Although they are not true hibernators, they do enter a form of hibernation both physiologically and behaviorally. Fat put on during the fall is metabolized during the winter, since bears do not eat, drink, urinate, or defecate for up to six months. Their body temperature does not change dramatically and only lowers by a few degrees, allowing bears to easily be aroused from their winter sleep. Dens are used only during this period of extended sleep, otherwise grizzlies sleep outside the rest of the year. This denning period usually lasts from October or November to March or April.
Males are the first to emerge and females with cubs are last to emerge.
However fertilized eggs do not begin development until the start of the denning season because of delayed implantation. Cubs are born in January or February, and are about 1 pound at birth. Litter size is usually two cubs, but ranges from 1-4. Cubs will normally stay with their mother for 2-3 years. Home ranges are dependent on food availability, but usually average around 250 square miles in size.