Opossums

Earlier we spoke of those "living fossils," the Duck-billed Platypus and Spiny Anteater of Australia and New Zealand. Here in America our opossum is nearly as curious -- despite its being fairly common, and sometimes even surviving in towns and cities.

If you open just about any field guide to the mammals, the opossum will probably be the very first of the 380 or so mammal species listed for North America. That's because opossums are our only marsupial -- and marsupials are considered very ancient forms of mammals -- and field guides typically place the "primitive" organisms first. The vast majority of marsupials occur mainly in Australia, where kangaroos are the best known of the group.

You know that kangaroo mothers carry their young in pouches on their bellies; so do opossums. That's because marsupial babies are born premature -- after only thirteen days of gestation -- the time from when the sperm unites with the egg, until being born. At birth, newborn opossums are so tiny that an entire litter, consisting of up to 14 babies, can fit into a single teaspoon. They are so undeveloped that it's impossible for them to survive outside the mother.

However, their front legs are developed all out of proportion to the rest of their bodies, and are strong enough for the completely blind babies to pull themselves from their birth opening, across the mother's belly, into her pouch, or marsupium. They do this by instinct. If the mother is disturbed and moves during the babies' pouch-searching process, the babies may well lose their way, fall off, and perish. Once inside the pouch, the babies find a nipple, attach themselves to it very securely, and during the next two months ride in the mother's pouch continuing their development.

Why did this round-about manner of producing babies considered a primitive feature, and why did it evolve? One answer is that the general evolutionary trend among animals is toward producing few children, but taking good care of them so that a high percentage survive into adulthood; that's the way we humans do it. The opossum's 14 or so babies clearly represent the primitive approach of producing lots of offspring and maybe one or two will survive. However, if all 14 babies were to develop to full term inside the mother opossum, the mother wouldn't be able to move around much during her pregnancy. On the other hand, with the young settled in a pouch that stretches as the young grow, and which is emptied more and more as the young mature and begin learning about the outside world, it's not nearly such a strain on the mother.

The opossum's tail is long, thick, hairless, and very talented. It can wrap around things and hold onto them. Such useful tails are said to be prehensile. One of the most remarkable sights ever to hope for in our backyards is that of a mother opossum with all her babies riding on the mother's back, with their tails grasping hers, to keep from falling off.

Sometimes when opossums are cornered in a dangerous situation, they simply keel over into a dead faint. Many predators, such as dogs, then don't know quite what to do with a dead opossum. If the predator decides to just leave, in a few minutes the opossum will raise its head and look around, and if no danger seems to be threatening, it'll rush to cover. It's been "playing possum." Research indicates that the opossum isn't really "playing" when it does this. It actually seems to undergo a kind of nervous collapse. Since opossums aren't well equipped for fighting, their "delicate nervous system" may be an adaptation for surviving attack by predators such as dogs, who may be more interested in a good fight than in eating a sick-acting opossum.

Probably the main reason that opossums are so successful is that they aren't fussy about what they eat; they're real omnivores, consuming fruits, vegetables, nuts, flesh, eggs, insects, carrion, and just about anything that can be chewed and swallowed in the garbage can.