Seasonal patterns of body temperature in response to experimental photoperiod variation in a non-hibernating ground squirrel

Unlike numerous other members of the holarctic Tribe Marmotini of the squirrel family (Sciuridae) that typically exhibit spontaneous bouts of torpor that progress into an annual season of hibernation, members of the genus Ammospermophilus (antelope ground squirrels) do not enter torpor, and they remain active throughout the year in nature. We have experimentally evaluated seasonal patterns of variation in the circadian rhythm of body temperature in captive A. leucurus over a two-and-a-half-year period by exposing groups to either a constant daily photoperiod of 12 h light or a seasonally changing photoperiod that cycled between a summer maximum of 16 h per day and a winter minimum of 8 h; ambient air temperature was maintained at 26 °C. All squirrels showed continuous, year-round diurnal locomotor activity, and the group exposed to seasonally changing photoperiod adjusted onset and end of activity to changes in duration of the photoperiod. Animals in both groups showed a marked circadian rhythm of core body temperature with a typical daytime level of about 38 °C and nighttime level of about 35 °C for most of each year, but the group exposed to naturally changing daylength surprisingly reduced the level of its circadian oscillation by about 2 °C at the winter seasonal extreme of shortest daily illumination to a daytime level about 36 °C and a nocturnal level of about 33 °C. Despite this modest experimentally induced reduction in the level of the circadian rhythm of body temperature, we conclude that A. leucurus shows an overall stable annual pattern of circadian rhythmicity of its core body temperature that is consistent with a lack of any other evidence that the species engages in torpor or hibernation.