Basking in the sun: How large mammals survive winter in the mountains

Animals that live in mountainous areas face a large problem. While food may be plentiful in summer, the winters are generally long and harsh. Winter temperatures are extremely low, which means that the animals need to expend energy on keeping warm at a time when food is very hard to come by. Small mammals may hibernate but large animals generally do not, presumably because doing so would make them too vulnerable to predators. So how do they cope in winter?

This question has been addressed by Claudio Signer, Thomas Ruf and Walter Arnold of the Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, in collaboration with the Hunting and Fishing Services of Grisons, Chur, Switzerland. The researchers equipped a number of wild Alpine ibex with sensors for measuring heart rate (a good indicator of energy expenditure), temperature in the stomach (i.e. body temperature) and locomotor activity and monitored the results for a period of two years. At the same time, various climatic factors were recorded.

The first finding was that the ibex lowered their heart rate significantly during the winter. It seems, then, that the animals do not respond to lower temperatures by generating more heat (to keep warm) but instead by somehow reducing their energy needs. Furthermore, the decrease in heart rate was found to be greater than could be explained by lower activity and lower body temperature, indicating that ibex have another mechanism for conserving energy.

Interestingly, the body temperature was found to fluctuate in a daily manner, with lowest temperatures around sunrise and highest temperatures in the late afternoon. The fluctuations were almost twice as large in winter as in summer. Of course, the surrounding temperature and level of solar radiation also vary over the course of the day. The scientists noticed a relationship between the daily patterns of stomach temperature and "black bulb temperature" (which combines surrounding temperature and solar radiation and thus gives a good indication of the temperature perceived by the animals). It seemed as though the animals "bask," or sunbathe, until noon to help increase their body temperature. Normal locomotor activity is not resumed until body temperature peaks in the early afternoon. In the early morning, there is almost no locomotor activity. This begins to increase just before sunrise, indicating that the animals move from their night shelter towards the nearest sunny spot.

Basking is well known for reptiles and had previously been observed in some small mammals, such as the rock hyrax, but was not thought to be used to such an extent by large animals. As Arnold says, "the ability to take advantage of external sources of heat may be common to all mammals. It may form part of our heritage from reptiles, enabling us to conserve body reserves during periods when food is scarce."