Behavior

Gila monsters and beaded lizards spend more than 95% of their time hidden within shelters (rocky crevices, burrows, pack rat middens, and trees). When active on the surface, however, they can travel long distances—more than 0.6 mi (1 km) in search of food and mates. Field studies using ra-diotelemetry have shown that both species are primarily diurnal. The specific timing of activity varies among individuals, seasons, and geographic locations.

During the breeding season, Gila monsters and beaded lizards perform spectacular ritualized male-male combat behaviors that are strikingly similar to those of many monitor lizards (Varanus). For the beaded lizards, combat consists of the formation of a high arch posture, with bellies pressed together and snouts, forelimbs, and tail tips forming contact points on the ground. Pressure exerted by the combatants eventually collapses the arch, and the dominant lizard emerges

Three defensive postures of the Gila monster (Heloderma suspectum). (Illustration by John Megahan)

on top. Combatants may repeatedly form the arch in matches that may continue for several hours. A typical combat session demands considerable physical effort and leaves both participants exhausted.

The combat of Gila monsters also consists of a series of ritualized wrestling matches, whereby combatants straddle each other, then perform a body twist in an effort to gain the superior position. Gila monsters do not form the arching postures performed by the beaded lizards, probably because their tails are too short. Each bout ends when pressure exerted by the body twist causes the lizards to separate, but bouts can be repeated many times over several hours. Two fighting males observed in southwestern Utah performed at least 13 individual bouts over nearly three hours of continuous exertion.

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