The inner clock is always ticking: Svalbard reindeer remain rhythmic even in the constant light of the midnight sun and in the permanent dark of the polar winter

Svalbard reindeers stick to their natural circadian rhythm despite midnight sun and polar winter. (Photo: © dieter leve / pixelio.de)

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All animals have an internal clock with a cycle length that deviates slightly from 24 hours, and that synchronizes based on cues from external "timers", usually the daily change from day to night. It was uncertain whether this internal clock also functions in the Polar Regions, where there is continuous darkness for months during winter and steady light in the summer. A research team led by Walter Arnold from the Research Institute of Wildlife Ecology at the Vetmeduni Vienna examined this question for wild reindeer on Svalbard, using for the first time a high-resolution telemetry system. They studied not only the behavior, but also the physiology of the animals. They found that - contrary to earlier findings - a circadian rhythm persists throughout the year.

Living in extreme conditions

Living conditions at the Polar Regions are extreme, especially for herbivorous mammals and birds. Maintaining their high internal body temperature is an energetic challenge during the long winters with cold temperatures and storms. There is hardly any food, and the few plant remains left over from summer are hidden under snow and ice. Herbivores are therefore dependent on their fat reserves in winter. In order to grow them sufficiently, the animals must eat as much as possible in the short time that forage plants are growing.  During the short Arctic summer, they cannot afford to take rest periods, as would be dictated by their circadian rhythm.   The Svalbard reindeer also seemed to be lacking such daily rhythms.  Scientists had so far assumed that natural selection shut down the inner circadian rhythms in the steady light of summer in order to guarantee 24-hour food intake.

The inner clock keeps ticking regularly

Together with Norwegian and British colleagues, Walter Arnold and Thomas Ruf of the Research Institute of Wildlife Ecology (FIWI) of the University of Veterinary Medicine, Vienna equipped free-living reindeer in the Arctic archipelago Svalbard of Spitsbergen with a telemetry system developed at FIWI. Unlike previous studies, this system did not just record the animals´ overall movement activity, it also recorded their feeding activity, body temperature and heart rate as a measure of the animals' energy expenditure.  Thomas Ruf established a particularly selective statistical method for biological applications such as this one, which was able to analyze whether these measured values change according to a daily rhythm.


The results were surprising: all measurement series showed a clear daily rhythmical organization. The intensity of this rhythm decreased considerably in the summer, not because of the steady light of the midnight sun, but because of the availability of freshly growing plant food. To eat as much as possible the animals evidently largely ignore their inner clock. Even in winter, when the reindeer lower their metabolic activity to less than half the summer level - a record among wild ungulates – they had a weakened circadian rhythm. In addition, during the polar night, the cycle length slightly deviated from 24 hours, with some differences between individuals and depending on the measurement parameters. The fact that inner rhythms “run free” in permanent darkness due to the lack of an external light rhythm stimulus was in itself the ultimate proof that the inner clock of the day never stops ticking, even in Svalbard reindeer.

Link to the press release and the scientific article; 2

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