Thermogenic Capacity at Subzero Temperatures: How Low Can a Hibernator Go?

Hibernation in mammals is a physiological and behavioral adaptation to survive intervals of low resource availability through profound decreases in metabolic rate (MR), core body temperature (T-b), and activity. Most small mammalian hibernators thermoconform, with T-b approximating ambient temperature (T-a); arctic species are an exception, since they must actively defend what can be large thermal gradients between T-b and T-a. Here we compare the thermogenic capacity of the arctic ground squirrel (Urocitellus parryii) to that of the golden-mantled ground squirrel (Callospermophilus lateralis), a temperate-zone montane hibernator. We allowed animals to reenter torpor at sequentially lower T-a's and found that arctic ground squirrels maintained steady state torpor at T-a's as low as -26 degrees C, through a 36-fold increase in torpid MR (TMR), compared to their minimum TMR, exhibited at a T-a of 0 degrees C. Golden-mantled ground squirrels are able to maintain steady state torpor at T-a's at least as low as 28 degrees C, through a 13.5-fold increase in MR, compared to their minimum TMR at a T-a of 2 degrees C. In a second experiment, torpid animals were exposed to continuously decreasing T-a's (0.25 degrees C/30 min); individuals of both species increased their metabolism while remaining torpid at low T-a's (as low as -30 degrees C for arctic ground squirrels and -10 degrees C for golden-mantled ground squirrels). Although the capacity to hibernate at subfreezing T-a's is not unique to arctic ground squirrels, their large body size, greater torpid metabolic scope, and previously ascribed capacity to supercool allow them to occupy much colder hibernacula for prolonged seasons of hibernation.