Helodermatids range from the southwest deserts of the USA southward to southern Mexico and Guatemala. There are two living species in the genus Heloderma, but the fossil record suggests the family was more widespread, and inhabited Nebraska, Colorado, and Wyoming. Ecological studies have shown that the gila monster, Heloderma suspectum, spends almost the entire year underground. It emerges in the early mornings of the spring and summer, and the evenings of late summer, particularly after a storm. Helodermatids feed on the eggs of reptiles and birds as well as neonate mammals. They have a low metabolic rate which allows them to go for long periods of time without eating, and extra calories in the form of fat are stored in their tail. Venom is produced in modified dental glands and flows along open grooves in their teeth. Recent molecular studies suggest that helodermatids are part of an ancient clade of squamates that included the ancestral snake (see Fry et al., 2006 Nature 439:584-588). For more details on helodermatids see Beck 2005, Biology of Gila Monsters and Beaded Lizards, University of California Press.
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