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Basking in the sun – how large mammals survive winter in the mountains

[Translate to English:] Ein Sonnenbad bei Temperaturen unter dem Gefrierpunkt klingt nicht besonders einladend, aber es stellt einen wichtigen Teil der Überlebensstrategie von Steinböcken im Winter dar. Diese überraschende Erkenntnis stammt aus einer aktuellen Studie der Gruppe von Walter Arnold am Forschungsinstitut für Wildtierkunde und Ökologie der Vetmeduni Vienna. Die Ergebnisse wurden kürzlich online in der Zeitschrift „Functional Ecology“ veröffentlicht.

Pflanzenfressende Tiere in alpinen Regionen stehen einem großen Problem gegenüber: Während im Sommer ausreichend Nahrung zur Verfügung steht, sind die Winter rau und das Nahrungsangebot äußerst knapp. Wegen der tiefen Temperaturen müssen die Tiere aber mehr Energie für die Aufrechterhaltung einer hohen Körpertemperatur aufwänden. Kleine Säugetiere lösen dieses zweifache Problem, indem sie in den täglichen Ruhephasen in Kältestarre fallen oder gar Winterschlaf halten. Große Tiere tun dies üblicherweise nicht, wahrscheinlich weil die Gefahr zu groß wäre, Fressfeinden zum Opfer zu fallen. Wie also meistern sie den Winter?

Überleben auf Sparflamme

Sunbathing in sub-zero temperatures may not be everybody’s idea of fun but it forms an important part of the strategy of Alpine ibex for surviving the winter.  This surprising finding comes from recent research in the group of Walter Arnold at the Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna.  The results have recently been published online by the journal Functional Ecology.

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.”

The paper Hypometabolism and basking: the strategies of Alpine ibex to endure harsh over-wintering conditions by Claudio Signer, Thomas Ruf and Walter Arnold was published on 10 January by the journal Functional Ecology.

Full text version of the article online

About the University of Veterinary Medicine, Vienna

The University of Veterinary Medicine, Vienna is the only academic and research institution in Austria that focuses on the veterinary sciences.  About 1000 employees and 2300 students work on the campus in the north of Vienna, which also houses the animal hospital and various spin-off-companies.  The Graf Lehndorff Institute for Equine Science in Neusdadt (Dosse), Germany belongs to the University.