Unimodal daily distribution of activity in antelope ground squirrels under stable environmental conditions

Abstract

Like many other animals, white-tailed antelope squirrels exhibit a bimodal daily pattern of activity in the field during the summer. Because seasons in the field involve not only changes in ambient temperature but also changes in photoperiod and food availability, the cause of the bimodal pattern is not known. In this study, behavioral activity of white-tailed antelope squirrels was studied under controlled environmental conditions in the laboratory. Without the rise in ambient temperature around noontime, the activity pattern was unimodal under a summer-like photoperiod, and this was so when activity was monitored either with running wheels or with infrared motion detectors. Some of the animals exhibited several bouts of activity each day, but the bouts were not uniform in timing or duration, so that the activity pattern of the whole group was unmistakably unimodal with a peak or plateau at noontime. Thus, the bimodal pattern observed in the field in this species is not due to an intrinsic property of the circadian system or to the presence of a long photoperiod but likely to the heat of summer noontime.