Pups are about long at birth and weigh about . They are born on the ice from mid-March to early April. They are born with a slate blue-grey coat with a pale cream color on the belly. They moult after about 14 months. Nursing of the pup lasts for an average of only 4 days. This is the shortest lactation period of any mammal. During the 4 days the pup doubles in size, gaining around 7 kg/day. This is possible because the milk that they drink has a fat content of 60%. The female pup will mature between ages 3 and 6. The male pup will mature between ages 5 and 7.