A marsupial is a type of mammal whose defining feature is how it reproduces: the young are born at a very early, underdeveloped stage and then continue growing while attached to a nipple, usually in a pouch or protective skin fold on the mother’s abdomen.
Key defining features
- Marsupial babies (joeys) have a very short gestation in the uterus and are born tiny, blind, and poorly developed compared with placental mammals.
- After birth, the joey crawls to the mother’s underside and attaches to a teat, which is often inside a pouch (the marsupium), where it stays for weeks or months to finish developing.
- Not all marsupials have a deep, bag-like pouch; in some species it is just a fold of skin, but in all cases the young develop externally while fixed to the nipples.
Anatomy and physiology
- Female marsupials typically have two uteri and paired vaginal tracts, and males often have a bifurcated (two-pronged) penis, reflecting their distinctive reproductive system.
- Many marsupials possess epipubic bones (extra bones at the front of the pelvis) that help support the pouch region.
- Compared with most placental mammals, marsupials generally have different tooth patterns and some differences in brain structure, such as lacking a true corpus callosum in many groups.
How they differ from other mammals
- Monotremes lay eggs, placental mammals keep the fetus in the uterus with a complex, long-lived placenta, but marsupials use only a brief placental connection and then rely on extended lactation in a pouch or on exposed teats.
- Marsupials are fully mammals: they have fur and mammary glands and produce milk, but their reproductive strategy is what sets them apart as a distinct infraclass (Metatheria).
Examples and distribution
- Well-known marsupials include kangaroos, wallabies, koalas, wombats, and opossums.
- Most species live in Australia and nearby regions, with a smaller number (such as opossums) native to the Americas.
At-a-glance comparison
Group| Birth and development| Typical example
---|---|---
Monotremes| Lay eggs; young lap milk from skin patches, no nipples.7| Platypus
Marsupials| Very short gestation; tiny young grow on teats, usually in
pouch.157| Kangaroo, opossum
Placental mammals| Longer gestation with full placenta; young born more
developed.67| Human, dog, whale
