Figure 1

Both environmental temperatures and activity affect thermoregulation in endotherms, though in potentially contrasting ways. Activity in nocturnal desert rodents is known to be strongly correlated to the lunar cycle, as rodents forage most intensely during periods when the moon is below the horizon and visual predators are less effective. The opposing trends in temperature and hours of darkness across the study period allow us to separate their respective effects on thermoregulation in banner-tailed kangaroo rats (Dipodomys spectabilis) on the Sevilleta National Wildlife Refuge. Temperatures generally increased across the study period (A), while the hours of darkness (defined here as the time between sunset and sunrise when the moon was below the horizon) generally decreased across the study period (B). Our study took place between roughly spring equinox and summer solstice, so the length of night decreased over the study period. At any point of the lunar cycle, there were more hours of true darkness at the beginning of the study period than at the end. For example, there were approximately 11.3 h of darkness on the night of the first new moon of the study period and 9.5 h on the night of the last full moon. Thus, if environmental temperature is more important in determining variation in body temperature, we predicted heterothermy should decrease across the study period (C; dashed line). If activity periods are more important, we predicted heterothermy should increase across the study period (C; solid line). We predicted the administration of corticosterone would strengthen the effect of moonlight but have little on the response to environmental temperatures.