The Mexican beaded lizard (Heloderma horridum) is a species of lizard in the family Helodermatidae, one of the two species of venomous beaded lizards found principally in Mexico and southern Guatemala. It and the other memberof the same genus, the Gila monster (Heloderma suspectum), are the only lizards known to have evolved an overt venom delivery system. The Mexican beaded lizard is larger than the Gila monster, with duller coloration, black with yellowish bands. As it is a specialized predator that feeds primarily upon eggs, the primary use of its venom is still a source of debate among scientists. This venom has been found to contain several enzymes useful for manufacturing drugs in the treatment of diabetes, and research on the pharmacological use of its venom is ongoing.
Threatened throughout its range by overcollection and habitat loss, it is a CITES protected species. The Guatemalan beaded lizard (H. charlesbogerti) is one of the rarest lizards in the world, with a wild population of fewer than 200.
Adult beaded lizards range from 57 to 91 cm (22 to 36 in) in length. They are substantially larger than the Gila monster, which only reaches lengths of 30 to 56 cm (12 to 22 in). The snout-to-vent length of a beaded lizard averages 33 to 48 cm (13 to 19 in). The average body mass of an adult beaded lizard is 800 g (1.8 lb), about 45% heavier than the average mass of a Gila monster, with large specimens exceeding 2,000 g (4.4 lb). Maximum weight known is 4,000 g (8.8 lb) Although males are slightly larger than females, the beaded lizards are not sexually dimorphic. Both males and females are stocky with broad heads, although the males’ heads tend to be broader. The beaded lizards’ scales are small, beadlike, and not overlapping. Except for the underside, the majority of its scales are underlaid with bony osteoderms.
Their base color is black and marked with varying amounts of yellow spots or bands, with the exception of H. alvarezi, which tends to be all black in color. The beaded lizards have short tails, which are used to store fat so they can survive during months of estivation. Unlike many other lizards, this tail does not regenerate if broken. Beaded lizards have forked, pink tongues that they use to smell, with the help of a Jacobson’s organ; they stick their tongues out to gather scents and touch them to the opening of the organ when the tongue is retracted.