Circadian Rhythms in Polar Bears By Dr. Thea Bechshoft, staff scientist at Polar Bears International
Do polar bears follow sleep-wake and other activity patterns like people do, even in the dark of the polar night and 24 hours of sunlight? Because the bears spend most of their time on the Arctic sea ice, this question has been surprisingly difficult for scientists to answer—until recently.
Over the past few years, satellite collars have advanced far beyond tracking movement data as the bears roam the ice. New technology now allows researchers to also gain insights into the frequency of swims, dive depths, ambient temperatures, and even daily activities through POV videos. Still, we did not have a good answer to one of the big questions out there: how do the extreme dark/light conditions of the polar night and the midnight sun affect polar bear behavior?
New study sheds light
Enter a recently published scientific paper by Dr. Jasmine V. Ware (Washington State University) and colleagues. The study covers an 8-year period, includes 122 female polar bears from the Chukchi and Southern Beaufort Seas, and is built on data collected via GPS satellite collars that were also fitted with accelerometers (small instruments that measure smaller scale changes in movement). The resulting data allowed the researchers to investigate the bears’ daily and seasonal patterns over the whole calendar year.
The findings are not only new, but also intriguing. First, it appears that polar bears, like humans and many other organisms, do indeed possess an inherent circadian (daily) rhythm that spans roughly 24 hours. Researchers often consider daylight one of the primary variables that control such daily rhythms, but in most of these bears the length of day only had a slight influence: the duration of their daily rhythm was 24.4 hours during constant daylight (June-August) and 23.9 hours during constant darkness (November-January). The number of active hours (on average, 11) in a day were also largely unchanged throughout the year. It’s important to understand though, that these are average values—in reality, the rhythmicity and activity was characterized by high variability within each bear as well as between bears.
Ringed seal pupping season shakes things up
The time of daily peak activity also stayed constant in the bears through most of the year, centering around noon to 2 p.m. This pattern, however, changed radically during the ringed seal pupping season, with activity generally spread out evenly over all 24 hours of the day. The pupping season (April-June) is an incredibly important time for polar bears. Although polar bears are active hunters year-round, the seal pups represent relatively easy and high-calorie meals that can be picked off by a bear crashing through the roof of the seal birth lair or sneaking up on young seals basking on the sea ice. Springtime is when polar bears pack on the pounds that keep them fat and healthy throughout the often-leaner summer months.
The advantage of being flexible
The results of the study are fascinating. Although we still do not yet know the mechanism behind it, the data show that polar bears retain their daily rhythms over the course of the year, while also maintaining a high degree of what is known as behavioral plasticity (the ability to adjust behavior to the current situation). While neither of these traits are unique to polar bears, it does speak to how well adapted they are at eking out a living in the constantly changing and often harsh environment of the High Arctic sea ice.
Thea has a Ph.D. in polar bear ecology/toxicology. So far, she has spent 15 years focusing on polar bears: studying and working in Denmark, Norway/Svalbard, and Canada; conducting polar bear fieldwork in remote locations in Greenland and Canada; publishing peer-reviewed scientific papers; and guiding polar voyages in Svalbard and Russia. Her work with PBI includes focusing on polar bear research and public outreach. In addition, she is the author of the popular Facebook page, “Polar Bear Questions”. Thea has a deep love of nature, especially the vast and remote Arctic regions. She currently lives in Aarhus, Denmark.