Let's celebrate Siblings Day by peeking at the antics of these playful grizzly bear cubs (while staying clear of their mother). Pregnant female grizzlies settle into their den in winter and give birth while hibernating, usually to two cubs. While their mother sleeps, the cubs nurse and grow quickly. When springtime arrives, the new family emerges from the den to search for food. The siblings will live with their mother for two to three years, after which they'll venture off on their own.