11/21/2012 6:25:03 PM
CSI Arctic: Understanding Polar Bears
Search "world's largest land carnivore" in Google and you may find a Wikipedia entry on polar bears. This distinction is a misnomer because polar bears are totally reliant on marine mammal prey and marine habitats. A recent study I led sheds new light on the unique nature of polar bear feeding ecology in the Beaufort Sea.
Studying polar bear feeding ecology is akin to being a detective at a crime scene. Most of our knowledge on polar bear feeding comes from observations after a kill has occurred. Identifying a seal kill site from a helicopter is straightforward: snow with blood. A dead seal is also an offer most polar bears cannot pass up, so tracks usually converge on a kill. We note the habitat and location, and seal jaws and claws are collected for analysis. Just like TV's CSI, laboratory DNA analysis confirms prey species and gender, while a tooth or claw is used to age the killed seal. Once all the data is combined, scientists can reconstruct the scene of the kill.
Spring is the prime polar bear feeding season, similar to grizzly bears gorging at salmon streams in autumn. Polar bears enter a heightened state of consumption called "hyperphagia", leading to large weight gains necessary for the lean months ahead. We wanted to know: what is it that makes this boom in feeding possible?
Without doubt, polar bears are ringed seal specialists: ringed seals comprised 90% of the kills. Ringed seals give birth in spring with a single pup born around mid-April in a subnivean lair (a snow-covered den built on the sea ice surface). Polar bears will crush the roof of the lair in an attempt to catch the pup inside. Trends in the seal kill data indicate that the sheer abundance of ringed seal pups contributes to the increase in polar bear feeding. Polar bears responded to the number of pups available from year to year, killing more pups when ringed seal reproduction was high. Ringed seal pups, however, are only part of the story.
The seal kill data revealed some unexpected results. More adult seals were killed than expected. Polar bears are thought to focus predominantly on younger seals and they do kill lots of pups, but adults were the most frequently killed age-class. Further, it was the oldest adults that were most vulnerable, with 50% of the adults killed over 20 years old. It seems that polar bears make a living off the young and tender and old and chewy. The proportion of male and female ringed seals was equal: no gender bias in the polar bear meal plan.
Is there a common thread that ties this all together? Possibly.
Breeding is stressful for both male and female adult ringed seals, and most are in worse body condition by the end of spring. Ringed seals also tend to use the surface of sea ice more often in spring than other times of the year. This combination may make adult ringed seals an easier target for polar bears. Ringed seals do not stop reproducing with age, and older animals may be less able to cope with the stress of breeding, making them more vulnerable.
Understanding what drives a polar bear's diet is vital to understanding the impacts of climate change. By being good detectives, we can further our understanding of one the world's largest and most enigmatic carnivores, and address urgent climate change questions. As for Google and Wikipedia getting the right information, we've got some work to do.