The Gila monster is a heavy slow-moving venomous lizard. It is the largest extant lizard native to North America north of the Mexican border. The name "Gila" refers to the Gila River Basin in the U.S. states of New Mexico and Arizona, where Gila monsters were once plentiful. The body of these lizards is covered with bumpy looking scales which are called osteoderms; they are black and orange or pink in color and are actually small bones under the scales.
Gila monsters are 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 or pine-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.
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. These lizards are generally solitary creatures but may gather in communal areas and share shelters. Gila monsters have poor eyesight and when they hunt, they use their extremely acute sense of smell to locate prey, especially eggs. Their sense of smell is so keen, they can locate and dig up chicken eggs buried 15 cm (6 in) deep and accurately follow a trail made by a rolling egg. Gila monsters may even climb trees and cacti in search of eggs. During cold winter months, these lizards hibernate in their burrows until spring.
Gila monsters breed in May and June and just before the mating season males have been observed engaging in combats. In this combat, 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. The male usually 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. Females lay eggs in July or August, burying them in sand 5 in (13 cm) below the surface. The clutch consists of 2 to 12 eggs: 5 is the average. The incubation lasts 9 months, as the hatchlings emerge during April through June the following year. The hatchlings are about 16 cm (6.3 in) long and can bite and inject venom upon hatching. The juveniles usually attain reproductive maturity at three to five years old. 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.
Illegal collection and habitat destruction due to urbanization and agricultural development have adversely affected Gila monster numbers. In 1952, they became the first venomous animal to be given legal protection.
The IUCN Red List and other sources don’t provide the number of the Gila monster total population size. Currently, this species is classified as Near Threatened (NT) on the IUCN Red List and its numbers today are decreasing.