Marsupials

If you’ve ever seen a kangaroo–whether in person, on the TV, or over the internet–you have probably noticed its distinctive pouch, where it holds its young. Have you ever wondered what purpose the pouch serves?

Kangaroos are marsupials, a group of Mammals whose offspring are born prematurely and develop outside the mother’s body. The pouch protects the vulnerable young, which are just about the size of a honeybee at birth.

Read on to learn more about the definition, characteristics, and evolutionary history of marsupials!

What is the Definition of Marsupials?

Mammals are vertebrates with mammary glands which produce milk for offspring. There are around 5,300 extant species of mammals on Earth.

Along with monotremes (egg-laying mammals, such as platypus and echidnas) and eutherians (placental mammals with a long gestation period), marsupials are one of the three major lineages of mammals.

The definition of marsupials is shown below.

What are Some Examples of Marsupials?

There are over 330 species of marsupial extant and around 250 of these species are native to Australia, New Guinea, and neighboring islands, where they make up most of the native mammals there.

These range in size from as small as a mouse, like the long-tailed planigale (Planigale ingrami), to larger than an adult human, like the red kangaroo (Osphranter rufus). Other examples of marsupials include bandicoots, cuscus, koalas, opossums, and wombats.

Around 70 species live in South and Central America, while one species–the Virginia opossum (Didelphis virginiana)–is native to the United States and Canada.

What are the Different Types of Marsupials?

Extant marsupials can be divided into three American orders and four Australasian orders (Fig. 1).

• The American orders are:

  • Didelphimorphia (opossums);

  • Microbiotheria (monitos del monte); and

  • Paucituberculata (shrew opossums).

• The Australasian orders are:

  • Dasyuromorphia (carnivorous marsupials);

  • Diprotodontia (kangaroos, possums, and koalas);

  • Notoryctemorphia (marsupial mole); and

  • Peramelemorphia (bandicoots).

While they are taxonomically less diverse than eutherians (which have a total of 19 orders), marsupial species are just as morphologically and ecologically diverse.

Marsupial structure varies according to the ecological Niches that they fill.

• Burrowing marsupials, like marsupial moles and wombats, have powerful foreclaws that they can use to tunnel into the ground for food and shelter.

• Gliders (or flyingpossums) have a membrane attached to the forelegs and hind legs, allowing them to glide down from a high perch. The wateropossum, which inhabits tropical rivers, streams, and lakes, is semiaquatic and has webbed feet.

Marsupial feeding habits are just as diverse as the Niches they occupy.

• Dasyurids, a group that includes the Tasmanian devil, feed primarily on insects and other small Animals.

• Wombats and many other marsupials feed only on Plants and plant materials. For instance, the honey possum, along with other marsupials, feeds on the nectar of flowers and serves as a pollinator.

What are the Distinguishing Characteristics of Marsupials?

A marsupial can be distinguished from other mammals by the premature birth of its young, which completes its development while nursing. The young–in its vulnerable embryonic condition–makes its way from the mother’s birth canal to the nipples, where it finds shelter, warmth, and nourishment.

It stays securely attached to the nipples for weeks or months, a duration roughly comparable to the latter stages of fetal development in a eutherian mammal’s womb. After this stage, they are weaned and begin to look after themselves.

The marsupial reproductive anatomy is also unique to the group:

• All female marsupials have a pair of lateral vaginae that each open into a common cavity called the urogenital sinus, which then empties into a short chamber called cloaca that connects to the intestinal tract. They also have two uteri (Fig. 2).

• In male marsupials, the testes are found in front of the penis.

Do all Marsupials have a Pocket?

In most species, the developing young are held in a maternal pouch called marsupium, which is a layer of skin that covers the nipples.

However, note that the marsupium is not a universal feature: while many marsupials have a prominent pouch, some only have remnants of a pouch, while others do not have a covering over their nipples at all.

How are Marsupials Similar to and Different from Monotremes and Eutherians?

Marsupials are similar to other mammals in that they have hair and mammary glands.

Marsupials share some characteristics with eutherians that distinguish them from monotremes: they both have higher metabolic rates than monotremes, they have nipples that provide milk, and, instead of laying eggs, they both give birth to live young, with the embryo developing within the mother’s uterus. They also both have a placenta.

Yes, that means, contrary to popular misconception, eutherians are not the only mammals with a placenta. Marsupials also have a placenta, but it develops late into pregnancy, giving them a shorter gestation period. Furthermore, their placenta is composed of tissues that are different from those of eutherians.

That means the difference between marsupials and eutherians is not one of having a placenta or not, but rather one of the relative importance given to placentation and lactation to nurture the developing young:

• In eutherians, the mother invests a roughly equal amount of energy in rearing its young before birth via placentation and after birth via lactation.

• In marsupials, gestation is brief, and the placenta develops late into pregnancy. Lactation is extended and takes place in multiple stages, with the milk composition changing as the offspring grows (essentially performing the function of the eutherian placenta).

What is the Evolutionary History of Marsupials?

Before true mammals emerged, there were synapsids: four-limbed terrestrial vertebrates that lacked hair, had a sprawling gait, and laid eggs. They had a single temporal fenestra, a hole found behind the eye socket on each side of the skull.

Over the course of 100 million years, mammalian features gradually surfaced in the jaw of succeeding lineages of early non-mammalian synapsids.

During the Permian period, synapsids evolved into massive herbivores and carnivores, and for a while they dominated the tetrapod community. Their diversity sharply declined during the Triassic period (around 252-201 million years ago), but more and more mammal-like synapsids began to emerge. These synapsids were small, hairy, and ate insects. They also had a relatively high Metabolic Rate. They also still laid eggs.

True mammals first emerged during the Jurassic period around 201 to 145 million years ago. During this time, they diversified into many short-lived lineages. They coexisted with dinosaurs in the Jurassic and Cretaceous periods.

However, these early mammals were different from the mammals we know today. They were neither dominant nor abundant. In fact, they were really small: most of them measured no longer than a meter. Their diminutive size could be attributed to dinosaurs occupying the ecological niches of large-bodied Animals.

In the early Cretaceous (around 140 to 100 million years ago), the three major mammal lineages emerged all over the world: monotremes, marsupials, and eutherians. Monotremes diverged from other mammals around 200 million years ago. Marsupials and eutherians diverged from a shared ancestor around 130 million years ago.

Ancestral marsupials are thought to have migrated to Australia and the nearby islands on land. Keep in mind that Australia used to be connected to South America and Antarctica as part of Gondwana, an ancient supercontinent that existed over 600 million years ago and broke up in stages starting 180 million years ago (Fig. 3).

Alongside the breakup of supercontinent Gondwana, another event drove evolutionary change in mammals: the Extinction of large non-avian dinosaurs, pterosaurs (flying Reptiles), and marine reptiles in the late Cretaceous period (around 100 to 66 million years ago).

Mammals underwent adaptive radiation--the process in which a group of organisms diversifies into forms to fill different ecological niches--giving rise to large predators and herbivores, as well as flying and aquatic species.

By 65 million years ago, the breakup of Gondwana had separated present-day Australia and New Guinea from other continents, resulting in the diversification of Australasian marsupials in isolation from eutherians that began adaptive radiation on other continents.

It is important to note that up until around 10,000 years ago, Australia, New Guinea, Sulawesi, and some neighboring islands were still connected as part of the Sahul continent. The Sahul continent was separated from Sundaland (present-day Java, Borneo, Sumatra, and neighboring islands) by water trenches, typically referred to as the Wallace Line.

For this reason, Australasian marsupials are typically found in Australia, Sulawesi, the Maluku Islands, and New Guinea, which are all east of the Wallace Line, but not in Borneo, Sumatra, or Java. In contrast, tigers, elephants, non-human apes, and rhinos are typically only found west of the Wallace Line.

Likewise, South and North America had been connected by the Panamanian isthmus (an isthmus is a narrow strip of land) up until 3 million years ago. This land bridge allowed for the movement of animals, including marsupials. This is believed to be the reason for the emergence of the sole North American marsupial: the Virginia opossum.