U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3061

April 2006

Gila Monster (Heloderma suspectum)

Printed on recycled paper

Figure 1. Adult Gila monster (Heloderma suspectum). USGS scientists 

initiated a research effort in Tonto National Monument beginning in 

2004 to provide information needed to make management decisions 

and improve scientific understanding of the species.

 Like the saguaro cactus (Carnegia gigantea) and the 

rattlesnake, the Gila monster is emblematic of the desert 

Southwest. The Gila monster is not only the largest lizard native 

to the United States, but also one of only two known species of 

venomous lizard in the Americas. This stout-bodied lizard can 

grow to 50 cm (20 in) and is covered with black and pink or 

orange markings and bead-like scales. The Gila monster’s range 

is centered in western and southern Arizona, continuing south to 

Sonora, Mexico. 

 Despite public fascination with the species, relatively little 

is known about the ecology and behavior of the Gila monster in 

the wild. For this reason, managers at Tonto National Monument, 

Arizona, contacted the U.S. Geological Survey (USGS) to 

investigate why Gila monsters were being seen in developed 

areas of the park, particularly crossing the main road. Managers 

were concerned about possible lizard-human conflicts and the 

risk of vehicle traffic killing Gila monsters. USGS scientists 

initiated a research effort in Tonto National Monument beginning 

in 2004 to provide information needed to make management 

decisions and improve scientific understanding of the species. 

Specifically, USGS scientists examined the movement patterns, 

range requirements, dietary habits, and use of developed areas 

by Gila monsters within the park. The 2004 research effort also 

extended a program begun in Tonto National Monument in 1994 

to recognize individual Gila monsters based on unique dorsal 

patterns identified from photographs.

Research Findings

 The research team searched for Gila monsters in Tonto 

National Monument during the spring and late-summer 

monsoon season in areas of the park that the animals were 

known to frequent. Gila monsters were also captured when 

they were seen outside of these periods by park staff. Captured 

Gila monsters were weighed, measured, and photographed. 

Additionally, each Gila monster was injected with a small 

passive integrated microchip transponder (PIT tag), which 

carries a unique identification code that allows scientists to 

identify the animal each time it is encountered. Adult Gila 

monsters were surgically implanted with radio transmitters that 

allow scientists to track these lizards from a distance.

 In Tonto National Monument, Gila monsters were found 

to begin emerging from their hibernation sites starting in late 

March, with activity peaking in early April. As expected, the 

USGS scientists found a number of Gila monsters crossing the 

main road below the visitor center parking lot. USGS research 

findings indicated that although Gila monsters use the entire 

monument as habitat, they seem to prefer rocky areas. Similar to 

the findings of other studies, the research team frequently found 

the radio-tracked animals below ground. These shelter sites 

were typically burrows that the lizards dug under boulders and 

small rock outcrops. 

 In terms of dietary habits, one of the Gila monsters 

captured by scientists regurgitated a freshly eaten clutch of six 

unbroken Gambel’s quail (Callipepla gambelii) eggs. Gila 

monsters also appear to be feeding in part on small mammals, 

including young desert cottontail (Sylvilagus audubonii). One 

of the radio-tracked Gila monsters was observed drinking from 

a leaking hose in the housing area. This observation is important 

because it shows the lizards are using human-modified habitat 

to meet basic ecological needs.

 Gila monsters exhibited a range of defensive behaviors 

when located by researchers. Of the 123 visual observations 

of radio-tracked animals, more than half (55%) of the Gila 

monsters moved under cover, ran away, or moved deeper 

into a burrow. In other cases, Gila monsters did not visibly 

react, froze before fleeing, or froze without moving when 

encountered by a scientist. These observations suggest that 

Gila monsters are not aggressive and prefer to avoid people. 

 USGS scientists compared the recent photographs of dorsal 

patterns on individual Gila monsters with pictures taken in 

the mid-1990s. Photo comparisons allowed the researchers to 
document the recapture of at least three Gila monsters. These 

findings were significant because they confirm the potential 

effectiveness of using noninvasive photography as a long-term 

mark-recapture method for this species. Additionally, the 

method allowed scientists to confirm that one of the recaptured 

Gila monsters had lived in the wild for at least 12 years.

Research Implications

 The USGS research provides some clues as to why Gila 

monsters frequent the main road and developed areas of Tonto 

National Monument. It is possible that the Gila monsters are 

moving across the road to reach preferred habitat in Cave Wash 

after emerging from their hibernation sites in nearby cliffs. Cave 

Wash is a site with relatively high humidity and Gila monsters 

have been documented to seek out moist sites during their 

active period. It is also possible that the lizards are following 

a traditional migration route. Additionally, the availability of 

water and food for prey species in the housing area may explain 

the presence of Gila monsters in developed areas during their 

active period.

 Because the research confirmed the peak periods in which 

the Gila monsters are likely to be crossing the road, it provides 

park managers the opportunity to focus management actions 

on these periods. For example, on peak emergence days in 

early April, park staff or volunteers could be stationed near the 

crossing location to ensure that Gila monsters can successfully 

cross the road. Periods when lizards are active also offer an 

opportunity to provide interpretive programs for park visitors, 

which may improve understanding of the animal and its needs.

 USGS research efforts in Tonto National Monument have 

increased scientific understanding of Gila monster ecology and 

behavior. Better understanding of the species can be used to 

improve management decisions and target limited resources 

in Arizona national parks. Knowing better how Gila monsters 

behave in the wild can also help people live in harmony with 

this colorful desert dweller.

More Information

Nowak, E.M., 2005, Why did the Gila monster cross the 

road?: basic research at Tonto National Monument: 2004 

annual report (04-15) and 2005 initial progress report (05-

12) submitted to Western National Parks Association, U.S. 

Geological Survey, Colorado Plateau Research Station.

Erika M. Nowak

U.S. Geological Survey

Southwest Biological Science Center

Colorado Plateau Research Station

2255 N. Gemini Drive

Flagstaff, Arizona, 86001

Tel: 928-556-7466 (ext. 239)

E-mail: enowak@usgs.gov

 Gila monster research is conducted by the Southwest 

Biological Science Center (SBSC) [http://sbsc.wr.usgs.gov], 

which is one of the 17 science centers that are a part of the 

U.S. Geological Survey’s Biological Resources Discipline. To 

address the research needs of the large and biologically varied 

Southwestern United States, SBSC research is conducted 

by scientists working at four research stations, including 

Canyonlands Research Station (Moab, Utah), Colorado 

Plateau Research Station (Flagstaff, Arizona), Grand Canyon 

Monitoring and Research Center (Flagstaff, Arizona), and 

Sonoran Desert Research Station (Tucson, Arizona). The 

mission of the USGS Biological Resources Discipline is to 

work with others to provide the scientific understanding and 

technologies needed to support the sound management and 

conservation of the Nation’s biological resources.

Figure 3. Scientists calculate the volume of a captured Gila 

monster’s tail in order to get a general indication of the individual’s 

condition because these lizards store fat in their tails. 

Figure 2. The research team, which included volunteers and park 

staff members, searched for Gila monsters in Tonto National 

Monument during the spring and late-summer monsoon season. 

Captured Gila monsters were weighed, measured, and photographed.