Marsupials, also called ‘pouched mammals,’ are a diverse group known for the presence of an external pouch in their body, the marsupium, where they nurture their newborns. They are one of the three groups of mammals, the other being the egg-laying monotremes and the placental mammals, including humans. However, unlike their placental cousins, marsupials give birth to relatively undeveloped offspring. They are also distinguished by their small birth size, unique reproductive structures, and a short gestation period.
They belong to the infraclass Marsupialia, which includes kangaroos, koalas, opossums, wombats, wallabies, and bandicoots, among many others. Most (around 334) extant marsupial species are found in Australia; however, a few are also spotted in the Americas, especially in South and Central America. Only a single species, the Virginia or North American opossum, is abundant in North America.
The smallest marsupial in the world, the long-tailed planigale (Planigale ingrami), is about 5 cm in length (1.9 m), whereas the largest living marsupial, the red kangaroo, grows up to 1.8 m (5 ft 11 inches) in height and weighs about 90 kilograms (200 lbs).
Although marsupials possess the characteristics of a mammal, including the mammary glands, three middle ear ossicles, and true hair, they also have distinct anatomical traits like a marsupium, a brain lacking a corpus callosum, presence of epipubic bones, and a bifurcated penis that set them apart from the placentals.
Marsupium
Most female marsupials have an external pouch called the marsupium, which contains multiple teats to nourish their young. While in most marsupials, this pouch is permanent, in some, like the shrew opossum, it develops only during gestation. Most kangaroos that walk or climb on all fours have a pouch opening at the back, while locomotive kangaroos have one at the front.
In some marsupials, like the water opossums, the males bear a marsupium that holds their genitalia while swimming or running.
Skull
Compared to placentals, the marsupial skull is small and more compact. Its frontal hole, the foramen lacrimale, is situated at the front of the orbital bone. Also, the brains of marsupials lack a corpus callosum.
Epipubic Bones
A pair of bones called the epipubic bones (ossa epubica) project forward from the pubic bones. In modern marsupials, the epipubic bones are often called ‘marsupial bones’ because they support the mother’s marsupium.
Reproductive Anatomy
Most males, except marsupial moles and macropods, have a bifurcated penis (separated into two columns) to correspond to the two vaginas in females. It is separate from the urinary tract, and the shape of the urethral grooves is often used to distinguish between certain species.
The only accessory sex glands found in males are the prostrate and 1 to 3 pairs of bulbourethral glands. Their prostrate glands are considerably larger than those in placentals.
Females have two lateral vaginas leading to separate uteri, though both open through the same orifice. A third canal (permanent or temporary) called the median vagina acts as the passage for giving birth to the young.
Several attempts have been made to interpret the phylogenetic relationships among the different marsupial orders. Didelphimorphia, which includes opossums, and Paucituberculata, which includes shrew opossums, are two orders of marsupials that have been proposed by different studies as the sister group to all other living marsupials.
German zoologist Johann Karl Wilhelm Illiger initially considered the current infraclass Marsupialia, a family under the order Pollicata. However, scientist James Rennie suggested that placing mammals, monkeys, lemurs, tarsiers, aye-ayes, and marsupials under a single order was unjustified.
Thus, in 1997, J. A. W. Kirsch and a few other scientists accorded Marsupialia the present-day rank.
They are abundant in Australasia, with about 70% of marsupial species concentrated in mainland Australia, Tasmania, New Guinea, and nearby islands like Maluku, Timor, Sulawesi, Solomon, and the Bismark Archipelago.
The remaining 30% are scattered across the Americas, primarily in South America (except central and southern Andes and parts of Patagonia) and Central America and south-central Mexico. Only one species, the North American opossum, is abundant in the eastern United States and along the Pacific coast.
While most Australian marsupials inhabit dry scrub and desert habitats, those in America live in tropical rainforests. They can occupy any niche in a forest habitat, ranging from tree tops to forest floors or even underground burrows.
During the breeding season, males and females aggregate and engage in shot-lived pair bonds, usually with different partners. They are viviparous and give birth to live young, just like placentals.
Marsupials have a very short gestation period, usually 12 to 33 days, after which a newborn called ‘joey’ is born. The infant is almost in a fetal state, too underdeveloped to venture out on its own. Unlike placentals, where the fetus is nourished in the womb by the placenta, marsupial mothers use their external womb or marsupium to nurture the newborn. They often lick the fur on their bodies and leave a scent trail to guide the infants to their pouches, where they attach themselves to the mother’s teat for food. The joeys have ‘oral shields,’ consisting of soft tissue that reduces their mouth to a round hole large enough to hold the mother’s teat. Unlike in placentals, where the genes responsible for fetal development are expressed in the placenta, marsupials express these genes in the mammary glands during lactation.
Joeys cling to the teats until they are grown enough to leave the marsupium for short periods, returning for further nourishment and protection. They usually stay in the pouch for up to a year or until the birth of the next joey. During this phase, the infant has not developed enough fur to regulate its body temperature, and thus, a temperature of 30–32 °C (86–90 °F) is constantly maintained in the pouch.
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