HIBERNATION VERSUS DAILY TORPOR IN MAMMALS AND BIRDS - PHYSIOLOGICAL VARIABLES AND CLASSIFICATION OF TORPOR PATTERNS

Hibernation and daily torpor are usually considered to be two distinct patterns of heterothermia. In the present comparison we evaluated (1) whether physiological variables of torpor from 104 avian and mammalia n species warrant the distinction between hibernation and daily torpor as two different states of torpor and (2), if so, whether this distin ction is best based on maximum torpor bout duration, minimum body temp erature (T-b), minimum metabolic rate during torpor, or the reduction of metabolic rate expressed as percentage of basal metabolism (BMR). I nitially, animals were grouped into species displaying either daily to rpor or prolonged torpor (hibernation) according to observations from original sources. Both cluster and discriminant analyses supported thi s division, and further analyses were therefore based on these two gro ups. Frequency distributions for all variables tested differed signifi cantly (P < 0.001) between daily torpor and hibernation. The average m aximum torpor bout duration was 355.3 h in hibernators and 11.2 h in d aily heterotherms. Mean minimum T-b's were lower in hibernators than i n daily heterotherms (5.8 degrees C vs. 17.4 degrees C) as were minimu m metabolic rates measured as rate of oxygen consumption (VO2; 0.037 v s. 0.535 mL O-2 g(-1)h(-1), and the metabolic rare reduction expressed as percentage of BMR (5.1% vs. 23.5%). Furthermore mean body weights were significantly, higher in hibernators (2384 g) than in daily heter otherms (253 g; P < 0.001). Thus, the comparisons of several physiolog ical variables appear to justify a distinction between the two torpor patterns. However, of all variables tested, only the frequency distrib utions of maximum torpor bout duration (1.5-22 h for daily torpor; 96- 1, 080 h for hibernation) showed a clear gap between daily heterotherm s and hibernators.