Gila Monster, Heloderma suspectum

Gila Monster, Heloderma suspectum. Photograph taken within the confines of the Arizona-Sonora Desert Museum, Tucson, Arizona, February, 2022.

The Gila Monster, Heloderma suspectum, is a member of the Helodermatidae family of Venomous Lizards. The Helodermatidae family consists of two species placed in a single genera. The Gila Monster is one of three species of extant venomous toxicoferan lizards, along with the Mexican Beaded Lizard, Heloderma horridium, and the Komodo Dragon, Varanus komodoensis. The Gila Monster is known in Mexico as Monstruo Gila.

Gila Monsters are large, heavy bodied lizards. Their dorsal scales have a beaded appearance due to the presence of osteoderms (small bones) under their scales. Their heads are large, with small, black, beady eyes. Their bodies are distinctly marked with black and pink or orange patterns. On the Reticulated Gila Monster, Heloderma suspectum suspectum, the southern subspecies, their light markings are broken up to form a reticulated pattern. On the Banded Gila Monster, Heloderma suspectum cinctum, the northern subspecies, their light markings generally form an unbroken band across their back. Their tails are short, thick, and typically have thick black bands that wrap around the entire tail. The lateral and ventral sides of their face are solid black in color. Their bright color and highly contrasted pattern serves as aposematic coloration, warning predators that they are venomous and can defend themselves. Gila Monsters grow up to 55 cm (21.5 inches) in length. They weight between 0.7 kg (1 lb 8 oz) to 1.5 kg (3 lb 0 oz). 

Gila Monsters mate in the spring from late April to early June, when food is most abundant. Male-male combat occurs during breeding season as males compete for access to females. This combat involves repeated sequences of strenuous grappling postures where individuals use their heads and torsos in attempts to subdue each other. The winner is typically the one who can repeatedly force his opponent into an inferior position during a series of bouts. Combat can range from one or two bouts lasting only a few minutes, to a series of bouts extending several hours. The goal of combat is not to severely injure the opponent, rather, it is to show dominance over the opponent. Courtship and mating in the wild are assumed to take place primarily within burrows or rock shelters, so the exact details of these processes are not well documented. Females lay 3 to 13 leathery eggs in a shallow hole, which they cover with dirt, typically in July or August. The eggs are not buried very deep, so the heat of the sun incubates them. Incubation lasts for approximately 10 months and their young hatch the following year, from April to June. Gila Monsters are the only North American lizards that incubate their eggs over the winter. Hatchlings are about 16 cm (6.3 inches) long and can bite to inject venom upon hatching. They reach sexual maturity at three to five years of age. They have lifespans of twenty years in the wild and thirty-five years in captivity.

Gila Monsters spend most of their time underground in burrows or in rocky shelters. They are active in the morning during the dry season (spring and early summer) and later in the summer, they may be active on warm nights or after a thunderstorm. They are primarily solitary, but they may gather in communal areas and share shelters. During cold winter months, they brumate in their burrows until spring. Gila Monsters can bite quickly and hold on tenaciously. They may hold onto a predator for more than ten minutes. Rather than injecting venom through hollow fangs like venomous snakes, Gila Monsters have enlarged, grooved teeth in their lower jaw. When they bite, their powerful jaws chew the venom in through capillary action along the grooves of their lower teeth. Their venom is as toxic as that of a Western Diamondback Rattlesnake. Although, a relatively small amount of venom is introduced in a single bite. There is no antivenom for Gila Monster bites. Gila Monsters often raid nests to prey upon small birds and eggs. Additionally, they consume small mammals, lizards, frogs, insects, and carrion. In one meal, they can eat up to one-third of their body weight. Gila Monsters store fat in both their tails and bodies. Their low metabolic rates and ability to eat large meals combined with their capacity to store fat, allows them to go long periods without eating. It has been suggested that Gila Monsters can consume all the calories they need for a year in three or four large meals. Their top speed is only 2.4 kilometers (1.5 miles) per hour. 

Young Gila Monsters may be mistaken for the Western Banded Gecko, Coleonyx variegatus (much smaller, reaching  a maximum of 15 cm (6 inches) in total length (including the tail). Additionally, Western Banded Geckos are much lighter in color and lack the dorsal bead-like scales that are found on Gila Monsters. 

From a conservation perspective, the Gila Monster is considered to be Near Threatened with their populations in significant decline that has been attributed to habitat loss due to urbanization and agricultural development throughout the majority of their range. Concrete-lined canals barriers to their movement and they are subject to becoming road kill on busy highways. They are also targeted by illegal commercial and private collectors.