Appendix W 

Status and Life History of the San Francisco Garter Snake 

(Thamnophis sirtalis tetrataenia) 

1.1 Species Listing Status 

The San Francisco Garter Snake (SFGS) (Thamnophis sirtalis tetrataenia) was listed as 

endangered on March 11, 1967 (32 FR 4001; USFWS 1967) by the U.S. Fish and 

Wildlife Service (USFWS) and was grandfathered under the Endangered Species Act 

(ESA) when it was signed into law in 1973. A Recovery Plan for the SFGS was 

approved by the USFWS on September 11, 1985 (USFWS, 1985). In addition, a 5-year 

review, which provides an updated life history for the SFGS, was published by the 

USFWS in September of 2006 (USFWS, 2006). 

1.2 Description 

The SFGS is a thin and colorful snake in the Colubridae family, which includes most of 

the species of snakes found in the western U.S. (USFWS, 2007). SFGSs range in length 

from 46 to 131 cm (18 to 51 inches) (USFWS, 1985 and Woodland Park Zoo). Female 

SFGS typically weigh about 8 oz. (227 g), whereas, males are usually smaller than 

females and can be half the length and weight of females (SF Zoo and Woodland Park 

Zoo). Garter snakes are usually about two inches in girth (Kaplan, 2002). Neonate 

snakes are 18 to 20 cm in length and disperse immediately after they are born. Juvenile 

SFGS grow quickly during their first year and spend most of their time feeding. These 

snakes are typically considered to be mature at two years of age when the males are about 

46 cm and the females are about 55 cm; however, some snakes take longer to mature and 

may not reach maturity until they are three years old. It is estimated that SFGS live 

between 8 and 15 years (Woodland Park Zoo and Goals Project, 2000). 

1.3 Distribution 

SFGSs are endemic to the San Francisco Peninsula and San Mateo County and were 

historically found from San Francisco to Santa Cruz (USFWS, 1985 and 2005), including 

the San Francisco Peninsula from just north of the San Francisco-San Mateo County line 

near Lake Merced south along the eastern and western bases of the Santa Cruz 

Mountains, to the Upper Crystal Springs Reservoir, along the coast south to Ano Nuevo 

Point, in San Mateo County, and Waddell Creek, in the extreme northern portion of Santa 

Cruz County (USFWS, 1985, 2006, and 2007). However, the findings of SFGS in 

Waddell Creek in Santa Cruz County may be questionable (Goals Project, 2000) due to 

the reported presence of hybrid species in the area. 

SFGS populations are thought to have primarily occupied habitat areas in the Buri Buri 

Ridge along the San Andres Rift Zone from near Pacifica, southeast to the Pulgas Water 

Temple, and in an arc from the San Georgio-Pescadero highlands west to the coast, and 

south to Ano Nuevo (USFWS, 2006 and Goals Project, 2000). Red-sided garter snake 

(RSGS) and SFGS hybrids have been found in eastern San Mateo County (southeast of 

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the Pulgas Water Temple) and in the extreme western part of Santa Clara County (Goals 

Project, 2000). The hybrids range from the Pulgas region near Upper Crystal Springs 

Reservoir south to Palo Alto (USFWS, 2006). 

The extreme northeastern part of the SFGS’s range may have been represented by a 

population at San Bruno Mountain; however, it is possible that this population has been 

extirpated (USFWS, 2006). This population may not be natural as individual SFGSs may 

have been moved here in an attempt to protect them because this area is protected under a 

Habitat Conservation Plan (HCP) (USFWS, 2006). It is possible that these attempted 

conservation methods were also used farther south, resulting in the population at Half 

Moon Bay (USFWS, 2006). 

The historic range of the SFGS may have extended as far south as Stanford in Santa Clara 

County (USFWS, 2006). This assumption is based on observations of hybrids of the 

SFGS and other garter snake species in this area (USFWS, 2006). The USFWS also 

believes that areas on the coast west of the Santa Cruz Mountains may be inhabited by 

the SFGS; however, this cannot be confirmed because this area is privately owned 

(USFWS, 2006). 

Historically, SFGSs were found in sag ponds, small seasonal ponds along the San 

Andreas Fault in the northern part of the San Francisco Peninsula (USFWS, 1985). Some 

believe that the purest SFGS populations were found on the coast around Sharp Park in 

Laguna Salada and along the ridge of the San Francisco Peninsula east of Sharp Park 

(USFWS, 1985). Habitat areas in the northern and eastern portions of the SFGS’s 

historical range have been destroyed by urbanization and development, including the 

SFGS population co-located with the sag ponds along the ridge of the San Francisco 

Peninsula east of Sharp Park (USFWS, 1985). 

Although the SFGS’s geographical range seems extensive, field surveys performed by the 

USFWS and the California Department of Fish and Game (CDFG) indicate that their 

range is limited to and localized within San Mateo County (USFWS, 1985). Six areas are 

known to contain significant populations of SFGS: 1) Ano Nuevo State Reserve (ANSR), 

2) Pescadero Marsh Natural Preserve, 3) San Francisco State Fish and Game Refuge 

(including both Upper and Lower Crystal Springs Reservoirs), 4) Sharp Park (Laguna 

Salada), 5) Cascade Ranch, and 6) Milbrae or West of Bayshore (near the San Francisco 

Airport) (USFWS, 1985). 

The current distribution of the SFGS is unknown because most of their historic range is 

now privately owned; however, it appears that the SFGS can still be found in much of its 

historic range (USFWS, 2006 and 2007). SFGSs were found in Upper Crystal Springs 

Reservoir and Mud Dam in 1998 (San Francisco Planning Department, 2001). They 

were also observed at “San Andreas Reservoir and in a sag pond between San Andreas 

and Crystal Springs” (San Francisco Planning Department, 2001). Regardless, the 

number of locations that previously maintained healthy populations of the SFGS has 

declined and SFGS have been extirpated from individual locations within their historic 

2
range (USFWS, 2007). Currently, wild SFGS populations are limited to coastal San 

Mateo County and other small pockets (USFWS, 2005). 

Some sources state that 65 “permanent” reproductive populations ranging from two to 

over 500 adults were found on the San Francisco Peninsula (Kaplan, 2002 and Goals 

Project, 2000). The total SFGS population is believed to be around 1500 snakes over one 

year old (Kaplan, 2002 and Goals Project, 2000). All young under one year old are not 

included in population counts because the population increases when the young are born 

and returns to around 1500 the following spring due to insufficient resources (Kaplan, 

2002). It is thought that half of the populations are protected to some degree by refuges 

including preserves and state parks (Goals Project, 2000). 

1.3.1 FWS Critical Habitat/ CNDDB Occurrence Data 

No critical habitat for the SFGS has been designated by the USFWS to date. 

1.4 Habitat 

Successful SFGS breeding populations are typically found in densely vegetated ponds 

near open hillsides where they can sun themselves, feed, and find shelter in rodent 

burrows; however, less ideal habitats can also be occupied (USFWS, 2007). These sites 

usually include emergent vegetation for cover, basking sites, food sources, upland 

hibernation sites, and corridors which allow for movement between the feeding grounds 

and the hibernation sites (USFWS, 1985 and 2007). SFGSs prefer shallow marshlands 

with emergent vegetation and breeding populations of both Pacific tree frogs and 

California red-legged frogs (CRLF) as well as open grassy uplands (USFWS, 2006). 

Breeding populations of SFGS are not known to exist where amphibians are not found 

(USFWS, 2006). 

SFGSs are usually observed near standing open water, including ponds, lakes, marshes, 

slow-moving streams, and sloughs; however, they are also observed at temporary pools 

including stock ponds, channelized sloughs, and reservoirs (USFWS, 1985 and 2007; 

NatureServe, 2007; Kaplan, 2002; and Goals Project, 2000). It is speculated that SFGS 

remain close to open water to retain proximity to anuran species, their preferred prey 

(USFWS, 2006). These snakes avoid brackish water because their preferred prey, the 

CRLF, cannot survive in saline water (USFWS, 2007). In addition, Pacific tree frogs and 

their larvae cannot survive in water with a salinity of 7.0 ppt or greater (USFWS, 2006). 

However, SFGS may use ditches influenced by the tides as “migration corridors between 

disconnected areas of freshwater wetland habitat” (Powers, 2003). 

Shallow water near the shoreline is required from May to July, to allow the anuran prey 

to hatch and metamorphose (USFWS, 2006); SFGSs are unable to capture their prey 

effectively in water that is more than 5 cm deep (USFWS, 2006). Shallow water also 

exposes rocks, algal mats, and floating vegetation found around the edges of the pond 

which the SFGSs use as basking sites (USFWS, 2006). These sites may also be used by 

the anurans and allows the SFGS greater access to their prey (USFWS, 2006). 

3
SFGS prefer emergent vegetation, including cattails, bulrushes, spike rushes, and water 

plantain, for cover (USFWS, 1985 and 2007); however, they will also seek cover in the 

water if necessary (USFWS, 1985). If these types of vegetation are not available, SFGSs 

can be found in aquatic areas surrounded by willow trees as long as there is only a small 

distance between the overhanging vegetation and the ground (USFWS, 2006). SFGSs 

sun themselves at the edges of ponds or streams and will also use the area between the 

aquatic habitat required for their prey and the grasslands where they aestivate (become 

inactive or dormant) for basking (USFWS, 1985). 

Upland grassy hillsides near ponds are also used for basking (USFWS, 1985). It is 

thought that SFGS may prefer hillsides that face south or west because they receive more 

sun (USFWS, 2006). Coyote bush, wild oat, wild barley, and other brome species may 

be found in the SFGS’s upland habitat (USFWS, 2006). SFGS “prefer a grassland/shrub 

matrix with brush densities ranging from 1 averaged sized bush/30 square meters to 1 

large bush/20 square meters” (USFWS, 2006). This mix provides both cover from 

predators and also exposed surfaces to allow the SFGS sun themselves, an important part 

of their thermoregulation (USFWS, 2006). The preferred brush to grassland ratios are 

maintained or can be achieved by allowing livestock grazing (USFWS, 2006). Large 

SFGS populations have been found to coincide with areas of land that are used for cattle 

grazing (USFWS, 2006). Gopher activity, which moves nitrogen-poor subsoil to the 

surface, has also been shown to stimulate early successional conditions (open grassland) 

required by the SFGS (USFWS, 2006). Although grazing appears to have a positive 

impact on the SFGS, overgrazing can be detrimental to this species as SFGS habitat may 

no longer be useful if the vegetative cover falls below 20 cm (USFWS, 2006). 

Rodent burrows and thick grass mats near the ponds are used for shelter and aestivation 

when the ponds become dry (USFWS, 1985 and 2007 and NatureServe, 2007). SFGSs 

may also forage for amphibians in the rodent burrows during the summer (Goals Project, 

2000). These burrows are also used for hibernation, since SFGSs found along the coast 

will hibernate during the winter (USFWS, 2006 and 2007). However, SFGS have been 

observed emerging from their burrows to bask in the winter indicating that SFGS may not 

truly hibernate (USFWS, 2006). This could also be due to the relatively warm climate of 

the San Francisco Bay area (USFWS, 2006). SFGSs that are farther inland are more 

likely to be active year-round (USFWS, 2007). 

Recapture studies have indicated that SFGS have small home ranges that are determined 

by the availability of food and cover (USFWS, 1985). Most individuals remain within 

one or two hundred meters of their aquatic and upland habitats (USFWS, 2006). 

However, some snakes may be able to travel up to two km or more in a short period of 

time (Goals Project, 2000). Although SFGS typically do not travel far, they may follow 

their prey to new areas (USFWS, 2006). It is in pursuit of their prey that SFGS may be 

adversely affected by creek channelization, removal of vegetation, and other flood 

control measures that are sometimes implemented in riparian areas (USFWS, 2006). 

Garter snakes are typically found from sea level to 2400 m (8000 feet) (Morey, 2005). 

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1.5. Life History and Reproduction 

SFGSs mate in the spring (March and April) and fall (September through November); 

however, mating is “concentrated in the first few warm days of March” (USFWS, 1985 

and 2006 and Goals Project, 2000). The increased mating in the spring is correlated with 

SFGSs emerging from their burrows and congregating at the aquatic feeding areas 

(USFWS, 2006). Aggregations of mating adult SFGSs have also been observed on open 

grassy slopes on warm sunny mornings in late October and early November (USFWS, 

1985). It is thought that the female’s role in mating is relatively passive. Females release 

pheromones to attract males, males actively seek out the females primarily using scent, 

and several males may court the same female simultaneously. These types of behaviors 

have been identified in other subspecies of the common garter snake (USFWS, 1985 and 

SF Zoo). Female SFGS can store the male’s sperm over the winter and can retain viable 

sperm for periods ranging from 3 to 53 months (USFWS, 1985 and Goals Project, 2000). 

Ovulation in the common garter snake typically occurs in late spring with pregnancy 

resulting in early summer (USFWS, 1985). The young are typically born about three to 

four months after successful mating (Woodland Park Zoo). SFGS are ovoviviparous 

meaning that the eggs hatch within the female’s body and the young are born live 

(USFWS, 1985 and SF Zoo). Female SFGSs give birth from June through September 

with young typically born in July or August (USFWS, 2007; NatureServe, 2007; and 

Goals Project, 2000). However, young can be born as late as early September (Goals 

Project, 2000). The number of young can range from three to 85 but are more typically 

in the range of 12 to 24 (USFWS, 1985). Most often around 16 young are produced 

(USFWS, 2007; SF Zoo; and Goals Project, 2000). 

The young are typically born in the upland areas near the aquatic feeding habitats. 

Neonate snakes are 18 to 20 cm in length and disperse immediately after they are born 

(Woodland Park Zoo and Goals Project, 2000). Juvenile SFGS grow quickly during their 

first year and spend most of their time feeding (Goals Project, 2000). These snakes are 

typically considered to be mature at two years of age when the males are about 46 cm and 

the females are about 55 cm; however, some snakes take longer to mature and may not 

reach maturity until they are three years old (Woodland Park Zoo and Goals Project, 

2000). 

1.6 Diet 

Newborn and juvenile SFGS prey almost exclusively on Pacific tree frogs in temporary 

pools during the spring and early summer (USFWS, 2006 and 2007). Juvenile SFGS 

may capture metamorphosed Pacific tree frogs in upland habitats; however, they 

primarily feed on the newly metamorphosed frogs once the temporary pools begin to dry 

up and the frogs begin to disperse (USFWS, 2006). A laboratory experiment performed 

using juvenile SFGS showed that Pacific tree frogs elicited the highest response rate out 

of all of the common prey items that were tested (USFWS, 2006). Observations at one 

SFGS site have shown that anuran numbers decreased during dry years with a subsequent 

decrease in the survival of SFGS juveniles (USFWS, 2006). This observation indicates 

5
that the SFGS may be so dependent on their anuran prey that they are not able to switch 

to other available prey sources if necessary to survive (USFWS, 2006). SFGS under 500 

mm snout-to-vent length (SVL) require Pacific tree frogs in various stages of 

metamorphosis, whereas individuals over 500 mm SVL can consume Pacific tree frog, 

CRLF, and bullfrog tadpoles and adults (USFWS, 2006). 

The main diet of adult SFGS consists of CRLF, which are listed as a threatened species 

(USFWS, 1985 and 2007). Adult SFGSs consume CRLFs in the late summer when they 

metamorphose (USFWS, 2006). The emergence of the CRLF in the late summer allows 

the adult SFGS to continue feeding after the Pacific tree frogs have left the drying 

wetland areas (USFWS, 2006). This observed behavior likely explains the high activity 

level of SFGSs in the late summer (USFWS, 2006). Adult SFGSs may also feed on 

juvenile non-native bullfrogs (Rana catesbeiana), but are unable to consume the larger 

adults (USFWS, 2007). Bullfrogs are known to be found in degraded habitats and it is 

thought that bullfrogs may allow SFGS to colonize areas where Pacific tree frogs or 

CRLF cannot exist (USFWS, 2006). Although bullfrogs may be consumed by SFGSs, 

captive SFGS often regurgitate them indicating that they may be undigestable, and that 

bullfrogs may not be consumed by wild SFGS populations (USFWS, 2006). 

Immature California newts (Taricha torosa), California toads (Bufo boreas halophilus) 

recently metamorphosed western toads (Bufo boreas), threespine stickleback 

(Gasterosteus aculeatus), and non-native mosquito fish (Gambusia affinis) are also 

known to be consumed by SFGS (USFWS, 1985). SFGS are one of the few animals 

known to eat the California newt without suffering the toxic effects of this organism 

(USFWS, 2007). Small mammals, reptiles, amphibians, possibly invertebrates, and some 

fish species may also be consumed by the SFGS if they can be captured in shallow water 

(USFWS, 1985 and 2006). Captive SFGS have been found to consume earthworms; 

however, some individuals will starve instead of eating them (USFWS, 2006). 

Earthworms may also not be readily available in the wild, further indicating that wild 

populations of SFGS may not prey on these animals (USFWS, 2006). 

1.7 References 

Goals Project. 2000. Baylands Ecosystem Species and Community Profiles: Life histories 

and environmental requirements of key plants, fish and wildlife. Prepared by the San 

Francisco Bay Area Wetlands Ecosystem Goals Project. P.R. Olofson, editor. San 

Francisco Bay Regional Water Quality Control Board, Oakland, California. Available 

online at: http://sfep.abag.ca.gov/pdf/speciescommunity/Species_and_Community_Profiles%5BPart3%5D.pdf

(Accessed on November 5, 2007). 

Kaplan, M. 2002. Not-so-common garters. Melissa Kaplan’s Herp Care Collection. Last 

updated April 19, 2007. Available online at: http://www.anapsid.org/garters.html

(Accessed on October 16, 2007). 

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Morey, S. 2005. R061-Common Garter Snake. California Wildlife Habitat Relationships 

System, a database of the California Department of Fish and Game (CDFG), in 

cooperation with the California Interagency Wildlife Task Group. Database version 8.1. 

Available online at: nrm.dfg.ca.gov/FileHandler.ashx?DocumentVersionID=2814

(Accessed on November 5, 2007). 

NatureServe. 2007. NatureServe Explorer: An online encyclopedia of life [web 

application]. Version 6.2. NatureServe, Arlington, Virginia. Available online at: 

http://www.natureserve.org/explorer/servlet/NatureServe?searchName=Thamnophis+sirt

alis+tetrataenia. (Accessed: October 15, 2007). 

Powers, D.J., and Associates, Inc. 2003. Environmental Constraints Analysis for the San 

Bruno Grade Separation Project (MP 10.8 to 12.0). Available online at: 

http://www.caltrain.org/pdf/SB_L.pdf (Accessed on October 25, 2007). 

San Francisco Planning Department. 2001. Peninsula Watershed Management Plan – 

Final Environmental Impact Report (EIR). EIR Certification Date: January 11, 2001. 

Available online at: http://sfwater.org/Files/Reports/3-Env_Set_Impacts.pdf

(Accessed on November 5, 2007). 

SF Zoo website. Available online at: 

http://www.sfzoo.org/openrosters/ViewOrgPageLink.asp?LinkKey=13516&orgkey=182

1 (Accessed on November 2, 2007). 

U.S. Fish and Wildlife Service (USFWS). 1967. Native Fish and Wildlife, Endangered 

Species (32 FR 4001). Washington, D.C. 

USFWS. 1985. Recovery Plan for the San Francisco Garter Snake (Thamnophis sirtalis 

tetrataenia). U.S. Fish and Wildlife Service, Portland, Oregon. 77 pp. Available online 

at: http://ecos.fws.gov/docs/recovery_plan/850911.pdf

(Accessed on October 16, 2007) 

USFWS. 2005. San Francisco garter snake returns to its namesake city. June 2005. 

Available online at: 

http://www.fws.gov/sacramento/ea/feature_stories/SFGS_returns_mediaevent.htm

(Accessed on October 16, 2007). 

USFWS. 2006. 5-Year Review: Summary and Evaluation for the San Francisco Garter 

Snake (Thamnophis sirtalis tetrataenia). U.S. Fish and Wildlife Service, Sacramento 

Field Office, Sacramento, California. 42 pp. Available online at: 

http://www.fws.gov/sacramento/es/documents/San%20Francisco%20Garter%20Snake%

205%20Year%20Review.FINAL.pdf

(Accessed on October 25, 2007 ) 

USFWS. 2007. Species Account: San Francisco Garter Snake (Thamnophis sirtalis 

tetrataenia). Sacramento Fish and Wildlife Office. Available online at: 

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8

http://www.fws.gov/sacramento/es/animal_spp_acct/sf_garter_snake.pdf (Accessed on 

October 16, 2007). 

Woodland Park Zoo website. Available online at: 

http://www.zoo.org/factsheets/garter_snake/sanFransiscoGarterSnake.html# (Accessed 

on November 5, 2007).