Gila monster

The Gila monster (Heloderma suspectum, /ˈhiːlə/ HEE-lə) is a species of venomous lizard native to the Southwestern United States and the northwestern Mexican state of Sonora. It is a heavy, slow-moving reptile, up to 56 centimetres (22 in) long, and it is the only venomous lizard native to the United States. Its venomous close relatives, the four beaded lizards (all former subspecies of Heloderma horridum) inhabit Mexico and Guatemala. The Gila monster is sluggish in nature, so it is not generally dangerous and very rarely poses a real threat to humans. Nonetheless, it has a fearsome reputation and is sometimes killed despite the species being protected by state law in Arizona.

The name "Gila" refers to the Gila River Basin in the U.S. states of Arizona and New Mexico, 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". The species epithet suspectum comes from the describer, paleontologist Edward Drinker Cope. At first, this new specimen of Heloderma was misidentified and considered to be a northern variation of the beaded lizard already known to live in Mexico. He suspected that the lizard might be venomous due to the grooves in the teeth.

The Gila monster is the largest extant lizard species native to North America north of the Mexican border. Its snout-to-vent length ranges 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 550 to 800 g (1.21 to 1.76 lb). They appear strong in their body structure with a stout snout, massive head, and "little"-appearing eyes, which can be protected by a nictitating membrane.

The Gila monster has four close living relatives, all of which are beaded lizards. There are three species in Mexico: Heloderma exasperatum, Heloderma horridum and Heloderma alvarezi, as well as another species in Guatemala: Heloderma charlesbogerti.

The evolutionary history of the Helodermatidae may be traced back to the Cretaceous period (145 to 166 million years ago), when Gobiderma pulchrum and Estesia mongolensis were present. The genus Heloderma has existed since the Miocene, when H. texana lived. Fragments of osteoderms from the Gila monster have been found in Late Pleistocene (10,000 to 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 distinct features indicate that Heloderma is better placed in a separate family.

The scales of the head, back, and tail contain little pearl-shaped bones (osteoderms) similar to those found in the beaded lizards from farther south. The scales of the belly are free from osteoderms. Female Gila monsters go through a total shed lasting about 2 weeks before depositing their eggs. The dorsal part is often shed in one large piece. Adult males normally shed in smaller segments in August. The young seem to be in constant shed. Adults have more or less yellow to pink colors on a black surface. Hatchlings have a uniform, simple, and less colorful pattern. This drastically changes within the first 6 months of their lives. Hatchlings from the northern area of the species' distribution have a tendency to retain most of their juvenile pattern. Gila monsters in areas with darker rocks and substrate will have darker colorations.

The heads of males are very often larger and more triangular-shaped than in females. The length of the tail of the two sexes is statistically very similar, so it does not help in differentiation of the sexes. Individuals with stout tail ends occur in both nature and under human breeding.

The Gila monster is found in the Southwestern United States and Mexico, across a range including Sonora, Arizona, and parts of California, Nevada, Utah, and New Mexico. No records have been given from Baja California. They inhabit scrubland, succulent desert, and oak woodland, seeking shelter in burrows, thickets, and under rocks in locations with a favorable microclimate and adequate humidity. Gila monsters rely heavily on the use of shelters and spend much of their time dwelling there. Often times these shelters are in rocky areas in Navajo Sandstone and basaltic lava flows. Gila monsters depend on water resources and can be observed in puddles of water after a summer rain. They avoid living in open areas, such as flats and open grasslands.