The Reclusive Gila Monster Packs a Venomous Punch

Gila monster
The Gila monster (Heloderma suspectum), which lives only in the southwestern United States, is the most venomous lizard we know of. Hal Beral/Getty Images

Rub against some manchineel trees and you're in trouble. Eat a wild blue dart frog and you're in bigger trouble. Both species are poisonous, meaning they administer toxic substances that can be inhaled, touched or swallowed.

Venomous creatures take a different approach. Jellyfish, rattlesnakes and other venom-wielders have to inject their targets with harmful cocktails, whether by fang, by stinger or by some other means.


For the Gila monster (Heloderma suspectum), a quick little bite won't do the trick. Instead, the largest lizard native to the United States envenoms attackers by chewing on them.

While Gila venom rarely kills human beings, the oral assault is still none too pleasant. After one scientist was gnawed on by a baby Gila monster, he compared the experience to getting repeatedly struck with a hammer.

The good news is Heloderma suspectum typically avoids people. Desert recluses through and through, Gila monsters spend most of their lives holed up in underground lairs. Yet the secretive creatures have been enlisted in the fight against diabetes, all thanks to the venom they carry.


Icons of the West

Gilas aren't totally unique. "The Heloderma genus may contain up to half a dozen species," biologist Bryan Fry, associate professor at the University of Queensland in Brisbane, Australia, says in an email. Besides the Gila monster, it includes outwardly similar reptiles called beaded lizards.

Herpetologists are struggling to get a definitive headcount. Many regions in North and Central America have distinctive-looking beaded lizard populations. To date, it's unclear if some of these animals represent separate species or not.


Various Heloderma lizards can be found from Guatemala to the Mormon Corridor. The Gila monster stands alone, however, as the only species on U.S. soil. This beast's natural range includes parts of New Mexico, Arizona, California, Nevada, Utah and the Mexican state of Sonora.

Oh and in case you were wondering, the name "Gila monster" is probably a nod to the Gila River in southern Arizona and New Mexico. But enough about geography.

Capable of growing up to 22 inches (56 centimeters) long, Gila monsters have tiny bones embedded in many of their scales, giving the lizards a pebbly appearance. As for the color scheme, it's classically southwestern, with orange to pinkish bands, dots or blotches popping out against a charcoal backdrop. Patterns may vary from one individual to the next.

In serpentine fashion, Gila monsters gather airborne taste particles on forked tongues. They share this trait with monitor lizards like the famed Komodo dragon.

Komodos and Gilas might have something else in common. You see, beaded lizards and Gila monsters are often considered the world's only venomous lizard species. However, Fry's research indicates that Komodo dragons (and other monitor lizards) pack their own form of venom. The idea has its critics, but wherever the truth may lie, reptilian oral secretions are well-worth studying.


Gila Monster's Venom: The Good and Bad

"Unlike [rattlesnakes], Gila monsters do not have fangs," says Arizona State University life scientist Dale DeNardo in an email. Rattler fangs are basically syringes, hollowed teeth designed to inject venom with rapid efficiency.

DeNardo explains that a Gila monster's venom is produced by "relatively large" glands around the lower jawbones. This toxic material comes oozing out as the animal chews. "Furthermore, to help [deliver] the venom components into the target, the teeth on the lower jaw are somewhat enlarged," he says. The pearly whites contain vertical grooves that help channel the substance.


"With each squeeze of the jaws, saliva — including venom — is released into the mouth and some of it moves via capillary action up the teeth and into whatever is being bitten," DeNardo says. "While not as sophisticated ... as rattlesnake venom delivery, it is still quite effective."

And here's a twist that should interest anyone with Type 2 diabetes. Among the many components of Gila monster venom is the peptide exendin-4. Chemically, it resembles GLP-1, a glucose-regulating hormone found in the human digestive tract.

The discovery of exedin-4 in Gila monsters led to the development of an FDA-approved drug for Type 2 diabetics. Sold as Bydureon or Byetta, this medication was first released in 2005 and is now used by more than 2 million people.

Gila monster
The venom of the Gila monster contains the peptide exendin-4, which chemically resembles GLP-1, a glucose-regulating hormone found in the human digestive tract. It's now being used to help treat people with Type-2 diabetes.
Mark Newman / Getty Images


Living Life Secluded

Wild Gila monsters are rather stingy with their venom. It's reserved for self-defense purposes, a weapon the lizards deploy against coyotes, birds of prey and overcurious humans. (Keep your hands to yourself, folks.)

On the flip side, there's usually no need for venom when Gilas go hunting. Because Heloderma suspectum eats things that can be subdued without it.


"Adult Gila monsters are near exclusive raiders of vertebrate nests ... eating quail eggs, rabbit pups, rodent pups, lizard eggs and tortoise eggs," says DeNardo. Less is known about the diets of baby Gilas, but they seem to have a taste for eggs laid by smaller lizards.

Hatchling Gila monsters were the focus of a 2018 paper DeNardo coauthored. The lizards are committed homebodies, rarely leaving the safety and comfort of burrows or rocky dens. Unable to warm themselves like humans do, Gila monsters derive heat from their surroundings. Their ideal body temperature is about 84 degrees Fahrenheit (29 degrees Celsius).

"In the summer, underground temperatures are very close to this ... throughout the day and night," DeNardo says. "Thus, the burrow provides a very good environment for the Gila monster."

Come wintertime, the thermometer plummets. Even though burrows tend to remain warmer than the outside air, they may drop to temperatures of just 54 degrees Fahrenheit (12 degrees Celsius) during the cold months.

These cold spells, while tolerable, pose logistical problems for infant Gila monsters. Females lay clutches of three to nine eggs in midsummer. Deposited underground, their eggs hatch during the fall — when the desert grows chilly.

By observing artificially warmed eggs and a wild clutch found near a construction site, DeNardo and his 2018 co-authors realized Gila monsters practice "overwintering." Food is hard to come by in the autumn season. So newborn babies stay put for months on end until the weather outside gets warmer and menu options increase. Only then do hatchlings seek out their first meals.

Now we'd be remiss if we didn't mention the prequel to egg-laying: mating behavior.

"If they come into contact with each other during the breeding season [early summer], males will fight," DeNardo says. "The battle is mostly a wrestling match, but they can inflict wounds with their bites, often on the tops of the head." As their bodies intertwine, each combatant tries to flip his rival over.

Full-contact sports aren't just for people, you know.


Gila Monster FAQ

What does a Gila monster eat?
The diet of a Gila monster is nearly exclusive to vertebrate nests including quail eggs, rabbit pups, rodent pups, lizard eggs and tortoise eggs.
Can a Gila monster kill a human?
A Gila monster typically avoids humans and rarely kills; however, their bite can be painful.
Are Gila monsters aggressive?
Gila monsters spend most of their time buried underground and typically don't approach humans. They are venomous creatures, but typically not aggressive.
How did the Gila monster get its name?
The name Gila monster is likely a nod to the Gila River in southern Arizona and New Mexico.
How long do Gila monsters live?
A Gila monster may live up to 20 years in the wild or even 30 years in human care.