Body Temperature Patterns and Energy Balance Hormones in Free-Living Thirteen-Lined Ground Squirrels (Ictidomys tridecemlineatus) from Different Latitudes.

AbstractThis article examines hormone concentrations and body temperature (T_b) patterns of free-living thirteen-lined ground squirrels (TLGSs) across the majority of their latitudinal range in the United States (from Texas to Minnesota). Free-living TLGSs ($n=40$) were implanted with T_bdata loggers in 2019 before they entered hibernation. Three adult female TLGSs, one each from Oklahoma (low latitude), Iowa (middle latitude), and Minnesota (high latitude), were recaptured in 2020 after the hibernation season. Although this provides an n of 1 for each location and therefore no statistically supported conclusions can be drawn, the hibernation season was longest in the animal from the highest latitude with coldest winter soil temperatures (Minnesota) and shortest in the animal retrapped at the lowest latitude (Oklahoma). Torpor bouts were generally longer when soil temperatures were lower. The Iowa and Minnesota squirrels had a prolonged period of short torpor bouts with T_b near 20°C at the beginning of the hibernation season. Concentrations of the orexigenic hormone ghrelin and the sex hormones estradiol and testosterone were also compared in populations from different latitudes. In general, Minnesota males had higher testosterone than males from other populations, possibly due to a later breeding season relative to other squirrel populations. Animals trapped in early summer had significantly lower concentrations of ghrelin than those captured in midsummer, potentially driving the fat-storing period before the hibernation season. Together, these results suggest latitudinal variation in physiological regulation of circannual rhythms.