After hearing the sad news about the passing of Galileo, the Museum’s resident, exhibit opossum, and talking with Keepers Sarah and Jessi about the longevity of said animals, I decided to do a little research.
I suppose I should have known opossums have a short life span. I mistakingly assumed they lived perhaps 4, 5, or even 6 years. I was incredulous when told they can only be expected to live about 2 years. Hamsters live longer than that!
In general, the larger the animal, the longer its life span. A White-footed Mouse or Short-tailed Shrew, both very small mammals, may live less than a year. A raccoon may survive 6 years. A Black Bear may have 10 – 12 years to accomplish what it needs to accomplish in life. Of course there are exceptions, a raccoon was known to live 17 years in the wild, a captive animal for twenty. A Black bear in the Adirondacks was recorded to have lived 41 years, although 30+ years is the realistic maximum for even a captive Black Bear.
So, it’s no wonder my amazement at finding that opossums have such a short life. Even in captivity they can’t be expected to live more than four years. I would expect an animal the size of an opossum to have a much longer life.
What is it about opossums that shortens their lives? They don’t seem to be very stressed. In fact they seem a bit too relaxed, almost as if in a stupor. They never seem in a hurry. Watching one amble across a road or rummage around the backyard, one might think they’re simply stumbling through life with not a care in the world.
They don’t seem to be as alert as they perhaps should be. Perhaps that’s why I see so many DOR (Dead On Road) opossums. One estimate puts the number of road-kills at between four and eight million opossums a year here in the U. S. alone.
Opossums also seem to take much abuse from other animals. Owls, hawks, coyotes, and even domestic dogs take their toll on the critters.
Most deaths occur during their first year of life, which brings down the average life span considerably. The fact is, though, that even if an opossum makes it through the first two years of life, gets past the predation, automobile encounters, doesn’t succumb to disease or parasites, it’s all down hill from there. The opossum begins to show the signs of aging such as cataracts, motor coordination problems, and weight loss.
With such a short life span the trend is to get it done early and often, reproduce, that is. Muskrats, about the same size as an opossum, live 3 – 4 years, can have 1 – 4 liters per year averaging 6 – 7 young per liter. That’s potentially 112 young in their lifetime. While opossums become sexually mature at 6 – 8 months and can have 2 liters per year with about 7 young per liter, they’re only likely to put out about 28 young in their lifetime. Something doesn’t seem right here.
Some authors suggest that marsupials (opossums are, as we all know, marsupials) have shorter life spans in general, but a Red Kangaroo can live for twenty years, and Koalas about 12 years. For their size, opossums seem to have the shortest life span of just about any other animal in North America. And, by the way, it’s the only marsupial in North America, outside of zoos.
Opossums are doing very well in North America despite their short lives and relatively low reproductive rate. Essentially a southern species (Central and South America), they had already occupied southeastern North America when Europeans arrived on the scene but have now spread as far west as eastern Colorado and as far north as Canada. Their range also includes western portions of California, Oregon, and Washington, as well as southern British Columbia, Canada. Apparently they don’t like the desert or the Rocky Mountains.
One interesting fact about opossums that would seem to increase their longevity and certainly their survival as a species, they are resistant to snake bite. That’s right, if a cooperhead or rattlesnake, or even a moccasin latches onto a opossum the only effect felt by the gray haired marsupial is local swelling, as if stung by a bee. And, it’s “you bite me, I’ll bite you back,” as far as snakes go. Besides just about every other edible member of the plant or animal kingdoms combined, venomous snakes are on the opossum’s menu.
Another thing in the opossum’s favor is the fact that they are resistant to several viral diseases, like distemper, parvovirous, feline hepatitis, and rabies. Those diseases do a number on other wildlife. Gray Fox, raccoon, and bobcats are all susceptible to at least some of those viruses, not so the opossum. Tough little animals, those opossums, at least until they reach that second year.
If you’re wondering where the opossum got its name, it comes from the Algonquin word, aposoum, opassom, or opussum, which means white animal, or white dog, depending on the source referenced. The Latin name Didelphis virginiana means double-wombed of Virginia. Double-wombed refers to the paired reproductive tract of this marsupial (marsupium is Latin for pouch) and the Virginia part comes from the fact that the animal was first described in Virginia.
And finally, a glimmer of hope. Although most opossums don’t make it past the two year mark, ocasionally a captive opossum will stretch it out to 10 years. You go, possum!
The source for much of the information and figures in this post
Krause, W. J.; Krause, W. A. (2006), The Opossum: Its Amazing Story, University of Missouri, Columbia, Missouri: Department of Pathology and Anatomical Sciences, School of Medicine.
Reid, F. A. (2006), Mammals of North America, 4th Edition, New York: Houghton Mifflin.
The College of Environmental Science and Forestry (SUNY-ESF) web site: http://www.esf.edu/aec/adks/mammals/