Marsupials of Australia and the Americas

When people think of marsupials, they think of the cute koala and the bouncy kangaroo of Australia. They may, if they are more knowledgeable, think of the many opossum species found in South America. But marsupials are not actually confined to Australia and South America. There are marsupials in New Guinea and the opossum Didelphis virginiana is native to North America.

It’s a pale, nocturnal, solitary creature about the size of a cat with a pointy head and an odd scaly rat-like tail. Opossums were brought west from their native east coast of North America, probably as food stock, and have also spread to the north in recent years. The name opossum comes from an Algonquian word, and the animals were cited in a message to London from the Jamestown colony as one of the wonders of the new world.

The Virginia opossum survives among the placental mammals by being a fierce fighter and by feigning death, playing possum, when overmatched. They also have a vigorous immune system compared to their placental competitors, will eat almost anything including carrion, and are at least partially immune to the venom of cottonmouths and similar snakes.

The 60 or more South American Didelphids, opossums, are also mostly nocturnal. They also “play possum”, becoming limp and immobile and emitting a rotting smell that may be associated by a potential predator with a sick or dead animal. In South America opossums range in size from tiny mouse-like forms to species as large as the Virginia opossum. They are mostly arboreal, tree-dwelling, though not all are. Their physical characteristics resemble those of the North American opossum. The Monito Del Monte is a South American marsupial which is not an opossum.

Most marsupials are found in Australia, though. They thrive there, and dominate the ecosystem the way placental mammals dominate in the rest of the world. This is believed to be partly because their metabolism, which is slower than that of placental mammals, is the best suited to the climate of desert Australia.

The reproductive system of the marsupial is very different from the placental as well and well adapted to conditions of scarcity. Marsupial young leave the uterus earlier than placental young do. They proceed to the famous pouch, where they will be nurtured until they are able to leave the mother. A kangaroo, to take an example, is immediately able to breed again, and may retain the new young thus created in the uterus with development halted at the blastocyst stage. Then, if conditions do not favor the survival of the young in her pouch, a replacement is farther along. This reproductive system, embryonic diapause, is well suited to cruel conditions.

Another reason marsupials have survived where they have is isolation. Imported dogs and cats, and other placental mammals, now threaten some marsupials in Australia. Australia’s and South America’s isolation after the breakup of Laurasia and Gondwana encouraged the development of these marsupial populations, but modern global conditions work against isolation.