The Difference Between Marsupials & Placentals

Pouch

• The most famous sign of a marsupial is the pouch; this is found on most marsupials, such as the kangaroo and the koala. A marsupial mother gives birth very early, typically after only a few weeks. The newborn animal then crawls to its mother's pouch and attaches to its mother's teat. The newborn lives in the controlled environment of the pouch for months, growing and developing until it is strong enough to survive on its own. Even after the newborn can live outside the pouch, it may continue to sleep in its mother's pouch or return there during times of danger.

Placentals

• The role of the placenta distinguishes marsupials from placental mammals. A placenta is a specialized organ found in female mammals that nourishes a developing fetus inside its mother. The placenta derives from the membranes also found in the eggs of birds and reptiles. While both marsupials and placentals have a placenta, its role is significantly limited in marsupials, and provides nourishment to a fetus for a very short time before the fetus is born and moves to its mother's pouch. The placenta nourishes the fetus of a placental mammal until the fetus becomes viable.

Female Reproductive Organs

• The female reproductive organs of marsupials differ from placental mammals. A female marsupial, like a placental female, has two ovaries and oviducts. However, in a marsupial, the oviducts lead to two separate uteri with separate cervices. These uteri lead to a single vaginal expansion, which splits into three separate vaginae; two lateral vaginae and a central vagina that acts as a birth canal. These three vaginae meet at a central opening called a cloaca. A placental mammal only has one uterus, cervix and vagina.

Male Reproductive Organs

• Marsupial males contain the same structures as placental males; however, they have some differences. A marsupial's scrotal organs rest in front of the penis instead of behind. In addition, the penis of most marsupial males splits into two. This is thought to be related to the two lateral vaginae in marsupial females. However, experts debate this conclusion as not all marsupial males have this feature.