Heloderma

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.