The Gila Monster (Heloderma suspectum) is a species of venomous lizard native to the southwestern United States and northwestern Mexican state of Sonora.
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. The tail is about 20% of the body size and the largest specimens may reach 51 to 56 cm in total length. Body mass is typically in the range of 350 to 700 g, with 11 males having been found to average 468 g. Reportedly, the very heaviest, largest specimens can weigh as much as 2,300 g.
The Gila monster has one close living relative, the Mexican 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.
Gila monsters use their venomous bites as a defensive weapon and will not bite unless agitated. Its sluggish behavior makes the Gila monster less of a threat to humans. Gila monsters are carnivores and will prey on a variety of food items that include small mammals, invertebrates, lizards, snakes, tortoises, carrion, and bird eggs. Eating up to three portions of its diet allow the Gila monster to survive a whole season. If two male Gila monsters cross paths, they will engage in a fight, with both contenders using their strong jaws as weapons and not letting go until one surrenders.