How do scientists learn what a bear does in winter? They followed 14 free-ranging brown bears over three winters. The scientists used tracking and monitoring technology that recorded movement, heart rate, body temperature, activity, air temperature and snow depth.
In the study, the bears' activity, heart rate and body temperature began to fall several weeks before they entered the den. They entered when snow arrived and the air temperature reached 0 degrees Celsius. Heart rate variability, which researchers used to track the nervous system's activity, dropped sharply once a brown bear entered the den.
What happens inside the den? Researchers did ultrasound of the heart (echocardiography) on 13 young free-ranging Scandinavian brown bears during late hibernation and in early summer. Heart rate 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.
How cold does a bear get? 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.
Bears lower their metabolism and heart rate in the den without harm to their hearts, which scientists hope could help treat heart disease in people. A denning bear also loses very little muscle and bone despite months of almost no movement, which interests medical researchers.
A bear in its den lives on the fat it stored up during the fall. The fat a bear stores for winter gives it energy and also keeps it warm.
Around Yellowstone Lake, grizzly bears feed on cutthroat trout that come up streams to spawn. A 2015 Yellowstone report estimated that cutthroat trout numbers were below 10 percent of their historical levels, while the trout eaten by grizzlies around the lake fell by 70 percent from 1997 to 2007.
A radio-collared grizzly was first seen feeding on moths in the Greater Yellowstone area in 1986. Army cutworm moths provide 8 kilocalories per gram, the richest documented food for grizzly bears there, and eating moths can supply a typical grizzly with about half of its yearly calories in a 30-day period.
How do scientists tell how fast a grizzly population is growing? USGS scientists estimate it by collecting hair at natural bear rubs along trails, roads, and fence and power lines.
Brown bears in the lower 48 states lost about 98 percent of their former range, and, as of November 2026, grizzly bears in the lower 48 states are protected as a threatened species under the Endangered Species Act.
Words to know
- echocardiography: heart ultrasound
- metabolism: body energy use
- spawn: lay eggs
