In obligate hibernators, torpor is interrupted by periodic warm-ups called what?

In obligate hibernators, torpor is interrupted by periodic warm-ups called euthermic arousals.

An euthermic arousal is a temporary return from torpor to a normal, or near-normal, body temperature and level of physiological activity. During the warm-up, heart rate, breathing and metabolism rise sharply before the animal may cool again and re-enter torpor.

Obligate hibernators enter this seasonal state each year, even when food and environmental conditions are relatively favourable. Their winter therefore consists not of one uninterrupted sleep but of repeated torpor bouts separated by arousals. Ground squirrels, some rodents, hedgehogs and several other mammals show this pattern.

Scientists still debate exactly why these warm-ups occur. Proposed explanations include restoring sleep lost during torpor, replenishing cellular energy supplies and supporting immune-system functions. Arousal is energetically expensive: studies of deep hibernators suggest that most of the energy used during hibernation may be spent during these brief returns to normal temperature.

The term “euthermic” refers to ordinary body temperature, distinguishing these events from seasonal moults or daily activity periods.