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

AbstractHibernation 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 (Tb), and activity. Most small mammalian hibernators thermoconform, with Tb approximating ambient temperature (Ta); arctic species are an exception, since they must actively defend what can be large thermal gradients between Tb and Ta. 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 Ta’s and found that arctic ground squirrels maintained steady state torpor at Ta’s as low as −26°C, through a 36-fold increase in torpid MR (TMR), compared to their minimum TMR, exhibited at a Ta of 0°C. Golden-mantled ground squirrels are able to maintain steady state torpor at Ta’s at least as low as −8°C, through a...