Journal of Comparative Physiology B

, Volume 173, Issue 5, pp 379–389 | Cite as

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

Original Paper

Abstract

A variety of definitions involving body temperature (Tb), 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 (Tsk), a measure that approximates but may not precisely reflect Tb. We assess the accuracy of Tsk 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 Tsk and core Tb at a range of ambient temperatures for clustered and solitary roosting animals, compared Tsk and Tb 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 Tsk. Tsk was significantly lower than Tb during normothermia but often greater than Tb during torpor. Flight increased the rate of warming from torpor. This effect was more pronounced for Tsk than Tb. 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 Tsk over a range of ambient temperatures.

Keywords

Torpor Skin temperature Body temperature Active temperature Bats 

Abbreviations

BAT

brown adipose tissue

MR

metabolic rate

Ta

ambient temperature

Tact

active temperature

Tb

body temperature

Tsk

skin temperature

Notes

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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Copyright information

© Springer-Verlag 2003

Authors and Affiliations

  1. 1.Department of BiologyUniversity of ReginaRegina, SaskatchewanCanada

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