Are Marsupials Mammals? Exploring Their Evolutionary Journey

Marsupials have evolved one of the most unique adaptations in the entire animal kingdom. These devoted mothers keep their offspring with them at all times, hidden away in an abdominal pouch. They are unlike any other animals on the planet. But does this mean marsupials are mammals, or something altogether unique?

These are the defining characteristics of marsupials and an exploration of whether or not they’re truly mammals.

Are Marsupials Mammals? Defining These Creatures

The marsupial is one of the three major groups of mammals, along with placental mammals and monotremes (egg-laying mammals). Some of the most well-known marsupials include wombats, wallabies, koalas, bandicoots, opossums, kangaroos (as well as tree kangaroos), and Tasmanian devils.

An estimated 70% of the more than 250 existing species are found exclusively in Australia. The remaining 30% are located in South America and Central America. Only a single species (the Virginia opossum) is found north of Mexico.

Despite their close association with Australia, marsupials are believed to have originated in South America and then spread to Australia via Antarctica. 70 million years ago, marsupials reached South America and Australia, which were nearly connected to each other via Antarctica during that time.

How Are Marsupials Different From (and Similar to) Other Mammals?

Marsupials differ from placental mammals in several important respects. The ability to nourish young in pouches is probably the most significant anatomical difference between marsupials and placental mammals.

They also have simpler social structures and less varied vocalizations. On the whole, placental mammals and marsupials have more similarities than differences. Let’s address the reasons why marsupials are classified as mammals.

Marsupials Have Mammary Glands

The ability to produce milk through mammary glands is a defining feature of all mammals. Even egg-laying mammals such as the platypus and echidna will feed their young with milk. In marsupials, milk is produced from the nipples, located within or sometimes above the pouch.

Some scientists have actually proposed a theory that prolonged suckling at the nipple limits the marsupial’s ability to evolve into different forms. Because they need to suckle so early on in their development, marsupials grow jaws before they even have fully functioning brains. This could be one of the reasons why marsupials are relatively limited in shape compared to other mammals.

Marsupials Have Fur

Mammals are among the few animals that grow a coat of dense fur primarily for the purpose of insulation. The color of the hair seems to directly influence the amount of heat retained within the body. The fur also serves many other purposes, including sensory feedback, camouflage, and protection for soft skin.

Wombats and Tasmanian devils also have fur that appears to glow under ultraviolet light, but it’s not clear what purpose this serves. Some marsupial species have such dense and luxurious fur that they’ve been hunted almost to the point of extinction.

Marsupials Are Warm-Blooded

“Warm-blooded” refers to the animal’s ability to maintain a warm and comfortable body temperature, even as the temperature of the surrounding environment falls. In order to maintain this temperature, marsupials tend to have a much higher metabolic rate and therefore need more energy to fuel it.

Marsupials Have Four-Chambered Hearts

The four-chambered heart is one of the major innovations of both birds and mammals. Whereas reptiles only have three-chambered hearts and other vertebrates only have two, mammals have developed a more complicated circulatory system.

This system enables the heart to more efficiently sort out oxygenated blood coming out of the lungs from depleted blood traveling to the lungs. This ensures that the animal’s cells receive only the most oxygen-rich blood so it can expend more energy for longer periods of time.

Marsupials Have Strong Diaphragms

The diaphragm is a muscle in the chest that serves the purpose of expanding and contracting the lungs. It also separates the heart and lungs from the abdominal cavity. Mammals arguably have the most advanced diaphragm in the entire animal kingdom (though birds have strong diaphragms as well).

Marsupials’ Red Blood Cells Have No Nucleus

Mammals are unique among vertebrates in having mature red blood cells that lack a nucleus, allowing them to carry more oxygen. The nucleus is the part of the cell that contains genetic information. Without a nucleus, the red blood cells are able to carry even more oxygen-rich hemoglobin around the body, thus fueling a mammal’s high-energy activities.

Marsupials Have a Mammalian Skeleton

Mammals differ from other types of animals in the evolution of their skulls. Whereas reptiles have a multi-piece jaw, mammals have a single-piece lower jaw bone that’s attached directly to the rest of the skull. The single bone allows mammals to cut and chew meat or grind down vegetable matter with incredible force.

Marsupials Have Complex Brains

Marsupial brains are not quite as complex as those in placental mammals. They lack the part of the brain that connects the two halves together. Their brains are also smaller relative to overall body size. Nevertheless, the relatively complicated brain structure, at least compared to many other animals, does enable them to exhibit complex behavior.