GILA MONSTER AND MEXICAN BEADED LIZARD: Helodermatidae

PHYSICAL CHARACTERISTICS

The two species in this family, the Gila monster and the Mexican beaded lizard, are both large, heavy-bodied lizards coated with small, rounded bumps that look like the beadwork on clothing. The bumps, which are actually pebblelike scales, cover the tops of the arms, legs, head, and tail, as well as the back and sides of the body. These lizards have rather short, but strong arms and legs and long, thin claws. The tail may be thin or thick, depending on how well-fed the individual is. This is because these lizards store fat in their tails. Beaded lizards have slightly longer tails than the Gila monsters. An average beaded tail is at least two-thirds the length of the entire body, but the typical Gila tail is about half the total body length. Unlike many other lizards, these two species also have thick, forked tongues. Members of the same species can look very different from one another. Some adults are brightly patterned, while others are faded and dull. The patterns may be made up of spots, blotches, circles, bands, or squiggles on a background of pink, orange, yellow, dark gray, or black. Juveniles are usually banded.

Gila monsters and Mexican beaded lizards are the only two venomous (VEH-nuh-mus) lizards in the world. Unlike venomous snakes that deliver venom from the upper jaw and through grooves in just the two fangs, these lizards store their venom in the lower jaw and deliver it through grooves in numerous teeth.

Adult Gila monsters and beaded lizards range from 12 to 18 inches (30 to 45 centimeters) from the snout to the vent, which is a difficult-to-see opening on the underside of the lizard at the beginning of the tail, or 14 to 39 inches (35 to 100 centimeters) from the snout to the tip of the tail. They weigh 1.0 to 4.4 pounds (450 grams to 2 kilograms). The beaded lizard can grow larger than the Gila monster.

GEOGRAPHIC RANGE

They live in North and Central America from the southwestern United States to northwestern Mexico, in Guatemala, and also in the state of Chiapas in southern Mexico.

HABITAT

Gila monsters and beaded lizards stay in deserts usually, although some make their homes in dry grassland, in shrubby forests on hillsides, or in nearby areas that have boulders or burrows where they can hide.

DIET

Gila monsters and beaded lizards wander through their habitat looking for young rabbits or rodents or snakes and lizard eggs to eat. They will even climb trees and cacti in search of bird eggs.

LIVING FOSSILS

The Gila monster and Mexican beaded lizard are often called "living fossils." This means that they have changed very little from the way their ancestors looked millions of years ago. Both lizards belong to a family that scientists have traced back 98 million years, long before the great dinosaur extinction of 65 million years ago. Most species in this family have disappeared, but the Gila monster and Mexican beaded lizard remain and display many of the characteristics of their long-gone relatives.

BEHAVIOR AND REPRODUCTION

These species spend about twenty-three hours of every day out of sight in burrows, within cracks in rocks, or in trees. When they do travel above ground, they wander about during the day looking for food or for mating partners, sometimes traveling more than 0.6 miles (1 kilometer). They also may come out in the evening. During the breeding season, the males of both species will fight one another, sometimes battling for two or three hours at a time. The male beaded lizards will arch their bodies and wrestle belly-to-belly until one comes out on top. The male Gila monsters wrestle by twisting the body to and fro. Courtship and mating occur in spring for the Gila monsters and in fall for the beaded lizards. Two to three months later, female Gila monsters lay two to twelve eggs, and female beaded lizards lay two to twenty-two eggs. Hatchling Gila monsters leave the nest the following spring in April, and hatchling beaded lizards appear in June or July as the wet season begins.

GILA MONSTERS, MEXICAN BEADED LIZARDS, AND PEOPLE

Although they are venomous, bites to humans are rare and typically only occur when a person tries to pick up one of these normally slow-moving Gila monsters or Mexican beaded lizards. Both can twist around quickly and deliver a hard bite. Their powerful jaws can remain clamped shut on a finger or hand for many minutes. While a bite can cause awful pain, swelling, and sometimes vomiting and sweating, it is hardly ever fatal. In fact, the last death due to a Gila monster bite occurred in 1930. On the positive side, scientists have found that substances in the venom may be useful in treating human diseases, such as diabetes.

CONSERVATION STATUS

The World Conservation Union (IUCN) considers both species in this family as Vulnerable, which means that they face a high risk of extinction in the wild. The destruction of their habitat, particularly as it is developed for housing or other human uses, is one of the biggest reasons they are at risk. Another reason for their low numbers is that humans sometimes illegally collect them for the pet trade or for personal pets.

GILA MONSTER (Heloderma suspectum): SPECIES ACCOUNT

Physical characteristics: The Gila monster is a slow-moving, heavy-bodied lizard with rather short, clawed arms and legs and upper skin that looks beaded. The beads are actually rounded scales that appear on the top of the head, back, tail, and limbs and down the sides of the body. The color of the skin and scales differs from individual to individual, but most have at least some pattern, which can be quite bright and beautiful, of squiggles, spots, blotches, circles, and bands. Colors range from pink, orange, and yellow to black and dark gray. Well-fed Gila monsters have thick tails, which store fat. This species and the Mexican beaded lizard are the only two venomous lizards in the world. Adult Gila monsters commonly grow to about 20 inches (50 centimeters) in length from head to tail tip.

Geographic range: The Gila monster makes its home in the southwestern United States and in Sonora, Mexico.

Habitat: Gila monsters live in warm habitats, including deserts, grasslands, and shrubby forests, sometimes on flat ground and sometimes on hillsides. They spend most of their time in underground burrows, inside large cracks in rocks, or in other hiding places, only coming above ground for about one hour a day.

Diet: Gila monsters need to eat only three large meals a year to survive. They store fat in the tail and then use it up between meals, which can be several months apart. Their favorite foods include lizard, snake, and bird eggs, as well as young cottontail rabbits and rodents.

Behavior and reproduction: They remain in burrows or other hiding places for all but about one hour a day when they venture out to look for food or mates. If they feel threatened, they will hiss and sometimes snap at or bite the attacker. Once they bite, they have a very strong grip and may hold it for five minutes or longer. Males and females court and mate from late April to early June, and in July and August the females lay eggs in a damp sand nest. Babies are about 6.5 inches long from snout to tail tip when they hatch.

Gila monsters and people: A Gila monster bite can be painful, but it is almost never fatal to humans. The last reported death from a bite occurred in 1930. Bites rarely happen, however, and usually result from a person's carelessness in picking up the lizard.

Conservation status: The World Conservation Union (IUCN) considers the species to be Vulnerable, which means that it faces a high risk of extinction in the wild. Habitat loss and illegal collection are the main problems the lizards face. ∎

FOR MORE INFORMATION

Books

Badger, D. Lizards: A Natural History of Some Uncommon Creatures— Extraordinary Chameleons, Iguanas, Geckos, and More. Stillwater, MN: Voyageur Press, 2002.

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.

Martin, James, and Joe McDonald. Poisonous Lizards: Gila Monsters and Mexican Beaded Lizards. Minneapolis, MN: Capstone Press, 1999.

Mattison, Chris. Lizards of the World. New York, NY: Facts on File, 1989.

Web sites

"Gila Monster." Animal Planet: Corwin's Carnival of Creatures. http://animal.discovery.com/fansites/jeffcorwin/carnival/lizard/gilamonster.html (accessed on October 16, 2004).

"Gila Monster." Enchanted Learning. http://www.enchantedlearning.com/subjects/reptiles/lizard/Gilamonster.shtml (accessed on October 16, 2004).

"Gila Monster." WhoZoo. http://www.whozoo.org/anlife2000/jamiebritt/gilaindexrev.html (accessed on November 3, 2004).

"Mexican Beaded Lizard." Lincoln Park Zoo.http://www.lpzoo.com/tour/factsheets/herps/beaded_lizard.html (accessed on October 16, 2004).