Venomous Lizards

The are only two venomous lizard species in the world and both live in this general region. The Gila Monster and the Mexican Beaded Lizard.

Gila Monster



The Gila monster (Heloderma suspectrum) averages 9- to 14 inches long, but can get up to 24 inches long. It ranges throughout the Sonoran Desert region below about 4,000 feet elevation. Its range included parts of Arizona, California, Nevada, New Mexico and northern Sinaloa, Mexico. It feeds on eggs of reptiles and birds, and takes small rodents, such as pack rats, and baby mammals such as rabbits and squirrels. Mating occurs in spring or early summer. Observations in the wild suggest an incubation period of 10 months, but captive lizards produce hatchlings in about 4 months. There are two subspecies of Gila monster: the reticulate Gila monster (H. suspectum suspectum) and the banded Gila monster (H. suspectum cinctum). The reticulate Gila monster lives in the southern region part of the range, while the banded Gila monster occurs primarily in the Mojave Desert. The reticulate Gila monster tends to have its lighter markings broken up by dark scales, giving it a reticulated pattern, while the banded Gila monster generally has more unbroken bands of lighter scales.

The Mexican beaded lizard (Heloderma horridum) averages 2- to 3 feet long. It ranges throughout southern Sonora. It feeds on birds, small mammals, other lizards, and eggs. It can climb trees. Little is know about mating in the wild. Captive lizards mate in August, produce eggs in October which hatch from January to March.

Both lizards have forked tongues which they use to sample the air to find prey. They are nocturnal hunters. The thick tails store fat. Both lizards are dormant during the winter, and both spend about 95% of their time underground. A young gila monster can eat the equivalent of 75% of its body weight at one meal; an adult can consume 50% of its body weight at one meal.

The venom is a neurotoxin similar to that of a coral snake. It is released from glands in the lower jaw and conducted through grooved teeth by capillary action and chewing. The lizards don’t have fangs; they just chew on you for a while, without letting go. The lizards are very tenacious. Study of the venom suggests it evolved as a defensive toxin. Bites to humans are rare. Victims have reported intense pain within five minutes of the bite and the pain may persist for several hours. Weakness, dizziness, perspiration, nausea, chills, and fever have been reported. No human deaths have been reported.

Research shows more than a dozen peptides and proteins have been isolated from the Gila monster’s venom, including hyaluronidase, serotonin, phospholipase A2, and several kallikrein-like glycoproteins responsible for the pain and edema caused by a bite. Four potentially lethal toxins have been isolated from the Gila monster’s venom, including horridum venom, which causes hemorrhage in internal organs and bulging of the eyes, and helothermine, which causes lethargy, partial paralysis of the limbs, and hypothermia in rats. However, the constituents most focused on are the bioactive peptides, including helodermin, helospectin, exendin-3, and exendin-4. Most are similar in form to vasoactive intestinal peptide (VIP), which relaxes smooth muscle and regulates water and electrolyte secretion between the small and large intestines. These bioactive peptides are able to bind to VIP receptors in many different human tissues. One of these, helodermin, has been shown to inhibit the growth of lung cancer.

Although Gila Monsters have a fearsome reputation, they are sluggish and pose little threat to humans, just don’t try to pet one or pick one up.

Myths about Gila monsters include: how the Gila monster is venomous because it lacks an anus and “all that stuff went bad in there.” Or about how “once they bite down, they can’t let go until sundown,” or “if one bites you, don’t worry, it has to turn upside down to get the venom in you.” (ASDM).

In 1952 the Gila monster became the first venomous animal in North America to be afforded legal protection; it is therefore illegal to collect, kill, or sell them in Arizona.


  1. Again, very interesting and informative, Jonathan.  We have the occasional Gila Monster passing through our yard.  While they move slowly even in the presence of humans, I noticed when I stood up quickly after crouching for a photo that they can move pretty darn fast when alarmed.  They are another reason we are blessed to live here.

  2. First heard of the gila monster by reading that cute children’s book, “Gila Monsters Meet You at the Airport” by Marjorie Weinman Sharmat.  Never seen either of these in the wild, only at the Arizona Sonora Desert Museum in those glass cages.

  3. Very good article!

    Now, let me ask you a question you might be able to help me with. I thought Gila Monsters were like Komodo dragons in so far as their mouth and saliva carry tons of bacteria as apposed to actually being venomous. But from all your info, this obviously isn’t true?

    1. Venom in the gila monster and venomous snakes is a derivation of saliva.  The venom starts the digestion process as well as paralyzing the prey.  The Komodo dragons just have dirty mouths full of bacteria.

      1. Hello and thank you for your reply…
        Okay, so then Gila Monsters and venomous snakes basically have a type of tube, if you will, that injects the venom, even though the gila monsters don’t actually have teeth or fangs? (sorry to keep bothering you, I’m just really into reptiles)
        Is their mouth similar to a blue tongued skink where they just have a bony plate in their mouth that crushes their prey, but, unlike the skink the bony plate extracts venom?

  4. I get Gila Monsters in my garage.  Being protected in Arizona, yes, Arizona protects the criminal, ops sorry.  I called an animal control person to pick up the monster. 

  5. The past couple of years they’ve determined the komodo dragons are in fact also quite venomous.
    In the past several years they have determined that there are many other lizards and snakes previously thought non-venemous that are in fact mildly venemous.  But the venom isn’t potent enough to effect humans. A guy studying evolution of all things determined this. Nobody had thought to look before.

  6. They most definitely have teeth. but they don’t have snake-like fangs, that inject venom.

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