Mammals life cycles can vary, especially in some varieties of mammals that may not be so obvious, such as dolphins and platypuses. The life cycle, while seemingly simple, can be fairly complicated and cause for many questions. There are some commonalities in every mammal's life cycle and also some exceptions to the rules.
All mammal life cycles start with mating. A male and female copulate, many times after a mating ritual, dance or call, and the cycle of life begins. Mating in mammals takes place inside of the female's body. The male inserts the male sex organ into an opening in the female and the mating process begins.
Meiosis is a term defining what happens post-mating. The two cells, or gametes, combine. Male gametes are sperm and female gametes are eggs. Most often, male gametes have flagella, similar to a tail, which helps the male reproductive cells travel to the female gamete. This all occurs within the female's body. This, much like the hard outer shell of a bird egg, is what protects the "precious cargo"—a new life. This new life is known at this point as the zygote, or the combined egg and sperm.
Gestation periods are the point from when the zygote is formed until the baby is developed birthed. In humans, this period is an average of nine months. During gestation, the cells are constantly multiplying. The cells, known as stem cells, form into different parts of the body and each stem cell has a special purpose in development.
Females give birth to the infants, which come in different numbers ranging from one to several, depending on the species. For example, except in rare occurrences when the cells split or fertility drugs are used, female humans give birth to one child. In the instance with cats, they can have litters ranging from two to six kittens. Also, birthing patterns are different. Female humans give birth to babies head first, as a way for the baby to be able to breath as soon as possible after the water has broken. In dolphins, babies are born tail first, so that they have ample time to surface to breath after being born.
Mammals do not go through stages of larvae, nymph or anything of the like. Mammals simply grow into larger versions of themselves at birth, until they are at an adult age and growth stopping point of maturity. Once reaching maturity, female cycles (or periods) begin and males begin seeking out females for courtship and possible mating. This is where the life cycle starts over again.
An important aspect of mammal development, and a key trait of mammals, is the ability to nurse the young. Baby mammals can find nipples on the mother's body (which vary in number based on species) and nurse the nipple to retrieve milk. Nursing milk is important to development, and the female's body knows when to change the nutrients in milk to make for a promising development.
Though nearly every mammal follows this pattern of birth, there is one odd-looking mammal that does not. The platypus is an odd-looking creature, with webbed feet, claws, a beak similar to a duck's, a beaver's tail and thick brown fur. A platypus is a monotreme, or a mammal that lays eggs. It's only similar mammal is the echidna, or a spiny anteater, that also lays eggs. The females of both species make nests, much like birds, to lay hard-shelled eggs into. They later sit on the eggs until they hatch. Baby platypuses and echidnas drink their mother's milk like other mammals for nourishment.