Temperature dependence of circadian rhythms in golden-mantled ground squirrels

We tested the hypothesis that the free-running period and phase angles of entrainment of circadian locomotor activity rhythms of golden-mantled ground squirrels (Spermophilus lateralis) vary as a function of body temperature (Tb). Animals were maintained in a light-dark cycle (LD 14:10), and at 6-week intervals ambient temperature (Ta) was changed from 20 degrees to 30 degrees C, or vice versa. Data were collected during the animals' homeothermic and heterothermic phases. Subsequently, squirrels were housed in dim constant illumination with the same alternating temperature sequence. In heterothermic ground squirrels (those capable of Tb less than 34 degrees C), a decrease in Ta from 30 degrees to 20 degrees C caused phase delays in activity onset, phase advances in activity termination, and a decrease in duration of the active phase; increases in Ta from 20 degrees to 30 degrees C produced the opposite effect on each of these parameters. The free-running period of the activity rhythm of heterothermic squirrels increased and decreased in response to 10 degrees C decreases and increases in Ta, respectively. Changes in Ta did not affect circadian parameters in homeothermic squirrels. Seasonal variations in circadian organization previously observed in this species appear to reflect the influence of endogenous fluctuations in body temperature. Tissue temperature has a major influence on circadian rhythms of golden-mantled ground squirrels.