Female brown bear gives birth to tiny cub national geographic

The female brown bear enters her den pregnant with one (sometimes two or three) baby bears. 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 bear wakes up in the spring, her baby (or babies in this case!) 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.

National Geographic