Heloderma suspectum (Cope 1869 [Heloderma, Wiegmann 1928])
Heloderma comprises the only two extant species in the Helodermatidae and the only two species of venomous lizard; the sole survivors of an ancient group of predacious lizards called the Monstersauria (see also Heloderma texana). In the southwestern United States (Arizona and parts of Nevada, California, New Mexico, and Utah) and northern Mexico (Sonora and the very northwest corner of Chihuahua) we find Heloderma suspectum, the Gila Monster. Two subspecies are recognized, H. s. cinctum – Banded Gila Monster, and H. s. suspectum – Reticulate Gila Monster. In Mexico H. horridum, the Mexican Beaded Lizard, is sympatric with H. suspectum in southern Sonora and continues down the Pacific coast of Mexico into Guatemala.
Heloderma suspectum is a large (maximum total length to about 56 cm), stout lizard with a large head and powerful jaws, short limbs with large toes and strong claws, and a robust tail used to store fat. The dorsal skin appears as bumps or beads underlain by bony osteoderms (see the animations above); the ventral belly scales are rectangular. Coloration is a striking pairing of black with pink, orange, yellow, or cream. The mottled pattern can provide camouflage in the broken shadows of desert vegetation, but the bright colors may also work as aposematic warning to potential predators of the venom contained in glands in the lower jaw of the Gila Monster. In contrast to helodermatids, venom in snakes is in glands in the upper jaw and often delivered via hollow fangs. In Heloderma the venom is delivered more slowly through grooves in teeth of the lower jaw during an often prolonged, even chewing bite. The venom has been described as excruciatingly painful, but rarely if ever fatal, to humans. A peptide found in the venom has become a promising new drug for treating type-2 diabetes.
The ancestors of extant Heloderma were tropical dwellers and, as a result, H. suspectum behaviorally avoids the hot, dry conditions that we associate with other desert lizard species. Gila Monster skin is more permeable to water than the skin of many other lizards and therefore microhabitat selection and microclimate, especially humidity, play an important role in survivorship. Individuals spend most of their time underground (up to 95% or more), becoming daytime active in the spring when food is abundant. They are also occasionally seen out at night and/or during monsoon activity in the late summer. Some individuals are known to have remained dormant underground for several years during an Arizona dry spell in the 1960s.
Distribution of Gila Monsters is from sea level to over 5,000 feet. Individuals will typically return to burrows year after year in rocky upland habitats during the winter and in cooler, more humid lowland sites in the summer. Individuals are known to travel several kilometers in just a few days. These long-lived lizards apparently recognize and interact with many individuals throughout their home range, being seen in burrows together in separate years.
Gila Monsters prey upon the eggs, newborn, and young of many species of small mammals, reptiles, and birds. Their active foraging strategy employs keen senses of smell and hearing, especially of ground vibrations, to find nests and juveniles in burrows, on the ground, and even in vegetation. Strong limbs and claws allow for rapid access to many subterranean food sources. Young H. suspectum may be able to consume up to 50% of their body mass in one meal; adults about 35%. Three or four large meals in one spring may provide individuals the energy reserves to make it to the next spring. Venom does not seem to be used often in foraging but is reserved for defense; bites are often preceded by gaping and hissing behaviors. Gila Monsters seem to be immune to their own or other individuals’ venom.
Reproduction begins with wrestling matches, often lasting several hours, between males for access to females. Selection for unusually high aerobic stamina, for a squamate reptile, seems to have been one result. Furthermore, males tend to have higher stamina and larger heads than females. Mating takes place in May and eggs (typical clutches are 2-12 eggs [mean = 5]) are laid in underground burrows in late summer and typically incubate until the following May.
About the Species
This specimen, a juvenile, was collected by Patrick Burritt on South Camino del Sol Road, Green Valley, Pima County, Arizona on 8 June 2003. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Kevin Bonine of The University of Arizona, Tucson. Dr. Bonine also donated the specimen to the Texas Memorial Museum (AZGF permit #SP745031). Funding for scanning and image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.
Dorsal view of specimen
Ventral view of specimen
About this Specimen
The head of the specimen was scanned by Matthew Colbert on 2 December 2003 along the coronal axis for a total of 735 slices, each slice 0.055 mm thick with an interslice spacing of 0.055 mm and a field of reconstruction of 26 mm.
Beaman, K. R., Beck, D. D., and B. M. McGurty. 2006. The beaded lizard (Heloderma horridum) and gila monster (Heloderma suspectum): a bibliography of the family Helodermatidae. Smithsonian Herpetological Information Service 136:1-66.
Beck, D. D. 2005. The Biology of Gila Monsters and Beaded Lizards. University of California Press, Berkeley. (Due for release May 2005; a comprehensive update, the first since Bogert and Martín del Campo. 1956. Bull. Am. Mus. Nat. Hist. 109:1-238).
Beck, D. D. 2004. Venomous Lizards of the Desert. Natural History 113:32-37.
Beck, D. D. 2004. Overview of the family Helodermatidae (for varanophiles) and species accounts for Heloderma horridum and Heloderma suspectum. In: E. R. Pianka and D. King: Varanoid Lizards of the World. Indiana University Press.
Beck, D. D., M. R. Dohm, T. Garland, Jr., A. Ramirez-Bautista, and C. H. Lowe. 1995. Locomotor performance and activity energetics of helodermatid lizards. Copeia 1995:577-585.
Crother, B. I. 2000. Scientific and standard English names of amphibians and reptiles of North America North of Mexico, with comments regarding confidence in our understanding. Society for the Study of Amphibians and Reptiles, Lawrence, Kansas.
Lowe, C. H., C. R. Schwalbe, and T. B. Johnson. 1986. The venomous reptiles of Arizona. Arizona Game and Fish Department, Phoenix, Arizona.
Pianka, E. R. and L. J. Vitt. 2003. Lizards: Windows to the Evolution of Diversity. University of California Press.
Stebbins, R. C. 2003. A Field Guide to Western Reptiles and Amphibians, 3rd edition. Houghton Mifflin, Boston, MA.
Dr. Mark Seward's Gila monster site
Gila monster page from the Arizona-Sonora Desert Museum
Gila monster pictures from The Gila Ranch
Three-dimensional volumetric renderings of the skull with osteroderms removed.