The Gila monster is the largest lizard in the United States, and one of the few species of venomous lizard in the world. It has a stocky body with a large head and a short, fat tail. The skin consists of many round, bony scales, a feature that was common amongst the dinosaurs but is unusual in today's reptiles. Gila monsters have a striking bright pink and black colouration and the two subspecies can be distinguished by their different patterns; the banded Gila monster (Heloderma suspectum cinctum) has a band of light markings along the back whilst in the reticulated Gila monster (Heloderma suspectum suspectum) these light marks are joined in a network. With their venomous bite and elusive nature, these lizards have inspired many myths over the centuries.
Classified as Near Threatened (NT) on the IUCN Red List.
This species is never very abundant, but its abundance varies greatly. It is represented by well over 100 collection/observation sites that are well distributed throughout the range (Campbell and Lamar 2004). The total adult population size is unknown but is probably at least several thousand; the species is fairly common in at least some parts of the range. Beck (1985) estimated that the population in Utah included 450 to 800 individuals, down from an estimated 2,000 to 5,000 before the 1930s. It is probably declining even more seriously in Mexico.
Occupied vegetation types include desert grassland, Mohave and Sonoran desert scrub, and thorn scrub (Sonora); less often oak or pine-oak woodland. In Mexico, it occurs on lower mountain slopes and adjacent plains and beaches, sometimes in irrigated areas. Canyon bottoms, arroyos (dry creeks), and rocky slopes may support relatively dense populations in some parts of Arizona and Sonora.
The majority of the Gila monster's range is in western and southern Arizona, south to southern Sonora in Mexico, although populations are also found in restricted areas of California, Nevada, Utah and New Mexico. Of the subspecies, the banded Gila monster occupies the northern extent of the species' range. The name 'Gila monster' is derived from the Gila River Basin in Arizona, part of this species' range.
As an adaptation to their harsh desert environment, Gila monsters spend a large proportion of their time underground in burrows, hibernating during the winter and sheltering from the midday sun in the scorching summer months. The lizards emerge from hibernation in spring and the majority of their activity occurs in the following three-month period. Mating may take place from April to June; males 'wrestle' to assert dominance. Females then lay their clutch of up to 12 eggs in late June or August. Eggs are laid in depressions dug into the soil and unusually remain incubating underground throughout the winter, hatching the following spring.
In springtime, Gila monsters are active during the day, although they are mainly above ground in the morning and late afternoon to avoid the midday heat. These lizards feed on eggs, young birds and rodents, as well as lizards; juveniles are able to consume over 50% of their body weight at one time. Gila monsters are able to survive for months without food as they store fat in their particularly large tail. The infamous venomous bite of the Gila monster is used as a defensive measure rather than to attack prey. If threatened, these lizards will back away hissing with their mouth open, and if provoked they attack surprisingly quickly with a bite that can be extremely painful to humans, although it is rarely life-threatening
Populations have been exploited (illegally) by commercial and private collectors, and they have suffered from habitat destruction due to urbanization and agricultural development (New Mexico Department of Fish and Game 1985). Concrete-lined canals are barriers to movement, as are busy highways. Mortality on roads likely is increasing as traffic volume increases on established highways and new roads are built. The most important reason for the decline is habitat loss resulting from development. It is probably decreasing in southern Sonora due to expanding commercial agriculture.
Gila monsters are protected throughout their range, first receiving protection in 1952 in Arizona when they were the first venomous reptiles to receive such legislation. Over 300 individuals exist in captivity in the United States, and with greater understanding of these elusive lizards many of the common myths and superstitions around them have been surmounted. It is hoped that conservation measures will allow this a colourful desert-dweller to persist despite its depleted habitat.