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First published online November 28, 2008
Journal of Experimental Biology 211, 3871-3878 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.023101

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Timing of the daily temperature cycle affects the critical arousal temperature and energy expenditure of lesser long-eared bats

Christopher Turbill^*,, Gerhard Körtner and Fritz Geiser

Centre for Behavioural and Physiological Ecology, Zoology, University of New England, Armidale, New South Wales, 2351 Australia

View larger version (17K): [in this window] [in a new window]   Fig. 1. Representative examples of skin temperature (Tskin; filled circles and dotted line) and metabolic rate (MR; solid line) of captive bats exposed to a diurnal fluctuation in ambient temperature (Ta; dashed line) with heating commencing (shown by arrow) at (A) 06:00 h, (B) 09:00 h or (C) 12:00 h. The photoperiod spanned from 06:00 h to 18:15 h.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Histograms showing times of arousal (filled vertical bars) and times of re-entry into torpor (open vertical bars) by bats over their rest phase during exposure to a diurnal fluctuation in ambient temperature (Ta; shown at the top) that commenced heating at (A) 06:00 h, (B) 09:00 h or (C) 12:00 h. Bats were placed into respirometry chambers in the evening (Ta: 13°C) and all had entered torpor during the night or around the time of lights on. All bats aroused in response to passive rewarming when heating commenced at 09:00 h or 12:00 h. When heating commenced at 06:00 h, bats (n=3) remained in torpor throughout on 4 of 15 (27%) days. The horizontal black and white bars above indicate the photophase.

View larger version (9K): [in this window] [in a new window]   Fig. 3. Ambient temperature (Ta) at the time of active arousal by bats receiving passive rewarming from exposure to a daily fluctuation in Ta when heating commenced at 06:00 h, 09:00 h or 12:00 h (individuals: filled circles and connecting lines; averages: open circles). Bats aroused at lower Ta and therefore after less passive rewarming when heating had commenced later in the day (repeated ANOVA: F2,22=14.4, P<0.001; Tukey's test: all P<0.05).

View larger version (6K): [in this window] [in a new window]   Fig. 4. Average torpid metabolic rate (TMR; ±1 s.d.) of bats during passive rewarming from heating of Ta (±1°C). Passive rewarming resulted in an exponential increase in average TMR [(in ml O2 g–1 h–1)=0.015x1.141Ta (°C); r2=0.78). Note that at Ta of 29.1°C (lower set-point of thermal neutral zone) TMR during passive rewarming extrapolated to 50% of BMR measured at same Ta (Geiser and Brigham, 2000).

View larger version (8K): [in this window] [in a new window]   Fig. 5. Energetic costs of active arousal from torpor as a function of ambient temperature (Ta) for arousals from minimum Ta without passive heating (triangles) and for arousals after diurnal passive heating (filled circles). Passive heating provided a linear reduction in the average cost of active arousal [solid line: energy expenditure for active arousal (kJ)=0.84–0.026xTa (°C); r2=0.87, P<0.001]. During torpor, passive heating resulted in a net increase in energy expenditure, which can be included as a component of the total energy expenditure for arousal [open circles; dashed line: total energy expenditure for arousal (kJ)=0.78–0.02Ta (°C); r2=0.82, P<0.001].

View larger version (7K): [in this window] [in a new window]   Fig. 6. Energy consumption over the rest phase (12 h) of bats exposed to a diurnal fluctuation in Ta when heating commenced at 06:00 h (open circles), 09:00 h (triangles) or 12:00 h (filled circles) as a function of time spent normothermic (0 h represents bats that remained in torpor). The time of commencement of heating had no significant effect on the slope of the relationship between energy consumption and time normothermic [general linear modelling (GLM) slope: F2,41=2.4, P=0.10], however, energy expenditure was greater overall on days when heating commenced at 06:00 h or 09:00 h, in comparison to 12:00 h [GLM, y-intercept: F1,43=287, P<0.001; fitted line for combined 06:00 h and 09:00 h: rest phase energy expenditure (kJ)=0.734.0+0.513xtime normothermic (h); for 12:00 h: energy expenditure (kJ)=0.261+0.513xtime normothermic (h); r2=0.90].

View larger version (9K): [in this window] [in a new window]   Fig. 7. An approximate model of the change in the critical threshold Ta that triggers arousal (thermal cue for arousal; thick dashed line) over the rest phase (daytime) in male N. geoffroyi. Torpor is entered before or near dawn. Arousal and active rewarming of body temperature (solid line) from torpor is predicted to occur if and when Ta (thin dotted line) reaches the thermal cue for arousal. Bats re-enter torpor in response to cooling of Ta. Torpid bats arouse near dusk.

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