Periodicity of desert rodent activity.

The radiation dose detected by microthermoluminescent dosimeters attached to pocket mice,Perognathus formosus, indicated the amount of time these animals were active on the surface of the ground. Radiation was from anelevated, partially shielded source in the center of the 8-hectareenclosure. The rodents are almost entirely inactive in midwinter but spend 30 to 40 percent of their time above ground in the summer months. Periods of activity increase gradually through the spring. These results support laboratory findings that members of this genus undergo periods of torpor in response to low ambient temperatures or food shortage. That this adaptation may enhance survival is indicated by the longevity of marked individuals of a related species.