Defining torpor in free-ranging bats: experimental evaluation of external temperature-sensitive radiotransmitters and the concept of active temperature

Abstract

A variety of definitions involving body temperature (T b), metabolic rate and behavior have been used to define torpor in mammals and birds. This problem is confounded in some studies of free-ranging animals that employ only skin temperature (T sk), a measure that approximates but may not precisely reflect T b. We assess the accuracy of T sk in the context of a recent definition for torpor called active temperature. We compared the active temperatures of individual big brown bats (Eptesicus fuscus), which aggregate in cavities, with solitary, foliage-roosting hoary bats (Lasiurus cinereus). In captive big brown bats, we compared T sk and core T b at a range of ambient temperatures for clustered and solitary roosting animals, compared T sk and T b during arousal from torpor, and quantified the effect of flight on warming from torpor. Hoary bats had significantly lower active temperatures than big brown bats despite having the same normothermic T sk. T sk was significantly lower than T b during normothermia but often greater than T b during torpor. Flight increased the rate of warming from torpor. This effect was more pronounced for T sk than T b. This suggests that bats could rely on heat generated by flight muscles to complete the final stages of arousal. Using active temperature to define torpor may underestimate torpor due to ambient cooling of external transmitters or animals leaving roosts while still torpid. Conversely, active temperature may also overestimate shallow torpor use if it is recorded during active arousal when shivering and non-shivering thermogenesis warm external transmitters. Our findings illuminate the need for laboratory studies that quantify the relationship between metabolic rate and T sk over a range of ambient temperatures.

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Abbreviations

BAT :

brown adipose tissue

MR :

metabolic rate

T a :

ambient temperature

T act :

active temperature

T b :

body temperature

T sk :

skin temperature

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Acknowledgements

Despite our disagreement with him on some of the issues raised above, we wish to express our admiration for the important contributions of Robert Barclay and his students to the study of torpor in free-ranging animals. We also thank Dr. Barclay for comments that improved an early draft of the manuscript. Andrew McKechnie, Don Thomas, and Chris Woods also provided helpful comments. Field and laboratory assistance was provided by Quinn Fletcher, Amanda Karst, Brianna Dobson, Renee Bendig, Desiree Idt, Christine Voss, Seb Martinez, Ryan Fisher, and Julie Adams. Jim Rusak provided invaluable statistical suggestions. This research was funded by Mountain Equipment Co-op, Saskatchewan Environment and Resource Management and by a Natural Sciences and Engineering Research Council (NSERC, Canada) research grant to R.M.B. and postgraduate scholarship to C.K.R.W.

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Correspondence to C. K. R. Willis.

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Communicated by L.C.H. Wang

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Willis, C.K.R., Brigham, R.M. Defining torpor in free-ranging bats: experimental evaluation of external temperature-sensitive radiotransmitters and the concept of active temperature. J Comp Physiol B 173, 379–389 (2003). https://doi.org/10.1007/s00360-003-0343-y

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Keywords

  • Torpor
  • Skin temperature
  • Body temperature
  • Active temperature
  • Bats