Mammalian Reproduction

Big Picture

Most mammals are viviparous and give birth to live young. Such mammals are therian mammals. Like all vertebrates, therian mammals have ovaries. However, therian mammals have two additional structures: the uterus and the vagina. Therian mammals can be split into two groups: placenta mammals and marsupials. Only two living species of mammals are not viviparous, and they are grouped together as montremes.

Key Terms

Therian Mammal: Viviparous mammal that may be either a marsupial or placental mammal.

Viviparous: The characteristic of giving birth to live offspring.

Uterus: Female reproductive organ in therian mammals where an embryo or fetus grows and develops until birth.

Vagina: Female reproductive organ that receives sperm during sexual intercourse and provides a passageway for a baby to leave the mother’s body during birth.

Placental Mammal: Therian mammal in which a placenta develops during pregnancy to sustain the fetus while it develops inside the mother’s uterus.

Placenta: Temporary organ that consists of a large mass of maternal and fetal blood vessels through which the mother’s and fetus’s blood exchange substances.

Marsupial: Therian mammal in which the embryo is born at an early, immature stage and completes its development outside the mother’s body in a pouch on her belly.

Monotreme: Type of mammal that reproduces by laying eggs.

Placental Mammals

Therian mammals have a uterus and a vagina. The embryo develops inside the uterus.

  • Most mammals are placental mammals; they are born as relatively large and developed as fetuses.
  • These mammals are sustained inside their mother by a structure called a placenta.
  • The placenta allows blood from the fetus and mother to exchange substances without actually mixing together. This prevents the mother’s immune system from attacking the fetus.
  • The placenta transfer oxygen, nutrients, and other substances to the fetus while the fetus passes carbon dioxide and other waste to the mother.
Figure: A fetus connected to its
mother by a placenta.
Image Credit: Convit, 2010, used under license from


A few mammals are marsupials, where the young are born and then develop inside the pouch of a mother.

  • Before they are born, they are nourished from inside the uterus by a small yolk sack that contains nutrients.
  • When the embryos are born, they enter their mother’s pouch and then cling onto a nipple.
  • Eventually, when the baby is mature enough, it leaves the pouch.
  • Examples of marsupials are kangaroos, koala bears, and opossums.


Mammalian Reproduction Cont.


There are five living svpecies of non-therian
mammals called monotremes. Monotremes are mammals
that actually reproduce by laying eggs.

  • Examples: platypus and echidna.
  • Females lack uterus and vagina. They have
    cloaca like reptiles and birds
  • Eggs kept inside the mother for a couple
    of weeks.
  • Mother provides the eggs with nutrients.
  • females lack nipples so they “sweat” milk
    on a patch on the belly.
  • Less risk to the mother, but offspring less likely to survive due to difficulty of protecting the eggs

Placental Mammals vs. Marsupials

Placental Mammals
Babies have a higher chance of survival since they’re well developed.
Mother’s immune system is less likely to attack the baby since the embryo only spends shot amount of time in mother’s uterus
Birth is very draining for the mother; she must eat more food and she loses mobility, making
her more susceptible to predation
The baby is extremely weak and has a lower chance of survival