Evolution's oddity found in our yards

Opossums grow up in a pouch, then make themselves at home in our neighborhoods

New York TimesAugust 1, 2011 

  • If cornered, an opossum may resort to its legendary talent for playing possum. This is an autonomic response, like fainting, in which the animal falls to its side with its mouth adrool, excretes droppings and a foul odor, and remains in a deathlike state of curled catatonia for minutes to hours, until finally it begins reviving with a twitch of the ears.

We found them in a corner of our garage: a half-dozen baby opossums, peeping like birds, squirming over one another and scratching the wall. They were cute and ghastly in an "Addams Family" sort of way, their long snouts tapered like wine stoppers, their black eyes bulging up from their pale fur like peppercorns from a bed of rice, and their tiny teeth as sharp and plentiful as a piranha's.

Alerted by the cries, the mother opossum quickly nosed in from the side. As she struggled to calm her babies, to mop up the bright chaos we'd inadvertently thrown her way, we quietly retreated.

A few weeks later, we were saddened to see, in the middle of our driveway, the corpse of the mother opossum. There were no signs of injury or disease. As it turned out, the opossum had simply followed her species' ruthless recipe for success in an overwhelmingly placental world: grow up fast, give birth to one or two large broods, and then, at a time of life when most comparably sized mammals have just reached their prime, die of old age.

The Virginia opossum, Didelphis virginiana, is one of the more familiar and widespread mammals in the United States, found coast to coast, up into Canada and down into Costa Rica, in fields and sheds, city parks and the urban alleys, and all too often as roadkill on highways. The opossum is generally lumped together in the public mind with raccoons, squirrels, skunks and other workaday wildlife of more or less housecat dimensions, but scientists emphasize that Didelphis is a fundamentally different animal, as singular in its evolutionary history as it is solitary in its habits.

A little piece of Australia

For one thing, it's our own private Australia, the United States' sole living example of a marsupial mammal: It gestates its young in a pouch, or marsupium, rather than in a uterus, as we placental mammals do. For another, new evidence suggests that the opossum is more deeply marsupial than such poster pouch-bearers as koalas, wallabies and kangaroos.

Analyzing fossils recently unearthed in Wyoming and elsewhere, scientists have proposed that the earliest marsupials, which date to the age of the dinosaurs, may well have resembled opossums, and that other marsupial species are all derived from the basic opossum format. If nothing else, scientists said, the new fossils show that opossums have changed remarkably little since their forebears nested in a Tyrannosaurus rex garage.

"Every time I see an opossum I get moved," said Ines Horovitz, of the department of ecology and evolutionary biology at the University of California, Los Angeles. "They've managed to survive all this time looking the same as their ancestors did 60 million years ago or more."

One key to the opossum's success is that it's low-key - a shy nocturnal forager that is nonterritorial and avoids fights. If cornered, an opossum may resort to its legendary talent for playing possum, an autonomic response, like fainting, in which the animal falls to its side with its mouth adrool, excretes droppings and a foul odor, and remains in a deathlike state of curled catatonia for minutes to hours, until finally it begins reviving with a twitch of the ears. Opossums are also willing to move surprisingly long distances. James Beasley and William Beatty of Purdue University have ear-tagged or radio-collared about 100 opossums and compared their activity and migration patterns with raccoons.

"Raccoons maintain very small home ranges," Beasley said, and they like to stay put. By contrast, some opossums the researchers captured would turn up two weeks later at a spot 15 miles away.

And while migrating mammals are often males in search of a mate, many of the far-ranging gypsy opossums proved to be females with young in their pouches. "They could be looking for more suitable dens," Beasley said.

50 teeth and an appetite

Or a better snack selection. Opossums are opportunistic omnivores, equipped with 50 teeth - a record among North American mammals. They'll eat fruits, insects, nuts, pet food, small rodents, Big Macs. "They're vacuum eaters," said Alfred Gardner, an opossum expert at the Smithsonian Institution. "They'll take in anything with nutritional value."

Because they are not as dexterous as raccoons at digging and foraging, opossums are not considered the sort of "subsidized predators" that clean out the nests of endangered songbirds and turtles. They can, however, catch and eat snakes, and they are among the few animals to be resistant to the venom of rattlesnakes and other pit vipers.

No matter what they eat, opossums are poor at storing body fat, and as a University of Massachusetts team discovered, they survive harsh winters only by taking advantage of the scraps and warm shelter that humans supply.

The most outstanding feature of a marsupial is its approach to childbearing. Embryonic opossums spend about 12 days in the mother's ill-equipped uterus before emerging as so-called joeys, which are roughly the size of rice grains. Those joeys must then crawl their way into the mother's pouch and latch onto a nipple, where they remain attached and nursing for the next couple of months.

After leaving the pouch they remain with the mother for another few weeks, then they're on their own. Most marsupials have large litters, which helps offset the brevity of their lives. Whereas raccoons can survive well into their teens, even in coddled captivity opossums are dead by age 4.

In the past, researchers thought marsupials were the primitive predecessors to placental mammals, but they have since learned that both mammalian groups arose about the same time, 125 million years or so ago, then evolved along independent tracks.

The placental plan appears to have the upper hand, however. "Wherever there has been contact, placentals have replaced the marsupials," Gardner said. Only in Australia and surrounding islands and parts of South America, where marsupials have been relatively isolated for long periods, have they managed to thrive.

Perhaps the placentals' advantage lies with their relatively larger brains, Gardner said. Horovitz suggested that because embryonic marsupials need well-developed forelimbs to crawl their way to a lactational lifeline, they lose the potential for the diversity of limb and body shape seen in placentals.

Yet we have our opossum, a living piece of the Cretaceous. If you're lucky enough to see one, say hello - and goodbye.

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