What's hot and what's not: defining torpor in free-ranging birds and mammals

Abstract

With the development of small implantable data loggers and externally attached temperature-sensitive radio transmitters, increasing attention is being paid to determining the thermoregulatory strategies of free-ranging birds and mammals. One of the constraints of such studies is that without a direct measure of metabolic rate, it is difficult to determine the significance of lowered body temperatures. We surveyed the literature and found that many different definitions have been used to discriminate torpor from normothermy. Many studies use arbitrary temperature thresholds without regard for the normothermic body temperature of the individuals or species involved. This variation makes comparison among studies difficult and means that ecologically and energetically significant small reductions in body temperature may be overlooked. We suggest that normothermic body temperature for each individual animal should be determined and that torpor be defined as occurring when the body temperature drops below that level. When individuals' active temperatures are not available, a species-specific value should be used. Of greater value, however, are the depth and duration of torpor bouts. We suggest several advantages of this definition over those used in the past.Résumé : Avec l'avènement d'appareils enregistreurs de données plus petits et de transmetteurs radio sensibles à la température portés à l'extérieur, il devient de plus en plus courant de déterminer les stratégies thermorégulatrices des oiseaux et des mammifères en nature. L'une des contraintes reliées à ce genre d'étude est qu'en l'absence d'une mesure directe du taux de métabolisme, il est difficile de déterminer l'importance d'une baisse de température corporelle. Dans la littérature, plusieurs définitions différentes sont utilisées pour distinguer la torpeur de la normothermie. Dans plusieurs études, les seuils de température sont arbitraires et la température normothermique du corps des individus ou des espèces n'est pas prise en considération. Cette variation rend difficiles les comparaisons entre les études et il se peut alors que des réductions de la température corporelle petites, mais significatives des points de vue écologique et énergétique, ne soient pas détectées. Nous croyons que la température normothermique du corps doit être mesurée chez tous les individus et nous proposons que la torpeur soit définie comme l'état qui prévaut quand la température du corps descend au-dessous de ce niveau. Lorsqu'il est impossible de mesurer la température de tous les individus, une valeur spécifique devrait être utilisée. Plus importantes encore sont la profondeur et la durée des périodes de torpeur. Nous indiquons ici plusieurs avantages de cette définition par comparaison à celles reconnues auparavant.[Traduit par la Rédaction] 1890 Notes

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

“…1A). The mean value of T b-onset for mammals 162 g was 35.3Њ ‫ע‬ , much higher than the 30ЊC cutoff used in most studies 0.4ЊC (Barclay et al 2001). A plateau in T b-onset above a threshold BM is not surprising.…”

“…Active temperature is measured when an individual under study is known to be behaviorally active. For the studies of bats discussed by Barclay et al (2001), this means recording T sk or T b immediately before dusk emergence each night of a study and then, to be conservative, defining T act (the threshold temperature) as the lowest dusk emergence T sk or T b value recorded for an individual during the study (Barclay et al 2001). The approach could also be applied to dusk emergence of nocturnal, fossorial mammals or morning observations of diurnal birds.…”

“…Such a threshold is essential if depth and duration of torpor bouts are to be calculated, but the subjective nature of the traditional definition for torpor (IUPS Thermal Commission 2003) has led to uncertainty about how it should be quantified. Barclay et al (2001) identified more than 20 values of T b and T sk used to define torpor bouts in the literature, as well as other definitions based on criteria such as differentials…”

“…The most popular value used in studies reviewed by Barclay et al (2001) was a conservative (though no less arbitrary) 30ЊC. A conservative threshold is important to avoid overestimating use of torpor, but a potential consequence of being overly exclusive is the failure to detect shallow torpor bouts.…”

“…Thus, shallow bouts are likely to be of great ecological and evolutionary importance because they result in large energy savings while potentially mitigating the selective costs of torpor, such as vulnerability to predation, reduced growth rate, and delayed reproduction. Barclay et al (2001) proposed an important step toward a standardized threshold with their concept of active temperature (T act ). Active temperature is measured when an individual under study is known to be behaviorally active.…”