Bears' mating systems vary by species; some, such as the spectacled bear, are monogamous. Others, such as the polar bear, are polygamous. Most bears mate in the spring or summer, but the fertilized eggs do not implant in the uterus and begin developing until fall. After this so-called delayed implantation, the eggs begin to develop and the females give birth in the winter. Some species, including the sloth bear, apparently mate year-round in especially warm climates, but due to delayed implantation, all give birth in the winter. Sun bears appear to have delayed implantation, but individuals in zoos have given birth at different times of the year.
Cubs are born small, naked and blind, having developed in the womb for only two to three months. Birth weight ranges from about 11 oz (325 g) in sun bears to 21 oz (600 g) in brown and polar bears. In most cases, females give birth from one to five cubs, although two is the most common litter size among ursids. Panda mothers generally rear only one cub, regardless of the litter size, and the others die. Among cooler-climate bears, the young are born while the mother is in winter dormancy. In warmer-climate species, such as the sun bear, the mother chooses a concealed site, perhaps under branches or thick vegetation, to make a nest for the cubs. Cubs are generally weaned within the first two to five months (pandas wean at about nine months), but remain with the family unit for two to four years, during which the cubs learn to find their own food and hunt while under the protective eye of their mother.
Sexual maturity generally occurs from four to seven years old, but the timing varies among species.
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