Polar bears are sexually active when they are 4 or 5 years old. Matings take place out on the sea ice in April and May. Cubs (generally 1 or 2 per litter) are mainly born on land between late November and early January. Female can only produce five litters in their lifetime, which is one of the lowest reproductive rates of any mammal. Mothers and bear cubs go back to the sea ice few months after the birth. Cubs are weaned after two and a half years of age.