Survival of the Fattest? Not for These Polar Bears

Global warming has already had a significant effect on the southern-most polar bear populations at western Hudson Bay, Canada. Researchers there have noted that polar bears are 10 percent thinner and have 10 percent fewer cubs than 20 years ago (Stirling et al. 1999). These Canadian researchers have been gathering data on the polar bears of western Hudson Bay for more than 30 years.



Polar bears hunt their prey, predominantly ringed seals, from the ice. Because the ice in this region melts completely each summer, the bears have adapted a strategy of coming ashore near Churchill, Manitoba, at the end of July and fasting until the ice again freezes in November. To overcome these months of fasting, the bears must have accumulated an adequate reserve of fat before the ice breaks up. The spring thaw is now occurring nine days earlier than it has in the past. The tragic result is that the bears arrive onshore about 40 pounds lighter for each week break-up occurs earlier. This translates into polar bears that have not stored up enough fat reserves to deal with the months of fasting and they starve to death, lose their cubs to malnutrition or even resort to cannibalism.

Ian Stirling, a research scientist for the Canadian Wildlife Service, and his colleagues have found not only an increase in cub mortality but also a decrease of 15 percent in birth rates. For these slow reproducing, long-lived animals this can quickly spell trouble.



The early break-up of the ice could be due to the fact that in some of the world's prime polar bear habitat, including Alaska and western Canada, average temperatures have increased 4 to 7 degrees Fahrenheit in the past 50 years, which is twice the global average. Scientists estimate that this pattern of increasing temperatures will result in the sea ice breaking up one week earlier for every degree the temperature rises.

The decline of the population of polar bears at the southern-most limit of their range serves as an indicator of things to come for other polar bears and for arctic wildlife as a whole. As the ice continues to shrink throughout the Arctic regions, we may soon begin to see similar results around the world.