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Gila Monster - Heloderma suspectum


The gila monster (pronounced HEE-la), Heloderma suspectum, is a species of venomous lizard native to the southwestern United States and northern Mexico. It is a heavy, slow moving lizard, up to 60 cm (2 feet) long, and is the most venomous lizard native to the USA. Its skin has the appearance of black, pink, orange, and yellow beads, laid down in intricate patterns. These beads are small bony plates that form scales, and are known as osteoderms. Until very recently, it was thought to be one of only two species of venomous lizard, the other being its close relative the Mexican beaded lizard. However research at the University of Melbourne, Australia and Pennsylvania State University has revealed that in fact many lizards in the iguanian and monitor families have venom-producing glands.

The name "Gila monster" refers to the Gila River Basin in Arizona. The generic name for Heloderma is from the Greek words Helos coming from the head of a nail or stud, and derma for skin, therefore Heloderma means studded skin. Suspectum comes from Cope's notion that the lizard might be venomous due to the grooves in the teeth.

Range & Habitat

Gila monsters live from southwest Utah to the southern Sonora and northern Sinaloa; extreme southwest New Mexico to southern Nevada and just into eastern California. Heloderma suspectum also occurs up to an elevation of 4,800 feet. The Gila monster is more common in the wetter, rockier paloverde-saguaro desert scrub association than the drier, sandier creosote bush-bersage association. Heloderma suspectum also seem to prefer rocky foothills and avoid open areas and agricultural regions. Gila monsters live in burrows dug by other animals or may construct their own.


There is moderate sexual dimorphism within this species, as males are larger, have a wider head, and a squarer frame than females. Female Heloderma suspectum tend to have an oval shaped body. Heloderma suspectum are relatively social creatures. Studies have shown that they recognize and interact with many individuals throughout their home range and have been seen in burrows together in separate years. In late April through late May, six or more individuals may occupy burrows at a time.

Gila monsters are oviparous, meaning that they lay eggs. Five eggs is the average clutch size, but can reach up to twelve eggs at a time. In southern Arizona, Gila monsters breed in May and June and lay their eggs in June and August of the following year. These eggs then incubate in burrows and develop from fall to the early spring, and young appear in April and June. Interestingly, no other egg-laying lizard in North America over-winters their eggs and hatches them the following year, like Heloderma suspectum does. When captively bred, the gestation period of a Gila monster is 42-55 days.

Gila monsters are adapted to eating large meals infrequently. In fact, an adult male Gila can consume its entire yearly energy budget in just a few meals. [1] This is because of the large meals they can consume as well as their limited food requirements. Their food requirements are reduced by a low metabolic rate, as well as the relatively cool body temperatures they maintain for most of the year.

Gila monsters specialize in feeding on the young and eggs in vertebrate nests such as the Spotted Barkev and the Blue Melodie. The Gila monsterís main activity period coincides with the availability of their main food source.

Additional Notes

Gila Monsters mate throughout the summer months, with the female laying 3 to 5 eggs in sandy soils, burrows or under rocks, during fall or winter.

The Venom

Unlike snakes which use hollow upper teeth (fangs), the Gila monster injects venom into its victim through grooves in the teeth of its lower jaw. The teeth are loosely anchored, which allows them to be broken off and replaced throughout their lives. The Gila monster produces only small quantities of its neurotoxic venom, which is secreted into the lizard's saliva. By chewing its prey, however, it tries to put as much of the venom into the bloodstream of its victim as possible. The Gila monster's bite is normally not fatal to humans (there are no confirmed reports of fatalities), but it can bite quickly and holds on tenaciously. When Gila Monsters bite, they hold on tightly and chew. This helps them work their venom into the bite. Gila Monster bites are not deadly, but it is very important to see a doctor as soon as possible if bitten.


Subspecies Common name Geographic range

H.s. suspectum

Reticulate Gila Monster

resides primarily in the Sonoran and Chihuahuan deserts. Adults are mottled and blotched.

H.s. cinctum

Banded Gila Monster

resides primarily in the Mojave Desert. Adults have a broad double crossband.