Many animals undergo phases of torpor by reducing body temperature and metabolic rate, which ultimately lowers their energy demands during harsh periods. Torpor use and especially hibernation, during which animals remain torpid in protected burrows or nests for months, are generally viewed as very advantageous survival mechanisms. Hibernating animals are less often subject to predation than non-hibernators and can withstand long periods when food is scarce. But surprisingly, individuals often vary in their use of torpor, leading to the question why do some individuals use less torpor than others if torpor is such a beneficial strategy?
Melanie Dammhahn, a researcher from the University of Potsdam in Germany, and colleagues from the Université du Québec à Montréal and McGill University in Canada analysed torpor patterns of free-ranging eastern chipmunks (Tamias striatus) to find out why some individuals forgo torpor when it has clear survival benefits. Eastern chipmunks are relatively small seasonal hibernators that regularly interrupt torpor bouts during winter to feed on food reserves stored in their burrows. The researchers collected skin temperatures of 55 chipmunks over five winters, analysed the amount of time that each chipmunk spent torpid and linked this to the animals’ survival and birth rates.
Dammhahn and colleagues found that, as expected, the use of torpor varied between individuals. Even more interesting, chipmunks maintained a stable torpor pattern over the whole hibernation season; animals that used less torpor at the beginning of the hibernation season also used less torpor later in winter. Linking the chipmunks’ skin temperature measurements with their survival rates, the scientists found that chipmunks using less torpor in autumn had higher death rates in years when food was plentiful, but not when food was scarce. Additionally, they found that the chipmunks that used more torpor early on and survived until the breeding season produced fewer offspring in spring.
While a lower birth rate in animals that use more torpor suggests that increased torpor use could be related to a slower pace of life – as animals increase their survival chances via torpor and spread their reproductive output over more years – the other results are not as easy to explain. Variation in torpor use might reflect individual personalities or could just be caused by differences in food availability in the animals’ habitats.
Extended torpor use may also be beneficial as individuals entering torpor early in autumn can spend more time underground and have a lower risk of predation, whereas animals that remain active have a higher risk of falling prey to a predator. In a year when food is abundant, an animal retreating underground early will probably have enough food stored to survive the entire winter season. However, in years when food is scarce, an animal that reduces its use of torpor – remaining active for longer in autumn – would probably have the advantage of establishing a larger food cache, thereby increasing its survival chances.
While it remains a matter of speculation whether the variation in torpor use is part of a pace-of-life syndrome or is simply caused by differences in food availability around burrows, one thing is clear: variation in torpor patterns within a population guarantees that some individuals will survive the winter regardless of the environmental conditions.