12 Fascinating Facts About Gila Monsters

They’re the largest native lizards in the United States. Their long-lasting bites are incredibly painful, and their venom is the basis for a popular type 2 diabetes medicine. Meet the Gila monster.

1. Venomous lizards like the Gila monster are more common than we once thought.

Earth is home to more than 4600 lizard species. Until recently, scientists believed that only two of these—the Gila monster and its close relative, the Mexican beaded lizard—produced venom. During the mid-2000s, biochemist Bryan Fry and colleagues identified venom-secreting glands within the mouths of various iguanas, alligator lizards, and monitor lizards. Even the Komodo dragon (a type of monitor lizard) is venomous.

2. Getting bitten by a Gila monster is not recommended.

A frightened Gila monster will open its purple-gummed mouth and hiss—a clear signal to back off. If an aggressor fails to retreat, Gila monsters will bite—and the venom’s delivery can be incredibly painful. With its powerful jaws, the Gila monster will clamp down on the victim and gnaw at the flesh for up to 15 minutes, which draws venom from glands situated in its lower jaw. Its venom is rarely fatal, but it can cause serious pain, swelling, nausea or vomiting, chills, fever, or fainting. If you ever get bitten by a Gila monster, the worst thing you could do is lift it off the ground—this will only make it hang on. Try dunking its head underwater instead.

3. Gila monsters store fat in their tails.

A Gila monster can’t lose and re-grow its tail as many other lizards do, but it’s still useful. They maintain fat reserves inside their tails. In conjunction with a low resting metabolic rate, their fat store enables Gila monsters to survive on as few as three or four large meals per year.

4. Gila monsters are the biggest native lizard in the U.S.

Adult Gila monsters can grow to 2 feet long and weigh 5 pounds or more, making it our biggest native lizard. But Gila monsters look puny next to invasive green iguanas and Nile monitors, both of which have become endemic in Florida and can grow up to 5 feet long.

Gila monsters are most commonly encountered in southern, central, and western Arizona. Its name stems from the Gila River basin, which encompasses much of the state. You might them in neighboring regions of California, Utah, Nevada, and New Mexico; and in Sonora and western Chihuahua in Mexico.

5. Eggs are one of the Gila monster’s favorite foods.

Gila monsters don’t use their venom to subdue prey (it’s mostly for defense). They typically eat small mammals, birds, and lizards. Eggs are also on the menu. Gila monsters swallow smaller eggs whole, but they break open larger ones to lap up the goo inside. They’ve been known to climb trees to get at nesting birds and hatchlings.

6. There may be two subspecies of Gila monster.

Not all Gila monsters look alike. Newborns bear alternating bands of black and pinkish-orange. Some individuals retain this pattern as adults. Others develop a marbled appearance, with blotches of black and orange. These two phenotypes likely represent subspecies, which scientists have named banded and reticulated Gila monsters.

7. Male Gila monsters like to wrestle.

From April through July, male Gila monsters wrestle to win the right to breed. Rivals will entwine their bodies and try to pin each other to the ground. After one of them succeeds, the contestants separate and then go several more rounds. Gila monster expert Daniel D. Beck once observed a duel that went on for 13 rounds over three hours.

Victors win a territory where they can seek mates. After mating, a female Gila monster lays up to 12 eggs in late summer, which hatch about 10 months later.

8. Gila monsters spend up to 95 percent of their lives underground.

Out of sight, out of mind. According to Beck’s research, Gila monsters pass the time underground, protected from the desert sun. Ideal shelters include abandoned mammal burrows, pack rat nests, and crevices beneath large rocks.

9. Gila monsters have been misunderstood for a long time.

In 1890, a Scientific American article suggested an explanation for Gila monsters’ supposedly awful breath: “The breath is very fetid, and its odor can be detected at some little distance from the lizard … It is supposed that this is one way in which the monster catches the insects and small animals which form a part of its food supply—the foul gas overcoming them.” ‘Why did Gila monsters need to kill prey with halitosis?’ you may ask. Back in those days, Gila monsters were thought to lack anuses, so they expelled waste through their mouths. (For the record, they do in fact have anuses.) Another popular myth suggested that when a Gila monster bit someone, it wouldn’t let go until either sundown arrived or a thunderstorm rolled in.

10. Specialized bladders help Gila monsters survive droughts.

Gila monsters evolved bladders that store urine for later use. During droughts, water stored in the organ recirculates throughout the body to maintain hydration. No other lizard species is known to have this ability, though some turtles and amphibians do. Because of this redistribution system, Gila monsters can go 81 days without drinking.

11. Gila monster venom led to a diabetes drug.

In 1992, endocrinologist John Eng found that Gila monster venom contains a peptide he called exendin-4. In humans, it increases the production of insulin. Though our bodies release a similar compound, it usually breaks down in a few minutes. Exendin-4 can keep working for hours.

Extendin-4 is the key ingredient in the popular diabetes drug exenatide (brand name Byetta). Eng is obviously a Gila monster fan. “It really is a beautiful lizard,” he once said. “Like many other animal species, it is under pressure from development and other environmental concerns. The question is, what other animal has something to teach us that can be of future value? And plants, too? We will never know their value if they are gone.”

12. The most iconic Gila monster movie of all time actually used another lizard.

The 1959 drive-in classic The Giant Gila Monster stars a 70-foot Gila monster that roams the countryside gobbling up hitchhikers, truck drivers, and amorous teenagers. It features a real lizard—a Mexican beaded lizard instead of an actual Gila monster. Audiences probably didn’t know the difference: the Mexican beaded lizard is a very close relative of the Gila monster. Both species belong to the same genus and have overlapping ranges.

A version of this story was published in 2016; it has been updated for 2021.