Seasonal thermogenic capacity in a hibernator,Spermophilus richardsonii

Summary The maximum thermogenic capacity (HPmax) and the maximum capacity for non-shivering thermogenesis (NSTmax) were assessed in a hibernator, the Richardson's ground squirrel at different times of year. The HPmax was elicited by exposing animals to He–O₂ (21% oxygen, balance helium) at –10 to –25°C. The NSTmax was estimated by i.v. infusion of isoproterenol in anesthetized animals at thermoneutrality. Non-hibernating phase adults were collected and tested in April and June, and youngs in June and August for effects of seasonal acclimatization; animals were also tested after acclimation to cold (5°C) or warm (20°C). Hibernating phase animals were tested both shortly after the onset of hibernation season and after several months into the hibernation season. Although HPmax differed significantly between the lowest 101 cal (wt^0.73·h)^–1 in the June-Young group] and the highest 142 cal (wt^0.73·h)^–1 in the June-Adult group], it was not significantly different between other groups regardless of hibernation status or temperature acclimation (Fig. 4). The NSTmax, however, increased from 40–50 cal (wt^0.73·h)^–1 in the Warm-Acclimated, August-Young, June-Adult, and April-Adult to 66.5 and 79.2 cal (wt^0.73·h)^–1 in the two hibernating groups (Fig. 3). No significant difference in NSTmax was observed between Cold- and Warm-Acclimated groups. Since HPmax was maintained essentially constant at different times of year or after temperature acclimation, the increase of NSTmax during the hibernating phase can best be viewed as an adjustment for facilitation of periodic rewarmings from depressed body temperature during hibernation rather than to counter cold.Abbreviations HP heat production - HPmax maximum heat production - NST non-shivering thermogenesis - NSTmax maximum non-shivering thermogenesis - ST shivering thermogenesis - T _a ambient temperature - T _b body temperature