Passive rewarming from torpor in hibernating bats: minimizing metabolic costs and cardiac demands

Abstract

Endothermic arousal from torpor is an energetically costly process and imposes enormous demands on the cardiovascular system, particularly during early stage arousal from low body temperature (Tb). To minimize these costs many bats and other heterothermic endotherms rewarm passively from torpor using solar radiation or fluctuating ambient temperature (Tₐ). Because the heart plays a critical role in the arousal process in terms of blood distribution and as a source of heat production, it is desirable to understand how the function of this organ responds to passive rewarming and how this relates to changes in metabolism and Tb. We investigated heart rate (HR) in hibernating long-eared bats ('Nyctophilus gouldi') and its relationship to oxygen consumption (Vo₂) and subcutaneous temperature (Tsub) during exposure to increasing Tₐ compared with endogenous arousals at constant low Tₐ. During passive rewarming, HR and Vo₂ remained low over a large Tsub range and increased concurrently with increasing Tₐ (Q₁₀ 2.4 and 2.5, respectively). Absolute values were higher than during steady-state torpor but below those measured during torpor entry. During active arousals, mean HR and Vo₂ were substantially higher than during passive rewarming at corresponding Tsub. In addition, partial passive rewarming reduced the cost of arousal from torpor by 53% compared with entirely active arousal. Our data show that passive rewarming considerably reduces arousal costs and arousal time; we suggest this may also contribute to minimizing exposure to oxidative stresses as well as demands on the cardiovascular system.