Up until 2009, scientists thought the only venomous lizards in the world were 2 closely related species of helioderms–the Gila monster (Helioderma suspectum) and the Mexican beaded lizard (Helioderma horridum). That all changed when an Australian scientist, Dr. Bryan Fry, discovered Komodo dragon (Varanus komodoensis) venom glands. Formerly, scientists assumed bacterial infection from a Komodo dragon’s bite doomed its prey, but Dr. Fry’s new studies found that gaping wounds combined with blood clot-destroying venom caused the Komodo dragon’s prey to expire. Moreover, he examined the bacteria in a Komodo dragon’s mouth and found fewer species of toxic bacteria than are found in a human’s mouth–debunking the old hypothesis that septicemia caused its prey’s death. Water buffalo bitten by Komodo dragons do die of toxic infection, but they get the infection from retreating into fecal contaminated water holes. Smaller prey such as deer or pig actually succomb to the Komodo dragon’s venom. Since this discovery, other scientists have discovered venom glands in close to 100 species of lizards including other monitors (varanids), iguanas (iguanids), and dragon lizards (adamids).
All the species of lizards that have venom glands share a close ancestry with snakes and prehistoric mosasaurs. In many cases such as the mostly vegetarian iguanas, the venom glands seem to serve no purpose and are probably just vestigial. The helioderms share a close ancestry with Old World monitor lizards, but enough differences in anatomy along with the wide geographic disparity between the 2 have convinced scientists that Gila monsters and Mexican beaded lizards should be classified within their own unique family.
Geographical range of the 2 subspecies of Gila Monster. Helioderms are confined to America while varanids are restricted to Africa, Asia, and Australia.
Gila monsters are surprisingly active and able to climb trees and raid bird’s nests.
Above link is a youtube video of a confrontation between a ground squirrel and a Gila monster.
Mexican beaded lizard. Note the swollen tail. Helioderms live in underground burrows most of the year and survive on fat stored in their tails. No wonder they survived the K-T impact that wiped out the dinosaurs.
The most notable physical characteristic distinguishing helioderms from other lizards is the bony ossicles in their skin, a feature no other lizards have. Like the varanids, they are large lizards. Gila monsters grow to 2 feet long, while Mexican beaded lizards grow even larger to 3 feet long. Both spend most of their lives in underground burrows but emerge seasonally to feed and mate. They only eat 5-10 times a year, primarily upon bird’s eggs and nestlings, but they will also take small mammals, frogs, other lizards, carrion, and insects. They will climb trees to raid bird’s nests. Their venom is not used in hunting. Most of their prey is small enough to simply be crushed between their jaws. Their venom is likely a defensive adaptation, and their distinctive coloration serves as a warning to predators. Nevertheless, they occasionally fall victim to coyotes and birds of prey.
Helioderm venom contains exanatide, an enzyme resembling the peptide that helps humans digest glucose. It’s an ingredient in a drug used to help manage diabetes. Helioderms rarely bite humans, and there have been no known fatalities, though the bite causes victims to suffer sickness and discomfort that generally lasts 5 days.
Species of helioderms lived during the Cretaceous when they probably fed upon dinosaur eggs and hatchlings. Various unknown and poorly known extinct species of helioderms lived all across southeastern North America and probably most of the continent during the Miocene. Fossils of Helioderma texana have been found in Texas. A jawbone and a few leg bones of helioderms were found at 2 sites in north Florida, but it wasn’t enough material to establish what specific species the specimens belonged to. At the Gray Fossil Site in Tennessee, scientists identified the bony ossicles of a helioderm–also insufficient evidence to identify the specimen at a species level. Seasonally subfreezing climates began to occur across most of North America early during the Pliocene. This eliminated much habitat for helioderms and probably resulted in the extinction of several species. Today, the only surviving species of this once more widespread genera live in relatively frost free regions of southwestern North America.
Fossil bony ossicles of a helioderm lizard found at the Gray Fossil Site in Tennessee.