What to Eat When There’s Nothing to Eat?

Bear ambassador Mi Ton Teiow finds relatively shallow snow, but no food, as the northern winter begins.

Bear ambassador Mi Ton Teiow finds relatively shallow snow, but no food, as the northern winter begins.

The answer? Nothing.

Most of us are familiar with the idea that some bears spend long periods of time in dens, inactive and not consuming significant amounts of food or water. Some bears in some locations survive eating nothing by doing almost nothing. They become inactive, which is sometimes called winter sleep or hibernation. Although you may be familiar with this aspect of bear ecology, have you thought about how incredible it is? These big mammals can go without eating or drinking for months, sometimes while birthing and nursing cubs, yet wake up without bedsores or weakened muscles! This is why the physiology of bears, including that of giant pandas and polar bears, has been a hot field of research.

The only way for a wild American black bear to survive for months without food in the northern U.S. is to save energy by hibernating in a safe, insulated den. The hole to the right of the tree trunk is the only obvious indication that there is an American black bear below the snow.

The only way for a wild American black bear to survive for months without food in the northern US is to save energy by hibernating in a safe, insulated den. The hole to the right of the tree trunk is the only obvious indication that there is an American black bear below the snow.

Although all female bears seclude themselves in dens to give birth to cubs, not all bears enter dens for long periods of time. There’s even variation within species in whether or not individual bears remain in dens or for how long. Researchers have found that in general, bears spend long periods in dens not to avoid cold temperatures, but to reduce their metabolic requirements when there is not enough food to survive environmental conditions. So, in the southern part of their range where their energy balance can remain positive, individual brown bears, Asiatic black bears, and American black bears may not den except to give birth. At last year’s meeting of the International Association for Bear Research and Management (see post, A Whimsical Bear Ambassador Arrives), Lorraine Scotson and Dave Garshelis reported that some sun bears might den for periods of time in the most northern parts of their range, meaning that non-reproductive denning may occur among at least half of the world’s bear species.

The holes melted in the snow are made by the black bear’s respiration.

The holes melted in the snow are made by the black bear’s respiration.

The giant panda is one bear species that has not been known to den in response to a relative lack of food, and perhaps it cannot do so. During the rare times when all the bamboo plants in an area have flowered and then died, the giant pandas have left the area in search of food; they have not entered dens. Perhaps this is because a bamboo die-off is unpredictable from the giant panda’s point of view, or perhaps this is because a giant panda eating bamboo cannot build up sufficient energy reserves to be able to wait out the lean time in a den, or maybe both factors play a role.

Adult polar bears also do not enter dens solely to avoid food shortages. Pregnant polar bears do spend prolonged periods of time in dens, but biologists think other adult polar bears don’t do so. However, polar bears in some populations regularly fast for extended periods when sea ice conditions don’t allow them to hunt. As for other bears, anything that causes a polar bear to expend more energy, whether inside or outside of a den, or to fast for a longer period of time, makes it less likely that the bear will survive. Climate change is doing just that by reducing the amount of sea ice available to polar bears: the bears expend more energy and go without eating for a longer period of time, creating a great challenge for the conservation of this species (see Polar Bears, Climate Change, and Mi Ton Teiow).

Hibernating bears often line their den with leaves, twigs, and grasses, insulating themselves from the cold ground and conserving additional energy.

Hibernating bears often line their den with leaves, twigs, and grasses, insulating themselves from the cold ground and conserving additional energy.

The Bear Specialist Group’s Ambassador Mi Ton Teiow recently made a visit to the northern US, where he found plenty of snow but little food for bears. After a short stay in this area, which receives an average of 45 inches of snow per year, the ambassador returned (fled?) back to sunny San Diego, where the last measurable snow fell on the city in 1967. The odds are good that Ambassador Mi will not be snow camping in San Diego any time soon.

Russ Van Horn is a scientist with the San Diego Zoo Institute for Conservation Research. Read his previous post, Courtship in Front of the Camera.

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