There are three different modes of reproduction used by mammals. The monotremes, whose extant members are the echidnas and duck-
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 placental 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 endothermic 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-
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.