ORDER DIDELPHIMORPHIA

Family Didelphidae - Opossums

Didelphis virginiana - Virginia Opossum

 

And, yes, Virginia, there is a NATIONAL OPOSSUM SOCIETY.

NATIONAL WILDLIFE (4/93) - Opossum are North America’s only marsupial, coming from South America over the Panamanian land bridge about 3 million years ago. 69 other species exist in South America. Of particular interest is the fur-lined pouch where the babies spend two months growing from bumble bee to chipmunk size. Nocturnal animals, they mark home ranges, pecking orders and can share nests. Smells have a role in the opossum society. They are unsurpassed omnivores (called "living vacuum cleaners"). They are even immune to rattlesnake venom, thereby adding rattlesnakes to it’s diet when available. Since food is found everywhere, no need for a permanent den. Opossum simply establish a temporary home range, forage throughout the area for several days using the same den, then, when the food has been used up, the opossum moves on to a new area.

Healthy female are more likely to give birth to males than females. In fact, using opossum as test animals (they are easy to catch---and recatch, and catch again), it was shown that well-fed opossum give birth to more male while older, less healthy females consistently give birth to more female. The reasoning for this is a male born from a weak mother will probably also be somewhat weak, cannot compete with healthy males, and therefore will not generate offspring. However, a weak female born from a weak mother, will still get inseminated by some male. Of course, a strong female will best insure survival of her genes by bearing a male who will inseminate a number of females (assuming polygynous habits---males taking on several females). This logic (theory) applies to all polygamous species.

Females can have two litters in a year, but rarely live to breed the following year. Almost never does an opossum live to be three years. Thus, broods are large, usually six in the southeast, up to 12 in the northeast, starting at age of 6 to 8 months.

Opossum are terribly near-sighted (one reason for so many road-kills). Also, when frightened, they freeze, or feign death (not effective against cars). Finally, they are often along roads looking for food in the form of road kills---often other opossum.

SMITHSONIAN (?) - John Seidensticker did a five year study with opossum at the Front Royal Conservation Research Center. He found they were very difficult to track, even with radio-equipped collars. With a life span of a year or two at most, the opossum must complete it’s life cycle in short order, i.e., it gets around. It can’t afford the leisurely life style associated with larger, longer-life spanned mammals. What it takes other mammals to achieve in 10 or 15 years to accomplish, the opossum must do in a year.

There is some question as to the origination of the marsupials. Although most say a South American origin, the oldest fossils have been found in Alberta, Canada, 85 million years old. In either case, the origins go back to the Cretaceous period (with dinosaurs dominating), when South America, Africa, Antarctica, and Australia were joined in the mega-continent called Gondwanaland. After Africa left, Australia left, then Antarctica (where marsupials are the only known fossils). Then, North and South America rejoined 3-5 million years ago, allowing passage south of better-equipped placental carnivores (bears, cats, dogs, weasels and others) that, in short order, wiped out all the Southern Hemisphere marsupial carnivores. Today, about 85 species of insectivorous and omnivorous marsupials exist in the Southern Hemisphere.

Only one marsupial, the Virginia opossum, came north from South America. The Virginia opossum evolved from the common (South American) opossum some 75,000 years ago and then came north. They have spread throughout US. The brain of the opossum, as all marsupials, is small. In like-sized mammals, the size of the opossum brain is a third to half the size of other mammals (Man comes in at 7.5x larger.) For example, they just don’t learn how to avoid being trapped. And it’s well known trait of feigning death, defecating, and emitting a foul-smelling greenish ooze from it’s anal glands are it’s best method of avoiding predation. Perhaps effective, but very limited in repertoire. Other than a three month period of interaction between mother and child, their is no social interaction. In the ranking of the complexity of mammalian social systems, the opossum earned a 2, on a scale that starts at 2 for "least complex" and ranges to 20 for "most complex". For their size, opossums are among the shortest-living mammal. In his Posey Hollow plot, 8% of female and 5% of the male lived more than one year. Records have one captive opossum lasting four years.

Food is mainly found in streams-small mammals, insects, snails, crayfish and other invertebrates. Sexual maturity is reached in eight months. Usually, a female will have two broods in her lifetime, with as many as 15 young (Posey Hollow averaged 9). The young travel the two inches to the 13 teats, feeding for the next three months. The advantage is obvious to the mother who can travel farther from the nest to feed, since the babies are always at the milk source. One study showed a range of 15-20 acres covered by a placental raccoon parent versus 35 acres covered by the same placental parent without young to nurse. Nor does the opossum need to return to the nest at night. Only for about a week before weaning, does the opossum mother leave the young alone in the den. After that, they again accompany her, this time riding on her body. No, opossum do not sleep hanging upside down by their tail. Opossum are super-omnivores, eating eggs, worms and carrion as well as whatever else they can find. (a SNP ranger has told me that when a deer carcass has been dined on in the woods, the opening at the rear end is invariably made by an opossum.) Females, with young, may travel several miles before finding a good foraging area, and then will spend several days denned in this area until the available food sources have been tapped, then she moves on. The male, will cover 25 acres compared to the female’s 10 acres in a single night. Thus, the range of a male may be quite large if food supplies are poor, or one male may spend it’s whole life in a single backyard if the supplies are adequate.