Meeting Abstract
Mammals that hibernate occur in 14 orders including monotremes and primates and vary in size from a few grams to several hundred kilograms. This talk will compare the energetics, thermoregulation, body temperature patterns and resulting constraints in two hibernators that overwinter in Alaska, the arctic ground squirrel and the American black bear. Q: Do bears hibernate in the woods? Some say no, since bears remain at relatively high body temperatures during winter. Ans: Of course they do. Hibernation is an adaptation of avoiding starvation by regulating metabolic demand at low levels; what happens to body temperature is then largely a matter of physics. Hibernating black bears and arctic ground squirrels have identical gram specific metabolic rates under common winter conditions, but because of the bear’s relatively low thermal conductance its body temperature averages 34 vs -2 deg C in the ground squirrel. From basal rates, ground squirrels can decrease their metabolism to minima of 2 vs 25% in bears, but both burn mostly fat and lose about 30% of their body mass overwinter. Hibernating ground squirrels but not bears require periodic arousals from torpor to euthermia, although bears display mysterious multi-day cycles of body temperature. Hibernating bears sleep almost all winter whereas sleep is confined to periodic arousals in ground squirrels. Female bears become pregnant, give birth, and lactate during hibernation, while reproduction occurs afterwards in ground squirrels, although males end torpor early to go through puberty. Whether hibernation among different mammal species is an ancestral or derived trait is controversial, but it works equally well in arctic ground squirrels and black bears to get them through long, cold, and dark winters.
