Metabolic rate and body temperature reduction during hibernation and daily torpor

A comparative review of measured metabolic rates and body temperatures across species that use daily torpor and species that hibernate, examining whether the two are points on one continuum or physiologically distinct states.

Studied in

Mammalia, Aves

Sample size

comparative review across heterothermic mammals and birds

Hibernation and daily torpor differ quantitatively in every measured dimension: bout length, minimum body temperature and the depth of metabolic suppression, which in deep hibernators falls to a few per cent of basal rate. Metabolic suppression is not simply a passive consequence of cooling — it begins before body temperature falls and is actively regulated.

• Comparative data are dominated by small mammals that hibernate conveniently in laboratories.
• Field metabolic measurements are far scarcer than laboratory ones and tend to be higher.
• Where exactly to draw the boundary between deep daily torpor and shallow hibernation remains a matter of convention.

• Hibernation

  A hibernating animal turns its metabolism down first, and gets cold as a consequence

• Hibernation

  Bears do hibernate — the old objection measured the wrong variable

• Seasonal survival

  Animals commit to winter before winter arrives, using day length rather than weather

• Dormancy

  A hibernating animal is not a stopped animal. Its metabolism falls further than the cold alone would explain, which means something is actively holding it down — and something is running the whole time to keep it alive.

• 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.