What Are Marsupials? Are Marsupials Mammals?

By Ferdinand Bada on September 24 2018 in World Facts

A Rock Wallaby with a joey in her pouch.

What Are Marsupials?

Marsupials are a group of animals whose members are thought of as pouched animals and whose young ones are born partly developed. The extant species of these animals, which belong to the class Marsupialia, are endemic to the Americas and Australasia (New Zealand, Australia, and nearby islands in the Pacific Ocean). The name “marsupial” is obtained from the pouch that these animals use to carry their young ones. The abdominal pouch is known as a "Marsupium" that derives from the Greek word “mársippos,” which translates to “pouch”. One of the most well-known marsupials is the kangaroo, but there are many other marsupials including the likes of possums, wombats, opossums, koalas, Tasmanian devils, and others.

Are Marsupials Mammals?

Marsupials are classified as mammals because they fulfill the criteria set forth by taxonomists for identifying mammals. One of the criteria is that the animal has to give milk, which is something that marsupials do for their young ones. Another reason is that they are warm-blooded animals, that is, they do not rely on external weather conditions to regulate their body temperatures. The animals also fulfill other conditions such as bodies covered by true hair instead of scales, three bones in the middle ear, and have one jawbone. However, marsupials are mammals in a class of their own as they have a number of differences from other mammals.

What Makes Marsupials Different?

One of the most obvious differences is the way they handle and gestate their young ones. Typical mammals do not have pouches for carrying young ones while marsupials do. For some animals, like the kangaroo, the pouch is located on the front while for others, especially those that walk using four legs, the pouches at the back. Since the young ones of most mammals are born fully developed, the gestation period is usually long relative to the size of the mammal. However, for marsupials, the undeveloped young ones are gestated for a period of between four and five weeks. For example, when a baby kangaroo is born, it is at the fetal stage of development. After that, it will make its way to the pouch and not come out for months until it is fully developed. The short period of gestation is due to the yolk-type placenta (which is considerably less complex) that marsupials have compared to other placental mammals.

Another difference comes in the structure and number of teeth. Compared to placental mammals, marsupials usually have more teeth. Whereas most placental mammals have three molars and four premolars in the upper and lower jaws, marsupials have four molars and three premolars in the same regions. Another difference relating to the teeth is that placental mammals usually have two sets of teeth namely the baby set and adult set that replaces the baby set during adulthood. On the other hand, marsupials can only replace certain teeth.

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