Fat and fed: frequent use of summer torpor in a subtropical bat
A widely held view is that torpor is avoided by mammals whenever possible because of potential costs associated with reduced body temperatures and slowed metabolic processes. We examined this hypothesis by quantifying use of torpor in relation to body condition of free-ranging northern long-eared bats (Nyctophilus bifax, approximately 10 g), a species known to hibernate, from a subtropical region during the austral summer when insects were abundant. Temperature-telemetry revealed that bats used torpor on 85% of observation days and on 38% of all nights. Torpor bouts ranged from 0.7 to 21.2 h, but the relationship between duration of torpor bouts and ambient temperature was not significant. However, skin temperature of torpid bats was positively correlated with ambient temperature. Against predictions, individuals with a high body condition index (i.e., good fat/energy reserves) expressed longer and deeper torpor bouts and also employed torpor more often during the activity phase at night than those with low body condition index. We provide the first evidence that use of torpor in a free-ranging subtropical mammal is positively related with high body condition index. This suggests that employment of torpor is maximised and foraging minimised not because of food shortages or low energy stores but likely to avoid predation when bats are not required to feed.
KeywordsBody condition index Chiroptera Insectivorous Nyctophilus Predator avoidance Torpor
Body condition index
We would like to thank Margaret Stawski for all her help with fieldwork. We are also grateful to Gerhard Körtner, Brad Law, Alexander Riek, Michal Stawski, Christopher Turbill, and Courtney Waugh for their contributions to this study. This research was undertaken in agreement with permits issued by New South Wales National Parks and Wildlife Service and the Animal Ethics Committee of the University of New England. The study was supported by grants from the University of New England and Bat Conservation International to CS and the Australian Research Council to FG.
- Arlettaz R, Ruchet C, Aeschimann J, Brun E, Genoud M, Vogel P (2000) Physiological traits affecting the distribution and wintering strategy of the bat Tadarida teniotis. Ecology 81:1004–1014Google Scholar
- Barclay RMR, Harder LD (2003) Life histories of bats: life in the slow lane. In: Kunz TH, Fenton MB (eds) Bat ecology. University of Chicago Press, Chicago, pp 209–253Google Scholar
- Bronner GN, Maloney SK, Buffenstein R (1999) Survival tactics within thermally-challenging roosts: heat tolerance and cold sensitivity in the Angolan free-tailed bat, Mops condylurus. S Afr J Zool 34:1–10Google Scholar
- Churchill S (1998) Australian bats. New Holland Publishers (Australia) Pty Ltd, SydneyGoogle Scholar
- Dausmann KH (2008) Hypometabolism in primates: torpor and hibernation. In: Lovegrove BG, McKechnie AE (eds) Hypometabolism in animals: hibernation, torpor and cryobiology. University of KwaZulu-Natal, Pietermaritzburg South Africa, pp 327–336Google Scholar
- Speakman JR, Racey PA (1986) The influence of body condition on sexual development of male brown long-eared bats (Plecotus auritus) in the wild. J Zool 210:515–525Google Scholar