Hibernation by tree-roosting bats
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ORIGINAL PAPER
Hibernation by tree-roosting bats Christopher Turbill · Fritz Geiser
Received: 30 September 2007 / Revised: 22 November 2007 / Accepted: 20 December 2007 / Published online: 22 January 2008 © Springer-Verlag 2008
Abstract In summer, long-eared bats (Nyctophilus spp.) roost under bark and in tree cavities, where they appear to beneWt from diurnal heating of roosts. In contrast, hibernation is thought to require a cool stable temperature, suggesting they should prefer thermally insulated tree cavities during winter. To test this prediction, we quantiWed the winter thermoregulatory physiology and ecology of hibernating tree-roosting bats, Nyctophilus geoVroyi and N. gouldi in the Weld. Surprisingly, bats in winter continued to roost under exfoliating bark (65%) on the northern, sunny side of trees and in shallow tree cavities (35%). Despite passive re-warming of torpid bats by 10–20°C per day, torpor bouts lasted up to 15 days, although shorter bouts were also common. Arousals occurred more frequently and subsequent activity lasted longer on warmer nights, suggesting occasional winter foraging. We show that, because periodic arousals coincide with maximum roost temperatures, when costs of rewarming and normothermic thermoregulation are minimal, exposure to a daily temperature cycle could largely reduce energy expenditure during hibernation. Our study provides further evidence that models of torpor patterns and energy expenditure from hibernators in cold temperate climates are not directly applicable in milder climates, where prolonged torpor can be interspersed with more frequent arousals and occasional foraging. Keywords Torpor
Arousal · Bat · Hibernation · Nyctophilus ·
Communicated by I.D. Hume. C. Turbill (&) · F. Geiser Department of Zoology, University of New England, Armidale, NSW 2351, Australia e-mail: [email protected]
Abbreviations MR Metabolic rate Ta Ambient temperature Tb Body temperature Tskin Skin temperature
Introduction Winter presents a critical energy bottleneck for small bats in temperate climates. Energy requirements are high for thermoregulation at a low ambient temperature (Ta), yet foraging is often unproductive because their insect food supply is reduced or seasonally unavailable (Williams 1961; Taylor 1963). In addition, bats cannot store food for long periods, nor can they carry the relatively large amount of fat needed to sustain high rates of energy expenditure at low Ta. It is well known, however, that temperate bats overcome this energy crisis by entering a period of hibernation over the temperate winter season. Hibernation consists of a series of prolonged torpor bouts lasting days or weeks, during which body temperature (Tb) and metabolic rate (MR) are substantially reduced, interrupted by arousals and brief periods of normothermic thermoregulation and perhaps activity (French 1985; Geiser and Kenagy 1988; Körtner and Geiser 2000). Over a wide range of Ta, torpid Tb is within 1–2°C of Ta and allowed to Xuctuate passively; however, at Ta below a torpor Tb set poin
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