Heloderma

Abstract: Activity patterns, behavior, food habits, and thermal biology were investigated by radio- telemetry in a population of banded Gila monsters in southwestern Utah. Twenty-seven Gila monsters were observed within a 2 km2 area. They fed on eggs and young mammals taken from nests. Quantities as large as 210 g, eaten in a single meal, did not appear to be envenomated. Activity peaked between late April and mid June, from 0800 to 1200 h. Distances traveled during activity bouts averaged 210 m (ap- proximately 50 min), although individuals occasionally traveled over 1 km. Lizards were active on less than 10 days/month during their 90-day activity season, spending over 95% of their time below ground in shelters. This low energetic investment to activity is contrary to traditional descriptions of activity of lizards that forage on patchy prey. Gila monsters had a relatively low activity temperature (x = 29.4 C) and at rest spent over 83% of the year at body temperatures of 25 C or below. Lizards occasionally basked near shelters in the spring. Several shelters were reused, some by more than one lizard, occasionally concurrently. Intraspecific interactions, including male combat, observed near shelters suggest that these helodermatids have a structured social system. Analysis of a 3-h fight between two large males revealed similarities with varanid lizard and crotaline snake combat, as well as similarities to combat in captive helodermatids.