What Is An Opossum?

W2Dopossum2In North America, many of us are familiar with the Virginia opossum, a medium-sized mammal about the size of a large cat. These are common in our North American southern states and are often seen as road kills along highways. Opossums are unique mammals–they are called marsupials and are most commonly found in Australia, Tasmania, New Zealand, adjacent islands and South America. Besides the opossum, other well-known marsupials are the koala (teddy) bear and kangaroo. Some American opossums have prehensile tails, that is, tails can be wrapped around limbs, supporting the main body. Marsupials are mammals with pouches (with milk glands) in which young are carried. Newly born opossums are very poorly developed (altricial) after less than two weeks of growth (or gestation). [Human babies require nine months of pre-birth development.] Fetal or baby mouse opossums are very small (about ¼ of an inch and abt. 2 ounces) and as many as 25 can fit into a teaspoon. At birth (parturition), young opossums rear legs are only stubs whereas front legs are better developed for when they migrate to the pouch (marsupiam), a distance of about 2 inches in a swimming motion, where they attach themselves to a pouch nipple for up to 100 days. Average numbers of female teats or nipples is 13, although 25 or so young may be born at a time. Only rarely are all available teats utilized. Those young not able to find and attach to a teat soon perish.

Ecology of the Virginia opossum* suggests occupation of a wide variety of habitats, from dry areas to streams and swamps. Home range, or area in which individual opossums move about is estimated to be from 12 to 58 acres. Opossums often inhabit burrows of other animals or are arboreal (tree dwelling). They appear to have few predators, though feathered enemies may be a major source of mortality for young animals less than a year of age. Opossums may live from 1 to 4 or more years or even more. Their diet is mostly omnivorous consisting mainly of insects, carrion (dead animals) and fruits/grains. External parasites are few, but internal parasites include nematodes and trematodes and opossums are a reservoir host for important communicable diseases. The Virginia opossum may feign death if severely threatened and often exudes a greenish solution from two anal glands which ostensibly discourages enemies. Unless attacked, after feigning death for 15 minutes or so, the opossum rouses itself and moves along on its way.

One very small marsupial opossum found from northern South America (Panama, Colombia, Venezuela) to the Islands of Trinidad, Tobago, Grenada is called Robinson’s mouse opossum (Marmosa robinsoni). This species lives in dry shrublands and deciduous forests. A recent article in the Journal of Mammalogy**, a leading scientific journal devoted exclusively to mammals, compared populations of Robinson’s opossum in various geographical areas to determine which are most closely related.  The authors compared cytochrome-b genes and a transferase proteins and found that, surprisingly, central and eastern populations in Venezuela are more closely related to Trinidad and Tobago populations than to those closer on the Paraguaná peninsula in northwestern Venezuela. The Paraguaná peninsula populations of Venezuela are more closely related to more distant populations to the west in Colombia and Panama. The article authors suggest that events of the Pleistocene glacial events lowered sea level, allowing land connections from islands to the mainland over which ancestral opossums emigrated.

I personally had some experience with Marmosa elegans,  the small elegant mouse opossum in Chile. As an international service team adult member at the World Scout Jamboree near Santiago, Chile, I helped scouts from many nations better understand exploitation of exotic animals. For instance, parrots and other attractive house birds are trapped in their tropical homelands and sold. However, the cost is immense, as an average of 9 birds die for every one that is successfully marketed! Often, Stately and valuable trees are felled for the sole purpose of capturing and subsequently selling birds from nests attached high up trees trunks.

Following the jamboree, I was prepared to trap small mammals and deposit them in a stateside university mammal collection which had poor Chilean faunal representation. I traveled with a colleague from the University of Chile to a university-owned wildlife reservation. There I trapped several species of small mammals and prepared them for preservation in the museum mammal collection. Several of my trapped specimens turned out to be the elegant mouse opossum (Marmosa elegans). They were small and but quite attractive. It took awhile for me to identify them as I prepared them as study skins.

The next time you see an opossum, remember that you are looking at a unique mammal, certainly quite primitive as compared to all other North American fauna.

*Didelphis virginiana. by John McManus. Mammalian Species, No. 40, 1974.
**Phylogeography of Marmosa robinsini: Insights into the biogeography of dry forests in northern South America by Eliécer Gutiérrez, Robert Anderson, Robert Voss, José Ochoa-G, Marisol Aguilera, and Sharon Jansa. Journal of Mammalogy, 2014,
95(6):1175-1188.