Heloderma horridum

The Mexican beaded lizard is one of the world's only two venomous lizards known to humans. The lizard is very similar in appearance and basic biological make-up to its venonmous, slightly smaller, American counterpart; the Gila monster (Heloderma suspectum). Mexican beaded lizards (as name implies) have scales which are similar in appearance to beads or pebbles and appear (depending on the subspecies) as a dull cream, dark gray, or black colour with or without yellow blotches. The scales are large and bony on the lizards' backs but flat and regularly arranged on the belly. The lizards' heads are broad and slightly flattened, connected to their elongated bodies by very short necks. The Mexican beaded lizards' tails are rounded and thick. The legs are short but relatively powerful, with five clawed toes on each for digging. The lizards appear to move slow and awkardly but, when provoked, can move quickly. The average length of a Mexican beaded is about one meter (three feet).

Also as the name implies, Mexican beaded lizards can be found in warm and dry habitats primarily in northern and southwestern Mexico. The lizards can also be found in northern Guatemala. During the day, the lizards burrow into sand to avoid the intense heat. The lizards can be frequently found in burrows created by small mammals that have since moved on (or died), usually near a permanent source of water.

Mexican beaded lizards have powerful jaw muscles, capable of crushing their prey, which they use to deliver the venom produced by two glands located in their lower jaw. The lizards flick their tongues into the air, similar to snakes, in order to smell their surroundings. The teeth of these creatures are located a bit inward from the edge of their jaws and are curved slightly backward. The lower jaw's teeth have grooves through which the venom the lizards create flows into whatever they bite. In order for the venom to be delivered in this manner, a Mexican beaded lizard must chew on its victim. As a result, people or large animals bit by a Mexican beaded lizard may find that the lizard is reluctant to let go of them. Their mouths can be pried off (remember though that they are quite strong) or the lizards can be induced to unlock their jaws by having a flame placed under their mouths. The effects of the venom of these creatures on larger animals includes bleeding (duh), swelling, and extreme pain in the part of the body biten, hypotension, excessive salivation, tearing from the eyes, frequent urination and defecation, and the inability to properly vocalize.

Fortunately, Mexican beaded lizards aren't aggressive creatures. Normally the reptiles are rather docile and only use their venom to immobilize their prey or defend themselves. If a Mexican beaded lizard becomes agitated it will give warning by hissing. Most reported bites are received by professional handlers of the creatures. Mexican bearded lizards feed on smaller reptiles, such as other lizards and snakes, the eggs of various wildlife, small rodents, ground-nesting birds, insects, and spiders. The lizards store fat in their large tails, which is metabolized in the event that food becomes scarce. Predators of the Mexican bearded lizard include large mammals and birds of prey.

Mexican beaded lizards mate in spring. In the summer, fertilized females will lay and bury between one and thirteen eggs about five inches in the sand. The incubation period of the eggs is between 117 and 130 days. These lizards can live to be almost 34 years old, which is quite a long time for a lizard. Unfortunately, the lizards are considered an endangered species under the Convention on International Trade in Endangered Species. Both the Mexican and Guatemalan governments have placed the animals under protection and a special permit is required to export the creatures. The primary cause of the Mexican beaded lizard's reduced numbers is the destruction of its habitat.