Long-term partnerships are quite rare among mammals. In many species, males compete for the right to mate with females, and the dominant male becomes the exclusive partner of a whole group of females. In other species, only the “queen” breeds.
And although the general rule is that mammal females give birth to live young, there are exceptions. The group of mammals known as monotremes actually lay eggs. While this is extremely unusual among living mammals, it’s normal for most other animals, such as fish, birds, and reptiles. And in fact, the ancestors of mammals laid eggs, too.
Another oddity in the mammal world is the group known as marsupials. While the vast majority of mammals are placentals—meaning their babies develop for a long time inside the mother’s womb—marsupials give birth to extremely immature young that continue to develop outside the womb in a pouch, a fold of skin, or attached to a nipple.
And speaking of nipples, not all mammals have them. Egg-laying monotremes ooze milk from the ducts of their mammary glands. It then collects in grooves on the mother’s skin, where nursing babies lap it up or suck it from tufts of fur.
Placental or Marsupial?
In placental mammals like humans, a complex placenta connects the fetus to the mother, supplying it with nutrients and removing waste products.
The placenta also may protect the fetus from the mother's immune system, which could otherwise attack the fetus because it has genes inherited from the father. The lack of this type of protective placenta may be one reason marsupial babies are born so immature.
There are more than 300 living marsupial mammals—so named for the characteristic marsupium, or pouch, in which many species carry their young.