Are Marsupials Mammals?

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 really like no other animals on the planet.

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.

The marsupial is one of the three major groups of mammals, along with placental mammals and monotremes (egg-laying mammals). The genetic evidence suggests they split off from placental mammals in the Cretaceous Period, around 125 million years ago, and evolved independently from there. Despite its close association with Australia, the marsupials actually originated in what is now North America, when it was still part of a larger supercontinent called Laurasia. By around 70 million years ago, the marsupials reached South America and Australia, which at the time were nearly connected to each other via Antarctica.

When Australia broke off from the other continents a short time later, the wildlife there evolved separately from anything else on the rest of the planet. Free of competition from placental mammals (which may have died out early or just never arrived), the marsupials thrived in the isolated Australian ecosystem. They may have also displaced the existing monotreme populations thereby competing against them for resources.

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. But on the whole, placental mammals and marsupials have more similarities than differences. This article will cover the top eight reasons why the 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 the egg-laying mammals such as the platypus and echidna will feed their young with milk. In marsupials, the milk is produced from the nipples, located within or sometimes above the pouch.

Although they can produce milk, the actual life cycle of the marsupial differs significantly from placental mammals. The newly born offspring (also known as a joey) is born at a very early stage of development, when it’s still small as a bean and lacking fur. After it’s about a few weeks old, the joey will crawl from the birth canal into the mother’s pouch and remain attached to the nipple until it is ready to begin learning how to survive on its own. This roughly corresponds to the end of what would be the fetal development of placental mammals.

While some marsupials do have temporary placentas, the marsupial development still differs dramatically from the development of placental mammals, despite the presence of mammary glands. Even after leaving the pouch, the joey will sometimes return there for safety and refuge until it’s fully independent.

Some scientists have actually proposed the 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 means (in scientific jargon) there is strong selective pressure to retain the current shape of the jaw. It cannot change and evolve easily in response to new environments. This could be one of the reasons why marsupials are relatively limited in shape, whereas placental mammals have evolved incredibly diverse forms, from whales to dogs.

Marsupials have fur

Mammals are one of 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 to make clothing and other apparel.

Marsupials are warm-blooded

Warm-blooded (also known by the scientific term of endothermy) 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, mammals tend to have a much higher metabolic rate and therefore need more energy to fuel it. Endothermy is very widespread in mammals and birds, but some fish are considered to be warm-blooded as well.

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 that 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). Combined with the four-chambered heart and fast metabolism, this enables them to use energy more efficiently than other animals (and therefore exploit a wider range of ecosystems).

Marsupial’s red blood cells have no nucleus

Mammals are the only class of vertebrates whose blood cells do not contain a nucleus of any kind. 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 the 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 known as the dentary 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.

Scientists now believe that at some point in early mammalian evolution, the bones from the lower jaw of reptiles moved further back in the head, forming the part of the auditory system that transmits sound to the innermost section of the ear.

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.

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