Gila Monster (Heloderma suspectum)

Photo by Jim Rorabaugh

Gila Monster, Maricopa Co., AZ. Photo by Jim Rorabaugh

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Within the 100-Mile Circle, the Gila Monster (Heloderma suspectum) is unique among lizards for its size (it is our the largest lizard, < 360 mm SVL, < 570 mm total length), its beaded skin, and it is our only venomous lizard. It is also the sole representative of the family Helodermatidae in Arizona and the USA.  The family also contains the Beaded Lizards of Mexico and Central America (Heloderma horridum was recently split into four species, see Reiserer et al. 2013 ). The Río Fuerte Beaded Lizard (H. exasperatum) occurs within about 275 km south of the Mexico border in east-central Sonora.  The family Helodermatidae is ancient, dating back almost a 100 million years.  At least six other genera in the family are extinct.  Until recently, the Gila Monster and Beaded Lizard were thought to be the only venomous lizards in the world.  However, recent work has shown that some Monitor Lizards (Varanidae) and Iguanas (Iguanidae) also produce venom.

An adult Gila Monster is unmistakable.  In addition to being large, they are stocky with a broad, thick head and short (<56% of SVL), thick tail.  Large, elongated venom glands are located under each side of the lower jaw and these lizards possess grooved teeth for delivery of venom.  Round, bead-like tubercles are found on the head, back, and sides of the body.  Each large tubercle is surrounded by tiny tubercles.  Two subspecies were recognized by Crother (2012), including the Banded Gila Monster (H. s. cinctum) and the Reticulate Gila Monster (H. s. suspectum).  The hatchlings and juveniles of both subspecies have 4-5 dark bands on the body and dark rings on the tail.  Essentially, the juvenile banded pattern is retained (to some degree) into adulthood in the Banded Gila Monster, which has a dorsal pattern of black chain-like double crossbands on a yellowish to salmon-pink background.  In the Reticulate Gila Monster, as the lizard ages, the juvenile dorsal pattern changes to a black reticulation on a rose, orange, or yellow background.  However, the geographic border between the two forms is not sharp, and either form can be found inside the range of the other.  For example, Gila Monsters from Maricopa County, Arizona and Grant County, New Mexico often show considerable banding, although both areas are within the range of the Reticulate Gila Monster (Beck 2005).   Furthermore, recent genetic work (Douglas et al. 2010) found no support for recognizing subspecies within Heloderma suspectum.  In all variations of the Gila Monster, the snout is black and blunt, and the top of the head is mottled black with rose, orange, or yellow.  The tongue is black and there are two or more enlarged preanal scales. Sexual dimorphism is quite subtle, but adult males have somewhat wider heads and proportionally shorter bodies than females. Adult females also have a postanal bulge that is absent in males (Figure 1).  Hatchlings are 110-141 mm SVL.

Photo by Roger Repp.

Adult male on the left, adult female on the right. The female has a postanal bulge that forms a crease a short distance posterior to the vent. That bulge is absent on the male. Males and females both have two or more enlarged preanal scales. Click on the image for a better view. Photo by Roger Repp.

Much folklore, myth, and legend surround the Gila Monster.  Such stories portray the Gila Monster as a spitter of its toxic saliva, its foul breath can kill men, Gila Monsters can control the weather, and its hide has healing powers (Brown and Carmony 1991).  According to the Seri people of Sonora’s central coast, the skin of the Gila Monster or “Paaza” can be heated and placed on a person’s forehead to cure a headache. In the past, Gila Monster tails were fried and eaten by the Seri (Nabhan 2003).

Although the Gila Monster is unique in many ways, Western Banded Geckos (Coleonyx variegatus) are sometimes mistaken for small Gila Monsters.  The uninformed sometimes refer to any large desert lizard as a Gila Monster or just “Monster”.

Gila Monsters are widespread in the 100-Mile Circle below about 1550 m elevation. There is an exceptional collection (UAZ 56030) from Highway 80 northwest of Bisbee in the Mule Mountains at about 1770 m. In the western portions of the Circle, they are most likely to be associated with the Arizona Upland Subdivision of Sonoran desertscrub, and only sparingly or in low densities in the Lower Colorado Subdivision. Moving eastward, Gila Monsters occur in semi-desert grassland, Chihuahuan desertscrub, and marginally into oak woodland.  To the south in the Sonora portion of the Circle, they are also found in foothills thornscrub.  Within these communities, the Gila Monster is typically encountered in rocky desert mountains, the foothills of the sky island mountain ranges, and in dissected bajadas with abundant arroyos. Valley bottom records are few, and usually where there is some structure, such as arroyos with cut banks and caliche caves or holes.  The 100-Mile Circle is entirely within the range of the Reticulate Gila Monster.  The Banded Gila Monster, as defined by Lowe et al. (1986), occurs to west and north.

Range wide, the Gila Monster occurs from the East Mojave in California, southern Nevada, and extreme southwestern Utah south and east through western and southern Arizona, southwestern New Mexico, and much of Sonora west of the Sierra Madre Occidental. Old, poorly documented records exist for southeastern California (Blythe, Chuckwalla Mountains, and Laguna Dam), and there is one specimen from the central, coastal lowlands of Sinaloa. It has been found as high as 1950 m in New Mexico (Degenhardt et al. 1996).

Most Gila Monsters in the Circle are found from April through mid-September, with peaks in April-May and then again during the summer monsoon season, but records for all months of the year exist.  For instance, one was collected in the Catalina Foothills on 9 December and a juvenile was taken in the Avra Valley on 29 January.  Diligent observers may find Gila Monsters basking at the entrances to caliche caves or holes in the banks of arroyos through the winter months.  Some foraging and feeding even takes place during winter. In the spring and fall, Gila Monster activity is primarily diurnal with peaks in the mid-morning and late afternoon.  As temperatures rise, morning activity occurs earlier and nocturnal movements become more common, especially in the Sonoran Desert.  At lower and hotter sites, nocturnal activity may predominate in mid-summer.  The Gila Monster is primarily terrestrial, but individuals may scramble short distances up into trees or shrubs.  Gila Monsters spend most of their time underground or in shelter sites, and thus are rarely seen above ground.  Some people live in Arizona for decades without seeing a Gila Monster, although they may become quite familiar with rattlesnakes, horned lizards, whiptails, and other more commonly observed reptiles.

Sperm maturation and most courtship and mating occur in late April through early June.  During this time, adult males may be found in spectacular combat wrestling matches in which their bodies twist and entwine together, and they may hiss and bite each other. These displays are sometimes mistaken for mating, but that activity apparently takes place underground and in shelter sites.  Clutches of 2-12 (mean = 5.7 near Tucson) eggs are laid in late June to early September, coinciding with the summer rains. However, hatchlings do not emerge until late April to early August of the following year. It is unknown whether they hatch out earlier than this but remain hidden, or if winter cold delays hatching.

The diet of the Gila Monster, both adults and juveniles, is largely the contents of mammal, bird, and reptile nests, such as young rabbits, ground squirrels, and pack rats; lizard, snake, and tortoise eggs; and the eggs and nestlings of birds, particularly ground nesting birds such as Gambel’s Quail.  However, Jerry Feldner (in Beck 2005) observed a Gila Monster consume two nestlings from a Curve-billed Thrasher nest, 3.6 m up in a Palo Verde tree.  Venom is hardly needed to forage on eggs and nestlings, and although it may aid in digestion, venom is probably most useful in defense against predators.  Gila Monsters are rather slow and therefore vulnerable to predation.  The venom, which is a complex of proteins and peptides, causes immediate severe pain, followed by weakness and a rapid drop in blood pressure.  When a Gila Monster bites, it tends to hang on, thus the predator is normally focused on trying to dislodge the lizard and alleviate the pain and discomfort.  Human bites occur when people pick Gila Monsters up or otherwise handle them inappropriately. Most bites are to hands and fingers, and cause severe pain and edema. Lacerations are a common symptom.  Death is highly unlikely, but any Gila Monster bite should be considered a serious medical emergency and the victim should get to a hospital as soon as possible.

The Gila Monster is listed as near threatened on the 2013 IUCN Red List, and collection and possession is prohibited in Arizona without special authorization from the Arizona Game and Fish Department. The species is eliminated by agricultural and urban development and it is highly sought after by poachers. Highways and concrete-lined canals create barriers to movement and fragment populations.

Suggested Reading:

Beck, D.D. 2005. Biology of Gila Monsters and Beaded Lizards. University of California Press, Berkeley.

Beck, D.D. 2009. Gila Monster, Heloderma suspectum Cope, 1869. Pages 499-502 in Jones, L.L.C., and R.E. Lovich (eds.), Lizards of the American Southwest: A Photographic Field Guide.  Rio Nuevo Publishers, Tucson, Arizona.

Bogert, C.M, and R. Martín del Campo. 1956. The Gila Monster and its allies.  The relationships, habits, and behavior of the family Helodermatidae. Bulletin of the American Museum of Natural History 109:1-238.

Brennan, T.C., and A.T. Holycross. 2006. Amphibians and Reptiles in Arizona. Arizona Game and Fish Department, Phoenix, AZ.

Brown, D.E., and N.B. Carmony. 1991. Gila Monster; Facts and Folklore of America’s Aztec Lizard. High Lonesome Books, Silver City, NM.

Campbell, J.A., and W.W. Lamar. 2004. The Venomous Reptiles of the Western Hemisphere. Vols. 1&2. Comstock Press, Ithaca, New York, NY.

Crother, B.I. 2012. Scientific and standard English names of amphibians and reptiles of North America north of Mexico, with comments regarding confidence in our understanding, seventh edition. Society for the Study of Amphibians and Reptiles, Herpetological Circular (39):1-92.

Degenhardt, W.G., C.W. Painter, and A.H. Price. 1996.  Amphibians and Reptiles of New Mexico. University of New Mexico Press, Albuquerque.

Douglas, M.E.;  M.R. Douglas, G.W. Schuett, D.D. Beck, and B.K. Sullivan 2010. Conservation phylogenetics of Helodermatid lizards using multiple molecular markers and a supertree approach. Molecular Phylogenetics and Evolution 55 (1):153-167.

Lowe, C.H., C.R. Schwalbe, and T.B. Johnson. 1986. The Venomous Reptiles of Arizona. Arizona Game and Fish Department, Phoenix.

Nabhan, G.P. 2003. Singing the Turtles to Sea: The Comcáac (Seri) Art and Science of Reptiles. University of California Press, Berkeley.

Reiserer, R.S., G.W. Schuett, and D.D. Beck. 2013. Taxonomic reassessment and conservation status of the beaded lizard, Heloderma horridum (Squamata:Helodermatidae). Amphibian & Reptile Conservation 7(1):74–96.

Authors: Jim Rorabaugh and Roger Repp

For additional information on this species, please see the following volumes and pages in the Sonoran Herpetologist: 1988-91 Collected Papers:47-52; 1996 Jun:59; 1997 Apr:44-45; 1997 Oct:106-110; 2005 Mar:30-31; 2005 Nov:129; 2012 May:42; 2017 June:43-46; 2017 December:70-71.


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