title: Thermogenic capacity at subzero temperatures: how low can a hibernator go?
abstract: Abstract 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 (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 13.5-fold increase in MR, compared to their minimum TMR at a Ta of 2°C. In a second experiment, torpid animals were exposed to continuously decreasing Ta's (0.25°C/30 min); individuals of both species increased their metabolism while remaining torpid at low Ta's (as low as -30°C for arctic ground squirrels and -10°C for golden-mantled ground squirrels). Although the capacity to hibernate at subfreezing Ta'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.
authors.affiliation: Department of Biological Sciences, University of Alaska Anchorage, Anchorage, Alaska 99508; 2Department of Biology and Wildlife, University of Alaska Fairbanks, Fairbanks, Alaska 99775; 3Department of Biology, Harding University, Searcy, Arkansas 72149; 4Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska 99775; 5Department of Biology, Colorado State University, Fort Collins, Colorado 80523.
authors.forename: M M
authors.initials: MM
authors.lastname: Richter
authors.forename: C T
authors.initials: CT
authors.lastname: Williams
authors.forename: T N
authors.initials: TN
authors.lastname: Lee
authors.forename: Ø
authors.initials: Ø
authors.lastname: Tøien
authors.forename: G L
authors.initials: GL
authors.lastname: Florant
authors.forename: B M
authors.initials: BM
authors.lastname: Barnes
authors.forename: C L
authors.initials: CL
authors.lastname: Buck
mesh_terms: D000818:Animals; D001110:Arctic Regions; D001481:Basal Metabolism / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology; D003080:Cold Temperature; D006605:Hibernation / Q000502:physiology*; D012589:Sciuridae / Q000378:metabolism*; D013045:Species Specificity