NOTES What’s hot and what’s not: defining torpor in free-ranging birds and mammals Robert M.R. Barclay, Cori L. Lausen, and Lydia Hollis 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 individu- als’ 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 me- sure 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 Introduction Torpor involves a controlled lowering of the thermo- regulatory set point, resulting in a drop in body temperature and metabolic rate (Hudson 1978; Wang and Wolowyk 1988). Endotherms can arouse from torpor without relying on exo- genous sources of heat. Among a wide range of mammals and birds, torpor is often classified as either shallow (daily) tor- por or hibernation (seasonal) torpor (Geiser and Ruf 1995). In either situation, captive animals have been used in numer- ous studies to investigate various aspects of torpor, including the resulting energetic and water savings. In many of these laboratory studies, the onset of and arousal from torpor, the depth and duration of torpor bouts, and energetic savings have been determined by measuring metabolic rates (e.g., Hiebert 1990; Morris et al. 1994; Coburn and Geiser 1996). In other cases, purely behavioural criteria have been used to determine when torpor occurs (e.g., lack of movement; Cranford 1978; Geiser and Masters 1994). With the development of small implanted or externally at- tached temperature-sensitive radio transmitters or data log- gers, increased attention has been given to the use of torpor by birds and mammals under natural, field conditions. These studies are particularly important if we are to understand the ecology and physiology of torpor, because animals frequently show different patterns of thermoregulation in the laboratory than in the field (Geiser et al. 2000). Defining torpor in the field is problematic. Behavioural criteria such as lack of movement are often not useful because the animals cannot be observed directly (but see Pierce and Can. J. Zool. 79: 1885–1890 (2001) © 2001 NRC Canada 1885 DOI: 10.1139/cjz-79-10-1885 Received February 9, 2001. Accepted August 13, 2001. Published on the NRC Research Press Web site at http://cjz.nrc.ca on October 26, 2001. R.M.R. Barclay, 1 C.L. Lausen, and L. Hollis. Department of Biological Sciences, University of Calgary, Calgary, AB T2N 1N4, Canada. 1 Corresponding author (e-mail: barclay@ucalgary.ca).