© Dr. Jon Aars/Norwegian Polar Institute
7/19/2021 2:27:59 PM
Polar Bear Home Ranges and Family Ties
By Clément Brun
In Svalbard, polar bears follow two different strategies in response to sea ice. Offshore bears predominantly roam along the marginal ice zone, following the sea ice as it expands and retreats through the seasons. In contrast, coastal bears remain within the Svalbard archipelago year-round, primarily using land-fast ice and glacier fronts rather than the drifting pack ice.
I recently took part in a study, published in Polar Research, that focused on female coastal bears in Svalbard, working with colleagues from the Norwegian Polar Institute and the Arctic University of Norway, including Dr. Jon Aars. We used GPS data collected over an eight-year period from 2011 till 2019 to investigate individual differences in site fidelity. The data provided us with a fascinating picture of the space use of each individual bear over multiple years. This allowed us to better understand the variability in site fidelity and to investigate a possible link between family relationships (that is, mothers/daughters/sisters) and home ranges.
Homebodies and wanderers
We discovered that the coastal female polar bears had consistent home range sizes from year to year. For example, one female had the two largest annual home ranges (about 20,000 square kilometers) while another one had the two smallest (about 100 square kilometers, one of the smallest home ranges described for polar bears).
Not only were females consistent in the size of their home range from year to year but they also stayed within the same areas of the archipelago: that is, we found that all the coastal bears studied were extremely loyal to specific areas over the study period! Interestingly, some very local females used the same fjord system over several years (possibly over their lifespan).
We also found out that some of the females had a strong fidelity in seasonal movement patterns, migrating to specific areas depending on the time of year. For example, one female consistently migrated to an island (Prins Karls Forland) in spring and spent her winters in a fjord on the largest island, Spitsbergen, in Svalbard (Van Mijenfjorden). Her tracks are almost identical from the year 2015/2016 and the year 2017/2018.
Moreover, we found out a strong relationship between spatial proximity and genetic relatedness in Svalbard coastal female polar bears. Related coastal bears utilized similar regions of the archipelago and their annual home ranges were much closer together than those of non-related individuals. When mapping the GPS tracks over the Svalbard archipelago, we can clearly see that different families (mother-daughter pairs) occupy different areas (see figure Fig.5 in the paper).
Photo © Dr. Jon Aars/Norwegian Polar Institute.
During the first years of life, cubs stay with their mother and learn how and where to find food as well as how to master hunting techniques and others important survival behavior. We discovered that when daughters reached adulthood, they remained within or near the home range of their mothers. This behavior is known as natal philopatry (low dispersal from the birth site). Adult females may benefit from remaining close to their mother’s home range by using some of their knowledge of the terrain and important foraging sites gained from their mother before weaning. Additionally, remaining close to their mothers might potentially decrease antagonistic behaviors (aggression) between related bears as they most likely recognize each other (through kin recognition).
It is important to increase our knowledge of the biology of polar bears to be able to better understand the species. I believe knowing that some of the polar bears in Svalbard have ridiculously small home ranges and remain within one area can change the natural perception most people have of them. While implementing rules and increasing conservation efforts for polar bears in Svalbard, it is important to take into consideration the differences which exist between offshore bears and coastal bears when it comes to space-use strategies.
Clément Brun recently graduated with a master’s degree in Polar Ecology from UiT, the Arctic University of Norway. His co-authors on the paper included Dr. Marie-Anne Blanchet, Dr. Rolf A. Ims, and Dr. Jon Aars.