Harp seals are born on pack ice. Because the ice is unstable, the harp seals have very short nursing periods - often only 12 days. They drink milk which can be 45% fat. Hooded seals also mate and breed on pack ice. They have the shortest nursing period of any mammal. After just four days, the pup is abandoned and left to fend for itself. This is a direct result of the threat of polar bears which has forced the females to give birth on unstable ice floes rather than on land.