Even though they live in a region known for its searing heat and arid conditions, Gila monsters prefer relatively cool temperatures when compared to other diurnal desert lizards.
In a group of Gila monsters in southern Utah (Beck, 1990), the mean monthly body temperatures at rest ranged from 28 C (82 F) in July to 12 C (54 F) in December. These Gila monsters spent more than 83% of the year at body temperatures of 25 C (77 F) or less, and over 50% of the year at or below a body temperature of 20 C (68 F). The body temperature during activity ranged from 24 to 37 C (75 F to 98 F). The air temperature during activity was 10 to 34 C (50 to 93 F) and the ground temperature was 21 to 32 C (69 to 90 F).
Basking behavior has also been observed in Gilas during their spring activity periods. Lizards have been seen exiting their shelters within an hour of sunlight hitting the entrance. They would press their ventral surface against the ground, flattening out their body. They would frequently enter, then exit, then re-enter the shelter many times over a 4-5 hour period as they regulated their body temperature. One particular animal, observed over several days in late April and early May, maintained a mean body temperature of 28.5 C (83.3 F) during basking periods, despite considerably lower environmental temperatures. Its’ body temperature would return to around 68 F overnight.
Gila monsters, like other reptiles, are ectotherms (cold-blooded). This doesn't mean they can't control their body temperature. It means they must use behavior and not internally produced heat to regulate their temperature. This springtime basking behavior is important for the proper development of sperm and eggs.
Gilas have relatively "leaky" skin. They avoid dehydration by spending most of their lives in relatively moist and humid shelters. They also use their bladder as a water storage container and are able to store up to a 90 day supply! A neonate Gila monster can have up to 40% of its total mass in a bladder full of water.