An adult grey seal usually eats about 5 kg of food per day, mainly fish. The fierce leopard seal also kills the pups of other kinds of seal, while one kind of sea-lion occasionally eats penguins. Walruses enjoy eating molluscs such as oysters, echinoderms such as starfish, and crustaceans such as shrimps. Some species of seals gather together in large numbers during the breeding season. These groups are called colonies or rookeries.
Islands are popular breeding grounds because there are fewer predators around to kill the young.
Grey seals give birth to their young in the autumn or winter. Usually, only one pup is born. Grey seals, like other earless seals, suckle their pups for just a short time. When she is suckling the pup, the mother does not feed, drawing on her reserves of fatty blubber to produce milk. Pups, however, grow rapidly and are ready to find their own food after about two weeks.