An Energy‐Based Body Temperature Threshold between Torpor and Normothermia for Small Mammals

Abstract

Field studies of use of torpor by heterothermic endotherms suffer from the lack of a standardized threshold differentiating torpid body temperatures (T b ) from normothermic T b 's. This threshold can be more readily observed if metabolic rate (MR) is measured in the laboratory. I digitized figures from the literature that depicted simultaneous traces of MR and T b from 32 respirometry runs for 14 mammal species. For each graph, I quantified the T b measured when MR first began to drop at the onset of torpor (T b-onset ). I used a general linear model to quantify the effect of ambient temperature (T a ) and body mass (BM) on T b-onset . For species lighter than 70 g, the model was highly significant and was described by the equation . This approach provides a standardized (0.040)T ϩ 31.083 a threshold for differentiating torpor from normothermia that is based on use of energy, the actual currency of interest for studies of torpor in the wild. Few laboratory studies have presented the time-course data required to quantify T b-onset , so more data are needed to validate this relationship.

Are torpid bats immune to anthropogenic noise?

“…For these, we only considered data points within 15 min of the onset of the playback. The T b distinguishing torpor from normothermy was calculated following the equation proposed by Willis (Willis, 2007) (T b,onset -1 s.e.). To be conservative, we used the minimum values for both body mass (M b , minimum value 21.5 g) and T a (minimum value 15°C) in our calculations.…”

A new cue for torpor induction: charcoal, ash and smoke

“…The start and end times for each activity bout that lasted for longer than 30 min were recorded. To define torpor bouts, we calculated the torpor T b thresholds (T b,onset ) from eqn 4 in Willis (2007), which were 32.5°C for females and 33.0°C for males. As maximum T b for antechinus during this study was ∼40°C, a drop of >7°C was deemed appropriate for defining torpor and, importantly, energetically significant decreases in T b may have been overlooked if the widely used threshold of 30°C had been used (Barclay et al, 2001;Brigham et al, 2011).…”

The importance of mammalian torpor for survival in a post-fire landscape

“…The torpor threshold (T b -onset) for antechinus was calculated as 31.58C using eqn (3) from [19], for bouts longer than 30 min. Activity periods were calculated from the time the individual's transmitter signal was absent from the receiver/logger and thus from the nest where torpor was expressed.…”