Heloderma horridum
Mexican beaded lizard

Heloderma horridum is found throughout central and western Mexico from latitude 25, southward to northern Central America.

Heloderma horridum is found in semi-arid rocky regions. The areas are sparsely vegetated canyon bottoms, open forest, and washes. These lizards are often found on rock ledges.

Adult female Mexican beaded lizards grow to about 76 cm in length and weigh 1.4 to 2 kilograms. Males are slightly larger, growing to 90 cm and weighing up to 4 kilograms The tail is the longest portion of the lizard, nearly 50% of the total length.

Beaded lizards have a cylindrical body with a long, thick tail. The head is wide and flat, and the legs are short and strong. Large, hard scales cover the top of the lizards' body, while the belly side is coated with soft scales.

The scales are bead-like and are predominately dark black or brown on the top and bottom of the lizard. Yellow spots are scattered along the tail and the neck. Younger individuals have thicker bands and larger blotches of yellow on their bodies when compared with the adults.

This species and the other species of Heloderma, H. suspectum (the Gila monster) are the only lizards known to be venomous. They have a few special grooved teeth to deliver their venom when they bite.

The grooved teeth are on the lower jaw bones, along with the venom glands, which are arranged at the rear edge. The venom passes from these glands through a channel to the roots of the grooved teeth. The venom is drawn into the wound by capillary action as the lizard chews its prey.

This species hides in self-dug or pre-existing burrows during the day and becomes active at night. When the lizard first leaves its burrow it moves slowly and clumsily but as the night progresses it becomes more aggressive. In this more active state, the Mexican Beaded Lizard can "turn and snap with the agility of an angry dog" (Ditmars, 1936).

The Mexican Beaded Lizard uses its venom not only to kill its prey, but also to subdue potential predators such as humans, coyotes, and raptors. The lizard also gapes and hisses to fend off its enemies.

In the adult stage, the Mexican Beaded Lizard is carnivorous. Its diet consists of small mammals, birds, lizards, frogs, insects, and eggs of birds and reptiles. All their prey is swallowed whole, except for eggs, which are broken first.

When food is scarce, the Mexican Beaded Lizard lives off fat reserves in the tail. Fat is stored in the tail of the lizard making it appear swollen. After the fat reserves are used up the tail appears thin again.

Even though a bite from Heloderma horridum is not usually life threatening to humans, the wound must still be medically treated. This can be considered a negative impact upon health care costs for treatment. Unprovoked attacks by this species on humans are extremely rare, and may never occur. Nearly all documented bites occured when the victim tried to handle or disturb an animal.

Beyond its role in its ecosystems, this species has little positive effect on humans. Some animals are sold in the pet trade.

Humans are not only a threat to the Mexican Beaded Lizard because they kill them for fear of their venom, but are also destroying the habitat upon which these lizards depend.

Therefore Heloderma horridum is protected under the Convention on International Trade in Endangered Species of Wild Flora and Fauna (Appendix II) and are classified as "Vulnerable" by the International Union for the Conservation of Nature and Natural Resources (IUCN).

When the Mexican Beaded Lizard inflicts a vigorous bite, the victim goes through many stages. The first stage is the inability to sit or stand. The animal becomes drowsy and then experiences paralysis. Respiration becomes slow and labored until the heart exerts an increase of activity. The arterial pressure begins to take a great fall due to vascular dilation and the prey dies. Although the venom may have these effects on its small prey, bites are rarely fatal to humans in good health.