1/19/2015 2:42:27 PM
How Different Are Individual Polar Bears?
Big white bears all tend to look the same—but are they? At first glance, even scientists who have studied polar bears in the field for decades can have a tough time telling two bears apart. Sure, scientists can sometimes see physical difference in things like size and scarring, but otherwise polar bears can seem pretty similar. Once we start looking below the surface, however, we very quickly see that no two bears are alike.
A paper published last fall in Polar Biology tracked female polar bears from the western Hudson Bay population over two decades, examining movements on the sea ice and how individual differences might help determine their home range size. Alysa McCall, our field programs manager and lead author on the paper, explains a few of the paper's findings:
First we compared annual home range sizes in the 1990s and 2000s, but found no differences between the decades. In both decades, though, home ranges were very large. For example, in the 2000s, the average home range size for a female polar bear from mid-November to early July was 353,557 km2 (136,509 mi2). That's larger than Germany!
While that was the average home range size, we found a lot of variation in home range sizes among individuals. Some variation depended on whether a bear had cubs. For instance, females with no cubs had smaller home ranges and stayed closer to land compared to females with cubs, who traveled larger distances (maybe because they needed to find more food?).
We also examined differences in what polar bears did at different times of the year, and we found individual variation there, too. For example, during the sea ice freeze-up and break-up seasons (when polar bear habitat is very dynamic) individuals acted very differently. Thus, the variation in our measurements was high. However, in the winter months, individuals acted more similarly, so there was less variation in our measurements. In the winter, ice habitat is more stable and bears are mainly focused on hunting seals, which might explain why the polar bears made more similar choices at that time of year. However, no matter the time of year or habitat conditions, no two bears ever followed the same pattern.
Another interesting result emerged when we plotted all the bear locations against the western Hudson Bay population boundary, basically an imaginary line that is used to manage this population. We expected many of the collar locations to fall within this boundary, but that was not the case. From 2004-12, the population boundary often encompassed 50% or fewer of the polar bear locations, with many bears traveling far outside of the boundary before returning. Researchers are currently examining whether the population boundary should be be readjusted.
Numerous factors influence what a polar bear does on the sea ice, and scientists are starting to get a better idea of what these factors are, though there is still a lot to learn. It is clear that sea ice conditions, season, and individual personalities all affect what polar bears do. It will be interesting to see how these factors influence the population in the future as the sea ice continues to change. Only time, continued research, and monitoring will tell.