Gila Monster

From Wikipedia, the free encyclopedia

Gila Monster[1][2]

Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
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. A heavy, slow moving lizard, up to 60 cm (2 ft) long, the Gila Monster is the only venomous lizard native to the United States, and only one of only two known species of venomous lizard in North America, the other being its close relative the Mexican Beaded Lizard (H. horridum).[3] Though the Gila monster is venomous, its slowness means that it represents little threat to humans. However, it has earned a fearsome reputation, and is often killed by hikers and homeowners, despite the fact that it is protected by state law in Arizona and Nevada.

The Gila Monster lives a largely sedentary lifestyle, its diet comprised mostly of eggs and small mammals and birds. The mating season runs from May to June, and two to twelve eggs are laid in July and August. They are incubated for nine months, and hatch around April to May the following year. The Gila Monster reaches sexual maturity in three to five years, and may live up to 20 years in the wild, or 30 in captivity.

Contents

[edit] Taxonomy and etymology

There are two subspecies of Gila Monster: the Reticulate Gila Monster (Heloderma suspectum suspectum) and the Banded Gila Monster (H. suspectum cinctum).[1] 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 have more unbroken bands of lighter scales.[4][5]

The Gila Monster has one close living relative, the Mexican 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 Monster refers to the Gila River Basin in Arizona, where they were once plentiful.[7] Heloderma means "studded skin", from the Ancient Greek words Helos — the head of a nail or stud — and derma, or skin. Suspectum comes from the describer, the famed paleontologist Edward Drinker Cope, who suspected that the lizard might be venomous due to the grooves in the teeth.[6]

[edit] Description

Closeup of a Gila Monster's skin, comprised of beadlike scales.
Closeup of a Gila Monster's skin, comprised of beadlike scales.

The Gila Monster is a bulky, sausage-shaped lizard with a length of 30-60 cm (12-24 in) and a weight of 1.3-2.25 kg (3-5 lb).[8] Although smaller than the Mexican Beaded Lizard, which can grow to be a meter long (3.2 ft),[9] the Gila Monster is the largest lizard in the United States.[8] It has a thick tail that is used to store fat, which it can live off of for months or, anecdotally, for years. Its tail is proportionately longer than that of the Mexican Beaded Lizard, a trait used to differentiate between the two. Unlike many other lizards, a Gila Monster's tail does not autotomize and cannot grow back if broken. The Gila Monster also possesses large forefeet and sharp claws, ideal for digging, as well as a thick black tongue, used to smell with the help of a Jacobson's organ.[9]

The Gila Monster's scales have the appearance of black, pink, orange, and yellow beads, laid down in intricate patterns. These beads are small bony plates that form scales, and are known as osteoderms.[10] The colors of its osteoderms are more vivid than those of its close relative, the Mexican Beaded Lizard. The scales on the belly are rectangular. It was the beaded skin of the Gila Monster that caught the attention of Native Americans, the vivid designs of which are recreated in Native American art and basketry.[11] The scales are most vivid when the Gila Monster is young; as it ages, the colors become paler and more reticulated.[9] The Gila Monster sheds its skin in small patches, such that they always seem to be in the process of shedding. Complete shedding of the skin is thought to occur once per year.[12]

It was mistakenly believed that the Gila Monster did not have an anus, when in fact the cloaca is concealed by scales. It was once believed that its waste contaminated its mouth and teeth, producing venom and "fetid breath."[9]

[edit] 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.
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.[4] They tend to 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.[13] They avoid living in open areas such as flats and farmland.[14]

[edit] 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 spring and early summer, the dry season, while later in summer may be active on warm nights or after a summer thunderstorm. On the surface, it maintains a body temperature of about 30°C (86°F).[9]

[edit] Diet

The Gila monster feeds primarily on bird and reptile eggs, and occasionally upon small birds and mammals.[15] Frogs, lizards, insects, and carrion are also consumed. The Gila Monster eats infrequently, but when it does feed, it may eat up to one-third of its body mass.[16] It uses its extremely acute sense of smell (thanks to its tongue and Jacobson's organ) to locate prey, especially eggs. Its sense of smell is so accurate that it can locate and dig up chicken eggs buried 15 cm (6 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. Prey is swallowed head first, and are helped down by muscular contractions and neck flexing. Unusually, after prey 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.[9]

[edit] Venom

“ I have never been called to attend a case of Gila monster bite, and I don’t want to be. I think a man who is fool enough to get bitten by a Gila monster ought to die. The creature is so sluggish and slow of movement that the victim of its bite is compelled to help largely in order to get bitten. ”

—Dr. Ward, Arizona Graphic, September 23, 1899

Venom is produced in modified salivary glands in the Gila Monster's lower jaw, unlike snakes, whose venom is produced in the upper jaw.[15] The Gila Monster lacks the musculature to forcible 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 their lives. Gila Monsters have also 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 also explain the Gila Monster's bright warning coloration.[5]

A Reticulate Gila Monster
A Reticulate Gila Monster

Although the venom is a neurotoxin as toxic as that of a Western diamondback rattlesnake, the Gila Monster produces only small amounts.[14] 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.[9] 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 also been isolated from the Gila Monster's venom, including horridum venom, which causes hemorrhage in internal organs and bulging of the eyes, 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.[17] 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][18]

[edit] Drug research

In 2005 the U.S. Food and Drug Administration approved the drug exenatide for the management of type 2 diabetes. It is a synthetic version of a protein, exendin-4, derived from the Gila Monster's saliva.[19] In a three-year study with people with type 2 diabetes, exenatide led to healthy sustained glucose levels and progressive weight loss.[20] The effectiveness is due to the fact that the lizard hormone is about 50 percent identical to glucagon-like peptide-1 analog (GLP-1), a hormone in the human digestive tract, that increases the production of insulin when blood sugar levels are high. The lizard hormone remains effective much longer than the human hormone, helping diabetics keep their blood sugar levels from getting too high. Exenatide also slows the emptying of the stomach and causes a decrease in appetite, contributing to weight loss.[20]

[edit] Life history

Three Gila Monsters at the Louisville Zoo, Kentucky.
Three Gila Monsters at the Louisville Zoo, Kentucky.

The Gila monster emerges from hibernation in January or February and mate in the months of May and June.[15] 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 her eggs in July or August, burying them in sand 5 inches (12.7 cm) below the surface. The clutch consists of two to twelve eggs with five being the average size.[4] The process of incubation lasts nine months as the hatchlings emerge during the months of April through June the following year.[21] The hatchlings are about 16 cm (6 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. Juveniles are sexually mature 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), until by November they have returned to hibernation.[5] The Gila Monster has a lifespan of about 20 years in the wild, although they may live up to 30 in captivity.[22]

Little is known about the social behavior of Gila Monsters, but they have been observed engaging in male-male combat, in which the dominant male lies on top of the subordinate one and holds in 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 bout 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.[23] 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 among lizards, allowing them to engage in intense aerobic activity for a sustained period of time. It has also been observed that males have a higher aerobic scope than females, presumably because of sexual selection for a trait advantageous in prolonged combat.[24]

[edit] Conservation

Gila monster at Bristol Zoo, England
Gila monster at Bristol Zoo, England

Though the Gila monster is venomous, its slowness means that it represents little threat to humans. However, it has earned a fearsome reputation, and is often killed by hikers and homeowners. Myths that the Gila Monster's breath is toxic enough to kill humans, that it can spit venom, or leap several feet in the air to attack are all false and are reflective of the Gila Monster's fierce reputation. In addition, urban sprawl, the pet trade, and habitat destruction has also affected their 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.[13][25][26] By the IUCN, Gila Monsters are listed as Near Threatened.[2] The San Diego Zoo was the first zoo to successfully breed Gila Monsters in captivity, doing so in 1963.[22]

Among Native American tribes, the Gila Monster enjoyed a mixed reputation. 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 Yaquai believed that the Gila Monster's hide had healing properties.[14]

[edit] References

  1. ^ a b Heloderma suspectum (TSN 174113). Integrated Taxonomic Information System. Retrieved on 19 May 2008.
  2. ^ a b Beaman (1996). Heloderma suspectum. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Listed as Vulnerable (VU A2cd v2.3)
  3. ^ Fry, Bryan G.; et al. (February 2006). "Early evolution of the venom system in lizards and snakes.". Nature 439 (7076): 584–588. doi:10.1038/nature04328. 
  4. ^ a b c d Stebbins, Robert (2003). Western Reptiles and Amphibians. New York: Houghton Mifflin, 338-339, 537. ISBN 0395982723. 
  5. ^ a b c d e Mattison, Chris (1998). Lizards of the World. London: Blandford. ISBN 0-7137-2357-2. 
  6. ^ a b c d King, Ruth Allen; Pianka, Eric R.; King, Dennis (2004). Varanoid Lizards of the World. Bloomington: Indiana University Press. ISBN 0-253-34366-6. 
  7. ^ Online Etymology Dictionary. Retrieved on 2008-06-10.
  8. ^ a b Animal Bytes - Gila Monster. Sea World. Retrieved on 2008-06-09.
  9. ^ a b c d e f g Greene, Harry L.; Pianka, Eric R.; Vitt, Laurie J. (2003). Lizards: Windows to the Evolution of Diversity. Berkeley: University of California Press. ISBN 0-520-23401-4. 
  10. ^ Bebler, John L. (1979). The Audubon Society Field Guide to Reptiles and Amphibians of North America. Alfred A. Knopf, 581. ISBN 0394508246. 
  11. ^ Netherton, John; Badger, David P. (2002). Lizards: A Natural History of Some Uncommon Creatures, Extraordinary Chameleons, Iguanas, Geckos, and More. Stillwater, MN: Voyageur Press. ISBN 0-7603-2579-0. 
  12. ^ Bogert, C.M.; R.M. del Campo (1956). "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: 1-238. 
  13. ^ a b (2001) Endangered Wildlife and Plants of the World. London: Marshall Cavendish, 629-630. ISBN 0-7614-7199-5. 
  14. ^ a b c Gila Monster Fact Sheet. Smithsonian National Zoological Park. Retrieved on 2008-06-06.
  15. ^ a b c Wilson, Don W.; Burnie, David (2001). Animal. London: DK, 419. ISBN 0-7894-7764-5. 
  16. ^ Christel CM, DeNardo DF, Secor SM (October 2007). "Metabolic and digestive response to food ingestion in a binge-feeding lizard, the Gila monster (Heloderma suspectum)". J. Exp. Biol. 210 (Pt 19): 3430–9. doi:10.1242/jeb.004820. PMID 17872997. 
  17. ^ Aird SD (June 2008). "Nucleoside composition of Heloderma venoms". Comp. Biochem. Physiol. B, Biochem. Mol. Biol. 150 (2): 183–6. doi:10.1016/j.cbpb.2008.02.012. PMID 18430599. 
  18. ^ Maruno K, Said SI (1993). "Small-cell lung carcinoma: inhibition of proliferation by vasoactive intestinal peptide and helodermin and enhancement of inhibition by anti-bombesin antibody". Life Sci. 52 (24): PL267–71. PMID 8389407. 
  19. ^ Bond, Aaron (July 2006). "Exenatide (Byetta) as a novel treatment option for type 2 diabetes mellitus". Baylor University Medical Center Proceedings 19 (3): 281-4. PMID 17252050. 
  20. ^ a b "Drug Derived From Gila Monster Saliva Helps Diabetics Control Glucose, Lose Weight", Science Daily, 2007-07-12. 
  21. ^ Seward, Mark (2002). Dr. Mark Seward's Gila Monster Propogation: How To Breed Gila monsters in Captivity. Natural Selections Publishing, 80. ISBN 0970139500. 
  22. ^ a b San Diego Zoo's Animal Bytes: Gila Monster. San Diego Zoo. Retrieved on 2008-05-19.
  23. ^ Beck, D.D. (1990). "Ecology and behavior of the Gila monster in southwestern Utah". Journal of Herpetology 24: 54-68. 
  24. ^ Beck, D.D.; M.R. Dohm, J.T. Garland, A. Ramirez-Bautista, and C.H. Lowe (1995). "Locomotor performance and activity energetics of helodermatid lizards". Copeia 1995 (577-585). 
  25. ^ Gila Monster - Heloderma suspectum. Sedgwick County Zoo. Retrieved on 2008-06-09.
  26. ^ Brennan, Thomas C.. Reptiles of Arizona - Gila Monster (Heloderma suspectum). Retrieved on 2008-06-10.

[edit] Further reading

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