Mexican beaded lizard
The Gila monster is a species of venomous lizard native to the southwestern United States and northwestern
Mexican state of Sonora. A heavy, slow-moving lizard, up to 60 cm (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). Though the Gila
monster is venomous, its sluggish nature means it represents little threat to humans. However, it has
earned a fearsome reputation and is sometimes killed despite being protected by state law in Arizona.
In this species, the largest extant lizard native to North America north of the Mexican border (non-natives
like green iguanas are larger), snout-to-vent length is from 26 to 36 cm (10 to 14 in). The tail is about
20% of the body size and the largest specimens may reach 51 to 56 cm (20 to 22 in) in total length. Body
mass is typically in the range of 350 to 700 g (0.77 to 1.54 lb), with 11 males having been found to average
468 g (1.032 lb). Reportedly, the very heaviest, largest specimens can weigh as much as 2,300 g (5.1 lb).
The Gila monster has one close living relative, the beaded lizard (H. horridum), as well as many extinct
relatives in the Helodermatidae, the evolutionary history of which 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 (10,000-8,000 years ago) deposits near Las Vegas,
Nevada. Because the helodermatids have remained relatively unchanged morphologically, they are occasionally
regarded as living fossils. Although the Gila monster appears closely related to the monitor lizards
(varanids) of Africa, Asia and Australia, their wide geographical separation and the unique features not
found in the varanids indicate the Gila monster is better placed in a separate family.
Distributión and habitat
The Gila monster is found in the Southwestern United States and Mexico, a range including Sonora, Arizona,
parts of California, Nevada, Utah, and New Mexico (potentially including 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. In fact, Gila monsters seem to like water and can be observed
immersing themselves in puddles of water after a summer rain. They avoid living in open areas such as flats
Gila monsters spend 90% of their time underground in burrows or rocky shelters. They are active in the
morning during the dry season (spring and early summer); later in the summer, they may be active on warm
nights or after a thunderstorm. They maintain a surface body temperature of about 30 °C (86 °F). Gila
monsters are slow in sprinting ability, but they have relatively high endurance and maximal aerobic
capacity (VO2 max) for a lizard. They are preyed upon by coyotes and raptors.
The Gila monster eats small birds, mammals, frogs, lizards, insects, and carrion. The Gila monster feeds
primarily on bird and reptile eggs, and eats infrequently (only five to ten times a year in the wild), but
when it does feed, it may eat up to one-third of its body mass. It uses its extremely acute sense of smell
to locate prey, especially eggs. Its sense of smell is so keen, it can locate and dig up chicken eggs buried
15 cm (6 in) deep and accurately follow a trail made by rolling an egg.
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 young in nests. Gila monsters are able to climb trees and cacti in search of
Although the venom is a neurotoxin as toxic as that of a coral snake, H. suspectum produces only small
amounts. The Gila monster's bite is not fatal to healthy adult humans. No reports of fatalities have been
confirmed after 1939, and those recorded prior to that year are possibly iatrogenic, or resulting from
attempts to treat the bite itself. The Gila monster can bite quickly (especially by swinging its head
sideways) and hold on tenaciously and painfully. 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.
Zoológico de Vallarta A. C.
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