The cold shoulder: Free-ranging snowshoe hares maintain a low cost of living in cold climates

The hypothesis that cold air temperatures (T_a) constrain the metabolic diversity of high-latitude endotherms is based on the observation among birds and mammals that mean field metabolic rate (FMR) increases, whereas the variability of FMR decreases, from the warm tropics to the cold poles. However, there is a paucity of FMR measurements from above 608 latitude and below 0 8C. We measured the daily energy expenditure of a high-latitude population of free-ranging snowshoe hares (Lepus americanus Erxleben, 1777) in Yukon, Canada, in winter (T_a-mean = -16.4 °C) and in autumn (T_a-mean = 0.5 °C). Doubly labelled water measures of FMR were approximately 20% lower in winter than in autumn, and were a similar, low multiple of resting metabolic rate in both seasons (2.04 and 1.94, respectively). The mass-corrected FMR of snowshoe hares in winter was only half the value predicted by extrapolating the relationship between FMR and Ta > 0 to -16.4 °C. These results contribute to an emerging pattern of a reversal in the relationship between FMR and Ta in free-ranging mammals from negative above 0 °C to positive below 0 °C. We refer to the positive, low Ta portion of this relationship as the cold shoulder, and suggest that it may reflect the general necessity for free-ranging mammals to use behavioural and (or) physiological means to conserve energy during long winters when cold conditions coincide with resource scarcity.