Daily torpor and hibernation in birds and mammals

Assembled measurements of torpor bout duration, minimum body temperature and metabolic rate for every species where they had been recorded, and tested statistically whether daily torpor and hibernation form one continuous distribution or two.

Studied in

Mammalia, Aves

Sample size

214 species with measured torpor or hibernation

The two states separate statistically rather than grading continuously: hibernators reach lower body temperatures and much longer bouts, and the distribution of bout lengths is bimodal. Both are widespread and neither is rare.

• Species are included because somebody studied them, so the sample is biased towards accessible temperate animals.
• Tropical heterothermy is substantially under-recorded relative to temperate.

• Torpor

  Daily torpor and hibernation are not the same thing at different lengths. Measured across two hundred species, bout durations fall into two groups rather than a continuous range.

• Torpor

  Torpor is not only a cold-weather response. Animals drop into it after a poor night’s feeding, in drought, or whenever food runs short — the same animal may be torpid one night and not the next.

• Torpor

  Hibernation is not continuous. Animals rewarm to near-normal temperature every week or two, and those arousals consume a large share of the energy the whole winter was meant to save. Why they do it is not settled.