Critter Corner No. 13
The Virginia Opossum
by George Sly
(image courtesy of Wikimedia)
In the last edition of Critter Corner I spoke of the coyote as a Goose Pond mammal which doesn’t garner the respect it deserves. To this list of unappreciated mammals I now add the Virginia opossum. Hardly anyone I encounter seems to have a good word for this curious and ancient denizen of Indiana’s fields and forests. The root of this problem seems to me to lie with the human tendency to anthropomorphize animals. This propensity to invest wild animals with human characteristics seems nearly universal. Thus the opossum is labeled as stupid, ugly, repulsive, or dirty. “It looks like a giant rat”, some will say. “The nasty things come up on my porch and eat the food I left for the cats”, says another. “Why do they make that hideous grin”, asks someone else? I think you get my drift. These little critters get less respect than Rodney Dangerfield ever did.
And yet I will argue that, like the coyote, there is much to be admired about the opossum In fact, I will go so far as to suggest that this is one of the most interesting mammals in all of North America. The opossum was the first and only marsupial encountered by the European colonizers of what was to become the United States. Their common name is derived from a Native American word, apasum, which means “white animal”. In 1792 Scottish science writer Robert Kerr examined the species and bestowed upon it the scientific name Didelphis virginiana. The species name is derived from the place the animal was first observed. The genus name is more interesting and is actually descriptive of the reproductive tract of the female. As you know, Greek or Latin is used in forming the root of scientific names. Didelphis comes from the Greek di meaning two and delphys, the term for the womb. The reproductive structure and physiology of marsupials like the opossum are among the most interesting aspects of their biology.
As the genus name suggests, females have a highly bifurcated reproductive system with two uteri as well as paired vaginas. Of course, like other marsupials, the female also has an abdominal pouch in which the young grow and develop. Incidentally, male opossums have a bifid (dual-forked) penis. This likely delivers spermatozoa into the branched female tract more efficiently. Such anatomy in the male has led to one of the more bizarre myths about the opossum. Since the only visible double openings on the female are the nostrils, colonists with a poor understanding of anatomy and physiology surmised that opossums must breed through the nostrils of the female. This myth was emboldened by the fact that, just before the birth of the young, the female may be seen grooming her pouch and abdomen. The conclusion followed that she was blowing or sneezing her babies into the pouch from her nostrils where they had formed. Needless to say, this story is literally 180o from accurate.
In reality, opossums mate essentially like other mammals although the course of development followed by their young is quite bizarre compared to placental mammals like us (and most all other mammals). Baby opossums are born in what is essentially an embryonic state after only thirteen days or so of gestation. This is an amazingly short period of time to go from a fertilized egg to a viable, and mobile embryo. Newborn opossums are about the size of a honeybee and weigh only a few thousandths of an ounce. Even after a few days of growth they are tiny as the following image illustrates.
(photo courtesy Opossum Soc. of the U.S.)
After emerging from the female at birth, the young pull themselves upward through her fur and move into the pouch. Here there are thirteen nipples to which the newborn attach themselves. Lasting several weeks, this attachment is semi-permanent as the nipple swells inside the mouth of the young. If more than thirteen young are born (the average is seven to nine) those who do not secure a nipple will not survive. It is interesting to note that the length of gestation in mammals is directly related to their size; the bigger the mammal, the longer the gestation period. For a placental mammal the size of an opossum we might expect a gestation period within the mother of around two months. Perhaps not surprisingly, this is the normal period of development within the pouch of opossums. Thus the marsupial and placental mammals have periods of embryonic and fetal growth relative to adult size which are very similar. But they use two exceedingly different evolutionary strategies to accomplish this. That is, placental mammals carry out much of their development within the mother’s uterus while marsupials complete their growth and maturation inside the mother’s pouch. After several weeks of development within the pouch, young opossums begin to venture out and, while accompanying their mother, to explore their environment. During this time, the young may be seen hitching a ride on mom’s back. By about four months after their birth the littermates will have taken up an independent and largely solitary lifestyle.Aside from their mode of reproduction, unique among North American mammals, opossums possess other interesting features. Their skull contains 50 teeth, a number higher than any of our other native mammals. Such a large number of teeth is considered a primitive characteristic in mammals. As an example more highly evolved skulls, such as those of mice, often have only 16 teeth. The hind feet of opossums have an opposable inner toe which is of great use in climbing. They also have a somewhat prehensile tail. This is used to stabilize themselves while climbing and is also used to gather leaf material in nest building. Adult opossums are often seen missing the tip of their tail due to a propensity for it to be damaged by frostbite. The behavior of opossums is also of distinctive interest. When confronted with danger, an opossum may gape the mouth exposing its teeth, hiss, growl, and salivate. This behavior is perhaps responsible for the reputation opossums have for being vicious. In fact they are fairly placid animals. Should its attempt at fierceness fail to deter a threat, an opossum will often “play dead”. This is the origin of the term playing possum. This behavior is not just an act. The animal’s feigned death is a neuromuscular reaction beyond its conscious control. Once this behavior kicks in the opossum is out for a period of time lasting minutes to hours. Since predators don’t normally utilize prey they find dead, the opossum may derive protection from potential enemies such as dogs, coyotes, and bobcats.
Opossums are thought to have evolved in South America and found their way into North America during the Pleistocene Epoch. The fact that they have been here for perhaps a million years should tell us that their survival skills are rather admirable. You may recall that in my essay about the coyote I characterized them as generalists, particularly in regards to diet. The same can be said of the opossum and, in both cases, these animals are also capable of using a wide variety of habitats. These are recipes for success in the animal kingdom. Opossums are notorious as omnivores. There is not much that they will not utilize as food. They will eat mice, shrews, their own kind, birds, snakes, lizards, earthworms, snails, ground beetles, grasshoppers, crickets, carrion, and plant materials such as persimmons, apples, and seeds. Given the opportunity they will happily feed upon our garbage. While often perceived as “stupid” by humans, opossums are experts at finding and remembering the source of food. Granted they do have a cranial capacity that is relatively small for their size, about a third of that of the house cat. However studies have shown that their ability to remember is on a par with rodents, cats, and dogs.
Perhaps another factor has worked in favor of the opossum’s survival as a species over immense spans of geologic time. Scientific studies have shown them to be unaffected by to two environmental resistance factors: venomous snake bites and viral pathogens. Experiments have shown that opossums are essentially immune to the bites of dangerous serpents such as the eastern diamondback rattlesnake and cottonmouth water moccasin. It has been determined that their blood contains a protein which binds to, and neutralizes, the venom of these snakes. This mechanism is being investigated in regards to a possible antivenin for use in humans; one which does not have the allergic potential of horse serum derived antivenins.
Likewise opossums are resistant to diseases such as distemper, parvovirus, and feline hepatitis which often affect both domestic and wild mammals. Opossums also appear to be seldom afflicted with rabies. A common hypothesis as to the source of this resistance has to do with the slightly cooler body temperature of the opossum compared to other mammals. In one study it was shown that opossums had an average diurnal body temperature of about 90oF. Most other mammals have an average body temperature closer to the 98.6oF average of humans. It is thought that the cooler body creates an environment unsuitable for viral growth.
If you are among those who have held a low opinion of the Goose Pond’s only marsupial mammal, I hope this essay has given you a new perspective. Like so many other native animals that we humans tend to only superficially contemplate, the Virginia opossum turns out to be a creature of rich and varied interest. It is my hope that you will now be more inclined to give the opossum looming in your headlights a little more time to skedaddle off the road. You might even turn a blind eye to the occasional pilfering of dog food from the back porch. Perhaps you will even feel motivated to share what you have learned with all those opossum-haters out there. After all, our most unique mammal is a species with its own role to play within its ecological world. And it most certainly needs all the friends it can get in today’s over-crowded world. In the end, let us remember that the opossum is an emissary from deep time. It is a species whose particular structures and behaviors have allowed it to survive for thousands upon thousands of generations. Surely this is deserving of a little respect on our part.
Resources for those interested in the Virginia Opossum
- The Opossum: Its Amazing Story. William J. Krause and Winifred A. Krause. web.missouri.edu/~krausew/Histology/Home_files/opossum.pdf