Bears in Winter

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As we head into the winter months of the Northern Hemisphere, bears across the globe are preparing for a change in weather. But not all bears respond to the season in the same way.

Wild brown and black bears are facing a bleak time of limited food availability in the coldest months of the year. For this reason, late in fall they engage in hyperphagia, compulsively eating anything they can get their paws on. This builds layers of fat that will be essential to keeping them warm and healthy through the upcoming winter. This fat is not only insulation against the cold, but the key to their ability to hibernate. While in their winter torpor, the bears draw on fat to keep their metabolism running, thus minimizing the wastage of their muscle while they fast. Adult female bears need an especially good store of fat to support the energetically demanding processes of birth and lactation while denned up over the winter months.

Pandas are Northern Hemisphere bears, but they do not experience torpor to the same degree as their North American cousins. Unlike the salmon, berries, and roots depended upon by brown and black bears, bamboo does not typically experience seasonal fluctuations in abundance. Only panda females den up, and the timing of their denning seems to coincide with the shooting of bamboo in some areas, making a more nutritious resource available at a time when a new mother needs it most. But the panda males do not experience torpor; they continue eating year-round.

Polar bear females den up in late fall and give birth in November or December. They emerge in the spring. But males don’t den up extensively; instead, the winter is an active time for the males, as the sea ice returns and they can break their fast by hunting on the ice.

Some zoos manage their animals differently, promoting the cycle of hyperphagia and torpor that exists in the wild. At the San Diego Zoo, our cold-weather bears don’t experience the same seasonal food variation as do their wild counterparts. This is why grizzly bear brothers Scout and Montana won’t spend their winter months waiting out the cooler weather; instead, they will be active year-round. Our polar bears, too, don’t have to worry about ice abundance, because the staff supplies them with year-round sustenance. Only our pandas will demonstrate a pattern of behavior typical of their wild counterparts in the Northern Hemisphere.

Of course, it’s a well-known truism around the panda facility: you can always count on Gao Gao to show interest in his bamboo, no matter what or where we are on the calendar.

Suzanne Hall is a senior research technician for the San Diego Zoo Institute for Conservation Research. Read her previous post, Panda Family Reunion?