What to Know About Mexican Beaded Lizards

Reviewed by Vanesa Farmer, DVM on November 24, 2022

Mexican beaded lizards are a species of large lizards closely related to Gila monsters. They live in Mexico, and like the Gila monster, have a venomous bite. Here are some fascinating beaded lizard facts.

The Mexican beaded lizard, Heloderma horridum, is a species from the beaded lizard family Helodermatidae. The Helodermatidae family has only one genus of lizards still alive today, the genus Heloderma. This genus contains Mexican beaded lizards, but also their more famous cousin, the Gila monster (Heloderma suspectrum).

Originally, Mexican beaded lizards and Gila monsters were the only species in the genus Heloderma, with the Mexican beaded lizard having several subspecies. After genetic testing, this was reassessed, and in 2013, each of the subspecies of the Mexican beaded lizards was promoted to its own species. These are: 

  • Heloderma alvarezi, formerly Heloderma horridum alvarezi, also called the black beaded lizard and Chiapan beaded lizard.
  • Heloderma charlesbogerti, formerly Heloderma horridum charlesbogerti, commonly called the Montagua Valley beaded lizard and the Guatemalan beaded lizard.
  • Heloderma exasperatum, formerly Heloderma horridum exasperatum, also called the Rio Fuerte beaded lizard.

Mexican beaded lizards are usually larger than their Gila monster cousins. Females can weigh up to nearly 4.5 pounds (2 kilograms) and grow to 30 inches long (76 centimeters). Males can weigh up to 9 pounds (4 kilograms) and grow to 35 inches long (90 centimeters). Nearly half the lizard’s length comes from its tail.

The bodies of Mexican beaded lizards are thick and cylindrical, with long tails. They have short, muscular legs and broad, flat heads. The top of their body is covered with hard, bead-like scales, while their belly is covered in softer scales. These lizards are mostly brown or dark black with yellow spots.

As its name suggests, the Mexican beaded lizard is native to Mexico. They prefer to live in thorny scrub and forests in central and western Mexico. Mexican beaded lizards can spend up to 95% of their time underground, often living in abandoned mammal burrows, in tunnels they dig, or under rocks.

Mexican beaded lizards are carnivores, meaning they eat meat. Their diets often consist of:

  • Birds
  • Eggs of birds and reptiles
  • Frogs
  • Insects
  • Lizards
  • Rodents
  • Small mammals

They swallow all their food whole, except for eggs, which they break open first. Mexican beaded lizards store fat in their tails. When food is scarce, they can metabolize this fat for energy.

Mexican beaded lizards mainly hunt at night. They use their forked tongues and the sensory cells in their nose to find prey. These lizards play a crucial role in desert pest control, helping keep these ecosystems balanced.

Mexican beaded lizards have a long lifespan, often more than 30 years. 

Both males and females usually reach sexual maturity when they’re about two or three years old. About two months after breeding, the female lays anywhere from three to 13 eggs, burying them in the ground. She then abandons the eggs. About six months later, the eggs hatch. The hatchlings fend for themselves. They have few natural predators, mainly coyotes and some raptors, giving them a good chance of survival.

Lizards in the family Helodermatidae, like the Mexican beaded lizard and Gila monster, are currently the only venomous lizards in the world. While both creatures deliver nasty bites with painful venom, these bites are normally not fatal.

Mexican beaded lizards don’t bite humans often, and when they do, it’s typically in self-defense. Before they bite, they’ll display warning signs, like gaping and hissing. Most bites to humans have happened when the human is handling the Mexican beaded lizard.

Unlike venomous snakes, Mexican beaded lizards don't have fangs. Instead, they have a few grooved teeth that deliver their venom. These teeth are on the lower jaw, near the venom glands. The Mexican beaded lizard must bite down hard on its prey, and the venom is delivered as the lizard uses a chewing motion. To pass the venom, the lizard needs to be able to hold on to its prey. As a result, the Mexican beaded lizard has a very strong bite.

When a Mexican beaded lizard bites small prey, the prey will first be unable to sit or stand upright. Then, it will start to feel sleepy and become paralyzed. It will become unable to breathe, and its heart will give out, causing it to die. 

In humans, Mexican beaded lizard bites are unlikely to be fatal but still require medical attention. Recorded symptoms of Mexican beaded lizard bites include:

  • Dizziness
  • Excessive sweating
  • Lethargy
  • Low blood pressure
  • Low heart rate
  • Low oxygen
  • Severe pain
  • Swelling around the bite as well as the lips and tongue
  • Tingling or prickling around the affected area

Treatment for a Mexican beaded lizard bite is supportive to help relieve symptoms. Doctors will monitor your vitals like your heart rate, blood pressure, and oxygen and may prescribe medications to treat pain and reduce swelling.

Most symptoms will resolve within a couple of days. If you’ve been bitten by a Mexican beaded lizard once and then are bitten again, you may be more likely to have an anaphylactic reaction. This is a severe allergic reaction that can cause you to go into shock.

Due to their venom, Mexican beaded lizards should not be kept as pets.

While the Helodermatidae family is down to one genus now, scientists have found evidence of other creatures that belong to this family but are now extinct. Some of these species may have existed as early as the Late Cretaceous, which ended 65 million years ago. These now-extinct species include:

  • Estesia: Estesia fossils have been found in the Gobi Desert. Its teeth indicate that it may have been venomous like its modern-day counterparts.
  • Eurheloderma: Eurheloderma fossils have been found in France.
  • Gobiderma: Gobiderma fossils have been found in the Gobi Desert like those of Estesia. It may have looked similar to modern Heloderma lizards but with a more elongated skull.
  • Lowesaurus: Lowesaurus fossils have been found in the U.S.
  • Paraderma: Paraderma fossils have been found throughout the U.S. and Canada.