Most species of bear come together in the spring or summer to mate. The she-bear becomes restless at the beginning of the mating season, expanding her daily expeditions. The male bear can tell whether or not she is ready to mate by the smell of her droppings and urine marks. During the first encounters, the female is initially cold towards the male and rejects him. He, in turn, attempts to impress the object of his desire by standing up and growling loudly. Finally, copulation takes place repeatedly over one or two days, triggering ovulation in the female. The bears often stay together for a few days or weeks afterwards.

As the females in some species of bear mate with various partners, it is possible for the young in a litter of cubs to have different fathers.

For a long time, the gestation period of bears was a mystery. In the case of brown bears, for example, a pregnancy period of six months was just as normal as one lasting nine months. The reason is that the development of the fertilized eggs is interrupted for different lengths of time. The so-called diapause ensures that – despite a mating season of many months – the young are not born until the she-bear is lying inside her secure winter den. If the she-bear has not eaten enough to put on sufficient fat to last her until the fall she will not become pregnant at all. Implantation of the ovule, and thus the beginning of embryonic development, which then lasts a mere six to eight weeks, only occurs if the female has a sufficient layer of fat for the winter.

The new-born offspring are tiny and helpless. Even the young of a 300-400 kilogram polar bear weigh just 500 grams and are hardly larger than guinea pigs. The little bears remain in the safety of the winter den for several months where the high fat content of their mother’s milk allows them to grow rapidly.