Gila Monster

The **Gila monster** (*Heloderma suspectum*) 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.

## Description, taxonomy and etymology

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.

The name "Gila" refers to the Gila River Basin in the U.S. states of New Mexico and Arizona, where the Gila monster was once plentiful. *Heloderma* means "studded skin", from the Ancient Greek words *helos* (ἧλος), "the head of a nail or stud", and *derma* (δέρμα), "skin". *Suspectum* comes from the describer, paleontologist Edward Drinker Cope, who suspected the lizard might be venomous due to the grooves in the teeth.

## Range 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 and farmland.

## Diet

Gilas are lethargic creatures that feed primarily on eggs raided from nests and newborn mammals as well as rogs, lizards, insects, and carrion. They may spend more than 95 percent of their lives in underground burrows, emerging only to feed and occasionally to bask in the desert sun. They can store fat in their oversized tails and are able to go months between meals.

## Toxicity

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. Gila populations are shrinking due primarily to human encroachment, and they are considered a threatened species.