Weird & Wonderful Creatures: Gila Monster

Heloderma suspectum. Photo Credit: H. Zell licensed under CC BY-SA 3.0, via Wikimedia Commons.

The Gila monster (Heloderma suspectum) is the largest native venomous lizard in North America. It grows up to two feet in length and five pounds in weight.

Native to deserts and near-desert areas of southwestern United States and northwestern Mexico, the Gila monster is named after the Gila River Basin in Arizona and New Mexico, where the lizards were once plentiful. They tend to be found in rocky brush and mountainous foothills and avoid open spaces. They live in burrows, where they spend 90% of their time. They hibernate during the winter and shelter there during the heat of the midday sun.

This slow, stout lizard has round, beadlike scales called osteoderms. They're mostly black with spots or bands of pink, orange, or yellow, and scientists theorize that these bright colors (as well as their loud hiss) may help warn off predators who might be tempted to accost them.

Gila monsters use their venom mostly for self-defense, against predators such as coyotes, raptors, and humans, rather than for hunting. They don't inject venom when they pierce another animal with their teeth, like a snake might. Instead, they make a mild neurotoxin in their lower jaw, which run through channels in their wide teeth. The lizards gnaw on their victim, drawing the toxin further into a wound and can hold onto their prey for up to 15 minutes. While the venom isn't fatal to humans, it is quite painful. It also contains a protein, exendin-4, that can stimulate insulin production when introduced to the human bloodstream. Researchers have created a synthetic version, which is now used as a popular Type 2 diabetes drug.

Gila monsters eat infrequently in the wild, only 3-4 times a year, at which time they will consume eggs, as well as small birds and mammals, lizards, frogs, insects, and carrion. When they do eat, they consume up to a third of their body weight in one sitting. They store extra fat in their stout tail for months when food is hard to come by. 

It's not only eating that Gila monsters do infrequently. As desert dwellers, they have evolved to have a specialized bladder that will retain water, which can recirculate water through the body. This allows them to go up to 81 days without drinking.

Gila monsters live up to 20 years in the wild. They tend to be solitary, mostly socializing during the spring mating season. During this time, males engage in feats of strength in pursuit of preferable breeding territories by wrestling about each other, which can go on for multiple rounds and last several hours. The female lizard lays 2-12 eggs, which they bury underground during the winter. After 4 months, they hatch and the young live on their own.

Gila monsters are classified as near threatened, predominantly due to human encroachment into their habitat. They are protected by state law in Arizona.

Learn more about lizard mating habits in the Lizard Mates Science Update, as well as the connection between their eggs and temperature.

You can find out about other types of lizards, including the Mwanza flat-headed rock agama and legless species. There is information on other reptiles, too. You can read up on World Turtle Day, Gecko Feet, Everglades Pythons, Giant Snake, and Snakes in a Cave.



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