Grizzly bears spend the winter sleeping in a den. A bear's winter sleep is not true hibernation, because its body temperature does not drop very far.

Scientists followed 14 free-ranging brown bears over three winters with tracking and monitoring technology that recorded movement, heart rate, body temperature, activity, air temperature and snow depth. Brown bears in Scandinavia are shallow hibernators: during the den period their body temperature drops only a little, to about 33 to 34 degrees Celsius.

Researchers measured the heart rate of 13 young brown bears in Scandinavia: it averaged 26 beats per minute in late hibernation and 71 beats per minute in early summer. In the den, a grizzly bear's heart beats much more slowly and its body uses less energy.

Hyperphagia is the time in fall when a grizzly eats far more than usual and turns the extra food into fat; it lasts two to four months before the bear enters its den. Brown bears can gain hundreds of pounds each fall to get ready for their winter sleep.

Whitebark pine seeds are high in fat and protein, which is why they are such a valuable food for grizzly bears. In September and October, whitebark pine nuts are the most important food for Yellowstone grizzlies in years when the seeds are plentiful. Grizzly bears mostly get whitebark pine seeds by raiding caches, called middens, that red squirrels have stored.

Insects are a big food for a grizzly. Eating moths can supply a typical grizzly with about half of its yearly calories in a 30-day period. Ants showed up in every grizzly bear scat study in the Greater Yellowstone area, from 1943 to 2009.

A bear in its den lives on the fat it stored up during the fall. All winter in the den, a grizzly bear does not eat or drink. A denning bear loses very little muscle and bone despite months of almost no movement, which interests medical researchers.

Words to know

• hibernators: winter sleepers
• hyperphagia: eating a lot
• middens: stored seed piles
• scat: bear droppings