10/27/2010 12:22:26 PM
Polar Bears Eat When They're Bored
This bear is spending part of his time onshore munching grass. It's likely not his favorite food, but it helps to pass the time. Polar bears onshore here in northeastern Manitoba often spend a lot of time eating grass, kelp, or other terrestrial vegetation. They can also feed on items like bird eggs, caribou, or small mammals.
Although these are nutritious foods for other species, polar bears are adapted to prey on marine mammals found on or near the sea ice. Polar bears are big-bodied predators and they got that way by eating energy-rich foods like seals and whales. Foods like grass and sedges provide little nutrition to polar bears, even when consumed in large quantities. As a result, polar bears in Western Hudson Bay lose nearly two pounds of body mass every day they spend on shore.
Polar bears do their most important feeding on the ice where they have access to seals and whales. Most of their diet is comprised of ringed seal, and to a lesser extent bearded seal, but bears in different areas also feed on locally abundant prey including harp seals, beluga whales, and bowhead whales. Polar bear diets also differ between males and females and even among individuals. But overall, polar bear populations rely on ringed seals.
Ringed seal pups are born in the spring and weaned in May and June. At this time of year, the young seals are very fat and not very good at avoiding polar bears. The spring season is critical for polar bears trying to build up their fat stores in preparation for the ice-free period when they'll be back on shore. When the sea ice breaks up early, it can cause problems; some seals may be forced into the water before their blubber layer is thick enough to keep them warm. Polar bears will also have less time to feed and will be forced to come on shore with less fat on their bodies. Unfortunately, this earlier break-up also extends the amount of time the bears have to fast on shore.
The availability of sea ice and the abundance of marine mammal prey is therefore critical to the long term survival of polar bears. In recent years, the ice in western Hudson Bay has broken up about three weeks earlier than it did 30 years ago. This has put stress on the population and caused declines in polar bear body condition, reproduction, and abundance - all because the bears have less time to feed on seals. Although they may spend a lot of time eating grass, polar bears won't turn into cows any time soon.
Photo credits: Top, ©Dr. Gregory W. Thiemann; second photo, ©Andrew Fore; third and fourth photos, ©Daniel J. Cox, Natural Exposures.