In the far North, at the Arctic Circle, the sun shines for 6 months during the summer and sets for 6 months in winter, creating days filled with 24 hours of sunlight or complete darkness. Reindeer have abandoned their familiar 24-hour biological rhythm to adapt to this phenomenon.
Typically, plants and animals set their biological clocks based on the sunrise and sunset. In humans, this endogenous cycle begins when we wake up, continues during our activities, and concludes when sleep arrives.
Many species can maintain their biological rhythms despite sudden changes, such as when migratory birds fly to areas with longer daylight hours. However, very few species can keep their rhythm when sunlight completely fades or diminishes, as occurs during seasonal changes in the Arctic.
“Maintaining a rhythm requires a ‘strong’ biological clock – one that can run independently of external cues,” says Karl-Arne Stokkan from the University of Tromsø, Norway. “We believe that reindeer, like other Arctic animals, possess a weak clock.”
What Do Reindeer Do?
Stokkan’s team monitored the daily behavior and foraging activities of two species of reindeer living at different latitudes: the mainland Rangifer tarandus tarandus in Norway (70 degrees North) and the Svalbard reindeer R. t. platyrhynchus at 78 degrees North.
At these latitudes, reindeer always experience winter in a twilight landscape, and conversely, the sun never sets during summer. In spring and autumn, the transition between these two states lasts several weeks, resulting in about 18 weeks each year with distinct day/night cycles.
During those few weeks, both species adhere to a 24-hour rhythm each day, Stokkan noted. However, in summer, they completely lose their day-night rhythm. The Svalbard reindeer specifically also lack a rhythm during winter.
With no daily body alarm clock, Stokkan hypothesizes that they occasionally take brief naps (though very rarely) interspersed with stressful activities.
In this context, possessing a weak biological clock seems to benefit them.
According to Stokkan, activities during winter and summer appear to be governed more by their digestive systems than by sunlight, and reindeer eat whenever the weather permits. This free-foraging diet aligns best with the reindeer’s unique digestion supported by bacteria and other ungulate species.
Reducing the influence of the biological clock may also enhance responsiveness and accelerate adaptation to changes in the light/dark cycle. This could play a significant role for migratory birds and hibernating mammals.
