Gila Monsters and Mexican Beaded Lizards (Helodermatidae)

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Gila monsters and Mexican beaded lizards

(Helodermatidae)

Class Reptilia

Order Squamata

Suborder Scleroglossa

Family Helodermatidae

Thumbnail description
Large, stout, venomous lizards with distinctive, beadlike scales (osteoderms) on the dorsal surfaces of head, limbs, body, and tail

Size
12–18 in (30–45 cm) snout-to-vent length; 14–39 in (35–100 cm) total length; 1.0–4.4 lb (450g–2 kg)

Number of genera, species
1 genus; 2 species

Habitat
Hot desert, tropical deciduous forest

Conservation status
Vulnerable: 2 species

Distribution
Desert regions of the southwestern United States and northwestern Mexico; Pacific drainages along the western slope of Mexico and southern Guatemala; and two Atlantic drainages in Chiapas and eastern Guatemala

Evolution and systematics

The helodermatid clan has a rich and diverse evolutionary history that dates back 98 million years across Europe, Asia, and North America to a time well before many dinosaurs had appeared. The fossil record shows that the remaining species of helodermatid lizards are relics of a more diverse lineage that included at least six other genera inhabiting subtropical desert, forest, and savanna habitats. Family members somehow managed to survive the great Cretaceous extinctions, which vanquished the dinosaurs 65 million years ago. Helodermatid lizards have undergone relatively little gross morphological change over this time, and may appropriately be regarded as living fossils. The genus Heloderma has existed since at least the early Miocene (about 23 million years ago).

Today only two species remain: the Gila monster (Heloderma suspectum) and the Mexican beaded lizard or escorpíon (H. horridum). The two species are readily distinguished from each other by the Mexican beaded lizard's proportionately longer tail (at least 65% of the body length; no more than 55% in the Gila monster). The escorpíon is a longer, lankier, more arboreal lizard than the Gila monster.

Heloderma horridum was first described by Wiegmann in 1829 in Huajintlán, Morelos, Mexico. Four subspecies are recognized. Heloderma suspectum was first described by Cope in 1869, on international boundary between the United States and Mexico, Sierra de Moreno, Arizona. Two subspecies are recognized.

No subfamilies are recognized.

Physical characteristics

Helodermatid lizards are named after their distinctively textured skin, consisting of rounded, bony bumps (osteoderms) on their dorsal surfaces. The name Heloderma is derived from the Greek for "studded skin." Their lumbering gait, thick forked tongues, robust skull architecture, and venom glands in the lower jaw give them a cumbersome appearance that some consider monsterlike. The body markings can be bright and colorful or faded and cryptic. Juveniles frequently have banded patterns, which break up with age into a variety of adult markings consisting of spots, blotches, or chainlike crossbands of black or yellow on a background of pink, orange, yellow, slate gray, or black. The limbs are relatively

short and strong; the clawed feet are reminiscent of tiny human hands. Fat reserves are stored in the tail, which may be plump in well-fed individuals but is often quite thin in wild-caught lizards. Individuals range in size from barely 6 in (15 cm) total length (hatchling Gila monster) to up to 3.3 ft (1 m) for a large beaded lizard, which can weigh more than 4.4 lb (2 kg).

Distribution

Gila monsters occur from near sea level to about 5,090 ft (1,550 m) from southern Nevada, southwestern Utah, and

southeastern California throughout much of Arizona and Sonora, Mexico, and part of southwestern New Mexico. The Mexican beaded lizards occur from sea level to about 5,250 ft (1,600 m) along the Pacific foothills of Mexico from southern Sonora to Chiapas, along Pacific drainages in southern Guatemala, and along two Atlantic drainages in Chiapas and eastern Guatemala.

Habitat

Gila monsters are primarily desert dwellers, although they also inhabit semidesert grassland and woodland communities along mountain foothills. They prefer canyons or adjacent rocky slopes and more rarely open valleys. Their occurrence is strongly influenced by the availability of suitable microenvironments (boulders, burrows, pack rat middens, etc.) used as shelters, where they spend most of their time. Mexican beaded lizards inhabit primarily tropical dry forests and thorn-scrub, occurring less frequently in lower pine-oak woodlands. They frequent relatively open sandy and rocky arroyos, as well as densely vegetated upland hilly terrain and plateaus.

Behavior

Gila monsters and beaded lizards spend more than 95% of their time hidden within shelters (rocky crevices, burrows, pack rat middens, and trees). When active on the surface, however, they can travel long distances—more than 0.6 mi (1 km) in search of food and mates. Field studies using radiotelemetry have shown that both species are primarily diurnal. The specific timing of activity varies among individuals, seasons, and geographic locations.

During the breeding season, Gila monsters and beaded lizards perform spectacular ritualized male-male combat behaviors that are strikingly similar to those of many monitor lizards (Varanus). For the beaded lizards, combat consists of the formation of a high arch posture, with bellies pressed together and snouts, forelimbs, and tail tips forming contact points on the ground. Pressure exerted by the combatants eventually collapses the arch, and the dominant lizard emerges

on top. Combatants may repeatedly form the arch in matches that may continue for several hours. A typical combat session demands considerable physical effort and leaves both participants exhausted.

The combat of Gila monsters also consists of a series of ritualized wrestling matches, whereby combatants straddle each other, then perform a body twist in an effort to gain the superior position. Gila monsters do not form the arching postures performed by the beaded lizards, probably because their tails are too short. Each bout ends when pressure exerted by the body twist causes the lizards to separate, but bouts can be repeated many times over several hours. Two fighting males observed in southwestern Utah performed at least 13 individual bouts over nearly three hours of continuous exertion.

Feeding ecology and diet

Gila monsters and beaded lizards are widely searching foragers that feed on the contents of vertebrate nests, primarily reptilian and avian eggs, and juvenile mammals. Among the most common food items are juvenile cottontail rabbits and rodents; snake and lizard eggs, especially those of the spiny-tailed

iguana (Ctenosaura pectinata) within the range of the beaded lizard; and quail eggs. Beaded lizards take a greater variety of food items. Gila monsters can fulfill their annual maintenance energy requirements with three large meals. Their relatively large size, low resting metabolic rate, and ability to take large meals make frequent foraging activity unnecessary for helodermatid lizards.

Reproductive biology

In Gila monsters, spermiogenesis, courtship, and mating occur in late April through early June. Eggs are laid in July and August, which coincides with the onset of the summer rains in the southwestern deserts. Hatchling Gila monsters do not emerge until the following April. The clutch size varies from two to 12, with a mean of 5.7. Hatchlings have a snout-to-vent length around 4.5 in (110 mm) and average 6.5 in (165 mm) in total length; they weigh 1.2 oz (33 g).

In beaded lizards, spermiogenesis, courtship, and mating take place in September and October. Eggs are laid between October and December and hatchlings appear in June or July with the onset of the wet season. The clutch size ranges from two to 22 eggs with a mean of seven to nine. Wild hatchlings have a snout-to-vent length of 4.5–5.0 in (115–127 mm) and weigh 0.8–1.0 oz (23–27 g).

Conservation status

Both Gila monsters and beaded lizards are categorized as Vulnerable by the IUCN. In addition, both species are listed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) as Appendix II species, which includes those for whom trade must be controlled to avoid overexploitation. They receive full legal state protection from collecting, transport, or killing throughout their ranges. Their greatest threat is from habitat loss, due to the development of their dry forest and desert habitats, and from unscrupulous collectors. The spotty distribution of Gila monsters in the Mojave Desert of the U.S. Southwest, and rapid urban and recreational development in that region, prompted the U.S. Fish and Wildlife Service to classify Gila monsters as a high-priority species that may be vulnerable for listing as a threatened or endangered species in the near future.

Significance to humans

Helodermatids are the only lizards known to be venomous. Their venom apparatus consists of multilobed glands that empty through ducts at the base of grooved, venom-conducting teeth. In contrast to snakes, the venom glands of Gila monsters and beaded lizards are housed in the lower jaw rather than the upper jaw. Their venom is used primarily for defense. A bite from a Gila monster or a beaded lizard causes excruciating pain, swelling, and, in more severe bites, a rapid drop in blood pressure, profuse sweating, and vomiting. Bites to people are rare and almost always result from careless handling. Despite numerous exaggerated accounts before 1950, there has not been a human death reported from a Gila monster bite since 1930. This is attributable more to improved accuracy of reporting and medical record-keeping than to reduced bite frequency or advances in treatment. No first aid measures are recommended aside from carefully cleaning the wound and seeking immediate medical attention. Several important biologically active peptides were discovered in the venom of helodermatid lizards in the 1990s. The best known of these, Exendin 4, is very effective at inducing insulin release in human subjects and has become a promising tool for the treatment of diabetes.

Resources

Books

Brown, David E., and Neil B. Carmony. Gila Monster: Facts and Folklore of America's Aztec Lizard. Salt Lake City, UT: University of Utah Press, 1999.

Campbell, Jonathan A., and William W. Lamar. The Venomous Reptiles of Latin America. Ithaca, NY: Comstock Publishing Associates, 1989.

Lowe, Charles H., Cecil R. Schwalbe, and Terry B. Johnson. The Venomous Reptiles of Arizona. Phoenix: Arizona Game and Fish Department, 1986.

Periodicals

Beck, D. D. "Ecology and Behavior of the Gila Monster in Southwestern Utah." Journal of Herpetology 24 (1990): 54–68.

Beck, D. D., et al. "Locomotor Peformance and Activity Energetics of Helodermatid Lizards." Copeia (1995): 577–585.

Beck, D. D., and C. H. Lowe. "Ecology of the Beaded Lizard, Heloderma horridum, in a Tropical Dry Forest in Jalisco, Mexico." Journal of Herpetology 25 (1991): 395–406.

——. "Resting Metabolism of Helodermatid Lizards: Allometric and Ecological Relationships." Journal of Comparative Physiology B 164 (1994): 124–129.

Beck, D. D., and A. Ramírez-Bautista. "Combat Behavior of the Beaded Lizard, Heloderma h. horridum, in Jalisco, Mexico." Journal of Herpetology 25 (1991): 481–484.

Bogert, C. M., and R. M. del Campo. "The Gila Monster and Its Allies: The Relationships, Habits, and Behavior of the Lizards of the Family Helodermatidae." Bulletin of the American Museum of Natural History 109 (1956): 1–238.

Doyle, M. E., and J. M. Egan. "Glucagon-like Peptide-1." Recent Progress in Hormone Research 56 (2001): 377–399.

Goldberg, S. R., and D. D. Beck. "Heloderma horridum (Beaded Lizard): Reproduction." Herpetological Review 32 (2001): 255–256.

Goldberg, S. R., and C. H. Lowe. "Reproductive Cycle of the Gila Monster, Heloderma suspectum, in Southern Arizona." Journal of Herpetology 31 (1997): 161–166.

Norell, M. A., and K. Gao. "Braincase and Phylogenetic Relationships of Estesia mongoliensis from the Late Cretaceous of the Gobi Desert and the Recognition of a New Clade of Lizards." American Museum Novitates 3211 (1997): 1–25.

Nydam, R. L. "A New Taxon of Helodermatid-like Lizard from the Albian-Cenomanian of Utah." Journal of Vertebrate Paleontology 20, no. 2 (2000): 285–294.

Pregill, G. K., J. A. Gauthier, and H. W. Greene. "The Evolution of Helodermatid Squamates, with Description of a New Taxon and an Overview of Varanoidea." Transactions of the San Diego Society of Natural History 21 (1986): 167–202.

Raufman, J. P. "Bioactive Peptides from Lizard Venoms." Regulatory Peptides 61 (1996): 1–18.