Scientists can read a bear's diet in its hair. Stable isotope analysis is a method that tells how much of a bear's diet came from animals and how much came from plants. Hair shows about a year of diet, while blood serum shows only the previous 10 to 14 days.

Most of what a brown bear eats is plants, and scientists have recorded grizzly bears in the Greater Yellowstone area eating more than 260 kinds of food. Ants showed up in every grizzly bear scat study in that area, from 1943 to 2009.

In Yellowstone, a grizzly's menu follows the calendar. From March through May, elk and bison make up a large part of a grizzly bear's diet, and around midsummer the bears start eating strawberries, huckleberries, whortleberries and buffaloberries. In summer, some grizzlies also climb to high mountain slopes where moths gather, and eat them: army cutworm moths fly long distances in early summer from farm fields on the Great Plains up to alpine slopes in the mountains. Bears feeding on moths on talus slopes can eat 40,000 to 60,000 moths in one day, and the moths provide 8 kilocalories per gram, the richest documented food for grizzly bears in the area.

In September and October, whitebark pine nuts are the most important food in years when the seeds are plentiful. Whitebark pine seeds are high in fat and protein, which is why they are such a valuable food, and grizzly bears mostly get them by raiding caches, called middens, that red squirrels have stored. Mountain pine beetles have killed trees in about 82 percent of whitebark pine stands in the Greater Yellowstone area, yet when the seeds were scarce, Yellowstone grizzlies seemed to eat more animal matter, and their body condition did not change in poor seed years.

Fall is the season for storing fat. Hyperphagia is the time in fall when a grizzly eats far more than usual and turns the extra food into fat, and it lasts two to four months before the bear enters its den. In the fall, a grizzly bear may gain as much as 3.64 pounds (1.65 kilograms) in a single day.

What starts the winter sleep, and what ends it? Scientists followed 14 free-ranging brown bears over three winters, using tracking and monitoring technology that recorded movement, heart rate, body temperature, activity, air temperature and snow depth. The bears' activity, heart rate and body temperature began to fall several weeks before they entered the den, and they entered when snow arrived and the air temperature reached 0 degrees Celsius. The study concluded that brown bear hibernation was started mainly by cues from the environment, but ended by cues from the bear's own body.

Inside the den, the body changes a great deal. Researchers measured the heart rate of 13 young brown bears in Scandinavia, and it averaged 26 beats per minute in late hibernation and 71 beats per minute in early summer. A denning bear loses very little muscle and bone despite months of almost no movement, which interests medical researchers.

As of November 2026, the estimated Greater Yellowstone grizzly population rose from 136 in 1975 to a peak of about 1,030 in 2024, and the U.S. Fish and Wildlife Service counts at least 1,923 grizzlies in the lower 48 states. USGS scientists used a 2004 genetic sampling effort and earlier captures to estimate 765 grizzlies in the Northern Continental Divide area, more than double the earlier estimate based on sightings of females with young.

A grizzly bear is one kind of brown bear, and scientists agree there is only one species of brown bear. Scientists once named many grizzly subspecies from body measurements, but genetic studies suggest there are fewer. For North America, two are currently recognized, the mainland grizzly and the Kodiak bear.

Words to know

• isotope: kind of atom
• serum: blood fluid
• subspecies: kind within species
• hyperphagia: fall overeating
• middens: stored caches