and others, 2007), and 0.11 (length 

SSD) and 1.0 (mass SSD) among dwarfed insular boas on Cayo Pequeño, Honduras 

(Reed and others, 2007). In Belize, insular boas did not exhibit appreciable SSD, but 

insular males had comparatively longer tails than did mainland males (Boback, 2006). 

Although preserved neonatal females from Brazil had shorter tails (Pizzatto and Marques, 

2007), Boback and Carpenter (2007) found no evidence of SSD at birth and no sex 

differences in any skeletal elements, including the skull. 

2.4 External Sexual Differentiation

As with most boas and pythons, the Boa Constrictor exhibits vestigial remnants of 

a pelvic girdle, including externally visible pelvic spurs. These are larger in males than 

in females (Pope, 1961). Very large individuals are virtually always female. In captive 
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individuals, male tail length averages 11.55 percent of total length, while the equivalent 

proportion for females is 9.85 percent (Smith, 1999).

3.0 Distribution in Space and Time

3.1 Native Range

Henderson and others (1995) presented what is perhaps the best-supported range 

map of Boa Constrictor for S. America; this map was based on museum and literature 

surveys. They noted that B. constrictor has a latitudinal range of 66 ̊, nearly twice that of 

any other New World boine, and that its altitudinal range is 0-1500 m. The species is 

present on a great number of islands in the Caribbean, Atlantic, and Pacific, including 

some populations that may be introduced (for example, Islas San Andres, Santa Catalina, 

and Providencia in the Atlantic, claimed by Colombia: Porras, 1999). Walls (1998a) 

provided a map showing the distributions of the traditional subspecies, and Köhler (2003) 

provided a fairly detailed range map for Central America. Steadman and others (1984) 

found fossils from B. constrictor on Antigua in the Lesser Antilles, indicating a former 

presence of the species—however, these authors cautioned that this could have been a 

human introduction to the island.

Argentina–Bertona and Chiaraviglio (2003) and Cardozo and Chiaraviglio 

(2008) studied B. c. occidentalis in the District of Pocho in the west plain of Córdoba 

Province of Argentina. This is close to the southernmost extent of the species (M. 

Chiaraviglio, pers. commun., 2007), although Chiaraviglio (2006) previously stated that 

the genus Boa is distributed from 30 ̊ N to 36 ̊ S. Chiaraviglio and others (1998) 

provided more information on geographic range in Córdoba Province, and DiCola and 

others (2008) modeled the distribution of B. c. occidentalis in the Gran Chaco (which 

extends from 22 ̊ to 31 ̊ S) of Argentina, at or near the southernmost extent of the 

species’ distribution. As a result of their model, they provided a dot-distribution map for 

93 B .c.occidentalis in Argentina, and a probability area for the occurrence of B .c.

occidentalis. They concluded that boas are continuously distributed across the plains of 

the Gran Chaco, and that suitable habitat extends from 60 to 1600 m. 

In contrast, T. Waller (pers. commun., 2009) states that the species does not 

exceed 1120 m in Argentina. Giraudo and Scrocchi (2002) also summarized the species’ 

distribution in Argentina, describing it as being present in the provinces of Catamarca, 

Córdoba, Chaco, Jujuy, Formosa, La Rioja, Mendoza, Salta, San Juan, San Luis, Santa 

Fe, Santiago del Estero and Tucumán. Chiaraviglio (2006) stated that B. constrictor

inhabits the Chaco, Monte, and Espinal phytogeographic provinces of Argentina, but 

within these it is largely restricted to Dry Chaco forests. Giraudo and Scrocchi (2002) 

stated that only B. c. occidentalis is present in Argentina and discussed the lack of 

specimens from La Pampa province, such that reports of the species’ presence south of 

about 34 ̊ S are suspect.
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Figure 7.2. Estimated native range of the Boa Constrictor (Boa constrictor), with approximate 

Mexican range in inset box. Intervening Central American range is nearly continuous at suitable 

elevations. Arrow indicates presence on St. Lucia and Dominica; species is present on many other 

islands, but this is not reflected on this map. Maps follow Henderson and others (1995) and 

references in section 3.1 of this chapter.
153

Belize–Present through most lowland areas. Neill and Allen (1962b) mentioned a B. 

constrictor caught on Cockroach Cay, Turneffe Group, Belize, and stated, "Boas are 

common on some of the cays, and on the mainland the species seems to find its optimum 

habitat in red mangrove swamp." They stated that mangrove boas from Belize have more 

dark red/cinnamon coloration than do lowland boas from Honduras. Boback (2005) 

collected B. constrictor from several mangrove islands off the Belize coast and from 

mainland areas around Belmopan. 

Bolivia–Present throughout the lowlands of Amazonia, Beni, and Chaco, including both 

B. c. amarali and B. c. occidentalis (T. Waller, pers. commun., 2009), but apparently 

absent from western highlands (Henderson and others, 1995).

Brazil–Found throughout the Amazon Basin and throughout much of the rest of the 

country, excepting only the extreme southeast (Henderson and others, 1995; Walls, 

1998a; Bartlett and Bartlett, 2003). Strüssmann and Sazima (1993) found two boas

during snake surveys in the Pantanal in western Brazil, but the species appears to be more 

common in the Manaus region (Martins and Oliveira, 1998). Andrade and Abe (1998) 

found color, morphology, and scutellation abnormalities in neonate B. c. amarali from a 

dam captured in the state of São Paulo that was bred in captivity and kept at 18-26o

C 

during gestation. They suggested that temperature may limit the southern extent of this 

subspecies’ range, although we note that their captive regime did not allow 

thermoregulation by the dam.

Colombia–Present in both Caribbean and Amazonian lowlands, Pacific Choco forest, 

some offshore islands (T. Waller, pers. commun., 2009; Henderson and others, 1995).

Costa Rica–Apparently present throughout, except for high elevations (Savage, 2002; 

Köhler, 2003). While some authors have claimed that the species reaches its altitudinal 

limits at about 900 m in Costa Rica (Pope, 1961; Leenders, 2001), Savage (2002) 

provided a dot-distribution map for Costa Rica and stated that B. constrictor reaches 1360 

m (this record is surprisingly similar to the 1370 m cited for Honduras by McCranie and 

others, 2006). 

Dominica–Lazell (1964) discussed B. c. nebulosus on Dominica, under the former genus 

Constrictor. He stated that boas are found throughout the island and to at least 350 m 

elevation, but that they are confined to wet ravines in the driest areas of the island. 

Ecuador–Apparently found throughout the lower elevations on both the Pacific coast and 

in Amazonia on the eastern slope of the Andes, including one possibly isolated 

subspecies in the Tumbes region (Price and Russo, 1991). Boas were considered 

uncommon at Santa Cecilia, Ecuador, at an elevation of 340 m (Duellman, 1978).

El Salvador–Historically, probably present through much of the country up to about 

1200 m. The fewer than 20 records in Köhler and others (2006) are from widely-
154

distributed lowlands (less than 600 m) and areas between 600 and 1200 m. Some of 

these records are higher than the limits proposed in earlier literature (for example, 650 m: 

Pope, 1961).

French Guiana–Documented by Chippaux (1986) in most of his field sites, although 

most of these were located along the coast [many of the localities given by Starace (1998) 

derived from Chippaux (1986)]. Probably found throughout the lower elevations.

Guatemala–Present along the Pacific coast and in the northern third of the country; 

possibly absent from much of Sierra Madre (Köhler, 2003). Stuart (1948) collected one

B. constrictor from Finca Canihor and 3 from Finca Chamá, Alta Verapaz, Guatemala. 

The species attains about 1200 m in the vicinity of Petén, Guatemala (Stuart, 1935; 

Campbell, 1998).

Guyana–Apparently found throughout the lower elevations, but poorly documented in 

the literature.

Honduras–Wilson and Meyer (1985) provided a dot-distribution map for Honduras, and 

stated that boas are found in Tropical Moist Forest, Tropical Dry Forest, and Subtropical 

Moist Forest at elevations to 800 m. McCranie and others (2006) extended the 

elevational range in Honduras to 1370 m and described the species as widespread 

throughout much of the mainland, as well as on the islands of Barbareta, Guanaja, 

Roatán, Utila, and Cayo Cochino Grande. Boas are present in the Cayos Cochinos (both 

Cayo Grande and Cayo Pequeño, referred to as Hog Island boas in the live animal trade: 

Lundberg, 2002; Reed and others, 2007), although Wilson and Cruz Díaz (1993) 

previously failed to find them on either island.

Mexico–Found in most of the tropical regions of the country, as well as north into 

portions of the Sonoran Desert and northeast into Tamaulipan scrub and on many islands 

(Oliver, 1937; Duellman, 1954; Martin, 1958; Pope, 1961; McGinnis and Moore, 1969; 

Lemos-Espinal and Ballinger, 1994; Lee, 1996; Walls, 1998a). In the northwest, 

localities include Hermosillo (Allen, 1933) and Guirocoba in Sonora (the latter at about 

453 m where they were described as common in valleys and rare in the hills), 79 km 

north of Hermosillo (Zweifel and Norris, 1955; Pope, 1961), and southeast of Alamos, 

Sonora, at around 250 m elevation (Myres and Eells, 1968), while the most northeasterly 

record is from Tamaulipas, Mexico, at 23 ̊23 ́25 ̋ N, 99 ̊30 ́30 ̋ W and 1040 m (Farr and 

others, 2007). Pope (1961) stated that B. constrictor reaches 792 m in Michoacán, 

Mexico.

Nicaragua–Apparently present throughout, possibly excepting a small portion of the 

north-central highlands (Gaige and others, 1937; Köhler, 2003). Holtzman and others 

(2007) studied B. constrictor on Ometepe Island, Nicaragua. A dwarfed form of B. 

constrictor is present on the Nicaraguan-owned Islas de Maíz, off the east coast of 

Central America (Barbour and Loveridge, 1929).
155

Panamá–Found throughout the lower elevations (Henderson and others, 1995; Köhler, 

2003).

Paraguay–In Paraguay, B. constrictor is present in most areas except the central 

wetlands (Waller and others, 1995); B. c. occidentalis is found in the west, and B. c.

amarali in the east, with little apparent intergradation even in the northeast in ecotones 

where the taxa abut one another (T. Waller, pers. commun., 2009). 

Peru–Found through much of the country except at very high elevations in the 

departments of Amazonas, Huánaco, Lambayeque, La Libertad, Loreto, Madre de Díos, 

Pasco, Piura, San Martín, and Ucayali (Lehr, 2002; Bartlett and Bartlett, 2003, T. Waller, 

pers. commun., 2009). Lehr (2002) personally observed specimens between 320 and 900 

m above sea level. B. c. longicauda was described from east of Tumbes, Tumbes 

Province, Peru (Price and Russo, 1991), and, if the taxon is valid, it is likely confined to 

Tumbes Province, which is surrounded to the north, east, and south by mountains rising 

to 3000 m.

Saint Lucia–B. c. orophias is apparently confined to the leeward and windward coasts in 

the northern two thirds of the island, including moderately dry lowlands and to at least 

350 m elevation. Boas are absent from southern and central highlands and from the south 

coast (Lazell, 1964).

Suriname–Apparently found throughout the lower elevations (Henderson and others, 

1995), but poorly documented in the literature and represented by only a few museum 

specimens, despite being highly valued in the live animal trade (“Suriname red-tailed 

boas” are prized). Individuals are regularly encountered in the interior (N. Hawley, pers. 

commun., 2008). 

Trinidad and Tobago–Found throughout the major islands (Mole and Urich, 1894; 

Boos, 2001), as well as smaller outliers such as Gaspar Grande and Monos Islands, 

southwest of Trinidad (Boos, 1984).

Venezuela–Probably present through much of the country, but Kornacker (1999) did not 

provide localities. La Marca and Soriano (2004) stated that B. constrictor reaches 1500 

m in the Andes of Venezuela. B. constrictor can be found on the arid Paraguana 

Peninsula (GHR, pers. obs., 1983), and on Isla de Margarita (Brongersma, 1940).

3.2 Habitat Range

As part of a short review of the species, Greene (1983) stated that, “Boas inhabit a 

remarkable range of environments from sea level to 1,000 m, including wet and dry 

tropical forest, savanna, very dry thorn scrub, and cultivated fields.” Henderson and 

others, (1995) reported that B. constrictor uses all available vegetative/edaphic zones 

present in S. America (16 categories in all, based on overlaying their range map on the 

vegetation map of Campbell and Lamar, 1989), and that the species also occupies more 

“morphoclimatic domains” than any other New World boine. They concluded that it is 

“...a habitat and diet generalist that has successfully colonized many islands and remains 
156

common in human-disturbed habitat,” and also reported that B. constrictor has been 

reported by fishermen in Belize to float in the Caribbean between small islands. Both 

Porras (1999) and Henderson and others (1995) provided lists of occupied islands in both 

Central and South America, including islands of less than 0.5 km2. 

In Argentina, B. c. occidentalis were studied in the “driest territory of Occidental 

Chaco. It is characterized by a xeric forest of Aspidosperma quebrachoblanco and 

Prosopis nigra and shrublands of Larrea divaricata and Mimozyganthus carinatus” 

(Bertona and Chiaraviglio, 2003). Di Cola and others (2008) described boa habitat in the 

Gran Chaco as the “largest dry forest in South America and its vegetation comprises a 

mosaic of xerophytic forests, woodlands, scrubs, savannas, and grasslands,” and T. 

Waller (pers. commun., 2009) confirmed that Argentine boas are habitat generalists 

within the Dry Chaco forests. 

In Venezuela, B. constrictor was described as inhabiting all low and warm areas 

of the country, up to 1500 m elevation (Lancini, 1986), while boas are present in lower 

montane dry evergreen forest, humid submontane forest, and semideciduous montane 

forest of the Venezuelan Andes (La Marca and Soriano, 2004). Stuart (1948) stated that 

“in Alta Verapaz (Guatemala) this species is apparently restricted to the lower Tropical

zone, where it is more frequently taken in second-growth than it is in the virgin forest.” 

In Manaus, Brazil, and outskirts, boas were often found in disturbed areas (Martins and 

Oliveira, 1998); these authors also summarized previous literature and concluded that the 

species is found in multiple habitats, is primarily terrestrial, and may be active by day or 

night.

In Sonora, Mexico, Myres and Eells (1968) found B. constrictor in deciduous 

thornscrub with rainfall near 508 mm per year. The rainy season at this site is in late 

summer (August–September), but winters are relatively dry. Martin (1958) stated that in 

southern Tamaulipas, Mexico, B. constrictor “...has not been found north of Tropical 

Deciduous Forest, its typical habitat in the Gómez Farías region.” However, Farr and 

others (2007) reported that B. constrictor inhabits Tamaulipan tropical thornscrub in the

Jaumave Valley to the north, where the thornscrub blends with Chihuahuan Desert. 

There were no differences in apparent boa abundance among the three main 

vegetative types on Cozumel, leading to the conclusion that these invasive boas are 

habitat generalists. Overall, more boas were seen in uninhabited areas than in inhabited 

areas on Cozumel (Romero-Nájera and others, 2007). Boos (1984) described the habitat 

of Gaspar Grande Island, apparently inhabited by B. constrictor, as extremely dry and 

scrub-covered. At the Deering Estate at Cutler (Miami-Dade County, Florida), invasive 

B. constrictor were found in multiple habitats, including tropical hardwood hammocks, 

dirt roads/trails, landscaped areas, and pine rocklands (Snow and others, 2007b), while 

invasive boas are established in sparsely vegetated arid habitats across Aruba (Quick and 

others, 2005; RNR pers. obs., 2008). 

3.3 Climate Range

The Boa Constrictor inhabits a wide range of climate spaces (Figure 7.3). While 

the mean monthly temperatures experienced by most South, Central, and North American 

boas generally range above 15o

C, the Argentine Boa (B. constrictor occidentalis) inhabits 

several localities with mean monthly winter temperatures well below 10 o

C (Di Cola and 

others, 2008). Multiple localities in various parts of the range exhibit a pronounced dry 
157

season, suggesting that boas do not require aseasonal conditions even in South American 

localities. The more recent results of Di Cola and others (2008) extend the climatic limits 

described by Henderson and others (1995), who stated that, “mean winter temperature of 

10oC appears to determine the northern and southern geographic limits for the species,” 

and that the species occurs in areas that receive fewer than 500 mm of rainfall annually 

(Peninsula de Paraguana, Venezuela), in areas receiving 100-200 mm of rain in evergreen 

shrubland of western Argentina, and in areas receiving less than 100 mm in western Peru. 

In contrast, a survey by T. Waller (pers. commun., 2009) recorded no Argentine boas in 

areas with less than 250 mm annual precipitation. 

Figure 7.3. Climate space for the Boa Constrictor (B. constrictor) with reference lines and 

localities as introduced in Figure 2.1, based on 131 known localities from the native range. Climate 

loops color-coded as follows: Yellow = Central America; Dark blue = Mexico; Turquoise = South 

America; Red = Argentina and all other B. c. occidentalis.

3.4 Microhabitat

Generalizations about the macrohabitat use of the Boa Constrictor are easy to 

come by in the available literature, but microhabitat descriptions are infrequent. This is 

partially due to the low sample sizes of boas in most mainland herpetological studies, 

such that microhabitat observations tend to be anecdotal and based on only a few 

individuals. These anecdotes are further complicated in that microhabitats are only 

recorded for snakes that are seen by humans, in places that may not typify microhabitats 

actually selected by boas. In contrast, radiotelemetric studies on several islands have 

yielded relatively more information on actual microhabitat use over longer periods of 

time.

Based on the observation that boas consume bird species that occupy all forest 

strata, from the ground to the canopy (Boback and others, 2000), it may be safe to assume 

that boas also inhabit these strata. On small mangrove offshore islands of Belize, 96 
158

percent of boas were found in trees and tree hollows, compared to 18 percent of mainland 

boas (Boback, 2006). On Ometepe Island, Nicaragua, most radiotelemetered snakes used 

overgrown fields and forest environments; resident snakes were more likely to use rocky 

outcrops, while displaced snakes were more likely to use trees/logs. With one exception, 

all locations in trees were less than 1 m above ground (Holtzman and others, 2007).

Juvenile B. c. orophias from Saint Lucia were often found in trees, once as high 

as 12 m (Lazell, 1964). B. c. nebulosus from Dominica (mostly small individuals) were 

also frequently found in trees, and boas occurred throughout the island to elevations of at 

least 350 m, but in arid areas their habitat appeared to be confined to wet ravines (Lazell, 

1964). Vandeventer (1992) found B. c. nebulosus on Dominica aggregated in tunnels or 

rock piles in the vicinity of hot springs and sulfur gas vents along volcanic mountain 

streams and stated that these dens hold snakes year round.

In the Cayos Cochinos, Honduras, B. constrictor captures resulting from visual 

surveys were highest in areas where snakes were most detectible, rather than where 

snakes spent most of their time. For example, radiotelemetered snakes tended to occupy 

microhabitats with higher leaf and branch density and smaller diameter trees than did the 

individuals detected by sight (Reed and others, 2007). 

Chiaraviglio and Bertona (2007) found that reproductive female B. c. occidentalis

in Argentina preferred sites with greater arboreal cover. Reproductive females used this 

heavy cover, but also basked more frequently than did nonreproductive females or 

reproductive males, and thus required thick vegetative cover interspersed with open 

areas. Overall, reproductive females maintained higher body temperatures than 

nonreproductive females or reproductive males, and they appeared to be better 

thermoregulators (Chiaraviglio, 2006). All B .c. occidentalis studied using 

radiotelemetry were located under heavy cover (logs, caves, etc.) over 50 percent of the 

time (Chiaraviglio and Bertona, 2007), and all reproductive individuals were associated 

with burrows.

Twelve neonate boas were captured in 1996 by children along the oolitic 

limestone walls of the C-100 canal adjacent to the known invasive population at the 

Deering Estate at Cutler, Miami, Florida, and B. constrictor were observed to use gopher 

tortoise burrows in pine rocklands (Snow and others, 2007b).

3.5 Introduced Ranges

The Boa Constrictor has established more introduced populations than any other 

boa or python species of which we are aware, with at least three known populations. 

There are also multiple records and anecdotes of individual boas found far from their 

native range all over the world, largely as a result of released or escaped pets or 

individuals transported by another means. As an example of the latter type of pathway, at 

least seven B. constrictor specimens currently held in museums in the United States were 

found among bananas shipped from Central America or Mexico (RNR, unpubl. data). 

There are clearly multiple means by which the species could be transported to an area 

outside its range, with risk of establishment varying with factors such as propagule 

pressure, climate match with native range, etc.

The invasive population on Cozumel, off the coast of the Yucatán Peninsula in the 

state of Quintana Roo, Mexico, was described by Romero-Nájera and others (2007) and 

Martínez-Morales and Cuarón (1999). This population apparently became established as 
159

a result of boas being discarded in 1971 by a film crew after their use in a movie being 

filmed on the island. The founding population may have consisted of two to six large 

individuals. Boas achieved high densities by 1989-1990 (Martínez-Morales and Cuarón, 

1999). Of the seven boas observed by Martínez-Morales and Cuarón (1999) during 

approximately 400 km of pedestrian transects, all but one were “...about 1.5 meters long, 

the exception being about 2.5 meters.”

Snow and others (2007b) reported on a population of B. constrictor on and around 

the Deering Estate at Cutler, a Miami-Dade County park property. Ninety-six individuals 

were captured between 1989 and 2005, including an individual found 2.1 km to the 

southwest of the park. Other individuals have been found in areas within 2.5 km of the 

park, and a large female was found dead after a recent prescribed burn in the park (S. 

Snow, pers. commun., 2008). However, most (around 70 percent) of the Deering snakes 

were found in 1996, when at least two females must have given birth in the park. The 

longest of 91 B. constrictor from the invasive population at the Deering Estate was 2.79 

m (2.51 m SVL), and 66 percent of individuals were young-of-year. Snow and others 

(2007b) also provided records of three individual B. constrictor found elsewhere in 

southern Florida and gave a list of other large constrictors that have been found in the 

Miami area.

A population of B. constrictor has rapidly colonized the entire island of Aruba in 

less than a decade (Quick and others, 2005). The first boa was discovered in 1999, with 

annual capture rates increasing from 36 in 2000 and 40 in 2001 to 74 in 2002 and 118 in 

2003 (Quick and others, 2005); population growth rates may have slowed since then (H. 

Reinert, pers. commun., 2008). Three scenarios have been proposed for how the boas 

arrived on Aruba: (a) natural dispersal from the mainland (the arid Paraguana Peninsula 

is only 17 km away); (b) stowing away in ornamental plants from the mainland; or (c) as 

a result of released/escaped pets (Quick and others, 2005). These boas tend to exhibit the 

small body size of island boas but exhibit head morphology and color pattern more 

typical of mainland boas (RNR, pers. obs., 2008). Further examination of the 

morphology and/or genetics of island and nearby mainland populations might be able to 

distinguish among these hypotheses to some extent, as boas from captivity might not 

exhibit traits from the nearby mainland, instead representing a more distant or inland 

source population.

3.6 Seasonal Activity

In Córdoba Province, Argentina, at the southern end of the species’ range, 

average annual temperatures vary from 18-23 o

C and rainfall varies from 300-550 mm 

with “distinct dry (April to September) and wet (October to March) seasons” (Capitanelli,

1979: cited by Bertona and Chiaraviglio, 2003). Maximum summer temperatures exceed 

40 o

C, and “...frosts occur commonly from May through early September” (Chiaraviglio 

and Bertona, 2007). Chiaraviglio and others (2003) found 125 of 153 B. constrictor

during the dry season (local winter), and 56 percent of individuals captured in the dry 

season were found between 1300 and 1500 hr. Wet season air temperature during 

captures was 28 ± 5 oC, dry season was 22 ± 4 oC. Overall mean annual temperature was 

19.5 oC, and annual activity appears to be primarily driven by temperature rather than by 

precipitation (T. Waller, pers. commun., 2009).
160

There were no differences in apparent activity of invasive boas by month or 

season on Cozumel (Romero-Nájera and others, 2007) as determined from road-cruising, 

but this conclusion was based on a small sample size. These authors found most boas at 

night while road-cruising, but they “did not systematically survey during daytime,” and 

noted that “we know that boas are also diurnally active on Cozumel.”

On Saint Lucia, B. c. orophias was considered extremely sedentary, and “...a 

specimen seen a week, two weeks, or even a month previously is usually in the same 

vicinity” (Lazell, 1964). Some boas were rumored to use the same den site for multiple 

years, but young boas were less predictable. Similarly, B. c. nebulosus from Dominica 

were considered “basically lethargic and prone to remaining in the same place for long 

periods of time” (Lazell, 1964).

In a series of short-term experiments, McGinnis and Moore (1969) used captive 

snakes, mesocosm trials, and radiotelemetry (n = 1) to examine thermoregulation in B. 

constrictor from Nayarit, Mexico. Juveniles in a mesocosm could maintain body 

temperature (Tb) as much as 7 oC below shaded ground temperature by moving to 

subterranean retreats, while conversely these retreats allowed Tb to remain above ambient 

levels during cool rainstorms. No basking was observed, although basking was inferred 

via radiotelemetry in a free-ranging 2.3 m B. constrictor which experienced a 15 oC rise 

in Tb over a 2.5 hour period. This snake experienced a peak voluntary Tb of 38.5 oC, but 

mean Tb was 26.4 oC. 

In Sonora, Mexico, Myres and Eells (1968) found an aggregation of 5 boas (4 

males and 1 larger female) in the foothills of the Sierra Madre Occidental, with body 

temperatures of 25.8-30.4 o

C at the time of capture. Tb was higher than substrate or air 

temperatures. Temperatures as low as 11 oC were recorded in this area during 17-20 

December 1965.

Brattstrom (1965) reported that the Tb of a basking boa on Barro Colorado Island, 

Panamá, was 34 oC (air: 27.0, log: 30.8) at 1030 hrs. A larger B. constrictor emerging 

from a stream had a Tb of 26.0 o

C (water: 25.5, air: 26.0). Boas in the Cayos Cochinos, 

Honduras, were captured at ambient temperatures between 24.5 and 36.2 oC, with most 

captures occurring between 29 and 30.9 o

C. Body temperatures of 7 adult females 

averaged 29.3 ± 1.8 oC (range 24.4-34.9), and body temperatures averaged 1.3 oC below 

ambient temperatures (Reed and others, 2007). Beebe (1946) found most of his B. 

constrictor at night on trails or low branches, and 88 percent of captures were “at the 

height of the long rainy season, May through July.”

Snow and others (2007b) suggested that the invasive population at the Deering 

Estate at Cutler may be limited by climate, and that reproduction may be successful only 

during years with especially warm winters, such as occurred in 1996; they support this 

idea by saying that the boas appear to be of northern South American stock and thus 

unlikely to be adapted to cooler temperatures.

Rivera and others (2005) stated that high dispersal capability is postulated for B.

c. occidentalis based on measures of gene flow between populations and sexes. They 

found low levels of protein polymorphism between two populations separated by 200 km 

in Córdoba, Argentina, but no evidence of inbreeding.
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3.7 Daily Activity

Daily activity is best studied by radiotelemetry because opportunistic observations 

of boas are typically biased by human activity cycles—this can result in artificially high 

numbers of observations of diurnal activity. Despite this potential bias, numerous records 

from the native range indicate that boas can be found active during virtually any time of 

day. However, records of daily activity cycles for B. constrictor are few, and once again 

the majority of studies are from islands or from a well-studied population in Argentina.

A single 2.4-m, 11.3-kg B. constrictor on Barro Colorado Island, Panamá, moved 

a total of 135 m over 12 days (Montgomery and Rand, 1978), moving from burrow to

burrow. Tracking at hourly intervals showed that the snake did not make intradiel 

excursions from burrows and then return. Boback (2006) found most mainland boas at 

night, either crossing roads after rains or moving in garbage dumps.

Holtzman and others (2007) radiotracked 6 translocated and 14 untranslocated 

snakes on Ometepe Island, Nicaragua. Mean distance per move was 241 m for displaced 

snakes and 73 m for resident snakes, and all displaced snakes moved directionally back 

towards their capture sites. Overall, translocated snakes moved 1872 ± 388 m after 

relocation, but resident snakes moved only 887 ± 322 m (both groups’ dispersal 

quantified from release point to position 5-10 months later).

In the Cayos Cochinos, Honduras, during the dry summer season, a small number 

(n = 4) of boas implanted with radiotransmitters moved on 23 percent of days they were 

located. Movements averaged 13.7 m from the previous location (range 1.6–59.1 m), and 

the total distance moved over the course of the approximately five week study ranged 

from 18.5 m to 208 m (Watson, 2004).

In Argentina, during the breeding season, radiotelemetry revealed that over 6-8 

weeks (4-22 fixes) reproductive female B. c. occidentalis moved a total of 41 ± 19 m 

(range 0-97, n = 5), while non-reproductive females moved 628 ± 238 m (range 0-1598, n 

= 6), and reproductive males moved 388 ± 172 m (range 0-1362, n = 8; Chiaraviglio and 

Bertona, 2007). 

3.8 Foraging Mode/Sensory Modalities

The Boa Constrictor is typically described as an ambush predator, using a sit-andwait strategy to capture prey. However, there is considerable evidence that boas spend 

significant time searching for optimal ambush sites, such as burrows used by prey 

species, logs or other runways used by prey, or waterholes that might be visited by prey. 

For example, Montgomery and Rand (1978) reported a sit-and-wait hunting strategy in a 

radiotelemetered boa on Barro Colorado Island, Panamá. The snake used four different 

mammal burrows over 12 days and was seen once about 5 m from the mouth of the 

burrow, facing out. A boa was sighted regularly in a Cecropia tree for one month before 

it was seen to capture a blue-grey tanager (Greene, 1983), and a snake coiled at the 

entrance to a tree cavity captured a bat that was clambering up the inner wall of the cavity 

(Arendt and Anthony, 1986). Juvenile B. constrictor in captivity have been reported to 

engage in caudal luring behavior when a live mouse was introduced to their cage 

(Radcliffe and others, 1980), but the frequency with which this tactic is used in the wild 

remains unknown.

B. constrictor has 3 visual pigments, with peak absorbances at 495 ± 2 nm (rod), 

549 ± 1 nm (cone), and 357 ± 2 nm (small rare cone); the latter has sensitivity in the 
162

ultraviolet (Sillman and others, 2001). These authors suggest that ultraviolet vision may 

improve hunting efficiency, and that, “...on the basis of cone packing density, it is 

reasonable to conclude that the photopic visual resolution of B. constrictor is similar to 

that of the cat.” Ultraviolet vision might allow snakes to see scent trails left by prey and 

would increase the contrast of lizards and birds with ultraviolet reflectance (Sillman and 

others, 2001).

B. constrictor also has dense arrays of infrared-sensitive neurons beneath the 

surface of labial scales (Grace and Matsushita, 2007). Bullock and Barrett (1968, cited 

by Goris and others, 2007) found that infrared stimuli produce responses in branches of 

the trigeminal nerve; Goris and others (2007) reviewed other evidence for infrared 

reception in B. constrictor. However, de Cock Buning (1983) opined that Boa 

Constrictors could likely detect a mouse at only 50-100 mm using thermal cues only, and 

hypothesized that substrate-borne vibrations from prey are more important in detecting 

prey in the absence of light.

Secor and Ott (2007) found that mass of the small intestine approximately 

doubled in B. constrictor between 24 and 48 hrs post-feeding of a meal approximately 25 

percent of snake mass, and nutrient uptake rates went up several-fold. These trends are 

similar to those observed in pythons, and support the notion of a generalized intestinal 

response among large-bodied, infrequently-feeding snakes.

In the Cayos Cochinos, Honduras, large females are apparently highly susceptible 

to starvation; it is possible that they require at least one large ctenosaur or iguana per year 

to meet reproductive and maintenance requirements (Reed and others, 2007).

4.0 Life History

4.1 Reproductive Mode

Boa Constrictors are live-bearing. The extent of post-parturient parental care in 

the wild is unknown, although Köhler (2003) described finding a spent female in a log, 

associated with neonates. Similarly, a female on Crawl Cay, Belize, was found in 

association with seven neonates (S.M. Boback, pers. commun. 2008).

4.2 Reproductive Phenology

4.2.1 Seasonality

There are few records of observed mating or parturition in the native range of B. 

constrictor, possibly because females appear to seek refugia during latter stages of 

ovulation and pregnancy and are thus largely unavailable for observation or capture. 

Much of the available information on reproduction is thus based on captive animals, 

which may experience highly artificial conditions. 

Because Boa Constrictors experience such a dizzying range of latitudes, habitats, 

and climatic regimes across their native distribution, it is fruitless to attempt to 

pigeonhole the species into generalities such as “winter breeder, summer parturition.” 

Overall, boas at higher latitudes tend to breed in the cool season and give birth in the 

warm season, but seasonality in precipitation may be more important in low-latitude 

areas (Table 7.3). The primary message is that reproductive seasonality is incredibly 
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variable across the range of Boa constrictor. Furthermore, extended gestational durations 

mean that an entire reproductive cycle may take at least a year for females, considering 

follicular growth, mating, ovulation, gestation, and parturition. 

Copulation is rarely observed in the field, and it is difficult to tell whether groups 

of boas constitute mating aggregations. For example, Myres and Eells (1968) found an 

aggregation of boas (four male, one larger female) in the foothills of the Sierra Madre 

Occidental, Sonora, Mexico on December 18, 1965. They inferred that the aggregation 

was for thermoregulatory purposes, but we suggest that a mating aggregation cannot be 

ruled out. The absence of data from the field leaves us to be somewhat reliant on data 

from captive individuals and museum specimens, but neither of these is without its 

drawbacks. Ronne (1996) gave an overview of reproduction in captivity, including data 

on the number of days between post-ovulatory shed and parturition (104-111, n = 6), a 

table of gestation periods for Colombian boas, and the observation that parturition 

seemed to occur on days when the barometer drops appreciably. 

The best data on reproductive seasonality come from the Argentine boa, which 

inhabits the coolest climate occupied by the species. Reproduction in Argentina is 

strongly seasonal—reproductive females were found in the dry season (April-August, 

winter), with large ovarian follicles (Bertona and Chiaraviglio, 2003). Reproductive 

males were found in March to August, with low testicular volume during March-May 

compared to June-August. However, only 50 percent (19 of 38) of adult females and 64

percent (29 of 45) of adult males were reproductive – the overall operational sex ratio 

was thus 1.53:1. While reproductive females were in better body condition than nonreproductive females, no similar effect was noted in males (Bertona and Chiaraviglio,

2003), although males with good body condition had larger testes (Cardozo and 

Chiaraviglio, 2008). Overall, reproduction occurs in winter and is synchronized, 

resulting in mid-spring ovulation. Although based on a small number of neonates, 

parturition is inferred to be at the end of the wet season (Feb-March: Monguillot, 1988; 

Bertona and Chiaraviglio, 2003); Waller (pers. commun., 2009) extends this period 

through April.
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Table 7.3. Recorded and inferred seasonality of reproductive activity in Boa constrictor from various parts of the range. Mating and parturition 

seasons marked with an * are inferred using an estimated average gestational period of 110 days. 

Location Latitude Mating Parturition Source Notes

Miami, FL 25°N Apr-May* Aug-Sep Snow et al 2007b Many infertile ova/embryos

Yucatán, Mexico ~18-21°N Mar-Apr* July-Aug Dundee and others 1986; Lee 1996

Guatemala ~13-17°N Jan-Apr* May-Aug Smith 1994; Campbell 1998

Cayos Cochinos, Honduras 15°N Apr* Aug S. Green, pers. commun. 2007 1 litter

Dominica 15°N Mar-Apr Jul-Aug Vandeventer 1992 Wild-caught females

Trinidad 10°N Feb-Mar Jun-Jul* Mole and Urich 1894

Iquitos, Peru 3.5°S July-Oct* Nov-Feb Dixon and Soini 1986

São Paulo, Brazil 23°S Apr-Jul Aug-Nov* Pizzatto and Marques 2007

Brazil ~5°N-18°S Jun-Feb Sep-Apr Pizzatto and Marques 2007 B. c. constrictor

Brazil (SE) ~14-29°S Mar-Oct Aug-Jan Pizzatto and Marques 2007 B. c. amarali

Argentina ~21-32°S Jun-Oct Feb-Apr

Monguillot 1988; Bertona and 

Chiaraviglio 2003, Waller, pers. 

commun. 2009

B. c. occidentalis. Some 

disagreement over timing b/w 

Chiaraviglio and Waller
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4.2.2 Reproductive Behaviors

Mole and Urich (1894) were apparently the first to note that males used spurs 

during courtship, and stated that “the claws scratching the scales of his mate make a noise 

which can be distinctly heard two yards off.” Chemoreception of conspecific cues is 

likely important during identification of potential mates. Chiaraviglio and Briguera 

(2001) showed that male B. c. occidentalis respond much more strongly (higher rate of 

tongue-flicking) to female skin and glandular chemical cues than to male cues, while 

females respond more strongly to cues from other females.

In Argentina (Bertona and Chiaraviglio, 2003), aggregations of one adult female 

and one to three adult males (presumed to be mating-related, with the first aggregations 

found in April and the last in August) accounted for a majority (43 of 84) of boas 

captured during the dry season. Of these, 33 were reproductively mature as indicated by 

ultrasonography. Most (16 of 19) reproductive females were found in aggregations. No 

male combat or copulations were observed. These authors concluded that reproductive 

activity in B. c. occidentalis is seasonal and associated (following terminology of 

Whittier and Crews, 1987), and that females reproduce less than annually. Additional 

potential mating aggregations have been observed on St. Lucia, consisting of one large 

female and five smaller males (Malhotra and Thorpe, 1999) and Sonora, Mexico (Myres 

and Eells, 1968).

Sillman and others (2001) suggests that B. constrictor might be able to use its 

vision in the ultraviolet spectrum to locate lipid trails left by conspecifics.

4.2.3 Vitellogenesis and Ovulation

Pizzatto and Marques (2007) examined 442 preserved B. constrictor from Brazil, 

including adults and juveniles. Although their text indicated that vitellogenesis occurs 

from summer (Jan-Feb) to winter (Jul-Sep) in both B. c. constrictor and B. c. amarali, an 

accompanying graph depicts significantly enlarged follicles during Jun-Sept (both with 

very small sample sizes). They observed a female reproductive frequency of 13.6 percent

of B. c. amarali and 16.7 percent in B. c. constrictor; such a low proportion of 

reproductive females is probably due to decreased detectibility of gestating females 

combined with the generally low frequency with which very large snakes are accessioned 

into museum collections. Among males, testes volume and deferent duct diameter were 

constant in B. c. amarali, but in B. c. constrictor testes appeared to be larger in Jan-Feb 

(summer) and duct diameter was larger in Apr-May (autumn). They concluded that 

female and male gametogeneses are asynchronous in B. c. constrictor, with mating 

season occurring after sperm production and possible short-term, over-winter sperm 

storage by males. As discussed in section 4.2.1, however, generating overall conclusions 

from such a vast area as Brazil is likely to swamp out regional differences in the seasonal 

timing of reproductive events, possibly giving the impression of extended reproductive 

periods even though such periods may be markedly shorter in any one location.

Vitellogenic follicles of captive Argentine females were 10-28 mm in diameter, 

while post-ovulatory follicles were over 28 mm and aligned in a row. Gravid females 

were not found in the field, probably because they stayed in Plains Viscacha burrows 

(Bertona and Chiaraviglio, 2003).
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4.3 Reproductive Effort/Fecundity

4.3.1 Clutch or Litter Size/Frequency

In a very general review of B. constrictor, Greene (1983) gave litter size as 20-64, 

with neonate size of approximately 0.50 m; these figures are generally concordant with 

those provided by other authors. In Mexico and Central America, litter sizes reported by 

Fitch (1985) averaged 17.8 ± 3.4 (range 10-36, n = 6: Fitch 1985); these numbers are 

corroborated for a record from Guatemala (Stuart, 1948: 11 eggs in a female) and 

mainland Belize (Boback, 2005; Boback and Carpenter, 2007: 11 litters averaged 30 ± 

18, with relative clutch mass of 17 percent of female mass). However, other reports 

exceed these numbers. For example, litter sizes are 20-50 in Chiapas, Mexico (Alvarez 

del Toro, 1960), and a female at La Selva, Costa Rica, gave birth to 52 young (Guyer and 

Donnelly, 2005). Insular boas in Central America tend to have reduced litter sizes. A 

female on Utila, in the Bay Islands of Honduras, was associated with 13 neonates with 

umbilical remnants, and insular females from Belize had litter sizes of 4.6 ± 1.9, with 

mean relative clutch mass of 15 ± 4 percent (Boback, 2005; Boback and Carpenter,

2007).

Litter sizes in South America are generally larger, as would be expected for the 

larger maximum sizes attained by South American females. Boas from Trinidad and 

Peru had litters averaging 30.3 ± 7.3 (range 6-63: Fitch, 1985). The maximum litter size 

of 63 appears to be derived from Mole (1924) for a snake from Trinidad. B. c. 

constrictor from Brazil had mean litter size of 27 ± 8.15 (range 18-41), while B. c. 

amarali from Brazil had mean litter sizes of 14.5 ± 6.9 (range 5-30: range listed by 

Pizzatto and Marques (2007) as 5-10 but cleared up by first author in a pers. commun. in 

2008). In the Iquitos Region, Peru, four adult females gave birth to 6-28 young (Dixon 

and Soini, 1986), and wild-caught B .c. nebulosa from Dominica gave birth to litters of 7-

11 after a few months in captivity (Vandeventer, 1992). Four dissected boas (of 

putatively South American origin) from the invasive population at the Deering Estate at 

Cutler had litter sizes between 24 and 47, most of which were infertile ova or slugs 

(Snow and others, 2007b). Litter sizes in Argentina averaged 25.05 ± 1.25 (range 11-33), 

and litter size was positively correlated with dam size (Bertona and Chiaraviglio, 2003; 

Chiaraviglio and others, 2003: dam sizes in the latter ranged from 1.92 to 2.7 m [1.73-

2.43 m SVL]).

Estimates of female reproductive frequency in free-ranging populations are rare. 

In Argentina, only 36 percent of females 1.82-2.28 m (1.64-2.05 m SVL) were 

reproductive, while 69 percent of females over 2.28 m (2.05 m SVL) were reproductive 

(Cardozo and Chiaraviglio, 2008). This implies variation in reproductive frequency 

among female size classes, even though the overall reproductive frequency might be 

close to 0.5. However, these observations are complicated by the observation that both 

body condition and clutch size of females decreased during the period 1995-2003, 

presumably as a result of deteriorating habitat conditions. The hypothesis of biennial 

reproduction among females in Argentina is supported by T. Waller (pers. commun., 

2009), who found 15 of 28 females with secondary follicles.
167

4.4 Growth

Growth rates of wild B. constrictor are almost entirely undocumented. Growth 

rates have been reported in the herpetocultural literature, but these rates typically involve 

sedentary snakes that are often fed large amounts of food, and are thus likely to be 

extremely poor indicators of growth in the wild. In a captive colony, growth of males 

and females slowed appreciably after three and five years of age, respectively (Smith, 

1999); based on Smith’s (1999) graphs and tables and assuming neonate length of 0.55 

m, male growth averaged about 45 mm/month for the first 24 months after birth, while 

female growth averaged about 40 mm/month for the first 40 months. In both sexes, 

growth in length slowed appreciably after these ages, but weight continued to increase for 

a longer period of time. Males reached their near-asymptotic weights at about 36 months 

(inferred average monthly weight gain of around 280 g/month over this period), but 

female continued a high rate of weight gain for over 48 months (inferred average monthly 

weight gain of about 380 g/month). 

Schuett and others (2005) found no effect of prey size on growth of the skull in 

captive B. constrictor, but found that females grew faster in terms of snout-vent and total 

length, while males grew faster in tail length. In captivity, mainland snakes grew faster 

than island snakes, and females grew faster than males in both SVL and mass, but there 

were no effects of prey diameter on growth of skull elements (Boback and Carpenter, 

2007).

4.5 Maturation

In a general review, Greene (1983) stated that, “Sexual maturity evidently occurs 

at a length of 1.5-2 m,” and this range appears to be a workable rule of thumb for most 

mainland populations of B. constrictor. However, males of some insular populations 

mature at considerably smaller sizes, and in some populations may fail to exceed 1.2 m 

even as adults (for example, Reed and others, 2007).

Pizzatto and Marques (2007) reported body size at maturity of Brazilian boas to 

be 0.74 (B. c. amarali) and 0.61 (B. c. constrictor) of mean adult male sizes, which were 

1.418 and 1.903 m respectively. Similar ratios for females were 0.85 and 0.70 of mean 

adult size (1.501 and 2.030 m). In the Córdoba Province of Argentina, males mature at 

1.73 m (1.49 m SVL), females at 1.82 m (1.64 m SVL: Chiaraviglio and others, 2003). 

Male B. c. longicauda from Tumbes Province, Peru, mature at 1.50 m in captivity (Price 

and Russo, 1991).

4.6 Longevity

We are aware of no longevity records from wild populations. Greene (1983) 

stated that longevity in captivity can be 38 years, 10 months. Snider and Bowler (1992) 

extended this record to 40 years, 4 months for a wild-caught adult B. c. constrictor and 

gave known longevity records for B. c. imperator (29 yrs), B. c. occidentalis (14 yrs), B. 

c. ortonii (18 yrs), and B. c. amarali (15 yrs); of these, all but the B. c. imperator were 

wild-caught adults. 
168

5.0 Diet/Trophic Role

5.1 Prey

The Boa Constrictor has a catholic diet, consuming prey from a wide variety of 

higher vertebrate taxa. Henderson and others (1995) reported that B. constrictor eats 

“fishes, lizards (iguanids, teiids), birds, [and] mammals (including Marsupialia, 

Chiroptera, Edentata, Lagomorpha, Carnivora, Artiodactyla)”, and Boback (2005) stated 

that 54 diet items were reported in the literature for mainland B. constrictor, from Greene 

(1983), Smith (1994), and Sironi and others (2000). Greene (1983) summarized prey 

known for the species up to that time, including ctenosaurs (Ctenosaura), Green Iguanas 

(Iguana iguana), opossums (Didelphis), Neotropical fruit bats (Artibeus), vampire bats 

(Desmodus), Eurasian rats (Rattus), agouti (Dasyprocta), prehensile-tailed porcupine 

(Coendou), rabbits (Sylvilagus), deer (Odocoileus/Mazama), coatis (Nasua), Ocelot 

(Leopardus pardalis), Small Asian Mongoose (Herpestes javanicus), and tamandua 

(Tamandua). 

Sironi and others (2000) reported on diet of 28 boas in the Chaco region of 

Córdoba Province, Argentina. Diet records were supplemented with interviews with 

rural inhabitants. Results included 11 species of mammals, 7 spp. of birds, and 1 reptile; 

Plains Viscacha (Lagostomus maximus) were an important prey item for boas of 2.35 m 

TL or longer, and their burrows provided refuge to snakes. Boas were used as viscacha 

control agents by farmers. Invasive boas were considered generalist predators on 

Cozumel (Gonzalez-Baca and Cuaron, 2003). Beebe (1946) found the following in boas

from Guyana and Venezuela: two large jungle whiptail lizards (Ameiva) and an Ameiva

tail, one small whiptail lizard (Cnemidophorus), a large antbird (Thamnophilidae), and 

two spiny rats (Echimyidae). 

Martins and Oliveira (1998) stated that no diet data are available for the Manaus 

region, but they summarized diet based on the literature and stated that a boa killed a 

South American Squirrel Monkey (Saimiri sciureus) and another constricted a Ninebanded Armadillo (Dasypus novemcinctus). In the Cayos Cochinos, Honduras, prey 

items of dwarfed insular boas included grackles (Quiscalus), Green Iguanas (Iguana 

iguana), and Black-chested Ctenosaurs (Ctenosaura melanosterna: Reed and others, 

2007); these authors posited that the historically high-density population of B. constrictor

was responsible for the absence of small mammals on these islands. Of 65 boas 

examined from the recently-established population on Aruba, 47 (72.3 percent) contained 

prey, consisting of birds (40.4 percent), lizards (34.6 percent), and mammals (25 percent: 

Quick and others, 2005). Some assertions on the diet of boas are of a dubious nature; for 

example, Goulding (1990) stated that boas eat fish and shrimp, based solely on the 

observation that boas are sometimes found in fish traps placed in flooded forests in the 

Amazon Basin. 

Additional diet records for higher vertebrate taxa follow:

5.1.1 Mammals

Borland (1855) described ingestion of a rabbit by a captive boa that “measured 

about 20 feet in length;” these observations include refutation of the myth that snakes lick 
169

their prey to coat it with saliva prior to ingestion. Bakkegard and Timm (2001) reviewed 

the rodent prey recorded for B. constrictor (Black rats [Rattus rattus], agoutis 

[Dasyproctus], prehensile-tailed porcupine [Coendou rothschildi]) and reported a Bigeared Climbing Rat (Ototylomys phyllotis) in a 0.96 m boa (0.84 m SVL, 450 g without 

prey) from Costa Rica. They also reported observing a Costa Rican boa consuming a 

Virginia Opossum (Didelphis virginianus). Records from El Salvador include another 

Big-eared Climbing Rat and a Deppe’s Squirrel (Sciurus deppei: Leenders and WatkinsColwell, 2003). Lazell (1964) considered B. constrictor to eat almost exclusively rats on 

Dominica, but stated that it probably takes agoutis as well. Davis and Smith (1953) 

reported on a large Rattus rattus taken from a specimen in Morelos, Mexico. 

Mole and Urich (1894) reported that Ocelots and young Wood-Brockets 

(Cariacus nemorivagus, a small deer) had been taken from boas on Trinidad, but that 

“their droppings contain evidences of the fact that they feed largely on agouti.” A boa 

imported from Panamá had porcupine quills imbedded in its mouth, and the snake later 

passed feces containing claws, quills and fur from a juvenile prehensile-tailed porcupine 

(Coendou rothschildi: species name misspelled in original paper: Tschambers, 1949). A 

snake in Brazil apparently died as a result of ingesting a tree porcupine of unknown 

species (Cherubini and others, 2003). A boa from the invasive population near Miami, 

Florida had eaten a Virginia Opossum (Didelphis virginianus), and captured adults 

accepted dead Raccoons (Procyon lotor: Snow and others, 2007b). In northwest Costa 

Rica, a boa killed but did not consume an adult White-Nosed Coati (Nasua narica: 

Janzen, 1970). A boa at La Selva, Costa Rica, consumed a Three-Toed Sloth (Bradypus 

variegatus: S.M. Boback, pers. commun., 2009). Mid-sized mammals are reported as the 

primary prey in Venezuela (Lancini, 1986), but this author pointed out that deer are not 

eaten. 

Arendt and Anthony (1986) reported on B. c. orophias (397 g, 1.1 m TL, head 

width 53 mm) eating an Antillean Fruit-eating Bat (Brachyphyllum cavernarum: 67 g) on 

St. Lucia. Bat remains were found in fecal samples from the same boa. A boa in the 

Cayos Cochinos, Honduras was observed falling from a palm tree while simultaneously 

constricting four phyllostomatid bats (possibly Artibeus phaeotis); the snake 

subsequently consumed two of them (RNR, pers. obs., 2007 G. Sorrell, unpub. data). A 

boa in the state of Mato Grosso, Brazil, inserted its head into a cavity in a Ficus tree and 

emerged with a Lesser Bulldog Bat (Noctilio albiventris) in its mouth (Esbérard and 

Vrcibradic, 2007); these authors also summarized previous bat prey known for B. 

constrictor, including Common Vampire Bats (Desmodus rotundus) and Jamaican Fruit 

Bat (Artibeus jamaicensis).

Tello and others (2002) reported attempted predation by a 1.5 m B. constrictor on 

a subadult male Moustached Tamarin (Saguinus mystax). Ferrari and others (2004) 

reported a B. constrictor (over 3 m long, approximately 8 m up in tree) killing a 2-3 kg 

female Bearded Saki (Chiropotes satanas) in Brazil, while Chapman (1986) reported on a 

2-m B. constrictor killing a 1.7-kg juvenile White-faced Capuchin (Cebus capucinus) in 

northwestern Costa Rica. Also in Costa Rica, Perry and others (2003) reported another 

attack by a 2-m B. constrictor attacking a 3-year-old C. capucinus, but the snake was 

driven off after being mobbed and bitten by the rest of the capuchin troupe. 
170

5.1.2 Birds

Boback and others (2000) reviewed passerine birds taken from B. constrictor, and 

added to this list the Bright-rumped Attila (Attila spadiceus: 39 g) from a 0.87-m (0.76 m 

SVL), 240-g (without prey) boa at La Selva, Costa Rica. On Cozumel, an invasive B. 

constrictor ate a Caribbean Dove (Leptotila jamaicensis: Martínez-Morales and Cuarón, 

1999), and a 1.50-m (1.32 m SVL), 1.1-kg B. constrictor constricted an adult Great-tailed 

Grackle (Quiscalus mexicanus) in Belize (Greene and others, 2003). Greene (1983) 

reported a Blue-grey Tanager (Thraupis episcopus) in the diet of a boa on Perico Island, 

Panamá, and boas at La Selva, Costa Rica, consumed an Orange-billed Sparrow 

(Arremon aurantiirostris: Guyer and Donnelly, 2005) and a Wood Thrush (Hylocichla 

mustelina: Wasko and Roberts, 2008). Boback (2005) found prey in 16 of 88 (18 

percent) of boas from small mangrove cays off the coast of Belize; 18 of the 19 prey 

items were Grey-breasted Martins (Progne chalybea). Mean prey mass was 7.3 percent 

of mean snake mass. A 6.8-kg B. constrictor in Belize contained feathers and bones of 

an adult Black Vulture (Coragyps atratus: Boback, 2004), and Argentine boas have been 

observed to climb 3-4 m into trees to consume juvenile Blue-fronted Amazon parrots 

(Amazona aestivus; T. Waller, pers. commun., 2009).

5.1.3 Reptiles

Lizards constitute the majority of the reptilian prey recorded for B. constrictor, 

especially large iguanid and teiid lizards. However, this may be due to the fact that 

boa/iguana interactions are likely to involve large individuals of both species and thus 

may be more apparent to human observers (RNR, pers. obs., 2005). Small lizards can 

also be difficult to feel when palpating boas for prey, so they may be overlooked as prey 

items. 

A 2.44-m, 6.85-kg B. constrictor from the mainland of Belize contained a 2.2-kg 

Green Iguana (I. iguana: Boback, 2004). Gutsche (2005) observed a 1.4-m boa fall from 

a tree while constricting a 0.192-m SVL, 235-g Utila Ctenosaur (Ctenosaura bakeri) on 

Utila, Honduras. Similarly, a 1.48-m (1.33 m SVL), 1267-g female boa fell from a tree 

while constricting a 0.316 m SVL, 996-g Black-chested Ctenosaur (Ctenosaura 

melanosterna) on Cayo Cochino Pequeño, Honduras; the snake subdued but did not kill 

the lizard, and two more C. melanosterna were reported in the diet of boas from the 

Cayos Cochinos (Reed and others, 2006). In Mexico, a 1.6 m boa killed a Mexican 

Ctenosaur (Ctenosaura pectinata: 0.34 m SVL, 1.03 m TL: Lemos-Espinal and Ballinger, 

1994). Bakkegard and Timm (2001) reported a Costa Rica boa that ate an adult Black 

Spiny-tailed Ctenosaur (Ctenosaura similis). Further south, Argentine boas regularly

consume Red Tegus (Tupinambis rufescens; T. Waller, pers. commun., 2009).

5.2 Predators

Due to their large body size, adult boas from the mainland are likely to have few 

predators, and records of predation on boas are rare. Likely predators of adults include 

Jaguars (Panthera onca), Pumas (Felis [Puma] concolor), caimans (Caiman) and 

crocodiles (Crocodylus), the largest raptors, feral hogs, and perhaps peccaries. Juveniles 

likely fall victim to a range of mesopredators, raptors, and ophiophagous snakes, but 
171

observations of such attacks are few. Insular boas typically exist on islands with low 

mammalian biodiversity, and predation rates on these snakes are likely even lower.

Savage (2002) stated that large birds, mammals, and crocodilians are the primary 

predators on boas. Gerhardt and others (1993) observed six boas among prey fed to 

nestling Giant Black Hawks (Buteogallus urubitinga) in Guatemala. Beebe (1946) 

reported on a Mussurana (Clelia clelia) attacking a small boa in South America. 

Campbell (1998) reported that a 2.95-m Black-tailed Cribo (Drymarchon melanurus) ate 

a 1.68-m B. constrictor, and Duellman (1963) reported on the same species eating B. 

constrictor in Guatemala. 

5.3 Diseases and Parasites

Pathogens of B. constrictor are largely known from captive individuals in the pet 

trade, but most of these are not specific to boas, or even to boas/pythons in general. For 

example, Russo (2007) has an entire chapter on parasites and pathogens of Boa 

Constrictors, but all are largely associated with captivity, and to our knowledge few of 

these ailments have ever been recorded in the wild for any snake. An apparently snakespecific parvovirus was isolated from a captive Boa Constrictor in Europe (Farkas and 

others, 2004). Lia and others (1999) described a pentastomid, Porocephalus clavatus, in 

a captive B. constrictor imported from Colombia to Italy, and heavy pentastomid loads 

are often observed in the lungs of captive B. c. occidentalis in Argentina (T. Waller, pers. 

commun., 2009). Wild-caught B. c. nebulosa from Dominica were found to harbor ticks, 

tapeworms, roundworms, protozoans, and pentastomes (Vandeventer, 1992), and both 

Central American (Ditmars 1931) and South American (M. Chiaraviglio, pers. commun., 

2008) boas are often heavily infested with ticks. Oda and others (1971) successfully 

transferred hemogregarine parasites from an infected B. constrictor to various other 

reptile species by feeding the recipients mosquitoes that had fed on the boa. 

Boas are apparently susceptible to Inclusion Body Disease, as are other boas and 

pythons (Schumacher and others, 1994). However, many putative cases of this fatal 

retrovirus-associated disease are diagnosed based on clinical symptoms and histology 

rather than isolation/identification of the causative agent (for example, Knotek and 

others, 2007). More information on parasites and pathogens can be found in the Risk 

Assessment portion of this report.

6.0 Demography

6.1 Population Density

Despite having one of the widest geographic distributions of any snake, there are 

few estimates of population abundance or density for B. constrictor. Mainland boas tend 

to be found in relatively low numbers, such that rigorous population estimates based on 

mark-recapture would require monumental effort. In the Pantanal of Western Brazil, for 

example, B. constrictor constituted only about 1 percent (two individuals) of the apparent 

snake community (Strüssmann and Sazima, 1993). Relative abundance of mainland boas 

may vary by site, however, as boas were described as “fairly common” at two sites in 

northern South America by Beebe (1946). T. Waller (pers. commun., 2009) described 
172

finding over two dozen Argentine boas near a village in a few days during periods when 

they were concentrated around viscacha burrows.

Insular boas, on the other hand, can achieve high densities. B. c. nebulosus on 

Dominica was described by Lazell (1964) as “amazingly abundant.” Boas were fairly 

common on small (4.5–24 ha) mangrove islets off the coast of Belize (Boback, 2006), 

with individual islets yielding between 4 and 40 boas. Reed and others (2007) roughly 

estimated population size of B. constrictor on Cayo Pequeño (0.64 km2

), Honduras to be 

632 boas (approximately 1000 boas/km2), and suggested that densities had been 

considerably higher prior to overcollection for the pet trade. On Cozumel, 49 invasive 

boas were found via road-cruising and haphazard encounters by Romero-Nájera and 

others (2007) over 17 months of intermittent searching. Road-cruising yielded 

abundance index of 0.11 ± 0.03 boas/10km, slightly lower than a previous estimate of 

0.18/10km (Martínez-Morales and Cuarón, 1999) based on diurnal line-transects along 

trails. Over 273 boas were captured on the 175 km2 island of Aruba in less than a decade 

after they became established (Quick and others, 2005).

6.1.1 Spacing and Aggregation

Most boas found on the mainland are singletons, and aggregations are rarely 

reported. Pizzatto and Marques (2007) stated that there are no records of aggregation in 

Brazilian boas but noted that reproduction in captivity only occurred when more than one 

male is present. Aggregations of individuals have been observed in Sonora, Mexico 

(Myres and Eells, 1968), and in Argentina during the local winter and at the onset of the 

reproductive period (Bertona and Chiaraviglio, 2003). It is notable that these 

observations are near the northern and southern range limits, respectively, for the entire 

species, and that aggregations were concentrated around refugia (rock crevice in Sonora, 

viscacha burrows in Argentina) that may be spatially limited. Boas in most of the 

tropical regions of the species’ range may not commonly need to find refugia from 

adverse climatic conditions. Boas on Dominica engage in communal “denning,” with 3 

to 12 individuals sharing a hollow log or tree stump (Lazell, 1964), and aggregations 

have also been observed on nearby St. Lucia (Malhotra and Thorpe, 1999). 

6.2 Size Distribution

To our knowledge, estimates of detectability for B. constrictor are available only 

for the insular dwarfed population in the Cayos Cochinos of Honduras, and those results 

are as yet unpublished (S. Green, pers. commun., 2009). Without estimates of 

detectability across ages, sexes, and sizes, any attempt to quantify size distribution based 

on opportunistic captures will likely be biased. In the Cayos Cochinos (without taking 

detectability into account), the distribution of female body lengths was highly rightskewed, due to a small number of extremely large females, while male body lengths 

largely fit a normal distribution. Similar trends were seen for snake mass, especially at 

the largest female sizes, and females were relatively heavier than males of the same 

length. Captures of invasive boas at the Deering Estate at Cutler, Miami, Florida were 

heavily biased towards neonates and juveniles, apparently due to a couple of large litters 

produced during one year (Snow and others, 2007b). Of 130 boas captured in Argentina, 

106 were considered adult (56 male, 50 female: Bertona and Chiaraviglio, 2003). 
173

Chiaraviglio and others (2003) divided captures into four size classes and found even sex 

ratios and relatively few small individuals (their Figure 1). 

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

Because of the historically large trade in skins of boas and pythons for decorative 

items, all of these species are listed under CITES Appendix II (and a few under Appendix 

I), requiring declaration of species type and individual numbers as a requirement of the 

import process among CITES signatories. Theoretically, this makes it easier to estimate 

the numbers of boas that are in the skin and pet trade, but in reality the level and quality 

of reporting by member states is highly variable. 

Schlaepfer and others (2005) reported that 8,182 wild-caught B. constrictor skins 

were imported to the United States between 1998 and 2002. B. constrictor represented a 

very small proportion of reptile skin products exported from Mexico, totaling 12 skins 

and 2,035 skin products during the period 1980-2001 (Arroyo-Quiroz and others, 2007). 

In contrast, Mexico exported 31,769 skins and 86,689 skin products of Python reticulatus

during this period. A large part of the skin trade in B. constrictor is not accounted for by 

CITES records, however; as an example, a huge variety of skin products from boas were 

available from curio vendors in Manaus, Brazil, during brief trips in 2003 and 2008 

(Reed and Rodda, pers. obs.). This indicates that international records of the boa trade 

seriously underestimate local trade, undeclared tourist curios, and cross-border trade in 

the native range that does not involve shipping.

B. c. occidentalis is in Appendix I of CITES, due to hunting for the skin and pet 

trade and habitat loss from intense farming and cattle ranching; this is the only New 

World boine to be assigned Appendix I status. Chiaraviglio and Bertona (2007) stated 

that forest cutting for charcoal production is likely to disproportionately impact 

reproductive females, which are dependent on forested areas. Cardozo and others (2007)

documented the severity of habitat loss in Argentina using GIS, with implications for 

levels of gene flow but apparently not for loss of genetic diversity.

7.2 Human Health Risks

Despite their fairly large size, wide distribution in their native range, and 

prevalence in the pet trade, we are unaware of any verified fatal attacks of a Boa 

Constrictor on a human being. Herpetoculturists have stated that any python over 8 feet 

(2.4 m) in total length may be capable of killing an adult human (Flank, 1997), and boas 

certainly exceed that length, so we do not exclude the possibility. A widely-disseminated 

news story (BBC News, 2006; Associated Press, 2006; and other sources) reported a fatal 

attack of a 3.96-m boa on its owner, an adult male residing in Cincinnati, Ohio. 

However, further investigation revealed that the snake had been misidentified and was in 

fact an underweight, 3.35-m Burmese Python (Python molurus: Damian, Arrowhead 

Reptile Rescue, pers. commun. to RNR, 2006). 

The largest, and thus presumably most dangerous, boas (B. c. constrictor from 

mainland S. America) have the reputation of being relatively docile in captivity, at least 

as compared to smaller northern (B. c. imperator) and southern (B. c. occidentalis) forms. 
174

We have additionally noticed that bites from both wild and captive B. c. imperator tend 

to consist of a quick defensive bite, but that ensuing attempts at constriction are either 

half-hearted or absent (Reed and Rodda, pers. obs., over many years). Only occasionally 

are bites from captive snakes of medical concern (for example, Kleinman and others, 

1998). Taken together, these observations tend to downplay the risk of fatalities from 

boas, although the risk does exist.

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

Due to its attractive coloration, impressive size, hardiness, and relative docility, 

Boa Constrictors have long been a popular animal in the pet trade. In recent years, 

literally dozens of locality-specific lines of boas have been available, along with 

selectively bred color morphs (Fogel, 1997; Bonny, 2007; Russo, 2007; Gilbert, 2008). 

These include albinos, dwarfs, and a myriad of other naturally occurring and artificially 

selected types.

United States importation records for the period 1989-2000 totaled 115,131 

individuals, a sum that was second only to Ball Pythons (Python regius: 366,808

individuals) among 24 species of boid/pythonid snakes imported (Reed, 2005). To this 

total can be added the 36,746 live individuals imported during 1983-1988 (Hoover, 

1998), summing to 151,877 individuals. However, imports were extremely variable 

among years, ranging from fewer than 2,000 to over 40,000 individuals (Fig. 7.4, also see 

Appendix). Based on similar sources of data, Franke and Telecky (2001) reported that 

21,209 B. constrictor were imported from Colombia and 7,326 from Nicaragua in 1997 

alone.

0

10000

20000

30000

40000

50000

60000

Number imported

Year imported

Figure 7.4. Annual imports of Boa Constrictors, Boa constrictor, to the United States during the 

period 1977-2007. Data from CITES. Number imported

 Year imported
175

Many of the boas imported from S. America are declared as “ranched” or “farm 

bred” individuals, especially those from Colombia. Ideally, these ranches collect 

pregnant females from the wild and hold them in captivity until they give birth, then 

release the females and sell the offspring. However, there are numerous questions about 

this practice. The estimated numbers of ranched boas imported are based on statements 

of the origin of snakes made by exporters and importers, and there are no checks on the 

validity of declarations of boa origins. Fogel (1997) stated that spent females are often 

slaughtered for their skins, and cautioned that farm bred animals were essentially wildcaught, as they represent neonates that would under normal circumstances contribute to 

population recruitment.

Reed and others (2007) reported on the population status and historical 

exploitation of insular boas from the Cayos Cochinos, Honduras, and concluded that at 

least 5,000 boas were removed from two islands, totaling 1.66 km2 in about a decade of 

collecting for the pet trade. They also claimed that poaching is ongoing, a claim that has 

been bolstered by the recent prosecution of an individual in Honduras selling wild-caught

boas from the Cayos Cochinos, as well as additional evidence of wild-caught Cayos boas 

for sale in the United States (S. Green, pers. commun., 2007). Wilson and Cruz Díaz 

(1993) gave additional history on exploitation of boas in the Cayos Cochinos.

The United States domestic trade in captive-bred boas is largely unquantified, 

thus complicating any attempt to estimate the number of boas in captivity in the country. 

Hoover (1998) reported 8,320 boas exported from the United States between 1983 and 

1995, but this total included an unknown number of re-exported individuals. Using 

USFWS Law Enforcement Management Information System (LEMIS) data, Reed (2005) 

concluded that import/export declarations of whether snakes were wild-caught or captivebred were largely unreliable. A prominent snake breeder estimates that “probably 

between one and two thousand common boas are now captive-bred and born in the U.S. 

each year” (Barker and Barker, 2008b); if true, then sales of domestically-produced boas 

are still dwarfed by sales of imported boas.

7.3.2 Other Uses

Major human uses of Boa Constrictors include meat, skins, medicinals, and the 

live animal trade. They may also be used in religious services in areas of the United 

States where use of venomous snakes for such purposes has been banned (RNR, pers. 

obs., 1996).

Management Profile

See the introduction to the management profile for Indian Pythons (chapter Four) 

for an explanation of the format and content of this section of the risk assessment.
176

8.0 Pathway Factors: Pet Trade

8.1 Volume of International Trade

Boa Constrictors are imported to the United States in huge numbers, usually in the 

tens of thousands per year. This species is imported at a long-term average rate higher 

than that observed for any other species addressed in this document. See section 8.1 of 

chapter Four for reasons why imported individuals may be of especially high risk as 

potential invasive species. Although many boas entering the United States are declared 

as captive-bred offspring of farmed or ranched individuals, the veracity of such claims is 

difficult to judge. 

8.2 Volume of Domestic Trade

The volume of domestic trade is poorly documented. If breeders produce only a 

couple of thousand boas per year as suggested by Barker and Barker (2008b), then 

domestic production is about an order of magnitude lower than the import volume. This 

would imply that production may be lower than that of Indian Pythons or Reticulated 

Pythons but higher than the giant African pythons or any of the anacondas. 

Breeders have produced numerous “designer morphs” based on aberrant 

individuals, including varieties that routinely sell for over $1000 per individual. Prices 

for these morphs tend to decrease over time as more breeders acquire adult stock, but 

usually stay above the priceline for individuals with wild-type patterns. Albino and 

similar genotypes may exhibit reduced fitness if released to natural habitats, but this is 

speculative.

9.0 Entry Potential (Survival in Transit – Meant Mainly for 

Unintentional Transport)

This category focuses on the prospect of a species surviving in a pathway that was 

not intended to convey the individual (stowaways). As the live animal trade is premised 

upon live animals, Boa Constrictors tend to be shipped using methods that ensure high 

survival, probably approaching 100 percent for such sturdy snakes. However, there are 

also numerous records of unintentional transport of Boa Constrictors, usually in fruit 

shipments, and the species appears to be hardy regardless of whether transportation is 

intentional or unintentional.

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

In the public mind, Boa Constrictors are considered a giant snake, but they are not 

particularly large in comparison to some of the true giants. As a consequence, we 

consider them somewhat less likely to be released due to reaching unmanageable sizes, or 

to escape substandard caging by brute force. However, the reduced per-animal risk is 

offset by the high volume of trade, as the latter increases the potential pool of keepers 

likely to exhibit poor husbandry techniques or to have a lower threshold for dumping an 
177

unwanted animal. Introduced populations in Florida and Aruba are likely the result of 

released or escaped pets (Snow and others, 2007b; H. Reinert, pers. commun., 2008), and 

free-ranging individuals are regularly reported in various parts of the United States.

10.2 Climate Match: Areas of the United States at Greatest Risk

We extrapolated the native-range climate space of Boa constrictor to the United 

States using (a) records from all localities (Fig. 7.5); and (b) all localities except those 

occupied by the Argentine Boa (B. c. occidentalis: Fig. 7.6). We include the latter map 

because the Argentine Boa is phenotypically distinct and is considered by some to be a 

candidate for description as a full species. The Argentine Boa is present in the live 

animal trade in the United States, albeit in low numbers, and we cannot predict whether 

adding genes from an escaped/released Argentine Boa to an established population of 

Boa Constrictor in the United States would allow the population to better withstand cold 

temperature. Because Fig. 7.5 depicts the United States extrapolation using climate space 

from the entire known native range (that is, the limits to which the species was capable of 

dispersing on its own) using the more inclusive species definition as used by a majority 

of current workers in the field, we use this map for the overall risk assessment in chapter 

Ten.

Using all localities, apparently suitable climate in the United States includes a 

large swath of land in the southern United States. This area includes mesic areas such as 

peninsular Florida and a corner of southeast Georgia, but also includes more arid zones 

including the southern half of Texas and portions of New Mexico and Arizona. Much of 

Hawaii and Puerto Rico also appear climatically suitable. When localities of Argentine 

boas are removed, however, the suitable area is much smaller and includes peninsular 

Florida south of about Orlando and extreme south Texas, as well as parts of Hawaii and 

Puerto Rico. 
178

Figure 7.5. Areas of the United States matching the climate envelope expressed by B.

constrictor in its native range (Fig. 9.3), based on 131 known localities. See chapter Two for 

details of analytical method.
179

Figure 7.6. Areas of the United States matching the climate envelope expressed by B.

constrictor in its native range (Fig. 9.3), excluding records for the Argentine Boa (B. c. occidentalis). 

See chapter Two for details of analytical method.

10.3 Prey Availability

The Boa Constrictor is capable of consuming prey from a variety of higher taxa 

and is unlikely to be limited by availability of suitable prey species in most areas of the 

United States with apparently suitable climate for the species. The introduced population 

on Aruba consumes many types of prey, including dogs, rats, birds, iguanas, etc. (Quick 

and others, 2005; H. Reinert, pers. commun., 2008), and introduced boas on Cozumel are 

considered to pose a threat to several endemic mammals and birds (Romero-Nájera and 

others, 2007).

The species is amazingly plastic in body size (Boback and Carpenter, 2007), with 

numerous examples of insular dwarfism evident in the lineage. Such dwarfism is likely a 

result of food limitation on species-poor islands, and boas are able to persist even on tiny 

islets by exploiting seasonally abundant prey such as migratory birds (Boback, 2005, 

2006; Reed and others, 2007). Reductions in body size may occur over a remarkably 

short number of generations, as evidenced by apparent dwarfism in boas introduced to 
180

Aruba only a couple of decades ago (Quick and others, 2005; RNR pers. obs., 2008). 

Moreover, dwarfed populations often exist at high densities and may exert top-down 

influences on prey species composition on these islands (Reed and others, 2007). The 

ability of boas to adapt to limited food by reducing body size and their propensity to 

achieve high densities in insular situations suggest that they may be able to become 

successfully established in insular states and territories including Puerto Rico, the 

Mariana Islands, Hawaii, etc.

10.4 Predation Risk

Because of their smaller body size and use of terrestrial/arboreal habitats, Boa 

Constrictors are likely to be exposed to a wider range of predators than some of the giant 

pythons or more exclusively aquatic species such as anacondas (see chapters Eight and 

Nine). A range of predators consume boas in their native distribution, and some of the 

same taxa (raptors, mesocarnivores, snakes, big cats, etc.) would be likely to at least 

occasionally consume boas introduced to the United States. None of these would be 

expected to specialize on consuming boas, however, and native predators have had no 

observable impacts on any invasive boa population. The potential for native predators to 

appreciably limit the growth or expansion of introduced boa population appears relatively 

slim, but is perhaps greater than the equivalent potential to limit Green Anacondas, 

Reticulated Pythons, or other giants.

10.5 Reproduction Requirements

Boa Constrictors do not exhibit specialized requirements for reproduction such as 

lekking, multiple male attendance of females, specific microhabitats for parturition, etc. 

Gravid females likely require access to basking sites for behavioral thermoregulation 

during embryonic development. Although herpetoculturists routinely expose boas to cool 

temperatures before breeding them, the species does not naturally endure such low 

temperatures in much of its range, and boas introduced to the United States would likely 

be naturally exposed to a cool winter anyway. 

10.6 Hibernation Requirements

So far as we are aware, Boa Constrictors do not engage in behaviors resembling 

long-term hibernation anywhere in their native distribution. Even in the coldest parts of 

the species’ distribution in Mexico and Argentina, individuals are observed engaging in 

reproductive, thermoregulatory, and/or other behaviors during the winter, although these 

are often conducted in the vicinity of a refugium to which snakes retreat at night or 

during particularly adverse weather. 

10.7 Tools for Eradicating a Colonization

In chapter Three, we reviewed the tools available for eradicating large continental 

populations of giant constrictors and concluded that no tools that are currently available 

are likely to be dependable for such a task. This conclusion is likely to hold for Boa 

Constrictors as well, and limited control activities on Aruba appear to have had little 

effect on the introduced population (RNR, pers. obs., 2008). Small, localized populations 

that can be easily accessed, such as the population near Miami, Florida, are more 
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promising candidates for eradication, and in at least two small islands in the native range, 

intensive visual searching by collectors for the pet trade has resulted in drastic declines of 

island populations (Reed and others, 2007; S.M. Boback, pers. commun., 2009). 

Developers of tools for eradicating a colony of Boa Constrictors would need to be 

especially cognizant of the need to target snakes that forage and refuge in arboreal 

microhabitats. 

11.0 Spread Potential

11.1 Dispersal Ability

Movement activity of Boa Constrictors has been studied in a few sites, but with 

generally low sample sizes and of relatively short-term durations. Furthermore, some of 

the radiotelemetric studies are of island populations for which dispersal is limited by the 

size of the island. It is therefore difficult to assess how far individuals in an introduced 

population might disperse. Genetic evidence from Argentine boas suggests that 

individuals disperse between distant populations within the state of Córdoba at fairly 

frequent intervals (Cardozo and others, 2007). The introduced population in south 

Florida has not dispersed widely over the past three decades, but it is unknown whether 

this is due to unsuitable surrounding habitat, whether we are currently observing the preexpansion lag phase typical of many invasive species (Crooks and Soule, 1999), or for 

some other reason. Maximal litter size is greater than 60, indicating that dispersal is 

unlikely to be limited by female fecundity. However, most boas produce fewer offspring 

than the maximal number, and even maximal boa litter sizes do not compare with 

equivalent maximal clutch sizes of giant pythons. 

11.2 Ability to Develop Races or Strains

There are no native species in the United States with which boas would be 

capable of breeding. However, certain aspects of the herpetoculture trade increase the 

odds of new strains developing in an introduced population. To a greater degree than for 

any other species assessed in this document, boa aficionados tend to seek out snakes from 

known localities, and many forms in the pet trade are known by the locality name (for 

example, Hog, Crawl Cay, Saboga, and Corn Island boas, Suriname red-tails, etc.). Such 

locality-specific individuals may be bred with boas of aberrant patterns to produce new 

variations, such as hypomelanistic boas produced from Colombian and Panamanian boas. 

These boas possess genotypes that are distinct from those found in the native range, and 

thus expand overall genotypic variation beyond that found in any one population across 

the massive geographic range of the species (for example, Kolbe and others, 2008). 

Lacking effective eradication tools, we consider it a high probability that new genes 

would eventually be incorporated into a large established population of Boa Constrictors, 

but the effects of such new genetic contributions to the population are largely unknown. 

11.3 Tools for Managing Spread

Existing control tools appear inadequate for the eradication of large established 

populations of Boa Constrictors (see chapter Three). In the absence of proven control 

tools, an effective interdiction program might hold the most promise for preventing 
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establishment (for example, Vice and Pitzler, 2002), combined with a well-organized 

rapid response capacity to respond to sightings of colonizers (Stanford and Rodda, 2007). 

A multi-agency rapid response capacity for giant constrictors was recently developed in 

the Florida Keys and resulted in capture of a large (over 3.0 m) female Boa Constrictor in 

early 2009 (A. Higgins, pers. commun., 2009), offering evidence that developing such 

capacity can be effective. However, interdiction within the continental United States 

would be difficult under current regulatory regimes, which largely fail to effectively 

monitor interstate transport of large reptiles. 

12.0 Economic Impact Potential

Regulatory agencies have not decided whether a formal economic analysis of the 

potential impacts of introduction of, and regulation of trade in, Boa Constrictors will be 

separately conducted pursuant to a variety of statutes and Executive Order 13272 (Proper 

consideration of impact on small business). Our narrative below is provided only to cite 

relevant biological literature and identify germane sections of the biological profiles.

12.1 Pathogen Vector

Very little is known of the pathogens of Boa Constrictors in their native range, or 

whether these pathogens exert demographic effects on boa populations. In captivity, boas 

are subject to a wide variety of ailments (Walls, 1998a; Mader, 1996; Smith, 1999; 

Russo, 2007), but many of these appear to be artifacts of captivity or exacerbated by 

captivity. For example, one of us (RNR) has handled well over 100 Boa Constrictors in 

the field in their native range and has never witnessed maladies common in captivity 

(including respiratory distress, “mouthrot”, skin blisters, etc.). Numerous ticks of 

potential medical or veterinary import readily use giant constrictors as hosts (see chapter 

Six for more information on these ticks and the economically-crippling pathogens they 

can vector), and readily transfer to new hosts in captivity. The common practice of cohousing multiple species of giant constrictors during importation, transport, or husbandry 

could facilitate such transfers. Overall, more knowledge of boa pathogens and parasites 

is required to understand the potential economic impacts of boa-vectored pathogens 

becoming established in the United States.

12.2 Predator on Livestock

Boas are unlikely to prey on large hoofstock, although juveniles (for example, 

lambs) may be taken occasionally. However, boas are perhaps more likely to impact 

domestic fowl such as chickens than are more aquatic species like anacondas, and the 

boa’s tendency to climb means that it is more likely to surmount fences or otherwise 

achieve ingress to domestic poultry operations. Boas that achieve high densities (similar 

to the situation on some Neotropical islands) could potentially have moderate impacts on 

poultry production; high densities are most likely to be achieved in areas with few native 

predatory competitors (for example, United States insular areas within the suitable 

climatic conditions).
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12.3 Predator on Pets

As with many of the large constricting snakes, boas would be capable of attacking 

and consuming a variety of companion animals, from caged birds to cats and dogs. We 

would not expect these attacks to be particularly common, but such an attack would 

almost certainly garner extensive media attention and widespread consternation among 

pet-owners.

12.4 Electrical Power Systems

The Boa Constrictor is perhaps the most arboreal of the species reviewed in this 

document, and in Central and South America we have observed boas of all sizes using 

habitats far above the ground. Boas may thus be the most likely species to cause power 

outages as a result of climbing electrical lines and spanning insulators. However, the 

areas of the United States where boas might conceivably become established already 

have fairly large arboreal snakes (genus Elaphe or Pantherophis) that have even better 

climbing abilities. We do not expect introduced Boa Constrictors to appreciably elevate 

the rate of snake-caused power outages above the background level caused by native 

snakes.

12.5 Traffic Accidents/Human Attacks

While an extremely large Boa Constrictor is probably capable of killing an adult 

human under some circumstances, we are aware of no legitimate attacks anywhere in the 

species’ native range. Children would be at higher risk of attempted predation than 

adults, but a boa attacking a human of any size would appear to be a very low-odds 

proposition. Loss of control of a vehicle after hitting a very large boa on the road is 

possible but unlikely, and the odds of losing control would approximate the odds 

associated with striking an alligator of similar girth. Overall, we judge boas to pose low 

risks to human health and safety.

12.6 Tourism

To many residents of the United States, the term “Boa Constrictor” brings to mind 

images of giant people-eating snakes, despite a complete lack of evidence that a boa has 

ever attempted to consume a human. Such misplaced fear is exacerbated by well-known 

ditties such as, “Oh, I’m being swallowed by a Boa Constrictor...” While it may be true 

that oh, gee, he could get to your knee, the odds of such an event happening are 

extremely low. Illogical fears about snakes being what they are, however, some tourists 

might be dissuaded from visiting an area with a well-publicized introduced population of 

boas.

Conversely, a small proportion of potential tourists might travel for the purposes 

of viewing introduced boas. However, the cryptic nature of the species would probably 

result in a very low reward rate, and we doubt that the desire to see boas would motivate 

many potential tourists.
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12.7 Impacts on Commercially Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

As compared to the giant pythons or Green Anaconda, boas are unlikely to 

frequently consume big game animals such as deer or pigs, although boas in the species’ 

native range have successfully consumed both these taxa. Waterfowl that are hunted by 

humans (primarily ducks and geese) would probably experience some level of predation 

by boas, but this snake is generally less aquatic than are anacondas and (probably) the 

large pythons, so the relative risk to waterfowl might be lower.

Among species of watchable wildlife, we consider birds to be at greatest risk of 

predation from introduced Boa Constrictors. Birds are an important part of the diet of 

some populations of boas, especially those on islands. On the mainland, the addition of 

predatory boas to the suite of existing stressors is most likely to impact species that are 

already of conservation concern, and boas would be capable of ingesting all threatened 

birds in Florida, from Cape Sable Seaside Sparrows to Wood Storks. Birds of 

conservation concern constitute an especially large proportion of the avifaunas of insular 

states and territories of the United States, and boas have proven capable of persisting on 

tiny islands by consumption of birds, even those that only briefly stop on islands during 

migration (Boback, 2005: this situation is repeated on several islands with high-density 

snake populations across the globe, for example, Shine and others, 2002). We consider 

boas to pose a significant risk to avian faunas that have not experienced snake predation 

in recent evolutionary time, including Hawaii and the Northern Mariana Islands. 

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture

We do not expect boas to significantly impact these industries. However, the 

arboreal habits of boas would make them more likely to be encountered in horticultural 

settings as compared to more terrestrial or aquatic snakes. Such encounters would be 

startling to a human, but would entail low risk.

13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

Boa Constrictors would be capable of consuming most of the species considered 

to be potentially at risk from Indian Pythons (see chapter Four, section 13.1), although 

boas rarely, if ever, exceed 4 m in length and would be less of a risk to the largest prey 

species. Among mammals, species from the size of rats to rabbits would be expected to 

compose the majority of prey for introduced boas, and unfortunately this size range 

includes a number of imperiled mammals in the areas of the United States (such as the 

Florida Keys) that appear to be climatically suitable for the snake. 

Along with medium-sized mammals, a wide variety of imperiled birds would be 

at risk of consumption by introduced Boa Constrictors. Among the species profiled in 

this document, boas are probably the most arboreal and take birds from all forest strata in 

their native range. Moreover, they manage to persist on tiny mangrove islets with little 

fresh water and low prey availability, primarily by targeting birds during some parts of 
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the year. This observation suggests that the list of potential prey provided in chapter 

Four, section 13.1 could be expanded to include all imperiled bird species that migrate 

through an area occupied by introduced boas, especially on small islands. 

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

As with the other species profiled in this document, it is unlikely that introduced 

boas would provide a net benefit as prey items to native predators. Intraguild predation 

would probably balance any benefit, and benefits would be further decreased by adding 

another predator to the ecosystem (see sections 10.4 and 13.2 of chapter Six). Although 

willing to forage in the water, boas are less aquatic than many of the other large 

constrictors, so crocodilians would probably encounter them at a low rate. 

13.3 Vector for Disease Spread to or Within Native Faunas

Our conclusions on the probability of Boa Constrictors vectoring pathogens to 

native faunas are subject to the same uncertainties described above in section 12.1, 

namely that there is insufficient evidence to rigorously assess risk. We can say with 

confidence that: (1) Parasites known or suspected to vector virulent vertebrate pathogens 

are found on large snakes in their native range; (2) These parasites readily transfer among 

reptile species and to mammalian hosts; (3) Multiple species of these parasites have 

survived importation to the United States on live reptiles; (4) At least some of these 

parasites can successfully switch to feeding on native reptiles; and (5) At least one of 

these species has become established in the United States as a result of the international 

trade in live reptiles (Oliver and others, 1988; Allan and others, 1998; Burridge, 2001, 

Burridge and others, 2000, 2006).

13.4 Species that Might be Adversely Affected by Control Measures

Control measures for Boa Constrictors would likely be similar to those envisioned 

for giant constrictors in general (see chapter Three), but would probably be deployed in 

fewer aquatic habitats, instead concentrating on terrestrial and arboreal habitats. Some 

possible avenues of control, such as toxicants, are likely to result in non-target mortality 

to native species. Because boas attain smaller sizes than the giant constrictors, it would 

be more difficult to design traps or toxicant delivery systems that would be accessible to 

boas yet repel native species. If control tools were to be used in arboreal habitats, it 

would be particularly difficult to design control tools that would thwart native climbing 

snakes. As no management tools have been adequately field-tested, however, it is not 

possible to estimate potential ecological costs.
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14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Production Affected?

Domestic production of Boa Constrictors probably exceeds 1,000 individuals per 

year (Barker and Barker, 2008b), although there are no means of verifying this estimate. 

Captive-bred boas are often from locality-specific lineages and/or “designer morphs” 

with aberrant colors and patterns, and are typically more expensive than imported boas 

from Central and South America. Individuals very often sell for hundreds of dollars, 

occasionally thousands of dollars, so the overall value of domestic production is likely to 

be a large sum relative to many herpetocultural activities. It is our subjective impression 

that domestic production of boas is lower than production of Indian Pythons or 

Reticulated Pythons, perhaps because of the continued availability of inexpensive wildcaught imports and slightly more involved husbandry methods required for predictably 

successful captive propagation of boas. Captive production is spread across far more 

breeders than is the case for the anacondas or the giant African pythons, and regulations 

on trade in Boa Constrictor would probably cause economic hardship for a greater 

number of breeders, but quantification of producer impacts would be better handled in a 

formal economic analysis.

14.1.2 International Trade/Alternate Pets

Boa Constrictors constitute one of the mainstays of the reptile import trade, with 

tens of thousands imported per year for at least the last three decades. Colombia is the 

declared origin of the bulk of imports, but CITES records include declarations from 

nearly every country in the native range. Many of these are juvenile snakes declared as 

“farmed” or “ranched” and described as the offspring from wild-caught and/or long-termcaptive dams. The veracity of these declarations has never been adequately investigated 

and much of the documentation associated with the live animal trade is unreliable 

(Blundell and Mascia, 2005). Regardless, implementation of restrictions on importation 

of Boa Constrictors would cause importers to lose the associated revenue stream. We 

cannot speculate whether such a loss could be compensated for by switching to different 

species, or whether a switch might actually result in importation of species with even 

more potential to be injurious as invasive species in the United States.
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Chapter Eight–The Yellow Anaconda, Eunectes 

notaeus, and DeSchauensee’s Anaconda, 

Eunectes deschauenseei

Biological Profile

1.0 Introduction

See section 1.0 of chapter Four for a description of species-specific and overall 

components of the risk assessment package.

1.1 Species

As currently understood, the yellow anacondas comprise two species with entirely 

disjunct distributions. The northern form, DeSchauensee’s Anaconda (E. deschauenseei), 

is known from a small number of specimens and has a limited range in northeast South 

America. The southern form, the Yellow Anaconda (E. notaeus), has a larger distribution 

in subtropical and temperate areas of South America, and has received more scientific 

attention. Subspecies are not recognized for either species (Peters and Orejas-Miranda, 

1986; McDiarmid and others, 1999), although the Yellow Anaconda exhibits significant 

genetic differentiation among disjunct populations that are within a few hundred 

kilometers of each other (Mendez and others, 2007). Because E. deschauenseei is so 

poorly known, general statements about yellow anaconda biology in this bioprofile refer 

to E. notaeus except where specifically noted as applicable to E. deschauenseei.

The literature on the Yellow Anaconda is fairly slim, and that on DeSchauenseei’s 

Anaconda even more so. In preparing this bioprofile, we leaned heavily on the work of 

Christine Strüssmann and colleagues (Strüssmann and Sazima, 1991, 1993; Strüssmann, 

1997) for information concerning Yellow Anacondas in the Pantanal of Brazil, and relied 

on the taxonomic review and associated literature on the genus Eunectes by Lutz Dirksen 

and colleagues (Dirksen and others, 1998; Dirksen and Böhme, 1998, 2005; Dirksen, 

1999, 2002; Dirksen and Henderson, 2002). The most ecologically revealing data on 

Yellow Anacondas, however, come from the research of Tomás Waller, Patricio Micucci, 

and their colleagues, who amassed a large sample size of anacondas from a harvested 

population in the Formosa Province of Argentina (Waller and Micucci, 1993, 2008; 

Waller and others, 1995, 2001, 2007; Waller, 2000; Micucci and others, 2006; Micucci 

and Waller, 2007). Many of our conclusions on body size, reproduction, and many other 

topics were drawn directly from their work.
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1.2 Common Names

In this report we will refer to the more widespread southern form, Eunectes 

notaeus, as the Yellow Anaconda, and the geographically limited northern form, 

Eunectes deschauenseei, as DeSchauensee’s Anaconda. We term them the yellow 

anacondas when describing features likely to be common to both species. 

DeSchauensee’s Anaconda is sometimes referred to as the Dark-Spotted Anaconda in 

English-language popular publications and websites, but we deem this name 

unsatisfactory as both E. notaeus and E. deschauenseei have extensive dark coloration, 

including spots. Because they are confined to smaller geographic areas than are wideranging species such as Boa Constrictor or Green Anacondas, the two yellow anacondas 

tend to have fewer common names ascribed to them in their native range. 

DeSchauensee’s Anaconda may be called Sucuri or Sucuriju in its native range (Dirksen 

and Henderson, 2002), but the former local name fails to distinguish it from the sympatric 

E. murinus. Common names for E. notaeus include Anaconda Amarilla (Uruguay: 

Achaval and Olmos, 2003), Curiyú or Boa Curiyú (Paraguay: Waller and others, 1995;

Argentina: Micucci and others, 2006), Sakobo, Aboma, Watra-Aboma, or couleuvre 

d’eau (French Guiana: Starace, 1998), Southern Anaconda (United States: Pope, 1961), 

Paraguay-Anakonda (Germany: Dirksen, 2002), and Kochokor (Paraguay: Waller and 

others, 1995). See Dirksen (2002) for an exhaustive list of common names.

1.3 Evolutionary Context

See the biological profile for Eunectes murinus for a discussion of placement of 

Eunectes in the New World boid snakes and hypothesized relationships of the four 

species within the genus. The two yellow anacondas addressed herein appear to represent 

a monophyletic lineage most closely related to E. beniensis and more distantly related to 

E. murinus (Dirksen, 2002; Dirksen and Böhme, 2005). In captivity, E. notaeus has been 

reported to readily hybridize with E. murinus, and hybrid offspring are reported by 

herpetoculturalists to exhibit a strong feeding response but to display unpredictable 

temperaments. 

The herpetocultural stance on hybridization is somewhat contradicted by Dirksen 

and Henderson (2002), who stated that most hybrid matings between Green and Yellow 

Anacondas produce no viable offspring and that reproductive competence of the few 

viable offspring is unknown. However, successful captive breeding of Green Anacondas 

appears to remain relatively uncommon, at least in Europe (Bisplinghoff and Bellosa, 

2007), such that it is difficult to tell whether low success of hybrid breedings is due to 

interspecific incompatibilities or to the reproductively intransigent nature of the Green 

Anaconda in the putative pairing. There are no records of hybridization between E. 

notaeus and E. deschauenseei, probably due to the rarity/absence of the DeSchauensee’s 

Anaconda in herpetoculture. Although the two species are almost indistinguishable in 

terms of scale counts and morphology, they exhibit divergent color patterns and unique 

genetic identities, and are allopatric; these differences have been used to justify their 

continued recognition as separate species (Dirksen, 2002).
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Figure 8.1. Schematic representation of the head of E. notaeus.

2.0 Description

2.1 Unique Morphological Features

The yellow anacondas typically exhibit a more complicated dorsal pattern than 

does the Green Anaconda (see Bioprofile of E. murinus), but to distinguish them from 

other large constrictors is still a relatively trivial task. Most casual observers would agree 

that the ground pattern of the Yellow Anaconda is strongly yellow across much of its 

range, as suggested by its common name. However, the ground color can range from 

olive yellow to brownish yellow both among and between populations across the species’

range (T. Waller, pers. commun., 2009). The yellow ground color is somewhat muted 

(sometimes to yellowish-green to light brown) in DeSchauensee’s Anaconda, and tends 

to be more confined to the lateral surfaces, but is still fairly visible and can be strongly 

yellow in some individuals. Strong yellow coloration is shared with very few non-albino 

constrictors, although “jungle” morph carpet pythons (Morelia spilota) and a few other 

taxa may exhibit considerable yellow coloration; the pythons, however, can be easily 

distinguished by their large thermoreceptive facial pits. 

After noticing the yellow ground color, an observer will likely notice the many 

black dorsal blotches and smaller lateral blotches present in the yellow anacondas. The 

number and size of blotches is usually, but not always, sufficient to differentiate between 

the species. DeSchauensee’s Anaconda has fewer (87-126) and larger dorsal oval 

blotches (usually solid) separated by two or three scales, as well as small lateral irregular 

blotches. The Yellow Anaconda has 101-175 dorsal blotches, separated by only one or 

two scales; the dorsal blotches tend to have lighter centers. This species also has 
190

numerous irregular lateral blotches which tend to form complete or incomplete ocelli on 

the upper flanks below the dorsal blotches, below which are numerous black flecks. The 

two yellow anacondas exhibit very similar scale counts, including 43-54 dorsal scale 

rows at midbody and 213-237 ventrals (Dirksen, 2002; Dirksen and Henderson, 2002; 

Dirksen and Böhme, 2005). In the field within either species’ native range, geography 

alone should be sufficient to allow determination of which species has been encountered.

Where either of the two species of yellow anacondas is sympatric with the Green 

Anaconda, the former can be reliably distinguished by any of the following: (1) the 

presence of fewer than 50 dorsal scale rows (Green Anacondas exhibit more than 50 

rows); (2) no scales present between the supralabials and the oculars (present in Green 

Anaconda); and (3) the presence of five dark head stripes (the Green Anaconda has only 

four; Fig. 8.1; Starace, 1998; Dirksen, 2002).

2.2 Size

The Yellow Anaconda attains considerably smaller body sizes than does its 

congener the Green Anaconda. A large sample (n = 908) from northeast Argentina 

included no males over 2.93 m (2.55 m SVL: Waller and others, 2007). The largest 

female was 3.80 m (3.35 m SVL; n = 647). Male mass in this study topped out at 10.5 

kg, while females attained 29 kg. Of the female sample, less than 9 percent were over 3.0 

m (2.6 m SVL), and Waller and others (2007) suggested that large females are rare. 

However, 40 percent of females were longer than 3 m in another region of Argentina (T. 

Waller, pers. commun., 2009), indicating regional differences in body size distributions, 

the causes of which remain poorly understood. Maximum lengths above were 

corroborated by Dirksen (2002), who surveyed museum specimens and wild-caught 

individuals from across the species’ range and found no females over 3.76 m or males 

over 2.34 m.

In the field or in semi-captive conditions in their native range, neonatal Yellow 

Anacondas are typically 0.48-0.65 m and 61-140 g (Waller and others, 1995, 2007); these 

values are appreciably larger than an estimate (0.40 ± 0.01 m, range 0.39–0.41 m) 

derived from a single Brazilian litter (possibly removed pre-term from a preserved 

female; Pizzatto and Marques, 2007). Neonates from captive females may be larger, with 

records of individuals ranging from 0.65 to 0.78 m total length and body masses up to 

210 g (Bisplinghoff and Bellosa, 2007), but it is possible that some of these were 

measured and weighed well after parturition.

Table 8.1. Total lengths and masses of typical E. notaeus (compiled from the literature sources 

listed in the text).

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 405-593 95-210 1.3-1.45 1.5-2.5 2.9 ~15

F: 440-540 95-210 1.4-1.85 2.3-5 4.25 ~50
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There may be a moderate amount of variability in body size in different parts of 

the distribution of the Yellow Anaconda, but few observers report wild-caught 

individuals over 4.0 m. The species was reported to reach 4.0 m in the Pantanal of 

western Brazil (Strüssmann and Sazima, 1993), and Dirksen (2002) similarly estimates 

maximum body size to be about 4.0 m; both of these estimates are only marginally larger 

than the published 3.79 m maximum size reported for Argentina by Waller and others 

(2007) and 3.95 m for a female from Corrientes, Argentina (T. Waller, pers. commun., 

2009). Most of the larger body sizes reported for the Yellow Anaconda (for example, a 

5-m specimen from Misiones, Argentina) are typically derived from skins, which can be 

stretched by as much as 30 percent over the actual length of the animal (Dirksen, 2002). 

Females occasionally exceed 4 m in captivity, and attain body weights in excess of any 

specimens known from the wild (approx. 50 kg; Bisplinghoff and Bellosa, 2007).

DeSchauensee’s Anaconda appears to be the smallest of the anacondas, although 

the extremely limited number of available specimens does not allow unequivocal 

determination of maximal body sizes. Dirksen and Henderson (2002) record maximum 

total length of available specimens as 1.92 m in males and 3.0 m in females; three 

females from Marajo measured 1.89, 1.91, and 1.60 m (Belluomini and others, 1959). A 

juvenile from French Guiana measured 0.75 m (Starace, 1998) and offspring from the 

three Marajo females mentioned above were 0.37–0.55 m (Belluomini and others, 1959), 

while dissections of gravid females in museums yielded a neonatal size estimate of 0.49 

m (0.42 m SVL, range 0.29-0.53: Pizzatto and Marques, 2007). 

2.3 Sexual Size Dimorphism

As with most giant boids, females attain larger body sizes (Strüssmann and 

Sazima, 1993; Waller and others 2007); see above for differences in male and female 

body size from one population in Argentina.

2.4 External Sexual Differentiation

Males exhibit proportionally longer tails and larger pelvic spurs (Dirksen, 2002), 

and adults can be visually sexed in the field with some reliability by examining the width 

of the tail base (wider in males) in combination with tail/body proportions (T. Waller, 

pers. commun., 2009). In northeast Argentina, male tail length averages 17.1 percent of 

SVL (range 14.3-20.0; equivalent to 15.1 percent of TL), while female tail length is 15.1 

percent of SVL (range 13.5-16.6; equivalent to 13.0 percent of TL); note that these 

ranges overlap to a significant degree, and the overlap among neonatal males and females 

was even greater (Waller and others, 2007). Examination of specimens from across the 

range of the species yielded tail lengths averaging 14.3 percent of total length among 

male Yellow Anacondas and 12.7 percent among females (Dirksen, 2002).
192

3.0 Distribution in Space and Time

3.1 Native Range

3.1.1 Yellow Anaconda, Eunectes notaeus

Waller and others (1995) described the overall range of the Yellow Anaconda as 

including the Pantanal in Bolivia and Brazil from 15 ̊S latitude, south through aquatic 

habitats of the Paraguay and Parana River Basins below 250 m elevation in Paraguay and 

Argentina, and reaching 32 ̊S latitude in the Parana Basin. The species’ range includes 

about 400,000 km2 overall (Micucci and others, 2006), but populations are confined to 

aquatic habitats within this range. Dirksen (2002) provided both dot-distribution maps 

for specimens and range maps showing the estimated extent of occurrence for both 

species.

Argentina–Found in the states of Corrientes, Chaco, Entre Rios, Formosa, Misiones, and 

Santa Fe over an area of 120,000 km2 (Giraudo and Scrocchi, 2002; Micucci and others,

2006; Waller and others, 2007), but Yellow Anacondas occupy only aquatic habitats 

within these extent-of-occurrence maps. The species reaches its southernmost range 

limits in Argentina at approximately 31°S latitude (Micucci and others, 2006), and 

persists in areas with an average annual temperature of 23°C, a maximum recorded 

temperature of 45°C and with occasional winter freezes (minimum temperature of -7°C 

(Waller and others, 2007).

Bolivia–Known from the northeast bordering Brazil (Itenex-Guaporé river drainage) and 

the eastern edge of the country south to the Paraguayan border associated mainly with the 

Pantanal (Paraguay River drainage; T. Waller, pers. commun., 2009). In the north, 

Yellow Anacondas appear to be somewhat sympatric with Green Anacondas (Dirksen, 

2002), but the degree to which they may overlap with the Beni or Bolivian Anaconda (E. 

beniensis; see biological profile for Green Anaconda) is unknown (Dirksen, 2002). 

Brazil–Present in the western Pantanal and in tributaries of the Río Paraguay in the states 

of Mato Grosso and Mato Grosso do Sul. Most Brazilian specimens are known from a 

region within a few hundred kilometers of the eastern Bolivian border, which extends 

south nearly to the Paraguayan border. However, an apparently disjunct population is 

located in São Paulo Province, well to the east of the rest of the rest of the known range 

(Dirksen, 2002); the two localities in this province may represent localized relictual

populations or may be part of a poorly-sampled population covering a wider area.

Paraguay–Widespread through the Paraguay, Parana, and Pilcomayo River Basins 

(Waller and others, 1995). In the Paraguayan Chaco, it is found only in wetlands of the 

Eastern Chaco region and tributaries around the Paraguay River, but in the Western (dry) 

Chaco the species is found only in the Pilcomayo floodplain (Norman, 1994; T. Waller, 

pers. commun., 2009).
193

Figure 8.2. Estimated native range of the Yellow Anaconda (Eunectes notaeus, shown in blue) 

and DeSchauensee’s Anaconda (Eunectes deschauenseei, shown in yellow). Blue triangles 

indicate apparently disjunct populations of E. notaeus. The range of the Beni Anaconda, E. 

beniensis, is shown in green for geographical reference; see chapter Nine for more information on 

the Beni Anaconda. All ranges follow Dirksen (2002: his Fig. 4.15) and Dirksen and Henderson 

(2002), with additional input from T. Waller (pers. commun., 2009).

?
194

Uruguay–Yellow Anacondas are found only in the far northern province (Achaval and 

Olmos, 2003), but this is apparently based on a single specimen from the delta of the 

Cuareim River. Individuals may be transported by floods into Uruguay down the 

Uruguay River, but appear to represent single waifs rather than established populations 

(Dirksen, 2002).

3.1.2 DeSchauensee’s Anaconda, Eunectes deschauenseei

This species has a much smaller range than does E. notaeus, and is largely 

confined to the Brazilian island of Marajó, nearby areas around the mouth of the Amazon 

River, and several drainages in French Guiana. The large gap between the Brazilian and 

French Guianan populations may be a result of undersampling or may reflect disjunct or 

relictual populations.

Brazil–Present in the states of Amapá and Pará, from Santarem to the mouth of the 

Amazon River (Chippaux, 1986; Starace, 1998; Dirksen, 2002). However, the species is 

known from few specimens, and its distribution may include smaller rivers draining to 

the north or south, and/or slightly higher reaches of the Amazon River.

French Guiana–Considered rare by both Chippaux (1986) and Starace (1998) and 

known from only three museum specimens and a few additional sightings/captures in 

French Guiana. The museum specimens were found in the Maroni, Mana, and 

Approuague drainages. These localities are widely spaced, including the northeast of the 

country near Kaw, the western border and a locality approximately 100 km inland from 

the coast. Although the Maroni borders Suriname, no specimens are known from the 

latter country. The area around Kaw is a large open wetland surrounded by mountains, 

with similarly isolated populations of Black Caiman (Melanosuchus niger; T. Waller, 

pers. commun., 2009), which suggests that Deschauensee’s Anacondas in this area 

represent a relictual population. 

3.2 Habitat Range

The Yellow Anaconda appears to be restricted to swampy, seasonally flooded, or 

riverine habitats throughout its range (Dirksen, 2002, Strüssmann and Sazima, 1993, 

Waller and others, 2007). In the Pantanal of Brazil, individuals spend the wet season in 

seasonally flooded habitats and spend the dry season near permanent water (Strüssmann 

and Sazima, 1993). In the Formosa Province of Argentina, the two primary habitats are: 

(1) palm savanna interspersed with creeks and rivers that abut marshes and riparian 

forest; and (2) the Paraguay River basin, which includes seasonally flooded forest, palm 

savannah and oxbow lakes (Waller and others, 2007). Although not as well known, 

DeSchauensee’s Anaconda apparently prefers swampy and riverine habitats that may be 

seasonally flooded (Starace, 1998; Dirksen and Henderson, 2002).
195

3.3 Climate Range

3.3.1 Climate Range of Yellow Anacondas

The Yellow Anaconda inhabits a fairly temperate climate range, including 

localities with cold-season monthly mean temperatures around 10oC and no localities 

with monthly means exceeding 30oC in the warm season. Most localities experience 

moderate variation in rainfall during the year, but overall the climate range of this species 

is considerably smaller than that of some of the other species reviewed in this document 

that range across large tropical and temperate areas. Yellow Anacondas are able to 

largely escape extremely cold temperatures in the native range by moving to deeper water 

that stays warmer than ambient water does; while parts of their range in Argentina 

experience winter lows below -5oC, individuals with implanted temperature data loggers 

did not display body temperatures below 6oC (T. Waller, pers. commun., 2009). 

Figure 8.3. Climate space for E. notaeus, with reference lines and localities as introduced in Fig. 

2.1. Based on 37 known localities from the native range. Climate loops color-coded as follows: 

Yellow = Argentina; Dark Blue = Bolivia; Green = Brazil; Turquoise = Paraguay; Red = Uruguay.

3.3.2 Climate Range of DeSchauensee’s Anaconda

DeSchauensee’s Anaconda is known from only a few localities in northeast South 

America, and its known climate range is accordingly very small. While the occupied 

range exhibits moderate variation in precipitation across the year, annual temperatures 

tend to range between 25o

C and 30oC. Whether the species could tolerate greater 

climatic variation is unknown.

0

5

10

15

20

25

30

35

40

0.01 0.1 1 10 100 1000 10000

Mean monthly precipitation (mm)

Mean monthly temperature (C)
196

Figure 8.4. Climate space for E. deschaeunseei, with reference lines and localities as introduced 

in Fig. 2.1. Based on six known localities from the native range. Climate loops color-coded as 

follows: Blue = Brazil; Red = French Guiana.

3.4 Microhabitat

Data that could be used to determine the type and frequency of microhabitat use 

have not been published for the yellow anacondas. Both species appear to spend a large 

proportion of their time submerged, resulting in few visual detections by humans. By far 

the best available ecological data for the species come from populations in Argentina 

(Micucci and others, 2006; Micucci and Waller, 2007; Waller and others, 2007), but the 

majority of these data originate with anacondas collected for the commercial skin trade 

(see 7.1, below), which is done by local hunters. These hunters would be expected to 

have the ability to predict when and where Yellow Anacondas of salable size can be 

collected using visual searching during the legal harvest season, but are perhaps less 

likely to know precisely where snakes spend their time during the rest of the year. 

During the winter harvest season when their habitats are flooded, anacondas from 

this population are collected from basking sites on emergent logs/palm trunks and 

vegetation mats, often covered with vines and other vegetation (Micucci and Waller,

2007; Waller and others, 2007). During the dry season (local spring and early summer), 

these individuals appear to congregate and seek refuge in pockets of dense vegetation 

(Typha latifolia) that retain moisture (Waller and others, 2007); overall, Argentine 

individuals spend virtually all of their time in heavily vegetated areas, especially along 

the shores of marshes and lagoons (T. Waller, pers. commun., 2009). Yellow Anacondas 

appear to select warm, shallow water for reproductive behaviors and fairly shallow water 

during spring and summer, but move to deeper water with more stable temperatures 

during the winter (T. Waller, pers. commun., 2009, based on unpublished radiotelemetric 

data). The microhabitats used for foraging, reproduction, and other activities have thus 

0

5

10

15

20

25

30

35

40

0.01 0.1 1 10 100 1000 10000

Mean monthly precipitation (mm)

Mean monthly temperature (C)
197

far not been identified for this population, and radiotelemetric studies would likely be 

required to quantify such microhabitat use. 

3.5 Introduced Ranges

We are aware of no established introduced populations of either species. An adult 

Yellow Anaconda was collected from Big Cypress National Reserve in south Florida in 

January 2007 (S. Snow, pers. commun., 2007), and another individual was photographed 

basking along a canal about 25 km north of this location in January 2008 (A. Flanagan, 

pers. commun., 2008). More recently (2008), an unnamed observer reportedly captured 

two anacondas that most closely fit the description of the Yellow Anaconda farther to the 

east near the Palm Beach County Line; these were reported to have been sold to a snake 

dealer/breeder (S. Snow, pers. commun., 2008).

3.6 Seasonal Activity

Dirksen (2002) concluded that the Yellow Anaconda in the Pantanal of Brazil was 

most active in the dry season (July-November), largely based on Strüssmann’s (1997) 

observation that 76 percent of prey removed from Yellow Anacondas was recovered 

during this period. However, anacondas are likely to be less detectible during the wet 

season when they are dispersed across seasonally-flooded wetlands, which may have led 

to under-representation of wet-season Yellow Anacondas containing prey in 

Strüssmann’s (1997) sample. 

Yellow Anacondas in Argentina are active year-round, including confirmed 

feeding activity throughout the year. However, individuals become somewhat sedentary 

in the winter and spend much of their time basking to elevate body temperatures above 

the threshold required for gonadal processes and digestion (Waller and others, 2007). 

Basking primarily occurs when water temperatures fall below 15o

C. Snake captures in 

Argentina decrease markedly when temperatures warm in the spring (October) and 

snakes reduce the amount of time spent basking. Radiotelemetered individuals move 

more than 2 km in a season and stay in one spot for several days before moving on (T. 

Waller, pers. commun., 2009). Adults of ca. 2 m body length exhibit activity ranges 

averaging 15 ha, while large (ca. 3.5 m) females have ranges of about 50 ha (T. Waller, 

pers. commun., 2009).

3.7 Daily Activity

Strüssmann and Sazima (1993) observed Yellow Anacondas actively foraging at 

night in Brazil, but the species appears to frequently exhibit both diurnal and nocturnal 

foraging behaviors (Strüssmann, 1997; Dirksen, 2002), with indications of crepuscular 

activity peaks in more temperate parts of the distribution (T. Waller, pers. commun., 

2009). In Argentina, individuals bask during the cold season and during cool days in 

summer, initiating basking around 10 AM; during extended cool periods, Yellow 

Anacondas move to vegetated deeper water and become inactive (T. Waller, pers. 

commun., 2009, based on unpublished radiotelemetric data). DeSchauensee’s Anaconda 

is reported as being nocturnal (Starace, 1998), but insufficient data are available to assess 

this conclusion. See the Green Anaconda biological profile for more discussion of the 

pitfalls of inferring diel activity based on opportunistic sightings of highly aquatic snakes.
198

3.8 Foraging Mode/Sensory Modalities

The Yellow Anaconda appears to employ both ambush predation and active 

searching for its prey (Dirksen, 2002), and may use active searching to a greater extent 

than is commonly credited. Although Yellow Anacondas are considered to be strongly 

aquatic in their foraging behavior, Strüssmann and Sazima (1991) reported observing 

individuals in Brazil taking birds from trees at heights of about 2.5 m.

4.0 Life History

4.1 Reproductive Mode

Both yellow anacondas are live-bearing. There is no evidence of parthenogenesis 

in these species, but we would not be surprised by its existence given anecdotal evidence 

for facultative parthenogenesis in three other genera of New World boid snakes (see 

Green Anaconda biological profile in chapter Nine).

4.2 Reproductive Phenology

Timing of reproductive events is likely to exhibit some variation among 

populations of the Yellow Anaconda, as would be expected of a species with a wide 

latitudinal range. In temperate parts of the species’ range, temperature appears to be the 

primary factor that drives reproductive seasonality, with most reptiles in the area 

exhibiting synchronized spring copulation and late spring/summer oviposition or 

parturition (T. Waller, pers. commun., 2009).

Females in Formosa, Argentina exhibited seasonal and synchronous reproductive 

cycles, with follicle size slowly increasing from March to June (while their habitats are 

flooding in the antipodean fall) and then accelerating rapidly until October. Formosan 

males were also largely synchronous in reproductive condition, with testicular 

recrudescence from February to April. Virtually all adult males contained turgid testes 

and/or sperm-containing efferent ducts during the pre-reproductive months of AprilSeptember (winter, flooded season). Argentine Yellow Anacondas bask on emergent 

logs and vegetation when water temperatures drop below 15o

C during the winter, and 

such basking is required for both sexes to complete gonadal cycles before the spring 

mating season (Micucci and Waller, 2007). 

Copulation occurs in October-November and parturition in February-March 

(Achaval and Olmos, 2003); these dates are apparently based on Brazilian data, as 

phenology is unknown for Uruguay. “Semi-captive” specimens (freshly captured) in 

Argentina copulated in September-November (onset of dry season, spring/summer), but 

mating behaviors dropped off sharply after October (Waller and others, 2007). While 

copulation was not observed in the field in Argentina, male efferent ducts attained 

maximal sperm scores from July to October, the tail end of which coincides with 

anecdotal reports of mating in the field as well as copulations in the semi-captive 

individuals (above). Field-collected females contained oviductal (fertilized) embryos 

starting in early October also. Anecdotal evidence supports the notion that Yellow 

Anacondas at least occasionally engage in breeding aggregations of multiple males and a 

single female (Waller and others, 2007), but the numbers of males involved and the 
199

duration of such aggregations have not been quantified as they have been for Green 

Anacondas (Rivas, 1999).

As with many other captive boids/pythonids, captive Yellow Anacondas are 

typically induced to mate by cooling them to 18-24oC for a couple of months 

(Bisplinghoff and Bellosa, 2007). Such experience with captives should not be used to 

justify any inference that the species requires ambient temperatures in this range to 

achieve reproductive readiness; whereas the lower temperature limit at which 

reproductive readiness can be achieved is unknown, the species appears to regularly 

experience much colder temperatures in parts of its native range (for example, Waller and 

others, 2007) and employs behavioral strategies (basking) to raise its body temperature 

above ambient during the pre-reproductive period.

The duration of gestation is 4-6 months, with a maximum recorded gestation of 9 

months (Waller and others, 1995). Gestation was 160-180 days in seven semi-captive, 

wild-caught females maintained in Argentina, with parturition occurring between 12 and 

28 April.

Reproductive phenology of DeSchauensee’s Anaconda is largely unknown. Based 

on dissections of five preserved females, vitellogenesis has been inferred to be from 

August through February and gestation from June into April (Pizzatto and Marques, 

2007). If accurate, these extended reproductive periods may be due to a less seasonal 

tropical climate producing more asynchronous reproductive periods, as compared to the 

Yellow Anaconda.

4.3 Reproductive Effort/Fecundity

4.3.1 Clutch or Litter Size/Frequency

Various authors give the number of Yellow Anaconda offspring as 12-40 

(Achaval and Olmos, 2003), 10-20 with a maximum of 33 (Waller and others, 1995), 10-

30 (Walls, 1998a), 13 (Belluomini and others, 1959), 12-22 (Norman, 1994), maximum 

of 37 (Williams and Scrocchi, 1994), “up to 56” (Bisplinghoff and Bellosa, 2007, citing 

an unpublished zoo report), etc. Many of the more general sources do not distinguish 

between wild and captive females (for example, the tabulated data of Bisplinghoff and 

Bellosa, 2007), even though litter sizes of the latter might be expected to be larger on 

average due to more generous feeding. Similarly, most sources provide little information 

on the relationship between dam size and offspring number. For example, captive 

females of about 2.5 m total length have been recorded to give birth to 13, 16, and 18 

young (Belluomini and others, 1959; Holmstrom, 1982; Luttenberger, 1984), but similar 

information is lacking for most wild populations.

As with other reproductive variables, the best available data on litter size come 

from the work of Waller and others (2007) in the Formosa Province of Argentina. Mean 

litter size among 11 wild-caught females that reproduced in “semi-captive” conditions 

was 19.5, with a range of 7-42. The upper end of this range is higher than the maximum 

recorded in all but one previous report, and higher than any litter sizes outside of 

captivity (see above). Female SVL was positively related to litter size, albeit with a small 

sample size of litters, and the increase in litter size with female body size appeared to be 

linear rather than allometric. Both the maximal number of vitellogenic follicles (around 

80) and the maximal numbers of oviductal scars (around 50) significantly exceeded 
200

maximal observed litter size, perhaps indicating that some follicles are resorbed and/or 

unfertilized. 

In the Formosa Province of Argentina, 54 percent of mature female Yellow 

Anacondas contained vitellogenic follicles, indicating that about half of females are 

capable of breeding in any given year (Waller and others, 2007). This proportion would 

be expected to exhibit temporal variability both within and between populations 

depending on local climatic, hydric, and other conditions. The presence in this 

population of virgin females well above the average size at maturity also implies that not 

all females capable of breeding end up producing offspring.

Clutch size of DeSchauensee’s Anaconda ranged from 3 to 27 (mean 10.6 ± 9.6) 

in a sample of five museum specimens (Pizzatto and Marques, 2007), a range far greater 

than reported in some general works (for example, 3-7 offspring; Walls, 1998a).

4.4 Growth

No mark-recapture data are available to assess individual growth rates in the wild, 

but anecdotal evidence suggests that growth rates can be rapid. Free-ranging Yellow 

Anacondas can double their body mass in less than a year after birth, and can mature in 

the second or third spring after birth (T. Waller, pers. commun., 2008; Waller and others,

2007). Growth rates can be even higher in captivity; captives can attain 1.8 m TL and 2.6 

kg within two years (Norman, 1994), thus roughly tripling their neonatal length and 

increasing mass by 20-40 times over the neonatal body sizes provided by Waller and 

others (1995). 

4.5 Maturation

In males from Formosa, Argentina, sexual maturity typically occurred between 

1.48 and 1.66 m (1.28 and 1.43 m SVL), and average length of the ten smallest mature 

males in a sample of several hundred individuals was 1.57 m (1.35 m SVL). Females 

exhibited a much wider range of body sizes at maturity, including a pre-reproductive 

individual of 1.69 m (1.47 m SVL) and a virgin non-reproductive female of 2.96 m (2.57 

m SVL). The latter, however, does not equate with immaturity as a mature female may 

not have mated in previous years (Waller and others, 2007). Using the presence of 5-mm 

follicles to determine maturity, Waller and others (2007) inferred maturity from 1.67 to 

2.13 m (1.45 to 1.85 m SVL), and proposed 1.9 m (1.65 m SVL) as a general threshold 

for maturity. Males in the Formosa population may mature as early as 17 months after 

parturition. Females may be physiologically mature in their second season after birth but 

require a second full year to attain energetic reserves required to reproduce, thus 

reproducing as early as 29 months.

A captive-born female appeared to be gravid at 3 years, 10 months of age, and 

gave birth to 18 young at 4 years, 2 months of age (Holmstrom, 1982). This is roughly 

concordant with Walls (1998a), who estimated maturation in captivity at “about” 4 years 

of age. A male captive reached 1.5 m and a female 2.1 m in their first year (Lederer, 

1942; Brandes, 1971: both cited in Waller and others, 2007), which places both 

specimens at or above the threshold of maturity identified by Waller and others (2007) in 

Argentina.
201

4.6 Longevity

We are aware of no records for longevity among free-ranging Yellow Anacondas 

or DeSchauensee’s Anacondas. In captivity, Yellow Anacondas have lived for over 20 

years, and DeSchauensee’s Anacondas for 17 years, 11 months (Snider and Bowler, 

1992).

5.0 Diet/Trophic Role

5.1 Prey

The yellow anacondas appear to be generalist predators on a range of vertebrates.

In the Pantanal of Brazil, the diet of the Yellow Anaconda includes fish, turtles, caimans, 

aquatic birds (ducks, herons, storks), rodents (Nutria and Capybara), and carrion 

(Strüssmann and Sazima, 1991; Strüssmann, 1997; Dirksen, 2002). This pattern was 

repeated among Yellow Anacondas in Argentina, which consumed fish, snakes, birds, 

and mammals (Waller and others, 2007); notable items in this sample included three 

False Water Cobras (Hydrodynastes gigas, a large colubrid snake), multiple Neotropical 

Cormorants (Phalacrocorax olivaceous), a Roseate Spoonbill (Platalea ajaja), a 

Capybara (Hydrochoerus hydrochaeris), and many rodents the size of rats. 

Both the Brazilian and Argentine samples included a preponderance of birds in 

the diet, accounting for over 50 percent of the Brazilian sample and around 50 percent of 

the Argentine sample. Fish did not figure prominently in either sample, but most of these 

snakes were collected from seasonally flooded wetlands where fish would be expected to 

be at low densities. Individuals in riverine habitats may eat more fish, and Strüssmann 

and Sazima (1991) mention a young Yellow Anaconda that was observed consuming 

small fish, while T. Waller and colleagues have observed anacondas scavenging dead fish 

from drying ponds during droughts (T. Waller, pers. commun., 2009).

The diet of Yellow Anacondas in Uruguay is unknown (Achaval and Olmos, 

2003), while in the Paraguayan Chaco an individual was observed consuming a Wood 

Stork (Mycteria americana: Norman, 1994). A series of Yellow Anacondas collected 

from the Paraná River during the winter contained rodents (Brazilian Guinea Pig [Cavia 

aperea] and Waterhouse’s Swamp Rats [Scapteromys tumidus], a Lutrine Opossum 

[Lutreolina crassicaudata], a Unicolored Blackbird [Agelaius cyanopus] and unidentified 

feathers: Waller and others, 2001). As with Green Anacondas, the Yellow Anaconda 

exhibits durophagy (eating hard-shelled items), which is relatively uncommon among 

snakes (Savitsky 1983); hard-shelled items recovered from Yellow Anacondas have 

included turtles such as the Pantanal Swamp Turtle (Platemys macrocephala), 

Vanderhaege’s Toad-Headed Turtle (Phrynops vanderhaegei), and eggs of the Limpkin, 

Aramus guarauna (Strüssmann and Sazima, 1991; Waller and others, 2007). This 

observation of egg-eating is noteworthy because the juvenile Yellow Anaconda (1.9 m; 

1.65 m SVL, 3.2 kg) had eaten three Limpkin eggs, each of which was only ca. 2 percent 

of the snake’s mass—these are very small prey items for a boid snake (Greene, 1997). In 

the Formosa province of Argentina, anacondas have consumed eggs of South American 

Screamers (Chauna torquata), as well as an aquatic snail (Ampullaridae) of about 5 cm 

diameter (T. Waller, pers. commun., 2009).
202

DeSchauensee’s Anaconda is reported to consume mammals, fish, and birds 

(Starace, 1998), and its overall diet may be similar to that of the Yellow Anaconda.

5.2 Predators

Predation on the yellow anacondas is poorly documented in the literature, 

although Yacare Caiman (Caiman yacare) are reported to consume them, and there have 

been rare observations of cannibalism (T. Waller, pers. commun., 2009). We assume that 

the documented and suspected predators of Green Anacondas (see biological profile of E. 

murinus, chapter Nine) would also be capable of subduing and consuming yellow 

anacondas, at least for those predatory taxa sympatric with the latter. As with other 

anacondas, E. notaeus and E. deschauenseei engage in protective balling posture when 

harassed by humans (Dirksen and others, 1998). 

5.3 Diseases and Parasites

While general information on the identification and treatment of reptile parasites 

and some viral/bacterial pathogens is readily available (Jacobsen, 1986a, 1986b; 

Klingenberg, 1993; Köhler, 1996; Mader, 1996), specific information for the Yellow 

Anaconda and DeSchauensee’s Anaconda appears to be lacking. Wild-caught anacondas 

imported to Europe are reported to be riddled with parasites (Bisplinghoff and Bellosa, 

2007), but the average veterinarian simply identifies parasites at a gross level (nematode, 

haemogregarine, cestode, etc.) and administers treatment according to gross taxon rather 

than identifying them to species. Similarly, while many ailments appear to be especially 

common among captive anacondas (respiratory infections, fungal problems, etc.), the 

demographic significance of such ailments in natural populations is unknown, and could 

range from minor (for example, Brown and others, 2006) to potentially severe (for 

example, Schumacher and others, 1994). 

6.0 Demography

6.1 Population Density

As discussed in the biological profiles for other constrictor species, population 

density is difficult to quantify without rigorous mark-recapture or intensive censuses of 

bounded areas. No such studies have been pursued for the yellow anacondas to our 

knowledge. 

In a collection of snakes from the Pantanal of western Brazil, the Yellow 

Anaconda was represented by 35 individuals, or 15.1 percent of the total number of 

snakes captured; it was second in abundance only to Hydrodynastes gigas (Strüssmann 

and Sazima, 1993). In Argentina, Waller and others (2007) amassed a sample size of 

over 1,500 individuals while sampling snakes harvested for the commercial skin trade, 

and Micucci and Waller (2007) estimated 30–60 anacondas per km2 at one study site. 

Neither of these studies used mark-recapture to estimate population size, nor were 

detectibilities estimated. Regardless, the relatively large numbers of snakes captured in 

each of the two aforementioned studies appear to reflect the species’ ability to achieve 

high population densities in suitable aquatic habitats (H.W. Greene, pers. commun., 
203

2008), as well as the ability to sustain viable populations in the face of moderate harvest 

by humans. 

6.2 Size Distribution

Although Strüssmann and Sazima (1993) reported capturing several dozen 

Yellow Anacondas in the Pantanal of western Brazil, they did not provide size 

distributions. The sole available data based on adequate sample sizes therefore come 

from the results of Waller and others (2007), obtained from northeast Argentina. Among 

449 individuals collected in 2002, the distribution was strongly unimodal, with a peak in 

the 1.86-2.09 m class (1.6-1.8 m SVL) and 72 percent of anacondas fell into the 1.62-

2.56 m size classes (1.4-2.2 m SVL). As expected for a snake with sexual size 

dimorphism, male size distributions differed from those of females. Males dominated 

(81% of individuals) size classes between 1.62 and 2.09 m (1.4-1.8 m SVL), but larger 

size classes (1.84-3.06 m, or 1.6-2.6 m SVL) are composed primarily of females, but 75 

percent of females falling into the 1.84-3.06 m size classes (1.6-2.6 m SVL). Juveniles

comprised only 13.4 percent of the sample, and female juveniles outnumbered male 

juveniles. Although these data were collected as part of an examination of anaconda 

harvest for the skin trade, snake hunters were paid equally for snakes of all sizes during 

accumulation of data for size distributions, thus minimizing the possibility that hunters 

were preferentially targeting large snakes. Any remaining biases are thus likely to be due 

to differences in detectability across individuals rather than due to preferential harvest of 

certain size classes.

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

Yellow Anacondas represented a large proportion of the historical trade in skins 

of New World boid snakes. From 1980 to 1999, several hundred thousand skins were 

exported from Paraguay and Argentina, mostly to the United States and Europe (Waller 

and Micucci, 1993; Dirksen, 2002; Micucci and others, 2006), and the trade was largely 

unregulated for over 60 years (Micucci and Waller, 2007). The Argentine trade was 

halted in 1999 due to conservation concerns, and an experimental harvest was initiated in 

the state of Formosa in 2002, overseen by biologists. This program resulted in harvest of 

16,517 skins averaging 2.6 m long during the first three years (Micucci and others, 2006; 

Waller and others, 2007). The program allows collection of snakes over 2.32 m (2.0 m 

SVL) during the winter (June-August) when they are basking above water (Micucci and 

Waller, 2007). Extended from 2002 through 2007, the long-term average harvest was 

3,800 skins per year (Micucci and Waller, 2007). This level of take has been deemed 

sustainable by the scientists supervising the harvest, who estimate that fewer than 5 

percent of adult females are removed annually (Waller and others, 2007), although this 

judgment has not been without controversy (Rivas, 2007; Waller and Micucci, 2008). 

The Argentine harvest is currently the only legal source of Yellow Anaconda skins in 

international trade (T. Waller, pers. commun., 2009).
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In French Guiana, killing DeSchauensee’s Anaconda is considered to bring 

misfortune on one’s family (Starace, 1998). We are unaware of any international trade in 

skins of DeSchauensee’s Anaconda.

7.2 Human Health Risks

Because they attain smaller sizes than do some of the truly giant constrictors, the 

yellow anacondas present a low risk level to humans. We are unaware of any welldocumented attacks by free-ranging yellow anacondas on humans. In captivity, the 

Yellow Anaconda is often considered irascible and less predictable than other large 

snakes (for example, Boa Constrictor from South America, Indian/Burmese Pythons; 

RNR, pers. obs. of captive animals), but injuries from Yellow Anaconda bites to their 

keepers are usually minor. However, given that some herpetoculturists consider any 

python over 8 feet (2.4 m) in total length to be capable of killing an adult human (Flank, 

1997), caution should be exercised when capturing or handling adult Yellow Anacondas, 

especially large females approaching 4 m in length and over 30 kg body mass.

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

The Yellow Anaconda is an attractive animal, and is considered by some to be a 

more appropriate choice as a pet than the related Green Anaconda; this judgment is 

generally due to the mammoth size attained by the latter. However, anacondas in general 

are relatively uncommon in captivity as compared to many members of the genera Boa

and Python. About 790 live Yellow Anacondas were imported to the United States 

during 1989-2000 according to USFWS Law Enforcement Management Information 

System records (Reed, 2005), an average rate of about 70 per year. Over a slightly longer 

period of time, CITES records revealed an average rate of 63 live Yellow Anacondas per 

year imported to the United States from 1977 through 2007. However, there was 

considerable variation among years (range 0-249 individuals/year). This is about 15 

percent of the rate at which Green Anacondas were imported during this time period, and 

less than 1 percent of the importation rate of taxa including Ball Pythons, Boa 

Constrictors, and Reticulated Pythons. 

Dirksen (2002) reported that exports of live animals from Paraguay fell off after 

1996, but that the volume of intra-European trade reflects continued imports to Europe 

and that there appears to be a thriving illegal trade. While CITES records documented 

only two individuals imported to the United States during 2005-2007, over 50 per year 

were imported over the previous decade; we also note that CITES says that 2005 is the 

most recent year for which data are complete. Captive breeding of the Yellow Anaconda 

is being accomplished more and more frequently in the United States and Europe, but 

overall production appears to be low (Bisplinghoff and Bellosa, 2007), and we are aware 

of no credible estimates of the volume of domestic production. We are unaware of any 

international trade in live DeSchauensee’s Anacondas.
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0

50

100

150

200

250

300

1977197919811983198519871989199119931995199719992001200320052007

Year imported

Number Number imported imported

Year imported

Figure 8.5. Annual imports of Yellow Anacondas, Eunectes notaeus, to the United States during 

the period 1977-2007. Data from CITES.

7.3.2 Other Uses

Major human uses of Yellow Anacondas appear to be limited to skins and the live 

animal trade, although their meat may be consumed in some parts of the native range.

Management Profile

See the introduction to the management profile for Indian Pythons (chapter Four) 

for an explanation of the format of this section of the risk assessment. 

8.0 Pathway Factors: Pet Trade

8.1 Volume of International Trade

The volume of international trade in the Yellow Anaconda is low, with a range of 

0 to about 250 snakes per year entering the United States. Within this range, trade 

volume changes drastically from year to year, implying that the few imports documented 

in recent years are not necessarily a good indicator of what to expect in the future. 

Furthermore, the observations of Dirksen (2002) regarding apparently high levels of 

smuggling in Europe may also be applicable to the United States import trade, although 

there is insufficient evidence. Section 8.1 of chapter Four outlines some reasons why 

imports may constitute higher risk as invasive species than captive-reared individuals. 
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International trade in DeSchauensee’s Anaconda appears to be virtually nonexistent, although the high volume of Green Anacondas exported from Suriname leads to 

questions of whether DeSchauensee’s Anacondas from next-door French Guiana might 

find their way into the export market at some point. 

8.2 Volume of Domestic Trade

We are unaware of any credible estimates of the volume of domestic sales or 

captive propagation of the Yellow Anaconda. Informal surveys of online and storefront 

reptile dealers lead us to suspect that only a few successful breedings of Yellow 

Anacondas occur per year in the United States and that domestic production is likely to 

be under 200 neonates per year, but we have no means of verifying this suspicion. 

9.0 Entry Potential (Survival in Transit—Meant Mainly for 

Unintentional Transport)

As with most species assessed in this document, survival of yellow anacondas in 

international transport is likely to be very high, as animals are packed and shipped via 

means designed to ensure their survival. While mortality rates of some reptiles have 

historically been very high during international transport (Christy, 2008), large, hardy 

species of snakes would appear to be among the taxa most likely to survive the travails of 

international transport. Once in the United States and purchased by an endpoint 

consumer, the anaconda is likely to be housed in conditions that are designed to meet its 

needs. Survival in captivity is likely to be high except for snakes possessed by extremely 

negligent owners.

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

Most of the factors promoting the release of pet Indian Pythons (see section 10.1 

of chapter Four) are likely to apply to yellow anacondas. The smaller size attained by the 

latter may result in a lower rate of escape by breaking out of cages by brute force, and 

lower rate of release due to individuals becoming unmanageably large in captivity. 

Indeed, Yellow Anacondas are occasionally promoted in the herpetoculture literature as 

being more suitable pets than their truly giant relatives capable of exceeding five meters 

in body length. Although the volume of domestic trade/production is unquantified, it 

may equal or exceed the volume of imports. If so, then a lower proportion of captive 

Yellow Anacondas would be expected to harbor high parasite/pathogen levels. 

Although the preceding factors may reduce the odds of captives escaping or being 

released, they may be countered by other factors. As compared to “calm” species like 

Indian Pythons and South American Boa Constrictor, Yellow Anacondas are often 

described as prone to biting and exuding a foul-smelling musk when handled (T. Waller, 

pers. commun., 2009). Furthermore, the necessity of providing water in which the snake 

can submerge greatly complicates the captive husbandry of the species; snakes can 

develop fungal or bacterial infections if the water is not cleaned regularly, and large 

Yellow Anacondas require a large amount of water. Both chronic illness and 
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expensive/complex caging requirements may increase the odds of a captive being 

released.

10.2 Climate Match: Areas of the Country at Greatest Risk

The Yellow Anaconda has a native-range distribution that includes highly 

seasonal and fairly temperate regions in South America. When projected to the United 

States, the climate space occupied by E. notaeus maps to a fairly large area, including 

virtually all of peninsular Florida and a corner of southeast Georgia (to about the level of 

Brunswick), as well as large parts of southern and eastern Texas and a very small portion 

of southern California. Large areas of Hawaii and Puerto Rico appear to exhibit suitable 

climates, and additional insular United States possessions (Guam, Northern Marianas, 

American Samoa, etc.) would probably be suitable as well. Within the areas deemed 

suitable, however, the Yellow Anaconda would be expected to occupy only habitats with 

permanent surface water. 

The Yellow Anaconda is able to escape temporary air temperatures below 

freezing in its native range by moving to deeper waters, and cold-season body 

temperatures of 6o

C have been recorded in free-ranging individuals. The ability of the 

species to similarly escape cold temperatures by behavioral means if introduced to the 

United States would likely depend on a poorly-explored interaction between the duration 

of such periods of cold body temperature and the absolute water temperature minima.

Rainfall has only an indirect effect on the life of an aquatic snake; it could be a 

misleading indicator of seasonal climate suitability. Much of the area described as “too 

wet” has a combination of rainfall characteristics and cool winter temperatures that are 

outside of the climate space that we bounded in Fig. 8.3. This zone includes mean 

monthly rainfall values between 100 and 300 mm and mean monthly temperature values 

below about 25oC (in other words, the area just to the right of, and below, the bounded 

climate space of Fig. 8.3). Although our climate space algorithm deemed this area 

unsuitable according to the boundaries we identified from native range climate records, 

we have no a priori ecological reason to believe that Yellow Anacondas would be 

adversely affected by additional rainfall up to a monthly maximum of 300 mm. 

Therefore, it seems very likely that the species could persist in areas with additional 

rainfall so long as these areas were within the temperature limits known from the native 

range. If so, then Coastal Plain portions of the area identified as too wet would actually 

be suitable. Similarly, areas in the Southwest deemed “too dry” under this climate space 

extrapolation could be thermally suitable so long as aquatic habitats are present. If these 

suggestions are valid, then Fig. 8.6 would be a significant underprediction of suitable 

regions of the United States.
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Figure 8.6. Areas of the United States matching the climate envelope expressed by E. notaeus

in its native range (Figure 8.3). See chapter Two for details of analytical method.

As would be expected of an exclusively tropical reptile with a small geographic 

range, DeSchauensee’s Anaconda appears to have a poor climate match with the United 

States (Fig. 8.7.). We identified no areas of the continental United States or Hawaii that 

appear to have precipitation and temperature profiles similar to those observed in the 

species’ native range, although the southern margin of Puerto Rico and adjacent islands 

(for example, Vieques) appear suitable. However, extending the climate match to the 

globe (not depicted) indicates that much of the Amazon Basin and some other tropical 

areas of the world appear to be climatically suitable. Such a result indicates that the 

native range of DeSchauensee’s Anaconda does not appear to be bounded by unsuitable 

climate, but may instead be due to other factors (biogeography, climate change, 

competition or other ecological factors, etc.). If the current range reflects historical or 

ecological limitations rather than climatic tolerances of the species, then Figure 8.7 could 

be a significant underestimate of actual suitable climate in the United States.
209

Figure 8.7. Areas of the United States matching the climate envelope expressed by E. 

deschauenseei in its native range (Figure 8.4). See chapter Two for details of analytical method.

10.3 Prey Availability

The anacondas in general exhibit among the broadest diet range of any snake, 

including ectotherms (lizards, crocodilians, turtles, snakes, fish) and endotherms (birds, 

mammals). All but the largest species considered at risk of potential predation by the 

Indian Python (see section 10.3 and Table 4.1, chapter Four) could be considered 

potential prey for the yellow anacondas, with the possible addition of more species of 

turtles, aquatic and semi-aquatic snakes, and perhaps a higher proportion of crocodilians.

The Yellow Anaconda appears to be especially prone to consuming birds, which 

are omnipresent in the aquatic habitats in the United States that appear to have suitable 

climate for the snake. Whereas aquatic and semi-aquatic organisms would likely 

constitute the bulk of the diet of the Yellow Anaconda were the species to become

established in the United States, we see no obvious limitations on prey availability. The 

ability to persist on smaller prey items implies that the species could fare better in insular 

situations than would the true giants, were adequate freshwater habitats to be available.
210

10.4 Predation Risk

There are no predators in the United States that are likely to specialize in 

consuming yellow anacondas. However, this species does not attain the stupendous 

proportions of the Green Anaconda or some pythons, so a larger proportion of an 

introduced population might be vulnerable to predation by native predators. Aquatic and 

semi-aquatic predators would be the most likely to consume yellow anacondas, and adult 

alligators in particular would likely be effective predators on all but the largest 

individuals. Avian predators might also consume some juveniles. However, the 

anaconda’s tendency to remain submerged much of the time would likely render it nearly 

invisible to most visual predators during much of the year. 

10.5 Reproduction Requirements

There appear to be no specialized reproductive requirements of this species, 

which is a live-bearer. Bearing live young allows pregnant females to use behavioral 

thermoregulation (for example, basking, shuttling between habitats with different 

temperatures) in order to maintain relatively high and constant body temperatures during 

embryonic development. Live-bearing anacondas may be able to successfully reproduce 

in cooler climates as compared to egg-laying giant constrictors with similar temperature 

requirements for embryonic development (excluding those species that employ shivering 

thermiogenesis).

10.6 Hibernation Requirements

So far as is known, the yellow anacondas do not hibernate to escape cold 

temperatures anywhere in their native ranges. In fact, individuals near the southernmost 

(coldest) parts of the range of E. notaeus engage in exposed basking to raise body 

temperatures during the coldest winter months, and this is the time during which they are 

most vulnerable to collection by humans. During longer periods (1-2 weeks) of cold 

weather or overcast skies, snakes submerge in deeper water and rest among aquatic 

plants. Prey items have been found in Yellow Anacondas year-round, indicating some 

level of activity even in winter. Individuals may seek out and congregate in moist 

vegetated refugia during the dry season in seasonally flooded habitats, and long-term use 

of such refugia may approach a physiological/behavioral state that could be considered 

aestivation, but not hibernation.

10.7 Tools for Eradicating a Colonization

Tools for eradicating a population of yellow anacondas are likely to be similar to 

those described in chapter Three and section 10.7 of chapter Four, but no tools have been 

developed specifically for highly aquatic snakes such as anacondas. In its native range, 

the species is targeted via visual searching while individuals are basking during the 

winter, but the limited harvest does not appear to reduce population size appreciably. 

Over 800 snakes per year are harvested from sites of 2000-3000 ha in Argentina, with no 

evidence of reduction after seven years of the harvest program (T. Waller, pers. 

commun., 2009). If the area occupied by an introduced population were a largely 

inaccessible wetland (such as the Everglades, Okeefenokee, etc.), we doubt that visual 

searching alone would be sufficient for eradication.
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11.0 Spread Potential

11.1 Dispersal Ability

We are unaware of data sufficient to evaluate dispersal ability in yellow 

anacondas, although Argentine individuals are reported to move over 2 km in an active 

season. Yellow anacondas are highly aquatic, and there is no reason to think that they 

would be less effective at aquatic locomotion than Burmese Pythons of equivalent body 

size (see section 11.1 of chapter Four). Tomás Waller (pers. commun., 2009) reported 

that Argentine individuals move extensively in response to the constantly changing water 

bodies in the dynamic Pilcomayo River floodplain, and that he considers this species “a 

very able disperser.” Litter sizes of yellow anacondas are lower than the Green 

Anaconda or the giant pythons, but the upper end of the known fecundity range (over 40 

neonates for E. notaeus) is well above the maximum for most species of snakes. 

11.2 Ability to Develop Races or Strains

There are no native species in the United States with which the yellow anacondas 

would be capable of interbreeding, negating the risk of developing new races or strains 

by that route. However, Yellow and Green anacondas appear capable of interbreeding, 

although the frequency and fertility of hybridization events is largely unknown and 

hybrids have not been observed in the native range to our knowledge. If hybrids are 

fertile and exhibit characteristics of both species (for example, cold tolerance of Yellow 

Anacondas but increased size from Green Anaconda genetic contributions), the resulting 

hybrid might represent higher risk as an introduced species. However, we judge such a 

scenario to be fairly unlikely. 

11.3 Tools for Managing Spread

Generic management of giant constrictors was addressed in chapter Three, 

including opportunities for eradication of incipient colonizations. That chapter 

concluded that eradication has not been achieved for any giant constrictor and is unlikely 

on the U.S. mainland with the technology and early detection infrastructure presently 

implemented. However, spread and local population density may be somewhat more 

readily managed, especially if the area to be protected from spread is geographically 

isolated. In areas where water bodies are isolated from one another, isolation and 

eradication of yellow anacondas colonies would be more feasible than for species with 

good terrestrial dispersal capacities. The converse of this conclusion is that containment 

of yellow anacondas would be extremely difficult in large wetlands such as the 

Everglades or Okeefenokee. There may be improved management opportunities 

associated with interdiction to islands such as Hawaii or the Florida Keys which tend to 

lack large wetland areas. 

12.0 Economic Impact Potential

Regulatory agencies have not decided whether a formal economic analysis of the 

potential impacts of introduction of, and regulation of trade in, the yellow anacondas will 

be separately conducted pursuant to a variety of statutes and Executive Order 13272 
212

(Proper consideration of impact on small business). Our narrative below is provided only 

to cite relevant biological literature and identify germane sections of the biological 

profiles.

12.1 Pathogen Vector

There is insufficient evidence to evaluate whether the yellow anacondas regularly 

vector pathogens of zoonotic, veterinary, or agricultural importance, or whether they host 

parasitic vectors of such pathogens. So far as we are aware, the native-range pathogens 

and parasites of the species are virtually unknown. The yellow anacondas are likely to be 

vulnerable to many pathogens known from boid/pythonid snakes, but their aquatic habits 

may reduce the rate at which they are colonized by ectoparasites such as ticks. Tomás 

Waller (pers. commun., 2009) has never seen a tick on a Yellow Anaconda, despite 

seeing them on Boa and Epicrates in nearby areas.

12.2 Predator on Livestock

Because yellow anacondas are smaller than the giant constrictors, they are less 

likely to attempt predation of domestic ungulates, although predation on lambs (Dirksen, 

2002) and young pigs (T. Waller, pers. commun., 2009) has been reported. Domestic 

fowl that are housed and maintained away from water are also unlikely to experience 

impacts from Yellow Anacondas (although locals in Argentina report loss of chicks; T. 

Waller, pers. commun., 2009), but domestic waterfowl would be more vulnerable. 

Commercial production of fish, turtles, or other aquaculture species could conceivably be 

impacted by yellow anacondas, and mitigation for these impacts would likely be 

expensive (draining ponds, erecting snake barriers, etc.).

12.3 Predator on Pets

Most yellow anacondas do not attain sizes sufficient to ingest large breeds of 

dogs, but would be capable of consuming many smaller companion animals. As 

compared to more terrestrial constrictors, water-dwelling yellow anacondas would appear 

to be less likely to regularly encounter household pets. However, this risk could be 

somewhat elevated for pets in neighborhoods that include extensive freshwater habitats 

(around golf courses, canals, water features, etc.).

12.4 Electrical Power Systems

Yellow Anacondas have been observed climbing trees to consume birds, but none 

has been observed more than a few meters above the water to our knowledge and adults 

are less likely to climb. The heavy build of this snake likely precludes it from being a 

particularly good climber, and we would expect minimal impacts to electrical power 

delivery networks.

12.5 Traffic Accidents/Human Attacks

The smaller size of the yellow anacondas as compared to the truly giant 

constrictors means that only the very largest individuals in an introduced population 

might be large enough to cause vehicle accidents if a snake were struck by a car moving 

at a high rate of speed. To our knowledge there are no records of attacks on humans in 
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the native range of this species, although large captives can inflict serious lacerations 

through defensive bites.

12.6 Tourism

The yellow anacondas attain smaller body sizes than do true giants such as 

Reticulated Pythons, and are responsible for no human deaths that we are aware of. It 

should by rights therefore have minimal impacts on tourism should the species become 

established in the United States. However, most citizens do not differentiate between 

Yellow and Green Anacondas and the word “anaconda” is associated with giant 

anthropophagic snakes in the public mind, such that potential tourists hearing of the 

establishment of Yellow Anacondas might reconsider their travel plans. Such fears could 

be allayed by education and outreach efforts, but these might be expensive. Overall, 

however, establishment of Yellow Anacondas would be unlikely to have major impacts 

on tourism. 

12.7 Impacts on Commercially Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

Predation by yellow anacondas is unlikely to result in population declines of 

upland game species such as deer or turkey, but the snakes would frequently come into 

contact with, and certainly consume, waterfowl such as ducks or geese. Of these, it is 

likely that dabbling ducks that forage in habitats with extensive aquatic vegetation would 

be more likely to fall prey to anacondas than would diving ducks. Whether introduced 

anacondas could attain densities capable of exerting demographic impacts on ducks is 

unknown, but does not appear out of the question. Yellow Anacondas are known to 

consume wading birds such as herons and egrets, and may climb trees in or near the 

water to forage for birds. They would thus be capable of impacting many species of 

“watchable wildlife,” which are the mainstays for birdwatching and other 

nonconsumptive uses of wildlife.

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture

We foresee no major impacts to these industries if yellow anacondas were to be 

introduced to the United States, although workers might be reluctant to venture into 

aquatic habitats containing a species with the word “anaconda” in its name (see section 

12.6, above).

13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

As generalist predators on ecto- and endothermic prey, the yellow anacondas are 

capable of ingesting the same kinds of prey as Indian Pythons (chapter Four, section 

13.1), albeit the former would be unlikely to consume the very largest species on this list. 

The highly aquatic habits of anacondas reduce predation risk for potential prey that do 

not require regular access to water. On the other hand, the anacondas are more likely to 
214

consume aquatic ectotherms including fish and turtles; we judge predation on fish to be 

of low demographic significance for the majority of fish populations, but regular 

predation on turtles could have demographic impacts on species of conservation concern. 

Bird species that nest communally near aquatic habitats (for example, Wood Storks) 

might be vulnerable to Yellow Anacondas, as these snakes are known to occasionally 

climb trees to forage for birds.

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

Introduced yellow anacondas would be suitable prey for native predators such as 

American alligators, and juvenile anacondas would likely fall prey to a range of predators 

ranging from ospreys to indigo snakes. However, such benefits would probably be 

balanced by anaconda predation on the native predators (for example, consuming juvenile 

and subadult alligators). Furthermore, adding predatory anacondas to United States 

ecosystems would likely reduce overall biomass available to a native predator by 

reducing populations of other prey species (see sections 10.4 and 13.2 of chapter Six).

13.3 Vector for Disease Spread to or Within Native Faunas

Insufficient data are available to evaluate the likelihood of transmission of 

pathogens or parasites to native faunas. The yellow anacondas are likely to be suitable 

hosts for some of the same ectoparasites known from other taxa (for example, the giant 

African pythons) and which are known or suspected to be carriers of zoonotic disease. 

However, because they spend much of their time submerged, anacondas might be less 

suitable hosts for such ectoparasites, as evidenced by an apparent lack of ticks on Yellow 

Anacondas from Argentina.

13.4 Species that Might be Adversely Affected by Control Measures

Control measures for yellow anacondas would likely be similar to those 

envisioned for giant constrictors in general (see chapter Three). Some possible avenues 

of control, such as acetaminophen toxicants, are likely to result in non-target mortality to 

native species, especially those species capable of accessing toxicant delivery devices 

designed for giant constrictors. Traps, drift fences, and other potential tools may also 

impede movements of non-target native taxa or result in their inadvertent capture. 

Because yellow anacondas attain smaller body sizes than do some of the truly giant 

constrictors, control measures could likely be designed to exclude large non-target 

species, but conversely it would be more difficult to exclude small-bodied non-targets. 

Control measures for yellow anacondas would largely be deployed in or adjacent to 

aquatic habitats, implying that species using such habitats are at highest risk of being 

adversely affected by control measures. Depending on geography, these species might 

include Alligators, American Crocodiles, aquatic snakes, River Otters, etc. As no 

management tools have been adequately field-tested, however, it is not possible to 

estimate potential ecological costs.
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14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Production Affected?

Domestic production of Yellow Anacondas is trivial compared to species such as 

Reticulated Pythons or Boa Constrictors, and production of DeSchauensee’s Anaconda 

appears to be non-existent. Regulations on trade in species would disproportionately 

impact a small number of breeders specializing in anacondas.

14.1.2 International Trade/Alternate Pets

The declared import rates of Yellow Anacondas are variable across years but are 

uniformly low as compared to other species of giant constrictors, and there are no 

declared imports of DeSchauensee’s Anaconda. If recent records are an indication of 

future imports, then regulating trade in Yellow Anacondas would have minimal impact 

on reptile importers.
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Chapter Nine–The Green Anaconda, Eunectes 

murinus, and Beni Anaconda, Eunectes beniensis 

Biological Profile

1.0 Introduction

See section 1.0 of chapter Four for a description of species-specific and overall 

components of the risk assessment package.

1.1 Species

As currently understood, the Green Anaconda is a single wide-ranging species, 

Eunectes murinus, and is the world’s heaviest snake. Although it has an extensive 

distribution in South America, its ecology is known only from a small number of study 

sites, such as the seasonally flooded savanna wetlands called the “llanos” of Venezuela 

(Rivas, 1999). 

Dunn and Conant (1936) described two new anaconda species from the island of 

Marajó in northeastern Brazil; the newly-described E. barbouri was very similar to E. 

murinus, and the former species was shortly thereafter considered invalid by Amaral 

(1944). After further examination, both Strimple and others (1997) and Dirksen (2002) 

concluded that E. barbouri is simply a color morph of E. murinus, and not a valid taxon. 

A species closely related to E. murinus has recently been described from Bolivia (E. 

beniensis; Dirksen, 2002); see section 1.3 Evolutionary Context, below, for more 

information.

A parallel situation exists within E. murinus itself, as the species has historically 

been considered to comprise two subspecies (E. m. murinus and E. m. gigas). However, 

based on morphology and color pattern, Dirksen and Böhme (1998) sank E. m. gigas into 

synonymy with E. murinus murinus, resulting in a single wide-ranging species with no 

subspecific differentiation. This German-language publication has failed to achieve wide 

penetration in the United States, where E. m. gigas is still often mentioned as a valid 

taxon, especially among herpetoculturists.

The vast majority of literature on the Green Anaconda consists of anecdotal 

observations of single individuals in the field, comments on small series of specimens, 

and observations on captive snakes. While the biology of anacondas has been reviewed 

in past decades (for example, Petzold, 1984; Strimple, 1993), two recent authors have 

made especially important contributions to our knowledge of the genus. Lutz Dirksen 

compiled a thorough review of the genus, re-defined the taxonomy of anacondas, and 

provided insights on a variety of topics (Dirksen and others, 1998; Dirksen and Böhme, 
217

1998, 2005; Dirksen, 1999, 2002; Dirksen and Henderson, 2002). Jesús Rivas conducted 

the only long-term study of any population of Green Anacondas, resulting in sample sizes 

at least an order of magnitude larger than any previously available and providing the first 

reliable data on myriad aspects of Green Anaconda behavior and ecology (Rivas, 1998, 

1999; Rivas and Burghardt, 2001; Rivas and others, 1997, 2007a, 2007b). We relied 

heavily on the literature produced by both of these authors during the production of this 

bioprofile, and we feel fortunate to have such sturdy shoulders upon which to stand.

1.2 Common Names

Most English-language publications refer to Eunectes murinus as the Green 

Anaconda (for example, Rivas and others, 2007a), although some call it the Common 

Anaconda (for example, Holmstrom, 1982). In cases where a species is simply called 

Anaconda, authors are typically referring to the Green Anaconda (for example, Pope, 

1961). Minton and Minton (1973) reported that “anaconda” is a Sinhalese word 

translating to “elephant killer” that was originally applied to Python molurus, and that the 

name was brought by the Portuguese to the New World. Regional names are various, and 

include Water Camoodie or Kamudi (Creole, Guyana: Beebe, 1946; Pope, 1961), Ow-oorah and Sal-urring mah (Akawai Indian, Venezuela/Guyana: Beebe, 1946), Sucuriú, 

Sucuri, Sicurí, or Sucurijúba (Brazil, Bolivia: Ditmars, 1931; Pope, 1961; Strimple, 

1993), Guio de Agua, Petaca, or Guio Petequero (Colombia: Strimple, 1993), Huilla or 

Huile (Trinidad: Mole and Urich, 1894; Mole, 1924), Culebra de Agua (Venezuela: 

Lancini, 1986), and many others (see Dirksen, 2002 for an extensive list). There is thus 

far no English-language common name for E. beniensis in wide usage, although some 

herpetoculturists refer to it as the Beni Anaconda (Bisplinghoff and Bellosa, 2007).

1.3 Evolutionary Context

Green Anacondas are members of the family Boidae, which includes species 

found in the Americas, Africa, Asia, Europe, and many islands. The genus Eunectes is 

part of the South American radiation of boid snakes, which also includes the genera Boa, 

Epicrates, and Corallus; this group is distantly related to North American boids of the 

genera Charina and Lichanura. Phylogenetic analysis of molecular genetic data places 

Eunectes as the sister group to Epicrates (Burbrink, 2005; Noonan and Chippindale, 

2006). There are four species within the genus Eunectes as currently understood. The 

yellow anacondas, E. notaeus and E. deschauenseei appear to be sister species (Dirksen 

and Böhme, 2005). Eunectes beniensis is a recently-described species (Dirksen, 2002) 

from northern Bolivia, previously considered to be contained within E. murinus. 

However, Dirksen and Böhme (2005) reappraised the relationships within the genus, and 

proposed that the members are related as: E. murinus (E. beniensis [E. deschauenseei, E. 

notaeus]). Because E. beniensis is known from only six specimens, is not known from 

the pet trade, and has a limited range, we will not consider it separately in this risk 

assessment. To the extent that the risk is dependent on the natural history traits of the 

species, E. murinus will be the best proxy for E. beniensis.
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Figure. 9.1. Schematic representation of the head of E. murinus

2.0 Description

2.1 Unique Morphological Features

The Green Anaconda is probably the easiest to identify among the giant 

constrictors due to its relatively simple color pattern. The ground color is typically olive 

green, upon which are scattered dark (black to blackish-brown) ovoid blotches of varying 

size; these blotches can be single, paired, joined, or alternating down the midline, 

depending on the individual. There are typically no saddles or other types of dorsal 

patterns, making for a very “clean” appearance of black blotches on a solid background. 

A second lateral series of irregular dark markings is typically present, sometimes 

presenting as small circles with centers that are lighter, usually yellow, than the ground

color. 

There is some color variation across the range of the species—for example, 

individuals from southern Brazil have more and smaller dark dorsal blotches than do 

those in Peru, dorsal blotches tend to be round in Peru and more ovoid elsewhere (L. 

Dirksen, pers. commun., 2009), and ground color can be dark brown to blackish-brown in 

the Manaus area (Martins and Oliveira, 1998). The ground color in the upper Amazon 

Basin was reported as bluish-grey, with spots that can be dark olive rather than black 

(Dixon and Soini, 1986), but some individuals from the same area appear olive green (L. 

Dirksen, pers. commun., 2009). Although Boos (2001) cites Fawcett (1953) in support of 

the notion that the eyes of Green Anacondas reflect light, this notion is dismissed by 

others with more field experience with the species (J. Rivas, pers. commun., 2009).
219

The massive build of large specimens is also indicative of the Green Anaconda, 

and large individuals can have midbody diameters exceeding 0.30 m (Coborn, 1991). In 

comparison to pythons and Boa constrictor, the eyes and nostrils of all anacondas are 

positioned more dorsally, as is typical of many semi-aquatic vertebrates. 

Dirksen and Böhme (2005) defined the Green Anaconda as having 239-269 

ventrals, 53-81 dorsal scale rows, 4 black head stripes, suborbitals between the oculars 

and supralabials, black dorsal blotches usually (our emphasis; look at the entire animal, 

as individual spots may well be more round than this standard) half as long as dorsal 

width (dorsal blotches sometimes lighter in center), and black lateral spots with yellowish 

centers that are lighter than the ground color. In contrast, the other three species of 

Eunectes all have five head stripes, no scales between the oculars and labials, and lower 

dorsal scale counts (Dirksen, 2002, pers. commun., 2009). For descriptions of additional 

morphological characters useful in defining boid snakes, see Kluge (1991; however, note 

that Kluge’s conclusions are incorrect with regard to E. deschauenseei vs. E. notaeus, due 

to errors in specimen identification; Dirksen, 2002).

2.2 Size

The Green Anaconda is truly a giant snake, with fairly reliable records of lengths 

over 7 m. Moreover, it is a very stout snake, such that very large anacondas are almost 

certainly the heaviest snakes in the world, even though more gracile species (for example, 

Broghammerus reticulatus) may attain greater lengths. Murphy and Henderson (1997) 

and Bellosa and others (2007) provide tabulated data on body size in Green Anacondas 

from a variety of sources. As with other giant snakes, many a tall tale has been related 

about maximum size. Ditmars (1931) considered the maximum length to be just over 25 

feet (7.62 m), based on a personal communication from A. do Amaral. In the apparent 

absence of confirmed specimens longer than Ditmars’ estimate, we consider his 

conclusion to be as valid today as it was nearly 80 years ago. One of the largest snakes 

measured in recent decades by a reliable observer (W.W. Lamar) was 7.49 m, taken from 

a shallow stream in Colombia in 1978 (Murphy and Henderson, 1997), while a specimen 

found crossing the road in Amazonas, Brazil was 6.2 m total length (Martins and 

Oliveira, 1998). 

Bisplinghof and Bellosa (2007) considered the largest verified specimen to have 

been “...just short of 9 m (29.5 ft.) and a maximum weight of 180-200 kg (397-441 

lbs.),” but they did not cite their sources for this statement and did not differentiate 

between captive and free-living individuals. Their maximum length of nearly 9 m may 

be based on Robertson (1998), who reported a Green Anaconda skin from Brazil that was 

nearly 29 ft. without the head and tail. As discussed for other species in their bioprofiles, 

however, we consider skins to be highly dubious sources of information on body size of 

giant constrictors, as skins typically stretch extensively during removal and tanning.

Size variation across the range of the Green Anaconda has not been adequately 

addressed. Some authors consider snakes in riverine rainforest habitat to attain greater 

lengths than those in the llanos (for example, Murphy and Henderson, 1997), but this 

may be due to more intensive study of the species in the llanos, producing more reliable 

and more realistic insights on maximum size. However, Rivas (1999) noted that several 

males from a Venezuelan river were larger than any of the several hundred males 

captured in the adjacent seasonally-flooded llanos, and males in captivity often exceed 
220

the maximum sizes observed in the llanos by Rivas (1999). On Trinidad, Mole and Urich 

(1894) stated that “specimens have been frequently killed 18 and 20 feet in length” (5.49, 

6.10 m); however, the longest specimen from Trinidad actually measured by Mole (1924) 

was 5.26 m and weighed 47.63 kg, while the heaviest was 5.03 m and 105.23 kg. Beebe 

(1946) shot a Green Anaconda measuring 5.23 m in Kartabo, Guyana, and stated that the 

maximum “accredited” size of the species is 8.8 m. At the other end of the scale, a 

specimen measuring 0.92 m total length weighed 280.4 g, while a 0.67 m specimen 

weighed 69 g (Beebe, 1946); the latter is somewhat smaller than the minimum size of 

0.73 m cited in a review by Martins and Oliveira (1998). 

In the field, neonates average 0.70-0.90 m total length and 200-300 g (Waller and 

others, 1995); French Guianan neonates are reported to be 0.65-0.85 m (Starace, 1998). 

Neonates from four captive females were 0.73-0.90 m total length and 192-303 g (but 

length and mass were generally more consistent within litters than between litters; 

Holmstrom, 1980). Variation in body mass by sex was not observed among male and 

female neonates in Venezuela (Rivas, 1999); several hundred neonates measured in 

Rivas’ study averaged 0.68 ± 0.04 (SD) m SVL, 0.790 ± 0.040 m total length, and 217 ± 

36 g at birth. Average neonate size from the Venezuelan llanos is somewhat larger than 

the mean of 0.618 ± 0.055 m SVL (range 0.435-0.685) recorded for embryonic Brazilian 

Green Anacondas (Pizzatto and Marques, 2007), perhaps because the latter had not fully 

completed their development. The majority of information on neonate sizes, however, 

comes from captive situations. Recent reviews of neonate body sizes based largely on 

captives (Dirksen, 2002; Bisplinghoff and Bellosa, 2007) reported total neonatal lengths 

from 0.508 to 0.965 mm and body masses from 153 to 454 g, indicating a potentially 

wider range of neonatal sizes in the Green Anaconda than in other giant constrictors such 

as B. reticulatus (see biological profiles in chapters Four to Eight).

Table 9.1. Total lengths and masses of typical E. murinus (compiled/estimated from the 

literature sources listed in the text).

Hatchling Maturation Max. reported

Sex Total (mm) Mass (g) Total (m) Mass (kg) Total (m) Mass (kg)

M: 508-965 150-400 1.8-2.8 2.5-7? 4.5 >50

F: 508-965 150-400 2.4-3.5 5-12? 8+ 180-200

2.3 Sexual Size Dimorphism

With the possible exception of the Australopapuan Morelia amethistina complex, 

all of the truly large snakes appear to exhibit female-biased sexual size dimorphism. This 

is certainly true of the Green Anaconda, in which females are capable of attaining much 

greater length and larger mass than do males. This has been best documented in the 

llanos of Venezuela, where females average 3.70 ± 0.71 m and 32.6 kg, while males 

average 2.63 ± 0.28 m and 7 kg (Rivas and others, 2007a). 
221

It is not unlikely that large female body size is a result of fecundity selection 

(large females can have larger numbers of offspring, possibly increasing their fitness) 

and/or selection to allow females to take advantage of large prey items (for example, 

Capybaras, caiman). Male size, on the other hand, may have undergone stabilizing 

selection, in which large males are favored during mating aggregations but smaller males 

are of an optimal size for consuming prey of moderate size classes, low predatory risk, 

and high abundances. See Rivas and others (2007a) and Rivas and Burghardt (2001) for 

extended discussions and alternative viewpoints on body size evolution in anacondas.

2.4 External Sexual Differentiation

Other than the massive size attained by some female Green Anacondas, there is 

no reliable means of determining sex of anacondas at a distance. In hand, males have 

relatively longer tails and pelvic spurs than do females (for example, Rivas and others, 

2007a).

3.0 Distribution in Space and Time

Despite its large geographic range and relative abundance in parts of this range, 

the Green Anaconda is not well represented in museum collections, and many parts of its 

distribution are assumed rather than documented by preserved material. In large part, this 

is because the large body size of the species necessitates large amounts of space and 

supplies in a museum collection, as is true for many of the giant constrictors. For 

example, Dixon and Soini (1986) stated, “Many specimens have been observed over the 

last 8 years but the large size of most specimens makes it impossible to maintain an 

adequate preserved series.” 

3.1 Native Range

Waller and others (1995) summarized the range of the Green Anaconda as 

including aquatic habitats in much of South America below 850 m elevation plus the 

insular population on Trinidad, encompassing the Amazon and Orinoco Basins, major 

Guianan rivers, the San Francisco, Parana and Paraguay Rivers in Brazil, and extending 

south as far as the Tropic of Capricorn in northeast Paraguay. Dirksen (2002) provides 

the most thorough review of the distribution of members of the genus Eunectes, as 

reflected in our frequent citation of his work below. Although anacondas are not present 

in Central America, Lutz Dirksen stated to us that he believes that this is solely due to 

their inability to surmount the Andes to access areas to the north (pers. commun., 2008)

Argentina–Probably not present. While a specimen of the Green Anaconda was reported 

from Argentina by Correa and Pautassi (1986, cited in Giraudo and Scrocchi, 2002), 

Giraudo and Scrocchi (2002) considered this to be an erroneous locality and stated that 

the Green Anaconda is likely not found in Argentina.
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Figure 9.2. Estimated native distribution of the Green Anaconda (Eunectes murinus, shown in 

blue) and the Beni Anaconda (Eunectes beniensis, cross-hatched oval). Distributions follow 

Dirksen (2002, his Fig. 4.15).

Bolivia–Known from the Itenez/Guaporé River along the border with Brazil, as well as 

the Baures River drainage in northeast Bolivia. Another species of anaconda with similar 

appearance, Eunectes beniensis, is known from a few specimens in the state of Beni 

(Dirksen, 2002).
223

Brazil–Likely throughout the Amazon Basin, as well as throughout rivers that drain to 

the eastern coastline from the state of Paraná northward and southward flowing rivers in 

the Mata Grosso do Sul (Dirksen, 2002; Bisplinghoff and Bellosa, 2007). 

Colombia–Known from a few specimens from lowland drainages to the east of the 

Andes, including upper reaches of the Amazon and Orinoco drainages (Dirksen, 2002). 

Although there are rumors of anacondas in the Magdalena Valley in northern Colombia 

(J. Rivas, pers. commun., 2009), their presence has not been confirmed by herpetologists 

familiar with the area (J. Maldonado, J. Daza, pers. commun., 2009).

Ecuador–A few specimens are known in the lowlands of extreme eastern Ecuador, 

including the Napo and Tiputini Rivers and other drainages to the Amazon (Dirksen, 

2002). 

French Guiana–Known from a few specimens and a larger number of sightings, 

including the Maroni and Oyapock (Oiapoque) Rivers that respectively border the 

country to the west and east, as well as several drainages in between (Dirksen, 2002). 

Probably historically present in most lowland drainages.

Guyana–Known from the Essequibo River and adjacent drainages along the coast, as 

well as farther south along the Takatu and upper Essequibo River (Dirksen, 2002). 

Probably historically present in most lowland drainages.

Paraguay–Presence confirmed only by a couple of specimens from the extreme northeast 

of the country east of the Paraguay River, plus anecdotal reports from this area (Waller 

and others, 1995). These authors surmise that the species may be present in the 

Concepción, Canindeyú and Alto Paraná Departments. 

Peru–Known from multiple specimens in northern and central Peru along the Ucayali 

and Marañon Rivers downstream to the upper Amazon River. In the south, specimens 

are known from the Madre de Dios River drainage near the Bolivian border (Dixon and 

Soini, 1986; Dirksen, 2002).

Suriname–Known from multiple specimens and sight records in drainages within 50 km 

of the coast, as well as scattered locations in the interior (Dirksen, 2002). A number of 

sight records from the vicinity of Kwakoegron, roughly 90 km inland (N. Hawley, pers. 

commun., 2008). Gaps in the known Suriname distribution are likely the result of 

incomplete sampling; the species may be present in most lowland drainages.

Trinidad and Tobago–Currently found in estuary lagoons and swamps of various parts 

of Trinidad, but not in apparently suitable habitat in the Caroni River drainage and now 

absent from some historically-occupied areas and rare elsewhere (Murphy, 1997; Boos, 

2001). Absent on Tobago. Boos (1984) reported a juvenile Green Anaconda found at 

sea on a floating mat of vegetation, and surmised that it was a waif that rafted from 
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Venezuela; Boos (2001) provided additional examples of rafting in the Green Anaconda. 

The species’ aquatic habits may facilitate such rafting events to previously uncolonized 

islands.

Venezuela–Found throughout the seasonally flooded areas (Rivas, 1999) and waterways 

of the Orinoco River system and other rivers of the states of Sucre, Guárico, Cojedes, and 

Portuguesa (Lancini, 1986), as well as Zulia, Amazonas, Bolivar, Anzoategui, Monagas, 

Barinas, and Apure (Roze, 1966; J. Rivas, pers. commun., 2009: anacondas in Zulia may 

represent a disjunct population). The species has received intensive study in the 

Venezuelan llanos of the state of Apure (Rivas, 1999). 

3.2 Habitat Range

Eunectes is Greek for “good swimmer,” and the habitat range of the Green 

Anaconda is largely defined by availability of aquatic habitats. Depending on location 

within the wide distribution of the species, these appear to include deep, shallow, turbid, 

and clear waters, and both lacustrine and riverine habitats.

3.3 Climate Range

The Green Anaconda inhabits areas delineated by a wide range of rainfall and 

precipitation regimes (Fig. 9.3). The coldest mean monthly winter temperatures (around 

14oC) are in several south Brazilian localities, while a few localities in Brazil, Venezuela, 

and Bolivia also experience periods with little rain. However, seasonal variation in 

rainfall is probably not as important to the Green Anaconda as is the year-round 

availability of surface water. Anacondas may also be able to escape both daily and 

seasonal temperature decreases by retreating to aquatic habitats when water temperatures 

exceed air temperatures. 

Figure 9.3. Climate space for E. murinus based on 77 known localities from the native range, 

and for E. beniensis based on three known localities. Reference lines and localities as introduced 

in Fig. 2.1. Climate loops color-coded as follows: Brown = Bolivia; Turquoise = Brazil; Green = 

Colombia; Dark blue = Ecuador; Black = French Guiana; Red = Guyana; Pink = Paraguay; Gray = 

Peru; Violet = Suriname; Dark Red = Trinidad; Orange = Venezuela; Yellow = E. beniensis from 

Bolivia (the latter is completely contained within the climatic envelope of E. murinus).
225

3.4 Microhabitat

In the llanos of Venezuela, radiotelemetered anacondas were found in the water 

86 percent of the time and at the water’s edge the remaining 14 percent. Most (92 

percent) locations of telemetered snakes were in shallow water covered in aquatic 

vegetation, and snakes apparently preferred still water with aquatic vegetation over areas 

with flowing water (Rivas, 1999). Basking behavior was observed in reproductive 

females and wounded or diseased individuals, but rarely among other age, sex, or stage 

classes.

Microhabitat use in rainforest habitats such as much of the Amazon Basin is 

poorly understood, and almost entirely reliant on chance observations of basking or 

otherwise visible anacondas. This trend is exemplified by a few records from central 

Amazonas, including a 4-m individual observed in tree roots near a stream, an adult 

female found crossing a road, two snakes observed basking by water, two more active 

along the aquatic margin, and two incidental captures in fishing nets (Martins and 

Oliveira, 1998). Radiotelemetric studies in Venezuela (above) suggest that the 

encounters in other parts of the species’ range are unlikely to represent “typical” 

anaconda behavior, and instead are the result of higher detectability to humans when 

snakes make infrequent jaunts to terrestrial or arboreal microhabitats. Nonetheless, such 

scattered anecdotes have often been used as evidence that the Green Anaconda is a 

nocturnal species that often exhibits diurnal basking. Around-the-clock monitoring of 

radiotelemetered snakes in the llanos of Venezuela showed no preference for nocturnal 

activity, and indeed showed very little activity from 0200 to about 0800 (J. Rivas, pers. 

commun., 2009).

3.5 Introduced Ranges

We are unaware of any established extralimital populations of the Green 

Anaconda. An individual (around 2.5 m total length) was found dead on US41 in the 

vicinity of Fakahatchee Strand Preserve State Park, FL, in December 2004, and there are 

reports of two medium-sized adults and a juvenile observed but not collected in this 

general area (S. Snow, pers. commun. 2008). 

3.6 Seasonal Activity

Green Anacondas exhibited marked seasonal changes in their activity ranges and 

movement behavior in the Venezuelan llanos (Rivas 1999), but equivalent information is 

lacking for any other population. In the llanos, activity ranges were small (about 25 ha) 

during the dry season when water bodies are isolated, but snakes migrated (1.3 km on 

average) to slightly higher elevations at the onset of the wet season, and activity ranges 

were somewhat larger during the latter (about 37 ha). Breeding females provided an 

exception to this trend, as their wet season activity range was very small (about 0.01 ha) 

and they spent much of their time basking on emergent ground (levees, river banks, etc.). 

Adult males increased their movement activity prior to the mating season, including 

movement during mid-day when other age and sex classes were inactive.
226

3.7 Daily Activity

Rivas (1999) opined that Venezuelan anacondas were somewhat crepuscular in 

their activity, but that movement, predatory, and reproductive behaviors were seen 

throughout the day and night. Most individuals in this study sought shelter during the 

warmest parts of the day, although male mate-searching activity was apparent throughout 

the day if skies were overcast. Nocturnal activity was not notably elevated above diurnal 

activity, and snakes often went a week or more without moving (Rivas and others, 

2007a).

In the Venezuelan llanos during the wet season, female Green Anacondas were 

often observed basking for extended periods during daylight hours, while 

radiotelemetered animals were usually found under vegetation or submerged (Strimple, 

1993; Rivas, 1999). This disparity may reflect judgments on daily activity based on 

where humans observe snakes (usually diurnal and basking) versus activity patterns 

gained from radiotelemetry (often submerged). Overall, conclusions that the Green 

Anaconda primarily exhibits nocturnal activity appear to be poorly supported.

3.8 Foraging Mode/Sensory Modalities

Mole and Urich (1894) noted over a century ago that anacondas ambush most 

prey from the water, a conclusion that has been largely corroborated by more recent 

observers. However, a 3-m individual was observed actively “chasing” a rallid bird in a 

flooded area in Brazil (Martins and Oliveira, 1998), and the extent to which the Green 

Anaconda uses chemosensory, visual, or other cues to locate prey is poorly known. 

The violence of the strike and the power of the subsequent constriction by large 

anacondas may be enough to kill prey by physical damage by dislocating or fracturing 

vertebrae, as evidenced by examination of prey species including Capybara, White-tailed

Deer, Spectacled Caiman, and White-lipped Peccary that had been killed by Green 

Anacondas (Valderrama and Thorbjarnarson, 2001; Rivas, 2004). 

As with Boa constrictor, anacondas lack the thermoreceptive labial/facial pits that 

are typical of most pythons, although B. constrictor exhibits labial thermoreceptors that 

are not housed in pits (de Cock Buning, 1983). We are unaware of unequivocal evidence 

for the presence of such receptors in Eunectes, but their presence would be relatively 

unsurprising given the prevalence of thermoreceptors among boid snakes.

4.0 Life History

4.1 Reproductive Mode

The Green Anaconda is live-bearing. While reproduction is typically sexual, a 

recent report noted that a female that was five years old in 1976 and kept in captivity for 

26 years without any access to males produced 23 female offspring in 2002 (only three of 

which were alive) and 20 atresic eggs (Lamonica and others, 2007). Although the 

authors did not comment on the implications of this event, it raises the possibility that E. 

murinus exhibits facultative parthenogenesis, as has been reported in female Python 

molurus, Epicrates cenchria, and several other snake species (including Boa 

constrictor)—these rare events typically produce only female offspring that are clones of 
227

the mother (Schuett and others, 1997; Groot and others, 2003; G. Schuett, pers. commun., 

2008).

4.2 Reproductive Phenology

4.2.1 Seasonality 

In the seasonally-flooded llanos of Venezuela (8° N), mating peaked before the 

onset of rains (late April–early May), with no breeding aggregations observed before 

mid-Feb (mid dry season) or after mid-June (early wet season: Rivas, 1999). Earlier in 

the year (January-March), males appeared to engage in long distance movements in order 

to locate females. It is possible that the timing of breeding in the llanos is due to the 

spatial predictability of females during the dry season.

Based on dissections of Brazilian (10°S - 5°N) Green Anaconda museum 

specimens, Pizzatto and Marques (2007) inferred vitellogenesis during late wet to dry 

seasons (roughly September to February), gestation from dry to early wet (January to 

July) and parturition from late dry to early wet (April to July). These inferences roughly 

correspond to observations by Mole (1924), who reported mating in December and 

January (early dry) on Trinidad (10.5° N) in a relatively aseasonal climate. Timing of 

reproduction in the Green Anaconda is apparently divergent from the pattern seen in a 

variety of boid and pythonid species, in which gestation/incubation primarily occur in 

wet/early dry and parturition/hatching in early-mid dry (Pizzatto and Marques, 2007). 

Instead, these authors suggest that parturition in Green Anacondas is related to low water 

levels during the dry season in much of Brazil (as contrasted with Rivas’ assertion that 

low water levels induced mating in Venezuela). 

We caution, however, that generalities on the timing of reproduction based on 

specimen dissections remain tenuous, as Pizzatto and Marques (2007) used specimens 

from across the range of the species in Brazil, and their sample sizes did not permit 

examination of seasonal variation by latitude or other factors that would be expected to 

vary across such a vast area (these authors also had inadequate material available for any 

inferences on male reproductive cycles). Timing of reproduction does, in fact, appear to 

exhibit a strong latitudinal signal, as populations in northern South America mate from 

December to April, while equatorial populations mate from June to September and 

southern populations mate in September and October (Dirksen, 2002; Bisplinghoff and 

Bellosa, 2007).

4.2.2 Reproductive Behaviors 

Breeding aggregations consisting of a single adult female and several males 

appear to be common in the Green Anaconda, at least in northern populations such as 

those in the llanos of Venezuela. Rivas and others (2007a) reported aggregations 

averaging 3.8 males per female, with a maximum of 13 males in attendance. 

Aggregations can occur in water or on land and were of surprisingly long duration, with 

none under four days duration and some lasting as long as 40 days (18 day average). 

Rivas (2007a; pers. commun., 2009) never observed male-male combat as described in 

some other giant constrictors, but observed tail wrestling and “pushing tournaments” 

among males in mating aggregations. Aggregations of Green Anacondas have also been 
228

reported from rivers in Brazil (Pope, 1961), indicating that the phenomenon is relatively 

widespread. On Trinidad, a single male courted a female from 24 December to 13 

January (Mole, 1924).

4.2.3 Vitellogenesis and Ovulation

Duration of gestation has been summarized as typically about six months, with a 

recorded maximum of ten months (Waller and others, 1995). The mean duration of 

gestation was inferred to be about nine months across the range of the species in Brazil 

(Pizzatto and Marques, 2007), but was only 202 ± 15 (SD) days or ~6.75 months in the 

Venezuelan llanos (Rivas, 1999).

4.2.4 Parturition

Parturition may occur on land or in the water, and was observed at the end of the 

wet season (October to late December) in Venezuela (Rivas, 1999). A female was 

observed giving birth on 01 July 1892 at the mouth of the Orinoco River in Venezuela 

(Mole and Urich, 1894). These are among the only verified records of parturition in the 

wild. In the upper Amazon Basin of Peru, a neonate was found in February, and two 

wild-caught females gave birth in early March (Dixon and Soini, 1986), whereas females 

from Guyana and French Guiana gave birth in January (Neill and Allen, 1962a; Starace, 

1998). Pizzatto and Marques (2007) report parturition during April through July (early 

wet). Given that these observations include parturition over ten months of the year 

(available data do not as yet support parturition in August or September), the timing of 

parturition is obviously highly variable. Further complicating the matter, a small 

individual that is judged to be a “neonate” may actually have been born several months 

previously but achieved little growth in body size, depending on its foraging success.

Females may consume infertile eggs and stillborn young after parturition (Neill 

and Allen, 1962a); Rivas (1999) observed post-parturient females (n = 8) moving their 

snout across the neonates, consuming those that did not respond by moving.

Many wild female Green Anacondas retain moderate fat reserves and decent body 

condition even after producing a large litter (Rivas, 1999). While some captive females 

accept food through all but the last month or so of gestation (Bisplinghoff and Bellosa, 

2007), wild-caught females held in Venezuela refused food before parturition, after 

which they readily accepted food (J. Rivas, pers. commun., 2009).

4.3 Reproductive Effort/Fecundity

4.3.1 Clutch or Litter Size/Frequency

The maximum recorded litter size is 82, removed from a Brazilian specimen 

(Belluomini and Hoge, 1958), but the typical range is 28 to 42 (Waller and others, 1995). 

Among 36 litters in Venezuela, mean litter size was 29.4, with litter size strongly 

correlated with mass of the dam, litter sizes ranging from 5 to 75, and total litter mass 

averaging about half the dam’s body mass (Rivas, 1999). Additional litter size data are 

available from wild and captive anacondas (Mole and Urich, 1894; Ditmars, 1931; 

Holmstrom, 1980, 1982; Dixon and Soini, 1986; Hero and dos Santos, 1987; Martins and 

Oliveira, 1998; Starace, 1998; Bisplinghoff and Bellosa, 2007). 
229

Reproductive frequency is poorly understood in the Green Anaconda. Only about 

half (57 percent) of females greater than 2 m SVL captured in the llanos of Venezuela 

were reproductive (Rivas, 1999), and all reproductive females exhibited high body 

condition, whereas all but a few non-reproductive females exhibited lower body 

condition. However, reproductive females were more detectible than were nonreproductive females, which may artificially inflate conclusions on relative abundances 

of breeders and non-breeders. These results suggest that females typically exhibit a 

biennial or longer reproductive cycle in this population, and that it takes females over a 

year to recoup energetic stores depleted by reproduction. Rivas estimated overall female 

reproductive frequency to be 0.38 among snakes captured in multiple years, but that 

breeding frequency decreases among very large females. There may be exceptions to the 

typical trend, as exemplified by two females that were inferred to be reproductive in 

successive years based on their condition index (these were not, however, observed to 

breed). Equivalent data are lacking for any other population of Green Anacondas.

4.4 Growth

Rivas (1999) provided the only available growth rates for free-ranging anacondas, 

from a sample of females captured from a population in Venezuela. Growth rates were 

highly variable; many individuals ranging from about 2.3 to over 5.5 m (2.0-4.75 SVL) 

exhibited zero growth between captures, while the highest growth rates (averaged over 12 

months) were observed in fairly large snakes (for example, the four highest rates were 

from females of 4.1-5.2 m (3.5-4.5 m SVL), whose growth averaged 34-49 mm/mo (29-

42 mm/mo SVL). In contrast, extremely slow growth (less than 3 mm/mo; 25 g/mo) was 

observed in an adult female originally captured in 1994 at 3.76 m (3.24 m SVL) and 21 

kg and recaptured in 2007 (thirteen years later) at 4.25 m (3.66 m SVL) and 25 kg (Rivas 

and Corey, 2008); these authors suggested that free-ranging anacondas may require more 

than a decade to reach 3.7 m (3.2 m SVL).

Growth rates can be high in captivity. A captive female grew from 1.42 m and 

1.35 kg to 3.2 m and 25.5 kg in just under four years (about 38 mm/mo: Holmstrom, 

1980), while a 0.60-m hatchling reached 2.05 m in 21 months (69 mm/mo), with mass 

increasing by roughly 30-fold from 160 g to 5000 g. Three captives held at a constant 

temperature of 26o

C grew from an average of 183 g and 0.717 m to 4.01 kg and 1.87 m 

over a period of 445 days, during which time they exhibited about 78.8 mm/mo growth 

(66.5 mm/mo SVL) and an assimilation efficiency of 42.5 percent of the prey mass 

consumed (Lamonica and others, 2007). Growth rates of 17-25 mm/mo can be sustained 

for up to a decade after a captive individual reaches 3 m total length (Bisplinghoff and 

Bellosa, 2007), a number which may also be reasonable for some free-ranging snakes 

when compared to the results of Rivas (1999), above.

The herpetoculture literature often discusses the slower growth rate of anacondas 

as compared to other giant snakes such as Reticulated Pythons; for example, Bisplinghoff 

and Bellosa (2007) stated that it takes 8-15 years for a captive Green Anaconda to reach 5 

m total length, but only 5-8 years for B. reticulatus or P. molurus to reach this length. If 

this difference in growth rate is real, it may perhaps be ascribed to the greater mass per 

unit length of anacondas, necessitating significant energetic investment in production of 

somatic tissue without appreciably increasing body length.
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4.5 Maturation

In the Venezuelan llanos, the smallest females observed participating in breeding 

aggregation were 2.42 m and 9.25 kgs (Rivas and others, 2007a; length and mass were 

not reported by individual, so these numbers may or may not represent the same 

individual). Equivalent numbers for males were 1.88 m and 2.45 kg. Most other 

reported sizes at maturation for free-living Green Anacondas appear to be crude 

estimates. Waller and others (1995) stated that individuals typically mature at about 3.2 

m after 4-6 years, while Pizzatto and Marques (2007) inferred maturity at 3.45 m (2.97 m 

SVL) in females (n = 6) and 1.54 m (1.33 m SVL) in males (n = 35); the latter appears to 

be an outlier for male reproductive size, as most authors state that males mature at body 

sizes larger than this.

Holmstrom (1982) reported that a captive female Green Anaconda (3.63 m, 25 kg 

post-partum mass) fed ad libitum reproduced at the age of three years and ten months, 

and this interval has been repeated elsewhere in the literature (for example, Ross and 

Marzec, 1990). This is the fastest time to maturity reported for the species, and is notable 

primarily because many of the other giant constrictors can reach maturity in a much 

shorter time, especially in captivity (see biological profiles for P. molurus and B. 

reticulatus). In contrast, more typical ages at maturity among captives were reported by 

the same author, who estimated sexual maturity for three females at 6 or 7 years of age 

and a male at 5 years (Holmstrom, 1982). 

4.6 Longevity

There are no data to document longevity of Green Anacondas in the wild (Rivas, 

1999) except for a single record of an adult female captured in 1994 and recaptured 

thirteen years later (Rivas and Corey, 2008). A wild-caught individual lived 28 years in 

captivity under zoo conditions (Snider and Bowler, 1992), and Waller and others (1995) 

stated that longevity can be at least 32 years; this record may be based on Stemmler 

(1963, cited by Strimple, 1993). Dirksen (2007) reported a captive that lived about 33.5 

years, based on a personal communication with the snake’s owner.

5.0 Diet/Trophic Role

5.1 Prey

Most reviews of the diet of the Green Anaconda indicate that the species is 

euryphagic (Beebe, 1946; Henderson and others, 1995; Murphy and Henderson, 1997; 

Martins and Oliveira, 1998). As examples, Strimple (1993) summarized the literature by 

stating that the species eats “fish, amphibians, reptiles (including turtles and caimans), 

birds, and mammals such as agoutis, pacas, peccaries, capybaras, tapirs, and deer,” and 

prey records on Trinidad included caiman, dogs, deer, tamandua, turtle, sheep, and 

“...rodents and almost any animal which comes down to drink of the water in which it is 

lying” (Mole 1924). Large prey items have included birds such as Jabirú (Jabiru 

mycteria) and mammals as large as tapirs (Tapirus terrestris), White-lipped Peccary 

(Tayassu pecari), a young zebu (Bos primigenius) and deer (Odocoileus and Mazama sp.: 

Martins and Oliveira, 1998 and references therein; Starace, 1998; Valderrama and 
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Thorbjarnarson, 2001; Rivas, 2004). The Green Anaconda can certainly be considered a 

top predator in South American ecosystems.

A partial analysis of dietary data from over 100 Green Anacondas in Venezuela 

yielded 63 individuals of 27 prey species, most of which were represented by only a 

single record (Rivas, 1999). Seventeen of the 27 species were birds, followed by reptiles 

(6 spp., including Green Iguana [Iguana iguana] and Common Tegu [Tupinambis 

teguixin]), mammals (3 spp.) and fish (1 spp.). Anacondas occasionally killed prey too 

large to consume, and there were reports and circumstantial evidence of carrion 

consumption. Small anacondas appeared to consume primarily birds, and as they 

matured underwent an ontogenetic prey shift to large mammals and reptiles (Rivas, 1999; 

Rivas and others, 2007a). Somewhat surprisingly, Savannah Side-Necked Turtles 

(Podocnemis vogli) were the most abundant (n = 10) among observed prey in the Rivas 

(1999) sample, followed by Northern Jacanas (Jacana jacana; n = 7), Spectacled Caiman 

(Caiman crocodilus; n = 5, with additional information on caiman predation in Rivas and 

others, 1999), Capybara (Hydrochaeris hydrochaeris; n = 5), whistling ducks 

(Dendrocygna sp.; n = 5) and White-tailed Deer (Odocoileus virginianus, n = 5). The 

large number of turtles observed may have been partially due to the ease of identifying 

osseous turtle remains in the voided contents of anaconda digestive tracts. 

The wide range of prey consumed by Green Anacondas in Venezuela is largely 

mirrored by diet records for other parts of the range, to which can be added Moustached 

Tamarin (Saguinus mystax: Greene, 1997), Neotropical Mud Turtles (Kinosternon 

scorpioides: Boos, 2001), jungle runner lizards (Kentropyx sp.: Martins and Oliveira, 

1998), domestic pigs (Sus scrofa: Macedo-Bernarde, 2006), Lesser Anteaters (Tamandua 

tetradactyla: Valderrama and Thorbjarnarson, 2001), a large dog (Neill and Allen, 1956), 

a Boa Constrictor (Boa constrictor: De Freitas, 2009), a Brown-Banded Watersnake 

(Helicops angulatus; Infante-Rivero and others, 2008) and occasionally conspecifics 

(Rivas and Owens, 2000). Despite the assertion by Ditmars (1912) that E. murinus does 

not eat fish, 27 fish, including sharp-spined catfish and armored catfish, were taken from 

the stomachs of three anacondas in Venezuela and Guyana (Beebe, 1946). Indeed, the 

regular inclusion of fish in the diet of the anacondas (including other members of the 

genus Eunectes) increases their dietary niche breadth in relation to the other giant 

constrictors, which very rarely consume fish.

5.2 Predators

Green Anacondas likely occasionally fall prey to a variety of predators, including 

felid, canid, procyonid, and mustelid mammals, birds including raptors and herons, and 

reptiles such as crocodilians and some large ophiophagous snakes. However, few such 

predation events are documented, and juvenile anacondas likely suffer most such 

predation. A Crab-Eating Fox (Cerdocyon thous) harassed a 1.87-m female Green 

Anaconda on dry land (Rivas and others, 2001), and the authors surmised that the fox 

would have eventually killed the snake had the authors not intervened. Two more 

radiotelemetered juveniles in this study may have fallen victim to a Common Tegu 

(Tupinambis teguixin) and a Crested Caracara (Polyborus plancus: Rivas and others, 

2001). Caimans were implicated in the deaths of at least a dozen Green Anacondas 

during a seven year study in Venezuela, with male snakes at particularly high risk of 

predation (Rivas, 1999; Rivas and others, 1999), but it was difficult to assign the 
232

predatory role to the players in some of these incidents, as a caiman may represent either 

predator or prey to an anaconda. The same author recorded numerous severe wounds on 

anacondas inflicted by prey species such as Capybara, and females were observed to have 

a higher injury rate than did males (Calle and others, 1994; Rivas and others, 2007a). 

Large female Green Anacondas occasionally cannibalize smaller males (Rivas and 

Owens, 2000), but this is unlikely to be of demographic significance unless predation on 

males occurs regularly (as is the case among some crocodilians, for example, Rootes and 

Chabreck, 1993). 

All members of the genus Eunectes appear to adopt a balling posture when 

attacked or when harassed by humans (Mole and Urich, 1894; Dirksen and others, 1998). 

This posture protects the head, which is held in the center of the ball. Anacondas may 

also burrow into mud to escape predators, including humans attempting to capture the 

snake (Strimple and Dirksen, 2001).

5.3 Diseases and Parasites

As with most of the giant constrictors, the range and impacts of pathogens and 

parasites of the Green Anaconda are known primarily from captive individuals (for 

example, McNamara and others, 1994; Miller and others, 2004). However, results of a 

health assessment of 24 wild Green Anacondas in Venezuela (Calle and others, 1994) 

provided more information for this population than is available for most giant 

constrictors. Observed parasites included ticks (Amblyomma dissimile; 6 individuals; 

note also that A. fulvum was described from a E. murinus: Keirans, 1972), tapeworms 

(Crepidobothrium sp.), subcutaneous nematodes (Dracunculus sp.), an unidentified 

trematode, and an intraerythrocytic protozoan (Hemoproteus sp.; all individuals). 

Cestodes and trematodes were abundant in two dissected individuals (Calle and others, 

1994), but Rivas (1999) noted that many individuals with known endoparasite loads 

survived and reproduced for years after the parasites were initially detected. A follow-up 

study revealed low or undetectable serum levels of antibodies for a variety of diseases 

including paramyxovirus, Leptospira, and Cryptosporidium among ten anacondas from 

the same site (Calle and others, 2001).

An apparent respiratory infection was observed in a free-ranging Green Anaconda 

in Venezuela, and another individual may have succumbed to lymphatic cancer (Rivas, 

1999; Rivas and others, 2007a), but no similar maladies were observed among the rest of 

nearly 800 animals examined during this study. 

6.0 Demography

6.1 Population Density

Rivas (1999) reported capturing 780 Green Anacondas during a seven year study 

in the Venezuelan llanos, which is by far the largest sample size reported for any 

scientific study of the species. Sample sizes increased during dry years, reflecting the 

fact that snakes were concentrated in smaller areas, and additional detection biases were 

noted for juveniles and females. One of the Rivas’ study sites yielded 550 snakes from 

about 1500 ha, for a density of 0.37 individuals/ha, but the low rate of recaptures at this 

site led him to believe that only about a third of resident snakes were captured. If correct, 
233

densities may have approached 1/ha. At a micro-scale, densities can be very high when 

suitable habitat is limiting, exemplified by 34 anacondas found in a 200 m2 pool after 

surrounding areas had dried (Rivas, 1999).

In the Iquitos region of Peru, the Green Anaconda was considered “...common in 

and around flooded forests, large and small streams, and bodies of water where aquatic 

vegetation is dense” (Dixon and Soini, 1986). In contrast, only one individual, found 

dead in a fishing net strung across a river, was recorded by Duellman (2005) during 

intensive studies at Cusco Amazónico in southeast Peru. However, little time was spent 

sampling riverine habitats or large snakes during this study and thus abundance was 

likely underestimated.

6.2 Size Distribution

Sample sizes are insufficient to produce reliable size distributions for most 

published reports on populations of the Green Anaconda, with the sole exception of the 

long-term studies of Rivas and colleagues in the llanos of Venezuela (for example, Rivas, 

1999). In Rivas’ study, the male size distribution was leptokurtotic (exceptionally 

peaked), with a strong peak at about 2.7 m (2.3 m SVL) and a maximum of about 4.1 m 

(3.5 m SVL). In contrast, the female size distribution was largely platykurtotic 

(exceptionally flat), with a weak peak around 3.1 m (2.7 m SVL) but numerous 

individuals in the 3.36-4.52 m (2.9- 3.9 m SVL) size classes and a maximum of nearly 

5.8 m (5 m SVL). However, even this distribution is biased by detectability issues, as 

small anacondas were very difficult to detect using any method and radiotelemetry results 

revealed differences in individual detection rates by size, sex, and season (Rivas, 1999).

Although Rivas (1999) did not provide a histogram of body mass, a regression of 

mass on length for 660 snakes (his p. 55) provided some insight on mass distribution. 

The majority of captured snakes were less than 20 kg, but about 60 individuals (roughly 9 

percent) were between 20 and 40 kg, about 35 (roughly 5 percent of sample) were 

between 40 and 60 kg, about 12 (roughly 1.8 percent) were between 60 and 80 kg, and 

about 3 were between 80 and 100 kg. These results are suggestive of the notion that 

females with very large body masses are a predictable component of natural Green 

Anaconda populations, rather than representing extremely uncommon giants.

7.0 Interactions with Humans

7.1 Human Utilization and Persecution

Populations of the Green Anaconda have been impacted by the leather and live 

animal trade in some areas (Strimple, 1993), but this conclusion does not appear to be 

supported by any rigorous population estimates. Green Anacondas are occasionally 

consumed by humans (Gasc and Rodriguez, 1980, cited in Strimple, 1993; Rivas, 1999). 

Anaconda fat may be used in traditional medicine, but demand in Venezuela is low 

(Rivas, 1999).

Anaconda skins were formerly exported in significant numbers from South 

America (ranging between 4,781 and 21,936 per year from 1980-1987), but this trade has 

effectively ceased since the species was listed by CITES (a maximum of 2,030 skins in 

1989 and only two in the period 1994-1997; Dirksen, 2002; Dirksen and Böhme, 2005). 
234

One of us (RNR, pers. obs.) observed no curios made from anaconda skins during visits 

to marketplaces in Manaus, Brazil in 2003 and 2008, although these markets included a 

wide variety of curios incorporating skins of Boa constrictor. Over 2,000 Green 

Anaconda skins were confiscated in Venezuela in 1988 and 1990 combined; continued 

trade in skins was suspected but its extent was unknown (Rivas, 1999). Dixon and Soini 

(1986) stated that “live anacondas and raw skins were shipped in considerable numbers 

from Iquitos to animal dealers and tanneries around the world” prior to their study, which 

occurred before the CITES listing.

In Trinidad, young anacondas are occasionally caught on fishing hooks baited 

with whole fish (Boos, 2001), and large individuals were recorded as by catch in fishing 

nets in Brazil and Peru (Martins and Oliveira, 1998; Duellman, 2005).

7.2 Human Health Risks

Although some authors state that several humans have been attacked or killed by 

anacondas (especially after provoking the snake: Lancini, 1986; Goulding, 1990), there 

are no well-documented fatal attacks by Green Anacondas on humans (Murphy and 

Henderson, 1997; Rivas, 1998). Mole (1924) cited a personal communication from a 

S.M. Klages, who said that he saw a young man in Venezuela killed by an anaconda that 

he had placed on his shoulders; this would not be considered a “legitimate” attack, as it 

involves a manipulated animal. Boos (2001) reported several incidents, some of which 

were anecdotal, in which anacondas attacked or attempted to attack humans. Rivas 

(1998) reported two apparently unprovoked attempted attacks on humans by Green 

Anacondas in Venezuela during the course of fieldwork on the snakes. The snakes were 

5.04 m total length and 54 kg and 4.45 m and 39 kg respectively, and the ratio of 

attempted human prey mass to snake mass was about 1.0 for the first attack and 1.46 for 

the second. Rivas (1998) concluded, “The potential exists for anacondas to prey on 

people.” 

7.3 Human Aesthetic Relations

7.3.1 Pet Trade

The Green Anaconda is relatively uncommon in the pet trade as compared to 

some of the pythons (P. molurus, B. reticulatus) or Boa constrictor. From 1989 through 

2000, about 1,400 live Green Anacondas were imported to the United States, an average 

rate of approximately 125/year (Reed, 2005). However, the import rate picked up in the 

2000s, and examination of CITES documentation revealed a longer-term average of 427 

per year (range 0-1,413). This average is about 10 percent of the average rate at which 

Reticulated Pythons are imported, and is an even smaller fraction of the annual trade in 

Boa Constrictor and Burmese Python. The major exporting countries are Guyana and 

Suriname, which together accounted for 98.6 percent of live exports during 1980-1997 

(Dirksen, 2002). 

According to Bisplinghoff and Bellosa (2007), most Green Anacondas offered for 

sale in Germany are exports from Guyana or Peru (the latter seems dubious based on 

CITES records), and captive breeding is relatively uncommon compared to many other 

giant constrictors. However, only a single live specimen was legally imported to 
235

Germany between 1980 and 1997 according to CITES documents, and most imports are 

smuggled from eastern Europe and are of largely unknown geographic origins (L.

Dirksen, pers. commun., 2009). Wild-caught and captive-bred juveniles in the United 

States typically sold for $150-225 in 2008, based on our review of online dealers. The 

relatively low popularity of anacondas as pets may be attributed to the necessity of 

providing enough water for the immersion of a large animal, combined with perceived 

predilections towards skin infections and unpredictable defensive behaviors including 

biting and exuding a vile-smelling musk.

Figure 9.4. Annual imports of Green Anacondas, Eunectes murinus, to the United States during 

the period 1977-2007. Data from CITES.

7.3.2 Other Uses

Major human uses of Green Anacondas appear to be limited to meat, skins, 

medicinals, and the live animal trade.

0

200

400

600

800

1000

1200

1400

1600

1977197919811983198519871989199119931995199719992001200320052007

Year imported

Number imported Number imported

Year imported
236

Management Profile

See the introduction to the management profile for Indian Pythons (chapter Four) 

for an explanation of the format of this section of the risk assessment. 

8.0 Pathway Factors: Pet Trade

8.1 Volume of International Trade

The number of Green Anacondas entering the United States represents a relatively 

low volume (an average of 427 individuals per year) as compared to species such as Boa 

Constrictor and Indian Python, and is somewhat lower than the import rate of the giant 

African pythons. However, the annual rate of imports in recent years has not exhibited 

drastic interannual swings as observed for some other taxa (for example, Yellow 

Anacondas), and the mean rate of imports over the last decade has increased as compared 

to the previous decade. Exporting countries are essentially limited to Suriname and 

Guyana; if these countries halt exports or other countries initiate exports, the numbers of 

individuals entering the United States could change dramatically. Section 8.1 of chapter 

Four outlines some reasons why imports may constitute higher risk as an invasive species 

than captive-reared individuals.

Imports spiked in 1997 as compared to levels in preceding or ensuing years. It is 

likely that this spike was related to the 1997 release of the horror movie Anaconda, in 

which larger-than-life anthropophagous anacondas consumed a variety of B-list movie 

stars. If the apparent relationship between the movie and import rates is more than a 

remarkable coincidence, such a spike implies that demand, not availability, drives the 

import rate of anacondas, and that suppliers can obtain more snakes from wild 

populations even within a short time period. 

The Beni or Bolivian Anaconda (E. beniensis) does not appear to be represented 

in international trade. 

8.2 Volume of Domestic Trade

We are unaware of any credible estimates of the volume of domestic sales or 

captive propagation of the Green Anaconda. Informal surveys of online and storefront 

reptile dealers lead us to suspect that Green Anacondas are infrequently bred in captivity 

in the United States and that annual domestic production may be less than the average 

number of individuals imported per year, but we have no means of verifying this 

suspicion. 

9.0 Entry Potential (Survival in Transit—Meant Mainly for 

Unintentional Transport)

As with most species assessed in this document, survival of Green Anacondas in 

international transport is likely to be very high, as animals are packed and shipped via 

means designed to ensure their survival. While mortality rates of some reptiles have 

historically been very high during international transport (Christy, 2008), large hardy 
237

species of snakes would appear to be among the taxa most likely to survive the travails of 

international transport. Once in the United States and purchased by an endpoint 

consumer, the anaconda is likely to be housed in conditions that are designed to meet its 

needs. Survival in captivity is likely to be high except for snakes possessed by extremely 

negligent owners.

10.0 Colonization Potential

10.1 Likelihood of Escape/Release

See section 10.1 of chapter Four for factors that might increase the rate at which 

captive snakes escape or are released; the same factors are likely to apply to Green 

Anacondas. The massive build of adult Green Anacondas increases requirements for 

caging size and strength, even above that required for pythons of the same body length, 

and this species regularly reaches body sizes that many owners would consider difficult 

to manage. As with Yellow Anacondas, the necessity of providing water in which to 

submerge complicates the captive husbandry of the Green Anaconda; snakes can develop 

fungal or bacterial infections if the water is not cleaned regularly, and large Green 

Anacondas require bathtub- or larger-sized basins. Both chronic illness and 

expensive/complex caging requirements may increase the odds of a captive being 

released. 

10.2 Climate Match: Areas of the Country at Greatest Risk

Much of peninsular Florida (roughly south of Gainesville) and extreme south 

Texas exhibit climatic conditions similar to those experienced by Green Anacondas 

somewhere in their large South American native range. Lower elevations in Hawaii and

all of Puerto Rico have apparently suitable climates, but the rest of the country appears to 

be too cool and/or too arid. Within the climate-matched area, however, anacondas would 

not be at risk of establishment in sites lacking surface water. Conversely, the areas of the 

American Southwest that are deemed “too hot” by our algorithm could conceivably 

represent suitable climatic conditions if aquatic habitats with cooler water conditions are 

available as thermal refugia. However, such habitats are few and far between in much of 

this region. Climate suitability is just one factor in the establishment of an invasive 

species—a necessary but not sufficient condition. As regards climate tolerance in 

southern Florida, Lutz Dirksen told us, “There is no environmental reason why anacondas 

could not survive in the Everglades” (L. Dirksen, pers. commun., 2009).

The Beni Anaconda is known from few specimens in a small part of Bolivia, and 

we judged the number of available localities to be insufficient for an attempt to delineate 

its climate space or extrapolate this space to the United States. Beni Anacondas are 

known from sites with fairly low seasonality (mean monthly temperatures approximately 

22.5oC-27.5oC, mean monthly precipitation about 50-300 mm; see Fig. 9.3 for an idea of 

the small size of this climate space as compared to that of E. murinus) and as such very 

little of the continental United States would appear to be climatically suitable (although 

insular states and territories may be at some risk). However, it is unknown whether the 

species’ native distribution is limited by factors other than climate; if the small native 

range is attributable to ecological (for example, competition with E. murinus), or 
238

historical (for example, climate change) factors, then our subjective estimate of the 

climatically suitable areas of the United States would represent underprediction.

Figure 9.5. Areas of the United States matching the climate envelope expressed by E. murinus

in its native range (Fig. 9.3), based on 77 known localities. See chapter Two for details of 

analytical method.

10.3 Prey Availability

Green Anacondas are highly euryphagic, and consume endotherms and 

ectotherms from a wide variety of higher taxa, including large prey such as deer and 

crocodilians. The regular inclusion of fish, turtles, and other aquatic organisms in their 

diet increases their range of prey even beyond that of Reticulated or Indian Pythons. 

Organisms that regularly come in contact with aquatic habitats are likely to be most 

commonly consumed by Green Anacondas, but this does not appreciably truncate their 

available range of prey. The mainland sites above would not appear to have any shortage 

of prey for Green Anacondas of a variety of body sizes, although in insular situations 

(Hawaii, etc.) adults may be limited by low availability of large-bodied prey.
239

10.4 Predation Risk

There are no predators in the United States that are likely to specialize in 

consuming Green Anacondas. Aquatic and semi-aquatic predators would be the most 

likely to consume Green Anacondas, but very large adults would be virtually 

invulnerable to attack by all native predators except in unusual situations (for example, 

animals stranded away from water that could be tired out by harassment), with the 

exception of very large alligators. Avian predators might also consume some juveniles, 

although submerged anacondas would be nearly invisible to visual predators except when 

snakes are basking above the waterline or when swimming near predatory wading birds 

(herons, etc.) or raptors. 

Any losses to native predators would probably be balanced by intraguild 

predation (for example, alligators eat anacondas, but the converse also happens), as 

discussed in section 10.4 of chapters Four and Six. The odds of terrestrial predators 

foraging in Green Anaconda habitat is probably low, but these same predators might be 

vulnerable to attack by anacondas when they traverse or otherwise need to use aquatic 

habitats; all in all the rate of predation on Green Anacondas would probably be 

outweighed by predation on native predators by Green Anacondas.

10.5 Reproduction Requirements

There appear to be no specialized reproductive requirements among Green 

Anacondas, which are live-bearing snakes. Bearing live young allows pregnant females 

to use behavioral thermoregulation (for example, basking and shuttling between habitats 

with different temperatures) in order to maintain relatively high and constant body 

temperatures during embryonic development. Live-bearing anacondas may thus be able 

to successfully reproduce in cooler climates as compared to egg-laying giant constrictors 

with similar temperature requirements for embryonic development (excluding those 

species that employ shivering thermiogenesis).

10.6 Hibernation Requirements

So far as is known, the Green Anaconda does not escape cold temperatures by 

hibernating anywhere in its range. Individuals may seek out isolated mudholes during the 

dry season in seasonally flooded habitats, and long-term use of such refugia may 

approach a physiological/behavioral state that could be considered aestivation.

10.7 Tools for Eradicating a Colonization

Tools for eradicating a population of Green Anacondas are likely to be similar to 

those described in chapter Three and section 10.7 of chapter Four, but no tools have been 

developed specifically for highly aquatic snakes such as anacondas. Because Green 

Anacondas spend much of their time submerged, visual searching is unlikely to be very 

effective for eradicating an introduced population unless most individuals need to bask 

above the waterline in cooler months and all individuals are accessible to human 

searchers. Meeting both of these requirements seems unlikely. 
240

11.0 Spread Potential

11.1 Dispersal Ability

Nothing is known of the dispersal rates of Green Anacondas in their native range, 

but it is likely that individuals could move efficiently through aquatic habitats. 

Green Anacondas are unlikely to engage in long-distance dispersal through entirely 

terrestrial habitats. This could potentially retard their rate of spread if introduced to the 

United States, especially in areas with widely-spaced bodies of water. Unfortunately, 

much of the potentially colonizable area (section 10.2) in the Southeast United States is 

characterized by numerous riverine, lacustrine, and/or seasonally flooded habitat. Green 

Anacondas exhibit large litter sizes; although these do not attain the maximal clutch sizes 

of some pythons, viviparity may allow high embryo survival as compared to oviparous 

snakes. 

11.2 Ability to Develop Races or Strains

There are no native species in the United States with which the Green Anaconda 

would be capable of interbreeding, negating the risk of developing new races or strains 

by that route. However, Yellow and Green Anacondas appear capable of interbreeding, 

although the frequency and fertility of hybridization events is largely unknown and 

hybrids have not been observed in the native range to our knowledge. If hybrids are 

fertile and exhibit characteristics of both species (for example, cold tolerance of Yellow 

Anacondas but increased size from Green Anaconda genetic contributions), the resulting 

hybrid might represent higher risk as an introduced species. However, we judge such a 

scenario to be fairly unlikely. 

11.3 Tools for Managing Spread

As with the other giant constrictors (see chapter Four, section 11.3), we judge 

existing control tools to be inadequate for the eradication of large, established 

populations of Green Anacondas. In the absence of proven control tools, an effective 

interdiction program might hold the most promise for preventing establishment (for 

example, Vice and Pitzler, 2002), combined with a well-organized rapid response 

capacity to respond to sightings of colonizers (Stanford and Rodda, 2007). Because 

interstate transport of anacondas and other giant constrictors is poorly monitored, 

interdiction within the continental United States would be difficult.

12.0 Economic Impact Potential

The Fish and Wildlife Service has not decided whether a formal economic 

analysis of the potential impacts of introduction of, and regulation of trade in, the 

anacondas will be separately conducted pursuant to a variety of statutes and Executive 

Order 13272 (Proper consideration of impact on small business). Our narrative below is 

provided only to cite relevant biological literature and identify germane sections of the 

biological profiles.
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12.1 Pathogen Vector

An amazing knowledge gap exists with regard to the ability of Green Anacondas 

to vector pathogens to native or domestic animals in the United States. A single health 

assessment of a few individuals in Venezuela (Calle and others, 1994, 2001) found Green 

Anacondas to harbor a number of internal parasites of unknown medical or veterinary 

importance, but failed to find antibodies to a limited range of diseases. Ticks known or 

suspected to act as pathogen vectors (see chapter Six, sections 5.3 and 12.1) which were 

observed using other giant constrictors as hosts could almost certainly infest Green 

Anacondas as well. While the highly aquatic habits of anacondas may reduce their 

susceptibility to external air-breathing parasites, at least two species of Amblyomma ticks 

are known to use Green Anacondas as hosts in their native range, as are species of ticks 

commonly found on cattle (J. Rivas, pers. commun., 2009).

12.2 Predator on Livestock

Green Anacondas are capable of preying on most species of domestic livestock, 

although reports of predation on cattle and horses are rare and often dubious. Domestic 

waterfowl and other livestock kept near water would appear to be at highest risk of attack 

from Green Anacondas. The tendency of adult anacondas to ambush large prey from 

water means that livestock victims may be quickly submerged and drowned without 

attracting the attention of human owners, thereby reducing the odds of rescue before the 

animal succumbs. Alligator and turtle farming might be somewhat impacted by 

established populations of Green Anacondas, but we would expect losses to be minimal 

to moderate.

12.3 Predator on Pets

Green Anacondas are certainly capable of successfully consuming nearly all 

species of companion animals, and have eaten dogs in their native range. As with the 

Yellow Anacondas, however, Green Anacondas would be most likely to attack 

companion animals in close proximity to aquatic habitats. Well-publicized attacks on 

companion animals would result in public concern that would almost certainly be out of 

proportion to the actual threat posed to such animals. 

12.4 Electrical Power Systems

Heavy-bodied adult anacondas are unlikely to climb high enough to impact 

electrical delivery systems, although a single 5 m long individual was observed 5 m up in 

a tree and attempted to climb higher when disturbed (De Freitas, 2009). Juveniles may 

occasionally climb (for example, several juveniles were observed in trees over water on 

Trinidad; R. Rozar, pers. commun., 2009), but this habit appears to be less common 

among Green Anacondas than among many giant constrictors, even as compared to 

Yellow Anacondas. 

12.5 Traffic Accidents/Human Attacks

Legitimate attacks on humans in the native range of Green Anacondas are 

incredibly rare, and poor documentation of such attacks makes their likelihood difficult to 
242

assess. A constricting aquatic predator of this size could conceivably overpower and 

drown an adult human without much associated noise or commotion, so it is possible that 

persons disappearing in the native range could be victims of anacondas without their 

disappearances being linked to this species. Compared to species with a betterdocumented history of attacks on humans (for example, Reticulated Pythons), the Green 

Anaconda appears to be a relatively low risk as an attempted predator on humans. 

Green Anacondas can attain huge sizes and girths greater than other constrictors 

of the same body length. They may therefore pose a slightly higher risk of causing auto 

accidents, should a fast-moving automobile strike a large anaconda on a road.

12.6 Tourism

As discussed above, Green Anacondas are fairly unlikely to attack humans. 

Perhaps more than any other species of giant constrictor, however, the Green Anaconda 

likely has the worst perception as a marauding terror among the general public, and 

“anaconda” is the common name most likely to evoke images of mammoth snakes. This 

misconception has been reinforced in recent years by a series of horror movies portraying 

gigantic (over 20 m) anacondas consuming entire boatloads of innocent and not-soinnocent humans. Establishment of Green Anacondas in the continental United States 

would likely have an overall negative impact on tourism, as the small proportion of the 

public who would be likely to visit an area to see anacondas would be far outnumbered 

by the ophiophobic majority who assume that their families would be in extreme danger 

from anacondas.

12.7 Impacts on Commercially Important Wildlife

12.7.1 Species Hunted with Gun or Binoculars

Upland game species such as deer, feral hogs, turkeys, etc. would be expected to 

largely escape predation by Green Anacondas. However, even upland species 

occasionally use aquatic habitats, and Green Anacondas would represent a novel largebodied predator of native game species. In the Southeast, such species would presumably 

be somewhat adapted to the presence of alligators, another large aquatic predator that 

often attacks from submerged ambush positions; if so, then Green Anacondas might not 

represent a completely novel class of predators. 

Waterfowl are the birds most likely to be jeopardized by established populations 

of Green Anacondas, as discussed for Yellow Anacondas in chapter Eight, section 12.7.1. 

Wading birds are a mainstay of birdwatching in Florida, and Green Anacondas could 

potentially hasten population declines of species that are already of conservation concern, 

such as Wood Storks. Duck hunting is an economically important activity in some parts 

of the Southeast; Green Anacondas commonly take anseriform birds in their native range, 

and would no doubt do so in the United States, but it is difficult to predict the 

demographic significance of this predation on huntable wildlife.

12.7.2 Species that Impact Forestry, Agriculture, or Horticulture

Green Anacondas would be unlikely to have drastic impacts on these industries, 

although safety considerations associated with the presence of giant introduced snakes 
243

might affect costs for business owners (see chapter Six, section 12.7.2), especially those 

overseeing work in aquatic habitats. 

13.0 Environmental Impact Potential

13.1 Species of Special Concern as Prey or Competitors

We consider all of the species of special conservation concern in Florida that are 

potential prey of Burmese Pythons (chapter Four, Table 4.2) to also be potential prey for 

Green Anacondas. Truly giant anacondas might be capable of eating even larger prey, 

but extremely large individuals would likely be uncommon in introduced populations.

13.2 Species of Special Concern as Predators (Giant Constrictors Augment Food 

Supply)

As discussed in this section in previous chapters, a range of native predators

would be capable of consuming Green Anacondas of varying sizes. Alligators are the 

most obvious of these, although panthers might take some adults and a variety of smaller 

predators could consume juveniles. However, such benefits to predators are likely to be 

balanced or outweighed by intraguild predation and/or overall energetic losses associated 

with eating prey that are themselves high on the food chain (see chapter Six, section 

10.4). There is no evidence to suggest that any native predator would experience a net 

demographic benefit from the presence of Green Anacondas.

13.3 Vector for Disease Spread to or Within Native Faunas

Insufficient data are available to evaluate the likelihood of transmission of 

pathogens or parasites to native faunas. The sole survey for infectious diseases in the 

species examined a sample of Green Anacondas from Venezuela and failed to find 

antibodies to any of a range of pathogens. Green Anacondas are likely to be suitable 

hosts for some of the same ectoparasites known from other taxa (for example, the giant 

African pythons) and which are known or suspected to be carriers of zoonotic disease. 

However, Green Anacondas might be less suitable hosts for such ectoparasites, as 

anacondas spend much of their time submerged. 

13.4 Species that Might be Adversely Affected by Control Measures

Control measures for Green Anacondas would likely be similar to those 

envisioned for giant constrictors in general (see chapter Three). Some possible avenues 

of control, such as acetaminophen toxicants, are likely to result in non-target mortality to 

native species, especially large snakes capable of accessing toxicant delivery devices 

designed for giant constrictors. Traps, drift fences, and other potential tools may also 

impede movements of non-target native taxa or result in their inadvertent capture. 

Control measures for Green Anacondas would largely be deployed in or adjacent to 

aquatic habitats, implying that species using such habitats are at highest risk of being 

adversely affected by control measures. Depending on geography, these species might 

include alligators, American crocodiles, aquatic snakes, river otters, etc. As no 
244

management tools have been field-tested, however, it is not possible to estimate potential 

ecological costs.

14.0 Perceived Impact (Social and Political Influences)

14.1 Pet Industry

14.1.1 Domestic Production Affected?

Domestic production of Green Anacondas by herpetoculturists is relatively 

miniscule as compared to some of the other large constrictors. Based on our examination 

of online dealers, stock available at reptile shows, and the herpetocultural literature, we 

estimate that domestic production is typically less than 250 neonates per year, but we 

have no means to verify this estimate. Breeding Green Anacondas is largely confined to 

a few specialty herpetoculturists. Prohibition on interstate transport or sales would 

disproportionately affect these individuals but would have a small impact on the overall 

domestic reptile trade.

14.1.2 International Trade/Alternate Pets

In a typical year, fewer than 500 Green Anacondas are imported to the U.S., and

most of these originate in northern South America. Trade at this volume would be worth

perhaps $50,000-100,000 annually to importers at the wholesale level, and two to three 

times that amount at the retail level. While a considerable sum, these estimates pale in 

comparison to species that are traded in the thousands (for example, Reticulated Pythons) 

or tens of thousands (for example, Ball Pythons), and represent a tiny fraction of the 

overall value of the reptile import trade. Were importation regulated, dealers would 

likely switch to other species to make up for the loss of Green Anacondas in the 

international trade, rather than experiencing major economic losses.
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Chapter Ten–The Risk Assessment

Introduction

The Risk Assessment is in two parts: (1) Four factors evaluating the risk of 

establishment, and (2) three factors evaluating the consequences of establishment. For 

each of these seven factors we begin by noting the relevant traits shared by the nine giant 

constrictor species, and then the traits that differentiate them. Each part concludes with a 

table summarizing the risk scores and producing an overall component risk score for each 

species. In turn, these will be combined into a single Organism Risk Potential (ORP) 

score for each species, presented in a table at the end of the chapter, following the 

guidance established by the Aquatic Nuisance Species Task Force (ANSTF, 1996).

Risk of Establishment

1. Organism with Pathway

This element establishes the likelihood that the species occurs in the pathway 

under consideration. As noted in the species accounts, the overwhelmingly dominant 

pathway for the entry of these giant constrictors into the United States is commercial 

trade in pets, consistent with the global pattern for reptiles (Kraus, 2009). The amount of 

commercial pet trade traffic is fairly easily assessed on the international scale, as the 

international regulatory authority CITES (the Convention on International Trade in 

Endangered Species) maintains the trade records we reported in each species account 

(tabulated in Appendix). However, there is a substantial additional but unknown amount 

of domestic trade, and a presumably small amount of clandestine trade (both international 

and domestic).

In aggregate, there is a very large trade in giant constrictors, with over one million 

of these snakes imported to the United States in the last 30 years and many of these bred 

domestically and traded in even larger numbers. Thus, as a group, the nine giant 

constrictors constitute a high risk relative to those taxa with lower trade volumes. But not 

all of the nine species are equally represented; about 60 percent of the trade in our focal 

group consists of Boa constrictor. On the basis of the number being traded, and the 

assumption that the proportion being released is constant from species to species, it 

would be possible to rank the relative risk of release, but to assign risk categories to each 

species we need to identify thresholds for distinguishing the high, low, and medium risk 

categories required. The ANSTF provides minimal guidance on this challenge, 

presumably because the thresholds vary among taxa (for example, giant constrictor 

snakes may all be higher than typhlopid [worm] snakes). It is possible that although the 

risks vary among the giant constrictors, all giant constrictor species would have an 

absolutely low or high risk. 
246

One of the rarely traded species (fewer than 2000 individuals total over 30 years) 

is the Yellow Anaconda; is the overall risk low because it is less frequently traded than 

most of the other giant constrictors? No unequivocal answer is apparent. We can get 

some indication from the appearance in Florida of multiple reports of Yellow Anacondas 

in or near the Big Cypress National Preserve. The occurrence of this species in the wild, 

in a location far from most human sources of this pet suggests that the risk of 

establishment of this rarely-traded species is substantial. If the risk of Yellow Anaconda 

“Organism With Pathway” is high (multiple individuals appearing at the same site in the 

same time period is a strong indication that enough are in the pet trade to constitute a 

significant pathway risk: Risk High), the species with greater trade volume would 

presumably be High also (Indian Pythons, Northern African Pythons, Reticulated 

Pythons, Green Anacondas, and Boa Constrictors). The level of certainty for these five 

species is Very Certain (certainty codes established by ANSTF, 1996, and reprinted on 

pp. 10-16).

The remaining species (De Schauensee’s Anaconda, Beni Anaconda, and 

potentially Southern African Python) have low levels of reported importation, and if 

these levels are accurate, and past performance is a guarantee of future prospects, the risk 

of their presence in the pet pathway would be Low. However, past performance is not a 

guarantee of future prospects, and importation reports for the United States wildlife trade 

are “notoriously unreliable” (Blundell and Mascia, 2005). In this case, the low levels of 

reported importation may be low by virtue of similarity of appearance (misreported or 

misidentified), but also because the closely related taxa of these lesser-known species are 

readily and inexpensively available in the pet trade. Should the species that currently 

dominate the trade become less readily available, one would expect a compensatory 

increase in sales and interest in the related taxa, as has been seen in response to export 

bans of particular species from several countries in recent decades. This is a question of 

human behavior, with the attendant high level of uncertainty. By virtue of similarity of 

appearance these species may warrant a High risk of presence in the pathway, though at 

present there appears to be only limited commercial traffic in these three species.

2. Entry Potential

This risk element is primarily relevant to unintentional pathways, for which the 

survival of the species in transit is variable or low. In the case of our nine giant 

constrictor species, transport is intentional and survival is therefore High, an assessment 

that is Very Certain.

3. Colonization Potential

Colonization potential depends on a wide range of elements, including propagule 

pressure (how many of the same species are released in the same area in a short period of 

time); the probability of release or escape; and the likelihood of the released pets finding 

suitable food, environmental conditions (for example, climate match: Bomford and 

others, 2009), lack of competition from similar species, and conditions suitable for 

reproduction (ANSTF, 1996). For a more comprehensive list of the elements 

hypothesized to promote the establishment of reptiles and amphibians see Table 3 in 

Rodda and Tyrrell (2008), which provides references for each of the hypotheses. The 

hypotheses are grouped into sets relating to the survival and reproduction of released 
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animals. For example, dietary characteristics primarily affect survival, and number of 

offspring primarily affects the species’ likelihood of successful reproduction.

Survival: Of the eleven hypotheses related to survival, ten apply with roughly 

equal force to all of the giant constrictors. Of these ten, two suggest that giant 

constrictors are not likely to establish: (1) Establishment is facilitated for insectivores 

(none of the giant constrictors are insectivorous), and (2) establishment is promoted for 

species that actively modify their environment in their favor (for example, digging 

tortoises). As far as is known, the giant constrictors’ requirement for burrows is 

dependent on other species digging them.

Four additional survival hypotheses are equivocal with regard to giant 

constrictors, at least with the information available: (1) Establishment is promoted for 

island/tropical/maritime destinations (equivocal because some American destinations are, 

and some aren’t), (2) establishment is more likely on islands (Kraus, 2009), at least in 

part because islands have maritime climates, unexploited niches, etc. (equivocal because 

some American destinations are islands and some are not), (3) establishment of adaptable 

species is facilitated compared to species that are more fixed in their ways (insufficient 

information about the behavioral adaptability of the giant constrictors), and (4) 

establishment of highly heterozygous species is more likely (insufficient information; 

although some of the giant constrictors are drawn from diverse localities and are 

therefore likely to have a deep gene pool, other giant constrictor individuals are likely to 

have been intensely inbred for purposes of obtaining “designer morphs”).

The four remaining hypotheses apply similarly to all of the giant constrictors and 

suggest that giant constrictors are likely to establish: (1) High longevity combined with 

early maturation leads to a high overlap between generations, with enhanced 

opportunities for benefitting from peers and finding a suitable mate, (2) empty niches at 

destinations make it unlikely that native species will outcompete the newcomers, (3) 

broad diets increase the opportunities for finding suitable prey, and (4) introduced snakes 

are likely to have very few parasites relative to the overall parasite fauna that afflicts the 

species in its native distribution, because there is a lack of suitable alternate hosts in the 

United States for the complete lifecycle of the parasite, and because many snakes are 

well-maintained and treated for parasites while in captivity prior to release. 

The one survival attribute that varies greatly among the giant constrictors is that 

of climate match. Some species (for example, Indian Python, Yellow Anaconda, 

Southern African Python, Boa Constrictor) would find suitable habitat over a broad swath 

of the American landscape, whereas others (Northern African Python, Reticulated 

Python, Green Anaconda), would likely be limited to the warmer fringes of the continent. 

The remainder (DeSchauensee’s Anaconda, Beni Anaconda) might be able to survive in 

very small portions of the mainland or on America’s tropical islands (Hawaii, Puerto 

Rico, American Samoa, Guam, Northern Mariana Islands, Virgin Islands). Although 

these islands are only a small proportion of the United States, they are especially 

important for biodiversity preservation.

We have summarized this assessment of survival prospects in Table 10.1.
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Table 10.1. Hypothesized attributes affecting survival of potential invaders during establishment 

(from Rodda and Tyrrell, 2008), as applied to giant constrictors. Y = yes; N = No; H = High; M = 

Medium; L = Low; - = equivocal; ? = insufficient information. Species names are coded by the first 

two letters of the genus and species name.

Hypothesized attribute PyMo BrRe PySe PyNa BoCo EuNo EuDe EuMu EuBe

Longevity Y Y Y Y Y Y Y Y Y

Climate match H M M H H H L M L

Tropical destination - - - - - - - - -

Island destination - - - - - - - - -

Empty niche Y Y Y Y Y Y Y Y Y

Adaptable ? ? ? ? ? ? ? ? ?

Heterozygous ? ? ? ? ? ? ? ? ?

Insectivorous N N N N N N N N N

Broad diet Y Y Y Y Y Y Y Y Y

Parasites absent Y Y Y Y Y Y Y Y Y

Modify environment N N N N N N N N N

Reproduction: The most obvious attribute influencing reproductive success in an 

incipient colonization is “propagule pressure” (how many breeders are present). The 

more individuals of the same species (or a closely-related species that when bred with the 

focal species produces viable offspring) that are present in an area, the greater the 

likelihood of successful reproduction (Reaser and others, 2008; Bomford and others, 

2009). The commercial trade statistics provide one component of propagule pressure 

(discussed above); another is the willingness of human owners to discard their pets, or the 

owners’ abilities to keep the pets from escaping. The more owners that are willing to 

release or unable to contain, the greater likelihood that two or more releases of the same 

species will occur in the same area. For most of the giant constrictors, there is a high 

likelihood of release of unwanted adult giant constrictors, as evidenced by dozens of 

media reports of individuals found across the country. Market value has some predictive 

value in this regard, as species that are inexpensive as juveniles are more likely to be 

dumped as juveniles. Most of these species lose market value as adults, because the 

demand for large adult giant constrictors of most species is far exceeded by the supply.

Rodda and Tyrrell (2008) enumerated eight reproduction traits that have been 

hypothesized to influence the establishment success of reptiles and amphibians. These 

eight are expressed to much the same degree in the giant constrictors under consideration:

Two giant constrictor attributes are potentially associated with reduced 

probability of establishment: (1) Gregariousness increases the chances of encountering a 

suitable mate at the season of reproduction (giant constrictors are not ordinarily 

considered to be gregarious, though they possess highly effective olfactory means of 
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finding suitable mates when needed), and (2) sexual monomorphism increases effective 

population size (number of individuals making a genetic contribution to the next 

generation) compared to species with strong sexual dimorphism or dominance hierarchies 

(most giant constrictors are sexually dimorphic, but the influence of this on gene 

transmission is unknown and may not apply at low population densities characteristic of 

incipient populations). This trait was hypothesized to influence invasive-species 

colonization primarily with reference to birds, for which monomorphism refers primarily 

to plumage coloration, and dimorphism connotes male dominance hierarchies (which 

diminish effective population size). It is not clear that this phenomenon is relevant to 

giant constrictors, for which sexual size dimorphism is primarily manifest in large female 

size to promote larger clutches or litters, a consequence that does not diminish effective 

population size.

Two traits are equivocal with regard to giant constrictors: (1) Ability to shift 

between r and K demographic strategies (Pianka, 1970) in response to prevailing 

conditions (intended mainly for fish that can adaptively alter their mating system; the 

application of this phenomenon to giant constrictors is uncertain—too little is known 

about their reproduction to exclude adaptive fecundity modification—but they are not 

known to switch sexes), and (2) short interclutch intervals expedite population expansion 

(insufficient information, though giant constrictors achieve the same fecundity increase 

with large clutch sizes).

Four reproductive traits of giant constrictors appear to facilitate population 

establishment: (1) Early maturation, (2) parthenogenesis (facultative parthenogenesis is 

firmly documented in Indian Pythons, suspected in Boa Constrictor and Green Anaconda, 

possible in the other species: see chapters Four–Nine), (3) sperm storage (probable, as it 

is present in many snakes but not known with certainty in these species), and (4) high 

fecundity (variable among these species, but with clutch/litter sizes that are all larger than 

the average for snakes).

We have summarized this assessment of reproductive attributes in Table 10.2.
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Table 10.2. Hypothesized attributes affecting reproduction of potential invaders during 

establishment (from Rodda and Tyrrell, 2008), as applied to giant constrictors. Symbols as in 

Table 10.1.

Hypothesized attribute PyMo BrRe PySe PyNa BoCo EuNo EuDe EuMu EuBe

Shift r to K ? ? ? ? ? ? ? ? ?

Early maturation Y Y Y Y Y Y Y Y Y

Parthenogenesis Y Y? Y? Y? Y? Y? Y? Y? Y?

Sperm storage Y? Y? Y? Y? Y? Y? Y? Y? Y?

Gregariousness N N N N N N N N N

High fecundity Y Y Y Y Y Y Y Y Y

Short interclutch interval ? ? ? ? ? ? ? ? ?

Sexual monomorphism N N N N N N N N N

4. Spread Potential

To quantify Spread Potential, the ANSTF (1996) guidelines mention: ability for 

natural dispersal, ability to use human activity for dispersal, ability to readily develop 

races or strains, and the estimated range of probable spread. Rodda and Tyrrell (2008) 

expand this list substantially, with specific reference to reptiles and amphibians, and point 

out that attributes that enable a species to spread aggressively are likely to be similar to 

traits that increase the success of the bearer at the expense of native wildlife. In other 

words, these traits correlate with the environmental impact potential (see subsequent 

section on Environmental Impact Potential). A potential additional complication is that 

traits that promote the speed of spread may be incompatible with the traits that promote 

the ability to spread (equal to environmental impact potential). In classical “r versus K” 

conceptualization of adaptation, a species that is adapted to rapidly occupy a vacant patch 

(r selected) may be poorly adapted for the competition inherent in behaviorally 

dominating resident individuals in an already-occupied patch. Evolutionary ecologists 

are divided on the relevance of the r versus K conceptualization, but it is notable that if 

that sort of trade-off has any basis in ecological reality, it may have a manifestation in a 

trade-off between speed of spread and ability to spread against biotic resistance (Rodda 

and Tyrrell, 2008).

In relation to the elements enumerated in the ANSTF (1996) guidelines, giant 

constrictors are probably similar to one another in the ability for natural dispersal (very 

high), ability to use human activity for dispersal (low as inadvertent stowaways; high if 

one includes intentional transport via the pet trade), and ability to develop races or strains 

(unknown, though hybridization between species is known in captivity and new 

genotypes could be formed by multiple releases of individuals from disparate parts of the 

native range, see Kolbe and others [2004] for an example). The estimated range of 
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probable spread differs among species, in keeping with the sizes of the climate-match 

areas outlined in the previous section (Colonization Potential).

Rodda and Tyrrell (2008) tabulate and describe 23 traits that have been 

hypothesized to influence spread (rate or success) in reptiles and amphibians. Twelve of 

these appeared in the two previous tables, and are worth reconsideration only to be 

reminded that traits that favor colonization may also favor spread: (1) Ability to shift 

between r and K strategies (insufficient information), (2) adaptability (insufficient 

information), (3) broad diet (Yes for giant constrictors), (4) heterozygosity (insufficient 

information), (5) sexual monomorphism (No for giant constrictors), (6) gregariousness 

(No for giant constrictors), (7) species that modify environment (No for giant 

constrictors), (8) insectivorous (No for giant constrictors), (9) climate match (varied by 

species), (10) tropical destination (equivocal: depends on which part of United States 

under consideration), (11) island destination (equivocal: depends on which part of the 

United States is under consideration), and (12) empty niches (Yes for giant constrictors).

In addition to the twelve repeated traits, Rodda and Tyrrell (2008) listed eleven 

novel attributes hypothesized to influence spread potential, of which eight are relatively 

consistent among the giant constrictor species: (1) Continental native range (Yes for giant 

constrictors) is thought to imbue the denizens of continents with more competitive ability 

(relative to island species), (2) disturbed habitat preference is thought to facilitate spread 

of invaders, probably due to a concurrent tendency to disperse rapidly to discover new 

habitat patches (insufficient information on giant constrictors), (3) high density in native 

range (insufficient information on most giant constrictors, but some species are capable 

of reaching high densities in some habitats), (4) fire-adapted (No for giant constrictors, 

except insofar as they refuge underground and in water and would survive fires in those 

refugia), (5) being a lizard (No for giant constrictors), (6) being a predator (Yes for giant 

constrictors), (7) large adult size (Yes for giant constrictors), and (8) arboreality opens up 

additional potential prey for predators (all of the giant constrictors are somewhat arboreal 

in their youth, and become less so as they gain weight).

The three remaining traits differ among the giant constrictors, and are scored in 

the following table (Table 10.3): (1) Wide latitudinal range of native range is thought to 

reflect wide ecological tolerances, (2) size of native range is another metric of ecological 

tolerance, and (3) tolerant of a wide range of climates is a third metric of ecological 

amplitude.
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Table 10.3. Eleven traits not previously mentioned that have been hypothesized to influence 

spread success in reptiles and amphibians (for source information see Table 5 in Rodda and 

Tyrrell 2008). Symbols as in Table 10.1. In this case arboreality is expressed primarily in juveniles.

Hypothesized attribute PyMo BrRe PySe PyNa BoCo EuNo EuDe EuMu EuBe

Continental native range Y Y Y Y Y Y Y Y Y

Disturbed habitat preference ? ? ? ? ? ? ? ? ?

High pop density in native range ? ? ? ? Y Y ? ? ?

Wide lat. range of native range H M M H H M L H L

Size of native range H H H H H M L H L

Wide climate range H M M H H H L H L

Arboreal Y Y Y Y Y Y Y Y Y

Fire adapted N N N N N N N N N

Lizard N N N N N N N N N

Predator Y Y Y Y Y Y Y Y Y

Large adult size Y Y Y Y Y Y Y Y Y

The overall guidance for the subscore Spread Potential is: “Estimate probability 

of the organism spreading beyond the colonized area” (ANSTF, 1996). This is relatively 

straightforward, as all of the giant constrictors most likely have the ability to spread 

beyond the colonized area if the colonized area is within the area that is climatically 

suitable. However, the ANSTF (1996) provides additional guidance to consider the 

estimated range of probable spread. On the one hand it is clear that an invasive species 

that is limited to a small part of the United States constitutes a lesser risk, all things being 

equal, than a species that could spread throughout the United States. However, not all 

things are equal, and some of the most egregious examples of invasive reptiles have been 

manifest in only a very limited area (albeit with catastrophic impacts in that limited 

region) (for example, Savidge, 1987; Romero-Najera and others, 2007; Cheke and Hume, 

2008). Consideration of the degree of impact is, however, more properly considered in 

the second part, Consequences of Establishment. Thus, for the purposes of scoring 

Spread Potential we focused on the overall guidance “Estimate probability of the 

organism spreading beyond the colonized area” with the implicit proviso that the 

colonized area was within the area deemed climatically suitable. We presume that a 

species would be unable to colonize areas that were not climatically suitable, although 

this is conservative given that many invasive species have expanded beyond the climatic 

profile of their native range (Williamson, 2006; Randin and others, 2006; Broennimann 

and others, 2007).
253

Probability of Organism Establishment

The ANSTF (1996) guidelines establish that the reviewer has considerable 

latitude in assessing the four factors—(1) organism with pathway, (2) entry potential, (3) 

colonization potential, and (4) spread potential—making up the probability of 

establishment, but the product of the four subscores rigidly determines the overall 

probability of organism establishment. Thus, the guidelines specify, “the probability of 

establishment is assigned the value of the element with the lowest risk rating (example: a 

high, low, medium, and medium estimate for the above elements would result in a low 

rating).” A separate evaluation of uncertainty is not computed for Probability of 

Establishment. In the following table (Table 10.4) we give our best estimates of the four 

factors and derive the final probability of organism establishment following ANSTF 

guidelines. Below each species’ subscore is our assessment of our uncertainty, using the 

intuitive abbreviations and verbal descriptions established in ANSTF (1996): 

VC = Very Certain (“As certain as we are going to get”)

RC = Reasonably Certain (“Reasonably certain”)

MC = Moderately Certain (“More certain than not”)

RU = Reasonably Uncertain (“Reasonably uncertain”)

VU = Very Uncertain (“A guess”).

Table 10.4. Probability of organism establishment for nine giant constrictor species, and constituent values. 

Constituent subscores are for the four specified factors: (1) Estimate probability of the nonindigenous organism 

being on, with, or in the pathway (Organism With Pathway), (2) estimate probability of the organism surviving 

in transit (Entry Potential), (3) estimate probability of the organism successfully colonizing and maintaining a 

population where introduced (Colonization Potential), and (4) estimate probability of the organism spreading

beyond the colonized area (Spread Potential). Uncertainty codes following the abbreviations given in the 

immediately preceding text. * = by similarity of appearance.

Species Organism With 

Pathway

Entry 

Potential

Colonization 

Potential

Spread 

Potential

Probability of 

Establishment

Python molurus High

VC

High

VC

High

VC

High

VC High

Broghammerus 

reticulatus

High

VC

High

VC

Medium

MC

High

RC Medium

Python sebae High

VC

High

VC

High

RC

High

RC High

Python natalensis High*

MC

High

VC

High

VC

High

RC High*

Boa constrictor High

VC

High

VC

High

VC

High

VC High

Eunectes notaeus High

VU

High

VC

High

RC

High

RC High

Eunectes 

deschauenseei

High*

VU

High

VC

Medium

MC

Medium

MC Medium*

Eunectes murinus High

VC

High

VC

Medium

RC

High

RC Medium

Eunectes beniensis High*

VU

High

VC

Medium

MC

Medium

MC Medium*
254

Consequence of Establishment

5. Economic Impact Potential

The Economic Impact Potential is the most difficult component to assess 

accurately, perhaps because the modern economy is so large and complex relative to our 

ability to accurately project ecological outcomes. Economic impacts that are negligible 

at normal snake population densities (for example, in native range) can become extreme 

at higher population densities, and ecologists have very limited ability to predict the 

densities that will be achieved during irruptions of introduced species. We defer to any 

potential economic evaluation to quantify the potential costs associated with giant 

constrictor colonization.

To appreciate the inherent uncertainty in economic analyses, consider the ability 

to judge the economic impact of the Brown Treesnake prior to its arrival on Guam. At 

that time, the snake was not known to be a threat to wildlife, children, pets, or electrical 

systems. It was known to eat domestic fowl. We now know that the negative economic 

impacts are greatest to electrical systems, and constitute a multi-billion dollar risk to 

Hawaii should it become invaded (Burnett and others, 2006). An equivalent economic 

threat darkens the prospects of tourism in Hawaii (Shwiff, 2008). By comparison, the 

economic risks for the other factors are relatively small, and at present the impact on 

Guam to the poultry industry is minimal, in part because the industry has been largely 

abandoned on Guam, reportedly in response to snake predation (Fritts and McCoid, 

1991). Similarly, the economic cost associated with zebra mussel impacts on power plant 

water intakes was underestimated prior to the mussel’s arrival in the Great Lakes because 

the mussel did not achieve such high densities in its native range (Benson and Boydstun, 

1995). Thus, all economic impact predictions have low certainty.

Based on the Brown Treesnake experience and the relatively weak arboreality of 

the giant constrictors, the largest economic costs are likely to be associated with damage 

to tourism. All of the giant constrictors would be unpopular with some potential tourists, 

though the impact is likely to be greater in places that are heavily dependent on tourism, 

such as Florida, southern California, or Hawaii. Though the uncertainty is great, the 

potential economic costs to tourism are also very high (see species accounts section 

12.6). If tourism impacts were based solely on an objective appraisal of the human health 

risks likely to be associated with interactions between tourists and giant constrictors, the 

tourism impact would be minimal and would be primarily associated with one species—

the Reticulated Python. However, fear of snakes is not entirely rational and one wellpublicized attack on a human could jeopardize considerable tourism business.

Although all of the giant constrictors are capable of climbing trees and some, such 

as the Boa Constrictor and Reticulated Python, readily forage in trees (at least when 

young), our impression is that these giant constrictors are unlikely to frequently climb 

power lines. Though we can claim only very high uncertainty with this prediction, we do 

not anticipate large power grid infrastructure losses due to giant constrictors (see species 

accounts section 12.4).

A potentially devastating impact to the nation’s agriculture could occur if the 

deadly cattle disease heartwater or some other tick-borne disease were to become 

established in the United States and be transmissible through reptile ticks (see species 
255

accounts section 12.1). African tick species that use pythons as hosts may be vectors of 

heartwater, and these ticks have been observed to transfer to other hosts, including other 

giant constrictors, other reptiles, and dogs. Because multiple python species are held 

captive together in the commercial trade, such transmission has ample opportunities to 

occur prior to retail sales.

Direct predation on livestock will occur if any of the giant constrictors become 

established in the United States (see species accounts sections 12.2). This prediction is 

very certain, in part because livestock losses have been widely documented in Florida (by 

Burmese Pythons, Northern African Pythons, and Reticulated Pythons). However, the 

extent of the damage is much less certain. Free-ranging, small–to-medium livestock, 

especially poultry, would be expected to be most heavily impacted. Pigs may be favored 

prey for some species, especially the Asian pythons. We anticipate that the anacondas 

would be relatively less detrimental to livestock, in part because of limited exposure of 

livestock to the anaconda’s preferred watery habitat. Most Boa Constrictors and yellow 

anacondas will be too small to greatly impact mammalian livestock, as will all but very 

large individuals of the remaining species.

Predation on pets is likely to be of limited economic importance, but acutely felt 

by the bereaved pet owner. If experience with alligators has predictive value for 

understanding other large reptile-human exchanges, encounters between companion 

animals and snakes have the potential to escalate into a human health risk (because 

aggrieved pet owners sometimes unwisely choose to defend their pets against the reptile).

With the possible exception of the Reticulated Python, most giant constrictors’ 

impact on human health will be in the realm of feared attacks rather than actual 

unprovoked events. Most of the area of the United States mainland that is likely 

inhabitable by Reticulated Pythons is lightly populated, except for cities and towns along 

the Florida and Texas coasts. Though the number of actual attacks is likely to be small, 

such events as do take place could significantly degrade the already angst-ridden 

psychological relationship between people and wetlands.

All of the giant constrictors could further endanger watchable wildlife species that 

presently constitute a significant draw for eco-tourists. Colonial bird rookeries (for 

example, Wood Storks) are a particular tourist draw, and are potentially vulnerable to 

depredation by a new nocturnal and opportunistically arboreal predator. Where their 

ranges overlap, all of the giant constrictors could significantly reduce stocks of 

economically-important fur-bearers such as beaver and muskrats. None of the giant 

constrictors seems likely to have a large direct impact on forestry, non-livestock 

agriculture, or horticulture.

6. Environmental Impact Potential

The natural history attributes that predict a species’ environmental impact were 

discussed under the section “Spread Potential” and summarized in Tables 10.3 and 10.4.

Based on experience with the Brown Treesnake (Fritts and Rodda, 1998) and 

other introduced snakes, the greatest environmental impact of invasion by giant 

constrictors would be predation on endangered species, either via further endangerment 

or outright extinction. Table 4.2 tabulates the large number of imperiled wildlife that 

presently occur in Florida and would be put at greater risk by the establishment in that 

state of any of the giant constrictors. The amount of damage probably correlates with the 
256

novelty of the invader’s predatory tactics: the larger giant constrictor species have 

predatory tactics that diverge more from the predator defenses of native wildlife than do 

the smaller species. For example, a native mouse may have evolved movement patterns 

and senses for detecting a native snake that is a sit-and-wait predator, but such skills are 

likely to be lacking in beaver, which have not evolved with such a threat. 

Giant constrictors would be novel predators in much of the area of the United 

States that is at risk of establishment. Because novelty is an important element of the 

predatory impact of giant constrictors, and the giant constrictors use largely the same 

predatory tactics as each other, a second colonization event (for example, if Reticulated 

Pythons were to become established in Florida in 2010, after Burmese Pythons had 

already spread throughout the area) would likely have considerably less impact on 

endangered species than the first colonization.

Novelty is an attribute that is hard to quantify; all large reptilian carnivores have 

some degree of overlap in their predatory tactics. Giant constrictors exhibit predatory 

tactics that partially overlap with a variety of native wildlife such as rat snakes, pit-vipers, 

and alligators. Alligators, for example, are also sit-and-wait predators on a very diverse 

suite of prey appearing near water. However, in comparison to most pythons and boas, 

alligators specialize on prey resistant to crushing (durophagy), such as turtles and 

mollusks; alligators tend to ambush from relatively deep water (especially in the summer 

when shallow marshes become too hot); and alligators rarely ambush prey on land 

(pythons and boas ordinarily do so). Juveniles of the giant constrictors will climb to 

extract prey from bird nests or capture perching or sleeping birds, a tactic not used by 

alligators. Nonetheless, some prey will obtain a degree of protection through their 

evolutionary experience with alligators and other large reptiles native to North America.

Although the novelty of sit-and-wait predation is likely to be most acutely felt by 

the larger prey species, the majority of giant constrictor individuals are juveniles or small 

adults. Thus, to the extent that predatory impacts reflect the numerical relationship 

between predators and prey (all things being equal, one expects a larger number of 

predators and predatory attacks, to result in a larger number of predatory successes), the 

potential harm to endangered species might be most immediately or acutely felt by smallor medium-sized mammals and birds. The Brown Treesnake eliminated or assisted with 

the elimination of most of the birds, bats, and lizards native to the island of Guam (Fritts 

and Rodda, 1998), with the smaller prey species showing the most immediate declines. 

Only after several generations of diminished recruitment did the larger prey species 

become extirpated (Wiles and others, 2003). 

A most difficult aspect of this threat is its irreversibility. Once an introduced 

giant constrictor becomes well-established, there is no known method for removing the 

threat (see chapter Three), and thus recovery of endangered species in an infested area is 

a prospect likely to be diminished or even eliminated permanently. In this respect, 

invasive species constitute a more enduring threat than pollution, overexploitation, or 

habitat degradation.

The giant constrictors are unlikely to appreciably supplement the diets of native 

wildlife. Too little information is available to assess the likelihood of giant constrictors 

vectoring new diseases or indirectly impacting native wildlife if control programs 

inadvertently harm non-target species. It is possible that giant constrictors could have a 

positive impact by reducing populations of exotic pest species such as rats or Nutria. 
257

However, our experience with Brown Treesnakes suggests that such non-native species 

allowed the snakes to achieve and maintain high densities, and snakes at these densities 

were able to cause the decline of many native species even though native species may not 

have comprised the majority of their diet (Fritts and Rodda, 1998). Therefore 

consumption of non-native prey species leads to more predation on natives, rather than 

less. 

7. Perceived Impact (Social and Political Influences)

The ANSTF risk analysis guidelines (ANSTF, 1996) state that perceived impacts 

may include aesthetic damage, consumer concerns, and political repercussions, and this 

category thus appears to be a catch-all for potential impacts that are not specifically 

environmental or economic in nature. The guidelines also consider political forces 

unleashed by regulatory action or control measures on the invader, not just the direct 

consequences of establishment. Were giant constrictors to become established, the 

potential dampening effect on tourism would have obvious social overtones, but these 

impacts can to some degree be better quantified by considering the economic impacts 

associated with drops in tourism (see the Economics Impact section of this chapter). 

Similarly, the loss of wading birds and other watchable wildlife would be an aesthetic 

impact, but the effect on birdwatching can be translated into economic terms and is more 

appropriately examined in that section. Political repercussions could include conflict 

between regional, state, and federal philosophies of invasive-species interdiction and 

control (such as occurred with Black Carp in the Southeast and Asian oysters in 

Chesapeake Bay), but we see no means of predicting the nature or intensity of such 

conflict.

Although it is difficult, or perhaps impossible, to fully quantify perceived impacts 

that have no overt economic or ecological impacts, it is notable that colonization by giant 

constrictors would affect human relations to the rural landscape significantly, and not in a 

good way. Perhaps a mother would no longer allow her children to explore the woods 

unescorted, or to swim in a creek. Perhaps a child would have fewer opportunities to 

experience the full range of native wildlife. Loss of these pivotal developmental 

opportunities comes at a cost that we can appreciate even if we cannot readily measure it.

In light of the preceding observations, we have largely addressed perceived 

impacts (both in the management profiles and here) as those that would accrue to the live 

animal trade, were regulations on this trade enacted by state or federal agencies in 

response to the risk of establishment of giant constrictors in the United States. These 

would best fall under the “consumer concerns” listed as a component of this section by 

ANSTF (1996), but we acknowledge that the concerns of importers, breeders, and 

retailers would also need to be quantified as part of any possible regulatory evaluation. 

As noted throughout this risk assessment, the only significant pathway for 

importation of live giant constrictors at present is the pet trade. Opposition to regulation 

of reptile commerce is likely to arise from both the commercial pet-trade interests and the 

pet-buying public. More-experienced buyers of exotic reptiles are often motivated by the 

opportunity to acquire a species or color morph that is unique. To such buyers, any 

restriction on the range of purchasing choices is likely to diminish the prospective scope 

of their acquisitions. There is also a widely-publicized fear that regulation of one type of 

import could facilitate subsequent regulations on other species. This concern may 
258

motivate regulatory resistance from ordinary buyers that do not have a strong personal 

stake in preserving purchasing options for the more unusual or gigantic forms.

Countries such as Australia and New Zealand limit reptile commerce to a modest 

number of species deemed unlikely to become invasive (Bomford, 1991, 2003, 2008; 

Shine and others, 2000). Several countries (for example, Brazil, Venezuela, Australia) 

also broadly limit the keeping of native wildlife (Shine, 1991; Honegger, 1993). 

Although there is widespread concern that these regulations may be excessive with regard 

to the keeping of native wildlife (Reed, 2002), less concern has been voiced regarding the 

diminished range of exotic species available.

Commercial trade interests have proven nimble with regard to redirecting imports 

among species and countries to accommodate export restrictions from countries of origin 

(Christy, 2008). Few doubt that giant constrictor import restrictions would not be 

accommodated in a similarly nimble manner. A much greater concern originates from 

those domestic producers who have invested considerable effort and resources in 

producing designer morphs of particular species whose sales might be proscribed. 

Although the novelty-seeking nature of this market dictates that new lines be introduced 

continually, most of the new lines are derivative from existing stock. Full prohibition of 

trade in selected species would greatly impact those producers who have spent years 

developing novel morphs in proscribed species.

The continuous search for novelty in exotic snake species also raises a regulatory 

challenge in that a black-list-only approach (identifying the worst offenders 

progressively, and prohibiting their importation and interstate transport through Federal 

regulation) is likely to drive a novelty-motivated market in unpredictable and potentially 

detrimental directions, possibly focusing trade on species that are more problematic than 

the common species previously considered. New Zealand and Australia reptileimportation regulators also maintain a white list of permanently allowable species, from 

which pet buyers can confidently identify choices that are not candidates for importation 

restrictions.

The Consequences of Establishment element in the ANSTF (1996) risk 

assessment are computed as the sum of the economic, ecological, and perceived impacts. 

Unlike the establishment factors, which are multiplied together (thus a zero in any 

contributory factor dictates a zero for overall establishment), the impacts are additive 

(thus a zero in any contributory factor merely diminishes the sum by that amount). Step 

2b in ANSTF (1996; see also Orr, 2003) directs how the Low, Medium, and High 

subscores are to be combined. Our subscores and confidence ranges are given in Table 

10.5, along with the Consequences of Establishment summation as specified in ANSTF 

(1996).
259

Table 10.5. Consequences of establishment for nine giant constrictor species, with their 

associated confidence ranges.

Species Economic 

impacts

Environmental 

impacts

Perceived 

impacts

Consequences of 

Establishment

Python molurus High

MC

High

VC

High

RC High

Broghammerus 

reticulatus

Medium

MC

Medium

RU

Medium

RC Medium

Python sebae High

MC

High

MC

Medium

RC High

Python natalensis High

MC

High

RC

Medium

RC High

Boa constrictor High

MC

High

RC

High

VC High

Eunectes notaeus Medium

RU

Medium

RU

Low

MC Medium

Eunectes 

deschauenseei

Low

VU

Low

VU

Low

VU Low

Eunectes murinus Medium

RU

Medium

RU

Medium

RC Medium

Eunectes beniensis Low

VU

Low

VC

Low

VU Low

Organism Risk Potential

The Organism Risk Potential (ORP) is an overall assessment based on the 

combination of Probability of Establishment (Table 10.4) and Consequences of 

Establishment (Table 10.5), following the algorithm of Step 2c (ANSTF, 1996), which is 

duplicated in Table 10.6. Table 10.7 restates the summary judgments from the preceding 

tables (10.4 and 10.5), and provides the ORP scores for each giant constrictor species. 

Most species warranted a “High” ORP, though two look-alike anacondas (Eunectes 

deschauenseei and Eunectes beniensis) and the Reticulated Python (Broghammerus 

reticulatus) were “Medium.” The two look-alike anacondas are readily distinguishable 

on the basis of differences in scalation and pattern from the regularly-traded anacondas, 

but based on their similarity of appearance may not be distinguished from other species in 

commerce. Based on the ANSTF (1996) guidelines, “mitigation” is warranted for species 

evaluated as High or Medium. However, the ANSTF guidelines provide no direction as 

to what form mitigation should take.
260

Table 10.6. Organism Risk Potential (ORP) algorithm adopted by the Aquatic Nuisance Species 

Task Force (ANSTF, 1996).

Probability of Establishment Consequences of Establishment Organism Risk Potential (ORP)

High High High

Medium High High

Low High Medium

High Medium High

Medium Medium Medium

Low Medium Medium

High Low Medium

Medium Low Medium

Low Low Low

Table 10.7. Organism Risk Potential (ORP) for nine giant constrictor species. * = by similarity of 

appearance.

Species Probability of 

Establishment

Consequences of 

Establishment

Organism Risk Potential 

(ORP)

Python molurus High High High

Broghammerus 

reticulatus Medium Medium Medium

Python sebae High High High

Python natalensis High* High High*

Boa constrictor High High High

Eunectes notaeus High Medium High

Eunectes 

deschauenseei Medium* Low Medium*

Eunectes murinus Medium Medium Medium

Eunectes beniensis Medium* Low Medium*
261

Acknowledgments

A project of this size and topical variety requires input and feedback from myriad 

colleagues, and the authors benefitted greatly from interactions with many of the 

foremost experts on both risk assessments and the biology of giant constrictors during the 

preparation of this document. We thank the many colleagues in the United States and 

abroad who provided valuable information and counsel.

First and foremost, we thank our USGS colleague C. Jarnevich, who extracted 

climate data from native-range localities and extrapolated these data to produce all of the 

climate match maps. We remain impressed by her understanding of issues related to 

climate matching, her technical competence, and her eternally sunny disposition. M. 

Eberhardt Frank cheerfully responded to far too many frantic last-minute literature 

requests, and we were dazzled by the speed with which she located and obtained copies 

of obscure documents dating back to the 19th century.

We are grateful to the following for providing thoughtful reviews of individual 

chapters, and in some cases the entire document: G. Alexander, M. Auliya, S. Boback, 

D. Broadley, E. Campbell, M. Chiaraviglio, L. Dirksen, K. Hart, R. Henderson, T. 

Hibbitts, F. Kraus, K. Krysko, B. Lardner, M. Lusk, J. Rivas, A. Roybal, R. Rozar, S. 

Snow, T. Waller, and G. Zug. We are responsible for any errors of commission or 

omission that remain in this document. 

For initiating this project and providing funding and support to see it through, we 

thank S. Snow and D. Kimball (National Park Service), P. Souza and A. Roybal (Fish and 

Wildlife Service), and S. Gross (U.S. Geological Survey). L. Bonewell ably assisted with 

manuscript preparation. We thank M. Auliya, C. Romagosa, F. Kraus, M. Largen, G. 

Schuett, J. Townsend, T. Waller, and G. Zug for providing valuable unpublished data on 

a variety of topics. W. Wonhong and M.-Y. Li assisted with translation of Chineselanguage documents. In addition to those listed above, we thank the following for 

providing various tidbits on snake biology, alerting us to important references, discussing 

distributional limits, verifying specimen identifications, and offering moral support: P. 

Andreadis, P. Andreozzi, E. Beltz, G. Best, J. Daza, H. Greene, J. Maldonado, J. 

Morisette, J. Renhifo, L. Rodgers, J. Seigel, N. van Wilgen, W. von Papineau, and E. 

Williams. Finally, the authors thank J. Reed and R. Rondeau for their patience during the 

preparation of this document.
262

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Appendix. Trade Volume of Giant Constrictor Snakes Imported to 

the United States.

Results reported here were extracted from records maintained by the Convention 

on International Trade in Endangered Wildlife (CITES). See chapter Two for additional 

discussion of these data, including caveats related to their reliability. CITES states that 

data after 2005 are incomplete, so data for 2006 and 2007 should not be considered 

definitive. Figures A.1, A.2, and A.3 are based on data from Table A.1.

Figure A.1. Number of Indian Pythons (P. molurus; yellow), Reticulated Pythons (B. reticulatus; 

pink), and African Pythons (Northern (P. sebae) and Southern (P. natalensis) not distinguished in 

trade; blue) imported to the United States during the period 1977-2007.

0

5000

10000

15000

20000

25000

30000

Number of individuals imported

Year
301

Figure A.2. Number of Green Anacondas (E. murinus; blue) and Yellow Anacondas (E. notaeus; 

pink) imported to the United States during the period 1977-2007.

Figure A.3. Number of Boa Constrictors (B. constrictor; blue) imported to the United States 

during the period 1977-2007.

0

10000

20000

30000

40000

50000

60000

Number of individuals imported

Year

0

200

400

600

800

1000

1200

1400

1600

Number of individuals imported

Year
302

Table A.1. Numbers of individuals of six species of giant constrictor snakes imported to the 

United States during the period 1977-2007, as reported by CITES.

YEAR P. sebae P. molurus B. reticulatus E. notaeus E. murinus B. constrictor TOTAL

1977 5 994 1926 26 0 4588 7539

1978 786 6683 9635 6 93 12242 29445

1979 1136 17309 16978 1 249 56707 92380

1980 335 5487 5983 25 318 19717 31865

1981 264 3536 4190 57 321 13786 22154

1982 200 5601 4425 10 169 4285 14690

1983 83 5443 4351 17 293 1683 11870

1984 111 12673 6744 21 337 8855 28741

1985 675 17102 10639 6 188 25362 53972

1986 191 13714 2340 57 544 8203 25049

1987 286 13962 7942 0 200 3326 25716

1988 813 9582 3851 0 390 4769 19405

1989 784 9412 3124 2 280 5750 19352

1990 1005 5982 3856 21 275 12503 23642

1991 1203 1065 3608 28 475 13391 19770

1992 1644 442 2948 76 364 37497 42971

1993 2579 308 3738 140 98 48107 54970

1994 2792 4161 2852 101 184 24724 34814

1995 1234 8530 2843 174 339 29626 42746

1996 1410 4192 2489 211 1413 28775 38490

1997 1651 7022 2498 249 487 34635 46542

1998 1233 23600 3958 163 83 28154 57191

1999 1881 16614 3381 50 645 14530 37101

2000 1364 24370 5479 122 583 11141 43059

2001 3488 15608 4068 78 596 22845 46683

2002 1181 18599 7519 216 807 21943 50265

2003 956 16948 3609 59 786 22549 44907

2004 1000 11395 3758 50 641 27569 44413

2005 1013 7266 3513 0 613 27512 39917

2006 706 5792 2321 2 714 22011 31546

2007 729 4051 2919 0 777 22087 30563

TOTAL 32738 297443 147485 1968 13262 618872 1111768

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