Fertilization is the fusion of an egg and sperm cell to form a zygote. Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother. This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth.