For polar bear mothers and cubs, den departure day in the High Arctic arrives in April or May—just in time for ringed seals to have their pups and the annual polar bear feast to begin. Ringed seals will start pupping in their own snow shelters above the sea ice but below a thick layer of snow.

The seal pups make a welcome snack for the hungry bears, but the real reward comes from catching adults. The pupping season provides a bounty that makes being a polar bear possible. The spring is the most important foraging period of the year and the bears will gorge themselves on huge meals of seal blubber. A polar bear can easily consume 20 percent of its body weight in a single meal: for an adult male, a meal this size would be roughly equivalent to eating over 200 blocks of butter. That might sound a tad fattening, and it is. Of those 200 blocks of butter, the equivalent of over 190 of them are going to be sacked right onto the bear.

For females that will be mating in the next few months, fat is where it's at and while they're only about half the size of an adult male, they'll be needing a huge store of fat. Some females triple their weight over the spring. In some populations, pregnant females will live off their fat stores for 8 months and eat nothing. While we visualize the most fit humans as those lean and muscular archetypes, the most fit polar bear female is the most Rubenesque. No polar bear ever looked at its reflection in the ocean and thought "I'm too fat." It must be nice—especially when polar bears seem immune to the effects of a high-fat diet. Clearly, omega-3 fatty acids are a bit easier on their system than butter is on ours. Sweet potato fries cooked in seal fat, anyone?

Dr. Andrew E. Derocher is a professor of biological sciences at the University of Alberta and a long-time scientific advisor to Polar Bears International.