Heloderma, or beaded lizards, are large, stocky, slow-moving reptiles that prefer semi-arid climates. Their tails are short and used as fat storage organs. They are covered with small, non-overlapping, bead-like scales, with osteoderms on the undersides of their bodies. All species are dark in color, with yellowish or pinkish markings. Heloderma is the only living genus of the family Helodermatidae. Beaded lizards are venomous. Venom glands are located in their lower jaws, unlike snakes' venom glands, which are located in their upper jaws. Also, unlike snakes, beaded lizards lack the musculature to inject venom. The venom is typically used only in defense, rather than in subduing prey, and the lizard must chew its victim to work the venom into the flesh. Venom glands are believed to have evolved early in the lineage leading to the modern helodermatids, as their presence is indicated even in the 65-million-year-old fossil genus Paraderma. Venom production among lizards was long thought to be unique to this genus, but researchers studying venom production have proposed many others also produce some venom, all placed in the clade Toxicofera, which includes all snakes and 13 other families of lizards. However, except for snakes, helodermatids, and possibly varanids, envenomation is not considered medically significant for humans. Helodermatids are carnivorous, preying on rodents and other small mammals, and eating the eggs of birds and reptiles. They are oviparous, laying large clutches of eggs.
Members of the genus Heloderma have many extinct relatives in the Helodermatidae whose evolutionary history may be traced back to the Cretaceous period, such as Estesia. The genus Heloderma has existed since the Miocene, when H. texana lived, and fragments of osteoderms from the Gila monster have been found in late Pleistocene deposits near Las Vegas, Nevada. Because the helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. Although the beaded lizards and the Gila monster appear closely related to the monitor lizards of Africa, Asia, and Australia, the wide geographical separation and unique features not found in the varanids indicates they are better placed in a separate family. The type species is Heloderma horridum, which was first described in 1829 by Arend Weigmann. Although he originally assigned it the generic nameTrachyderma, he changed it to Heloderma six months later, which means "studded skin", from the Ancient Greek words hêlos —the head of a nail or stud—and derma, meaning skin.
H. horridum, H. exasperatum, and H. suspectum are frequently found in captivity and are well represented in zoos throughout much of the world. They are often bred for the exotic pet trade and can command high prices. The other two species of Heloderma, H. alvarezi and H. charlesbogerti, are extremely rare, and only a few captive specimens are known.
