During the fall, a brown bear eats practically around the clock, stocking up for the four to seven months when it'll have to live off stored body fat. A grizzly may chow down on 90 pounds (40 kilograms) of food each day.

As the cold swoops in, the fattened bear waddles into a den among rocks or one it dug out among tree roots. The female brown bear enters her den pregnant with one (sometimes two or three) baby bears. Mama bear doesn't even wake up as her blind and hairless cub is born midwinter. The tiny bear, about the size of a chipmunk, is just strong enough to crawl into a position where it settles in to nurse. A female brown bear's milk is very rich in fat and calories, so the cub grows quickly.

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​By the time the adult grizzly wakes up in the spring, her baby is strong enough to follow her out of the den. Cubs live with their mothers for up to three years, and then they're usually ready to face life on their own.