What is a Placental Mammal?
Image Citation: OpenClipart-Vectors (artist). (2013). Embryo Human Infant [digital art]. Retrieved from https://pixabay.com/vectors/embryo-human-infant-pregnancy-159691/
To put it very simply, a placental mammal is any mammal that produces a placenta during pregnancy. However, that definition isn’t exactly helpful if you don’t know what a placenta is to begin with.
The placenta is an organ that develops during pregnancy in mammals that gestate their offspring internally and give live birth. This organ allows the mother to pass nutrients and oxygen to the developing fetus while also removing wastes from the fetus’s bloodstream. The placenta also works to prevent infection in the uterus and releases important pregnancy hormones.
Despite its very important role in pregnancy, if you were to see a placenta in real life you would probably think it looked like a clear plastic bag full of blood. The placenta is made of dense, squishy tissue and full of blood vessels that allow for exchange of materials between the mother and the fetus. It also includes the umbilical cord, which is where the fetus is connected to the placenta. Your bellybutton is where you were connected to the placenta as a baby!
When a female mammal gives birth, the placenta is also ejected from the body. It’s part of what some people call the “afterbirth”. This occurs because the temporary organ is no longer needed; many mammals consume their placenta in order to recycle its nutrients.
The vast majority of mammals are placental mammals. Exceptions include all marsupials (such as kangaroos, wallabies, and opossums) and monotremes (such as platypuses and echidnas). This means that humans are placental mammals too!
Most early mammals probably laid eggs similar to the monotremes, so the placenta is a relatively new adaptation in terms of mammalian reproductive strategies. It is found in so many mammalian lineages because carrying offspring internally confers a big evolutionary advantage; more offspring survive when they can be carried safe and sound inside their mother’s body.