Reproductive system

There are three different modes of reproduction used by mammals. The monotremes, whose extant members are the echidnas and duck-billed platypuses, lay eggs. The therians (marsupial and placental mammals) give birth to live young. Marsupial newborns are undeveloped (some mammalogists call them embryos). After only a short gestation period they must make their way to a teat outside the mother's body (a teat that may be in a pouch in species that have pouches) to finish development. The embryos of placental mammals remain in the uterus during development, and they have a nutritive connection with the mother through the placenta. The young of placental mammals are born more mature than the young of the other two groups.

The female reproductive tract in monotremes is very much like a reptile's. A cloaca (also found in amphibians, reptiles, and birds) is a common chamber for the digestive, urinary, and reproductive system. The eggs are conveyed from the ovaries through the oviducts where fertilization occurs. After fertilization the eggs are covered with albumen and a leathery shell produced by the shell gland. In therian females the reproductive organs are separate from the urinary and digestive systems. The marsupial female has two uteri, each with its own vagina. Eutherian females may have either a single uterus or paired uteri, but always a single vagina. The pla-cental embryo implants and develops in the uterine wall.

In all therians, the male urinary and reproductive systems share a common tract, the urethra. A problem for endother-mic mammals is that their body temperature may be too high to sustain viable sperm. This is not a problem for monotreme males because their body temperature is lower than that of therians, and their testes are contained in the abdominal cavity. The testes of therian males are typically contained in a scrotum, a sac-like structure that lies outside the body cavity. The testes may descend into the scrotum from the abdominal cavity only during breeding season or they may be permanently descended. The penis differs in the three main groups of mammals. The monotreme penis is attached to the ventral wall of the cloaca. The marsupial penis is directed posteriorly, contained in a sheath, and the glan penis (tip) is bifid, which accommodates the two vaginas in the marsupial females. The eutherian penis is directed forward. It may hang freely or be contained in an external sheath. In many species, including most primates, a bone called the baculum supports the penis.

Mammary glands (see also the discussion under integument) provide nourishment for the young mammal. While milk requires energy to produce, it also conserves energy for the mother: Mammals do not have to make numerous trips to find food and return with it to feed their offspring. Observations of bird parents making trip after trip in order to feed insatiable hungry mouths at the nest illustrate this point. A mammal mother obtains her food, returns to the nest or den, and can feed her young in comparative safety.

Pregnancy And Childbirth

Pregnancy And Childbirth

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