Being mammals, shrews can produce enough heat to keep themselves warm during the cold night, but this takes a lot of energy so they must eat almost continuously. As there is never quite enough food around, rivals fight to keep hold of a good territory. The winner will not just get the food there, but be able to mate with the local females, to whom the male is almost as aggressive as he is to a rival male. After testing the male's strength, the female accepts him as a mate. Two weeks later the female gives birth. As she has nourished them inside her womb, the young arrive comparatively well-developed. Caring for young is a winning part of the mammal's design and something that few reptiles do. A mother shrew will even quench her baby's thirst with her own saliva if necessary. Most importantly of all though, she provides them with that uniquely mammalian food - milk. The milk is so rich that it only takes two weeks for the young to be almost as big as their mother, by which time they are quite a handful and need to be weaned from the nipple despite their protests. She then leads them outside where the young walk in a line biting down on each others' tails to ensure they do not get lost.