Male polar bears sometimes congregate, but otherwise bears are solitary except during the mating season. Then they tend to congregate, pair off, and mate in seclusion. The male leaves the female soon after mating and plays no role in raising the young. Gestation periods vary, the fertilized egg remaining dormant in the uterus ( delayed implantation), which ensures the birth of young while the female is in the winter den and guarantees that the cubs will emerge from the den in the spring, when food is abundant. Ursids breed once per year at most, and many bears breed only every two to four years. The breeding season is usually in late spring or early summer. Delayed implantation results in most births occurring in January or February. Newborn bears weigh about half a kilogram (one pound) and are about 23 cm (9 inches) long from the nose to the tip of the short tail. Twins are most common in bears, but up to five young may be produced. The cubs nurse for a few months and stay with the female until the next breeding (about a year and a half or more after birth). Most young, however, can get along on their own by about six months of age. Bears reach breeding condition at three and a half to six years of age, males usually maturing later than females. Longevity of bears in the wild ranges from 15 to 30 years, but in captivity they can live considerably longer.