Gila monster

Gila monster
Conservation status
Near Threatened (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Superfamily:	Varanoidea
Family:	Helodermatidae
Genus:	Heloderma
Species:	H. suspectum
Binomial name
Heloderma suspectum Cope, 1869

The Gila monster (pronounced /ˈhiːlə/, HEE-la), Heloderma suspectum, is a species of venomous lizard native to the southwestern United States and northern Mexico (but not Baja California). A heavy, slow-moving lizard, up to 60 centimetres (2.0 ft) long, the Gila monster is the only venomous lizard native to the United States and one of only two known species of venomous lizards in North America, the other being its close relative, the Mexican beaded lizard (H. horridum).^[2] Though the Gila monster is venomous, its sluggish nature means that it represents little threat to humans. However, it has earned a fearsome reputation and is sometimes killed by hikers and homeowners despite being protected by state law in Arizona and Nevada.^[3][1]

Taxonomy and etymology

There are two subspecies of Gila monster: the reticulate Gila monster (H. suspectum suspectum) and the banded Gila monster (H. suspectum cinctum).^[3] The reticulate Gila monster lives in the southern region of the Gila monster's range, while the banded Gila monster is a northern subspecies occurring primarily in the Mojave Desert. The reticulate Gila monster tends to have its lighter markings broken up by dark scales, giving it a reticulated pattern, while the banded Gila monster generally has more unbroken bands of lighter scales.^[4][5]

The Gila monster has one close living relative, the beaded lizard (H. horridum), as well as many extinct relatives in the Helodermatidae whose evolutionary history may be traced back to the Cretaceous period. The genus Heloderma has existed since the Miocene, when H. texana lived, and fragments of osteoderms from the Gila monster have been found in late Pleistocene (8000-10,000 years ago) deposits near Las Vegas, Nevada. Because the Helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils.^[6] Although the Gila monster appears closely related to the monitor lizards (varanids) of Africa, Asia and Australia, the wide geographical separation and unique features not found in the varanids indicates that the Gila monster is better placed in a separate family.^[5]

The name Gila refers to the Gila River Basin in Arizona, where they were once plentiful.^[7] Heloderma means "studded skin", from the Ancient Greek words Helos (ηλος), "the head of a nail or stud", and derma (δερμα), "skin". Suspectum comes from the describer, paleontologist Edward Drinker Cope, who suspected that the lizard might be venomous due to the grooves in the teeth.^[6]

Distribution and habitat

A Gila monster's head. The bead-like scales can be observed, as well as the strong forelegs and claws suitable for digging.

The Gila monster is found in the Southwestern United States and Mexico, a range entailing Sonora, Sinaloa, Arizona, parts of California, Nevada, Utah, and New Mexico (but not Baja California). They inhabit scrubland, succulent desert, and oak woodland, seeking shelter in burrows, thickets, and under rocks in locations with ready access to moisture.^[4] In fact, Gila monsters seem to like water and can be observed immersing themselves in puddles of water after a summer rain.^[8] They avoid living in open areas such as flats and farmland.^[9]

Ecology

It is estimated that the Gila monster spends 95% of its time underground in mammal burrows or rocky shelters. It is active in the morning during the dry season (spring and early summer); later in the summer, it may be active on warm nights or after a thunderstorm. On the surface, it maintains a body temperature of about 30 °C (86 °F).^[10] Gila monsters are slow in terms of sprinting ability, but they have relatively high endurance and maximal aerobic capacity (VO₂ max) for a lizard.^[11]

Diet

The Gila monster feeds primarily on bird and reptile eggs, and occasionally upon small birds, mammals, frogs, lizards, insects, and carrion.^[12] The Gila monster eats infrequently (only five to ten times a year in the wild),^[13] but when it does feed, it may eat up to one-third of its body mass.^[14] It uses its extremely acute sense of smell to locate prey, especially eggs. Its sense of smell is so keen that it can locate and dig up chicken eggs buried 15 centimetres (5.9 in) deep and accurately follow a trail made by rolling an egg.^[5]

Prey may be crushed to death if large or eaten alive if small, swallowed head-first and helped down by muscular contractions and neck flexing. Unusually, after food has been swallowed, the Gila monster immediately resumes tongue flicking and search behavior, probably as a result of a history of finding clumped prey such as eggs and babies in nests.^[10] Gila monsters are able to climb trees and cacti in search of eggs.^[15]

Venom

Venom is produced in modified salivary glands in the Gila monster's lower jaw, unlike snakes, whose venom is produced in the upper jaw.^[12] The Gila monster lacks the musculature to forcibly inject the venom; instead, the venom is propelled from the gland to the tooth by chewing. Capillary action brings the venom out of the tooth and into the victim.^[6] The teeth are loosely anchored, which allows them to be broken off and replaced throughout life. Gila monsters have been observed to flip over while biting the victim, presumably to aid the flow of the venom into the wound. Because the Gila monster's prey consists mainly of eggs, small animals, and otherwise "helpless" prey, it is thought that the Gila monster's venom evolved for defensive rather than for hunting use. A defensive use would explain the Gila monster's bright warning coloration.^[5]

A reticulate Gila monster (Heloderma suspectum suspectum)

Although the venom is a neurotoxin as toxic as that of a Coral Snake, H. suspectum produces only small amounts.^[9] The Gila monster's bite is normally not fatal to adult humans (there are no confirmed reports of fatalities), but it can bite quickly and hold on tenaciously and painfully.^[10] If bitten, the victim may need to fully submerge the attacking lizard in water to break free from its bite. Symptoms of the bite include excruciating pain, edema, and weakness associated with a rapid drop in blood pressure. More than a dozen peptides and proteins have been isolated from the Gila monster's venom, including hyaluronidase, serotonin, phospholipase A₂, and several kallikrein-like glycoproteins responsible for the pain and edema caused by a bite. Four potentially lethal toxins have been isolated from the Gila monster's venom, including horridum venom, which causes hemorrhage in internal organs and exophthalmos (bulging of the eyes)[1], and helothermine, which causes lethargy, partial paralysis of the limbs, and hypothermia in rats. However, the constituents most focused on are the bioactive peptides, including helodermin, helospectin, exendin-3, and exendin-4.^[16] Most are similar in form to vasoactive intestinal peptide (VIP), which relaxes smooth muscle and regulates water and electrolyte secretion between the small and large intestines. These bioactive peptides are able to bind to VIP receptors in many different human tissues. One of these, helodermin, has been shown to inhibit the growth of lung cancer.^[6][17]

Drug research

In 2005, the U.S. Food and Drug Administration approved the drug exenatide (marketed as Byetta) for the management of type 2 diabetes. It is a synthetic version of a protein, exendin-4, derived from the Gila monster's saliva.^[18] In a three-year study with people with type 2 diabetes, exenatide led to healthy sustained glucose levels and progressive weight loss. The effectiveness is due to the fact that the lizard protein is about 50 percent identical to glucagon-like peptide-1 analog (GLP-1), a hormone released from the human digestive tract that helps to regulate insulin and glucagon. The lizard protein remains effective much longer than the human hormone, helping diabetics keep their blood sugar levels under control. Exenatide slows the emptying of the stomach and causes a decrease in appetite, contributing to weight loss.^[19]

Life history

Gila monster at the Bristol Zoo

The Gila monster emerges from hibernation in the months of January or February and mates in May and June.^[12] The male initiates courtship by flicking his tongue to search for the female's scent. If the female rejects his advances, she will bite him and crawl away. When successful, copulation has been observed to last from 15 minutes to as long as 2.5 hours. The female will lay eggs in July or August, burying them in sand 12.7 centimetres (5.0 in) below the surface. The clutch consists of two to twelve eggs, with five being the average clutch.^[4] The process of incubation lasts nine months, as the hatchlings emerge during the months of April through June the following year.^[20] The hatchlings are about 16 centimetres (6.3 in) long and are able to bite and inject venom upon hatching. The juveniles typically have larger bands of pink scales than adults, although the banded Gila monster (H. s. cinctum) has a tendency to retain the band pattern. H. suspectum sexually matures at 3–5 years. After egg-laying, adult Gila monsters gradually spend less time on the surface to avoid the hottest part of the summer (although they may be active in the evening), eventually starting their hibernation around November.^[5]

Little is known about the social behavior of H. suspectum, but they have been observed engaging in male-male combat, in which the dominant male lies on top of the subordinate one and pins it with its front and hind limbs. Both lizards arch their bodies, pushing against each other and twisting around in an effort to gain the dominant position. A wrestling match ends when the pressure exerted forces them to separate, although bouts may be repeated over a continuous amount of time. These bouts are typically observed just before the mating season. It is thought that those with greater strength and endurance win more often and enjoy greater reproductive success.^[21] Although the Gila monster has a low metabolism and one of the lowest lizard sprint speeds, it has one of the highest aerobic scope values (the increase in oxygen consumption from rest to maximum metabolic exertion) among lizards, allowing them to engage in intense aerobic activity for a sustained period of time. It has been observed that males have a higher aerobic scope than females, presumably because of sexual selection for a trait advantageous in prolonged combat.^[11] The Gila monster may live up to 20 years in the wild, or 30 in captivity.^[22]

Conservation status

Urban sprawl, the pet trade, and habitat destruction has affected Gila monster numbers. As a result, Gila monsters are protected by Arizona and Nevada state law; it is illegal to "harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect the Gila Monster." In 1952, they became the first venomous animal to be given legal protection.^[8][23][24] Gila monsters are listed as Near Threatened by the IUCN.^[1] The San Diego Zoo was the first zoo to successfully breed Gila monsters in captivity, doing so in 1963.^[22]

Relation with humans

Though the Gila monster is venomous, its laggard movement means that it poses little threat to humans. However, it has earned a fearsome reputation and is often killed by humans because of fear. Among Native American tribes, the Gila monster had a mixed standing. The Apache believed that its breath could kill a man, and the Tohono O'Odham and the Pima believed that it possessed a spiritual power that could cause sickness. In contrast, the Seri and the Yaqui believed that the Gila monster's hide had healing properties.^[9] The Gila monster has even starred as a monster in a B movie, The Giant Gila Monster. Myths that the animal's breath is toxic enough to kill humans or that it can spit venom or leap several feet in the air to attack are all false.^[22] Another myth held that the Gila monster did not have an anus and therefore expelled waste from its mouth, the source of its venom and "fetid breath".^[10]

References

Further reading

• Beck, Daniel D. (2005). Biology of Gila Monsters and Beaded Lizards. Berkeley: University of California Press. ISBN 0-520-24357-9. 
• Capula, Massimo; Behler (1989). Simon & Schuster's Guide to Reptiles and Amphibians of the World. New York: Simon & Schuster. ISBN 0671690981. 
• Carmony, Neil B.; Brown, David (1991). Gila Monster: Facts and Folklore of America's Aztec Lizard. Silver City, NM: High-Lonesome Books. ISBN 0-944383-18-1. 
• Cogger, Harold; Zweifel, Richard (1992). Reptiles & Amphibians. Sydney, Australia: Weldon Owen. ISBN 0831727861. 
• Ditmars, Raymond L (1933). Reptiles of the World: The Crocodilians, Lizards, Snakes, Turtles and Tortoises of the Eastern and Western Hemispheres. New York: Macmillian. pp. 321. 
• Freiberg, Dr. Marcos; Walls, Jerry (1984). The World of Venomous Animals. New Jersey: TFH Publications. ISBN 0876665679. 
• Roever, J. M.; Hiser, Iona Seibert (1972). The Gila Monster. Austin, Tex: Steck-Vaughn Co. ISBN 0-8114-7739-8.