In most species of animal, one or both parents abandon their offspring. It is very rare for both of them to share the caring. When only one looks after the young, this single parental care may come from either the father or the mother, making it a hard job. Although most birds raise their young jointly, the male purple jacana of Central America cares for his young alone. It is one of a few species where a single female mates with several males. Four or five males build nests within one female's territory, and she lays a clutch of eggs in each. Once the eggs are laid, the female has nothing more to do with them - the male has sole responsibility for incubating the eggs and protecting the young chicks until they can look after themselves. Usually it is the other way round - the females look after the young. With mammals, the mother protects the young within her body as they are growing, and provides them with nourishing milk once they are born. A female grey seal goes without food for 18 days in order to feed her pup. In that time the pup's weight increases three times - to nearly 50 kilograms - but the mother becomes very thin and eventually leaves the pup, who looks like a barrel of blubber, to go and feed. Only when the little seal becomes hungry does it enter the water and start looking for fish. The fat it stored from its mother's milk has to last until it can feed itself properly.