Marsupial

Marsupials are one of three groups of mammals defined by their method of reproduction.  Babies are called joeys and are born live after a short gestation period.  Monotremes are egg-laying mammals, and eutherians are live bearers where the baby is born completely developed after a long gestation period.  Marsupials are quite diverse, with over 300 species.  Two-thirds of all species of marsupials are found in Australasia, and the rest are native to South America, except one species found in North America.

After birth, crawling along the mother’s belly, the joey must find its way to her marsupium, an external pouch that keeps the joey warm and protected and contains the mother’s teats where the joey can get milk to complete her or his development.  The number of babies born at one time often exceeds the number that will fit in the mother’s pouch, and those who don’t get there first will perish. 

A few species including shrew opossum, Caenolestes sangay, a group in the Andes Mountains, and the red-tailed phascogale, Phascogale calura, do not have a real pouch, but several folds of skin around the mammary glands forming a pouch-like area.  Wombats have backward-facing pouches that stay free of dirt while the animal burrows underground.  Upon entering the pouch, wombat joeys start to suckle and the teats swells until they cannot let go.  In this way, the joey is locked safely into the pouch, unable to fall out, until it is big enough to let go and take care of itself.

Marsupials have excellent hearing and sense of smell; they are good climbers; and all species can swim.  Their brain is considered to be a simple form because it lacks a corpus callosum, the part of the brain found in all other mammal species that connects the right and left halves of the brain.  Lacking this structure is often associated with less intelligent species, and plays a major role in reducing or restricting vocal abilities.  Kangaroos use a series of grunts, growls, barks, and soft clucking to communicate with one another, but many marsupial species have no vocal ability.  There are few family connections outside of breeding time, other than mother and joey being closely associated until the joey leaves the pouch.  Only one species, the sugar glider, Petaurus breviceps, maintains a permanent family group.

Convergent evolution is a term used when two different types of animals, such as eutherians and marsupials, occupy the same niche in different locations.  A niche is their spot in a habitat based on diet, foraging strategy, defense, or other specifications.  In Australia, marsupials mainly live in desert or dry scrub, but they live in forested areas in North and South America.  Niches may also be defined by habits such as burrowing, grazing, or gliding or by body structure such as two ant-eating species with long snouts adapted for that purpose.  Pictured above is a Southern flying squirrel, Galucomys volans, a eutherian, and a feathertail glider, Acrobates pygmaeus, a marsupial, that can both escape a predator by gliding from a high perch downward.  A few of the most notable marsupial species found worldwide are highlighted below.

The Virginia opossum, Didelphis virginiana, is the only marsupial found in North America.  They eat fruits, vegetation, urban trash, and dead animals.  After the joeys are born, they remain in the pouch for only a few weeks before getting too heavy for the mom to carry them in her pouch, so they all ride on her back.  Female opossums can be fierce protectors, but another defensive strategy involves “playing possum,” an autonomic response to stress where the animal enters a state of immobility when threatened, appearing to be dead.  Breathing and heart rate slow considerably, and the animal may remain in this state for several hours.  

Joeys of red kangaroos, Osphranter rufus, spend 33 days in the pouch, but may return if they feel threatened at any time up to about 8 months old.  When they are born, the joey is the size of a jellybean and feeds on their mother’s milk for their first year.  Kangaroos have well-developed hind legs that not only allow for good speed when traveling, but can also be used in self-defense.  Red kangaroos have only one joey at a time, and if they become pregnant while they have a joey still using their pouch, they can delay the birth of the new joey until the current one has left.  This ability is known as embryonic diapause.

Bandicoots, a group covering 20-some species, range from one to two feet long with a stout body covered with coarse-hair.  They have long, pointed snouts and are excellent at digging for worms and grubs, their favorite diet.  This species breeds four times each year, with up to five joeys in each litter.  The bandicoot has the shortest pregnancy of any mammal: joeys are born after 11-12 days and spend another 50 days in the mother’s pouch before leaving.  The female will breed again within one to three weeks.  Typically, only one joey will survive the threats of urban development, vehicular traffic, and dog and cat encounters.

Marsupials are a fascinating group of mammals, well represented in zoos and other nature centers around the world.  Come visit the Jurica-Suchy Nature Museum or another nature center near you to learn more.

To learn more about the following species or topics, click on the name to link to a previous blog:

Egg-laying Mammals

Southern flying squirrel

Virginia Opossum-1

Virginia Opossum-2