The Surprisingly Social Loner

New research reveals the sophisticated social life and survival tricks of the solitary opossum

04-01-1993 // Doug Stewart

Opossums don't get any respect. "This morning, I was talking to my vet about a worming medication for our opossums," says biologist Donna Holmes, who studies the species at Harvard University. "He said it sometimes affected an animal's central nervous system. Then the vet laughed and said, `But with opossums we don't have to worry about that.'" Holmes is a courteous, mild-mannered sort, but she took offense immediately. "You know, opossums have brains, too," she responded.

As Holmes was reminded that day, the poor 'possum has never been held in very high regard. Purdue biologist Durward Allen once dismissed it as "a sluggish, smelly, disreputable critter without a semblance of character or self-respect." The biological literature itself is full of slights. Opossums are primitive "living fossils," we're told, barely differentiated from the earliest mammals. Opossums' social skills, we read, are rudimentary, more reptilian than typically mammalian. Says Steven Austad, associate professor of biology at Harvard and another expert on the species: "I think some biologists forget that opossums are mammals."

That may be changing now. Recent studies by Austad, Holmes and others are now arousing new respect, if not affection, for this country's only marsupial. At the heart of this research are clear indications that opossums are more highly adapted, more sociable and more sophisticated in their survival strategies than any scientists had suspected.

There are some 70 species of opossum in the New World, but only one is seen north of the Rio Grande-as often as not, lying flattened in the road at dawn. One of its earliest descriptions is also one of the most vivid: "An Opossum hath a head like a Swine & a taile like a Rat," wrote Jamestown's Captain John Smith in 1612, five years after his rescue by Pocahontas.

More modern biologists emphasize that marsupial wonder, the fur-lined abdominal pouch. Inside that pouch, suckling opossum babies in the course of two months grow from roughly the size of honeybees to about the size of chipmunks. Once weaned, the young opossums are essentially on their own, pursuing a largely solitary existence of sleeping by day, foraging by night.

But that doesn't mean they're rootless, uncommunicative, antisocial nomads, says Donna Holmes. "People used to say that opossums were so primitive, they did not even have a stable home range. They just stumbled around at night until they found a female and managed to copulate." To Holmes, that's just irresponsible opossum-bashing, due in large part to the fact that biologists in the field (who do stumble around at night) can neither see nor smell what the opossums they're tracking can see and smell.

Working with a small colony of captive opossums at Bowling Green State University, Holmes has shown that the animals are capable of social interactions long thought to be too complex for a mere marsupial. They mark home ranges, establish pecking orders and sometimes even share nests. Holmes has evidence that odors are the hidden language of the opossum's social world-used not only for marking home ranges but also for keeping tabs on one's fellows, perhaps even gauging a prospective mate's virility.

Opossums, fortunately, are highly odoriferous animals. "Opossums in the wild usually smell like dead fish or something," she says. "It can be pretty disgusting." Holmes speculates that each opossum may leave behind in its travels an olfactory signature, a unique "eau d'opossum" composed of substances both excreted and secreted. "It could be a very complicated bouquet of odors that let other animals identify that individual." Perhaps such findings don't vault the opossum into the gregarious company of whales and chimpanzees, but the discovery does indicate a capacity for purposeful interaction.

Another oft-heard knock against opossums is that they ramble around aimlessly when foraging; to humans, the looping, meandering night travels can seem confused, even dim-witted. Yet the people who know opossums best attribute this seeming aimlessness to an overlooked but highly evolved adaptation: unsurpassed omnivorousness.

"Opossums are essentially living vacuum cleaners-they eat anything," says Alfred Gardner, a U.S. Fish and Wildlife Service biologist stationed at the National Museum of Natural History. To opossums, square meals can include fruit, nuts, insects, earthworms, small mammals, snakes, bird eggs, garbage, dog food and the occasional chicken. The opossum has evolved an immunity to snake venom so that rattlesnakes are part of the animal's preferred diet. In addition, opossums have been known to gobble up soil, bits of paper, even cellophane. "I'm convinced a lot of this had insect eggs, blood or something else on it," says Gardner. The opossum swallows the whole thing, and its digestive tract extracts whatever nutrients are available.

An opossum is an omnivore with capital 0, in other words, which has survival value. Super-omnivory also helps explain the opossum's meandering travels: If every log, every puddle, every rotting carcass beckons the opossum with a tempting snack, then why stick to a trail like a deer or a wolf? Food is to he found everywhere.

The opossum's pouch, moreover, is indirectly responsible for some of this seeming waywardness. Put yourself in an opossum mom's place: If you could easily carry your kids around to fresh feeding grounds, would you keep returning to the same den every night? "The dogma in biology used to be that every animal had its own territory," says Gardner. Since opossums didn't seem to bother establishing a home territory, their behavior struck many observers as poorly developed. To the contrary, says Gardner. "Opossums will change their nest or burrow sites every few days. They park in an area, do their vacuum-cleaner bit, then move on. It's very effective."

Even if you can't bring yourself to admire opossums as opossums, say the biologists who know them best, you have to admire them as research animals. If the truth be known, the recent surge in opossum studies probably has more to do with the creature's suitability as a study animal, both in the lab and in the wild, than it does with any sudden urge by biologists to get to know the opossum better.

Opossums endear themselves to scientists by breeding quickly and having large litters. Full-grown animals are small enough to be handled easily, yet they're big enough to carry a radio collar. And last but not least, they're usually easy to trap.

"And re-trap," says Mel Sunquist, a wildlife biologist at the University of Florida who's worked with opossums for years. "In wildlife biology, there's always the question: If you catch it once, can you catch it again?"

Sunquist found out just how trappable an opossum is when he was in Venezuela in the early 1980s doing fieldwork with crab-eating foxes. Nearby, Steven Austad was studying the social behavior of Venezuelan wrens. Austad recalls that his colleague was unable to catch enough foxes. "His traps kept filling up with opossums. I said, 'Mel, you're studying the wrong animal. These opossums are trying to tell you something.- Before long, both researchers had switched to opossums.

Working together, Sunquist and Austad chose to use the common opossum, a close cousin of our species that inhabits Latin America, to test a controversial hypothesis put forward by biologists Robert Trivers and D.E. Willard. The latter had argued, in a paper published in Science in 1973, that polygynous mammals (whose males mate freely with several females) shouldn't bear male and female offspring equally.

Their theory was that a very healthy female will specialize in producing male offspring. The reason: These offspring will probably tend to be very fit too and can compete successfully to mate with many females and sire abundant grandchildren-the measure of reproductive success.

If females are less fit, however, the theory predicts they will not produce so many males. Male offspring of these less-fit females will probably turn out less-fit too and will just be pushed aside by other males in the competition for mates. The substandard mothers would do better to specialize in female offspring, which tend to find mates-or, more accurately, a mate finds them-whether they're fit or not. There's less chance of a complete flop, that is, zero grandchildren.

Trivers and Willard's hypothesis, based more on how evolution should work than on experimental observation, seemed too strange to be true. By what possible mechanism could an animal favor boys or girls in utero? Like many other biologists, Austad and Sunquist were highly skeptical. They decided to use opossums to test the theory once and for all. "It was an experiment waiting to happen," says Austad.

Working with radio-collared opossums in the Venezuelan wilds, the two biologists began leaving tuna fish outside the dens of some of their opossums-extra food to create extra-fit mothers. The result was a surprise.

The theory worked. "When we supplemented their food, the animals tended to produce sons," Austad says. "And when opossums became old and decrepit, they tended to produce mostly daughters."

"We were shocked," Austad says. "And although we didn't expect this result, it was much more exciting than if we'd just proven the hypothesis wrong." Austad and Sunquist's results, published in Nature in 1986, made a splash in the zoological world. Their findings are thought to be generally applicable to any mammal that isn't monogamous. The actual physiological mecha nism that makes adaptive sex ratios possible, however, remains a mystery.

Austad suspects that olfactory cues are somehow at work-"I can't imagine what else it can be." He's thought of an elegant experiment to explore the idea further: "I'd use the scent of very healthy captive opossums to scent-mark all over the place to see if that would make the opossums in the wild feel inferior and thus have more female offspring proportionally." Austad delights in the idea of tricking opossums for a good cause, but he admits the work involved in carrying out such an experiment would be prodigious.

At Harvard today, Austad and Holmesare working with opossums to explore the mysteries of aging. Understanding how and why aging takes place is a major research effort cutting across many disciplines and species, including our own, and opossums have become a key tool in the effort. Austad and Holmes's current work involves trying to determine whether diet-restricted animals live longer, as some scientists have hypothesized, and if so, why.

Opossums lend themselves to aging studies because they do it so precipitously. A female can have two litters in a year, but rarely survives to breed the following year.

If two-year-old opossums are a rarity, three-year-old opossums are virtually unheard of. "I liken them to pansies," says the Fish and Wildlife Service's Alfred Gardner. "They're planted one year, they bloom the next, then they die."

Yet, in one of the ironies of the zoological world, these short-lived marsupials are considered highly successful animals, successful because they're such efficient breeders. Opossums have large litters-half a dozen offspring is typical in the Southeast, a dozen or more in the colder North-and they have them quickly, starting at just six to eight months after a female's birth.

Austad and others believe that fast living and early dying must go hand in hand. "There seems to be a tradeoff between reproductive rate and longevity," he observes.

Even if the opossum ends up helping us live longer, healthier lives, we'll probably never embrace it with the same affection that we reserve for the dog, the guinea pig, or even the raccoon. There's that hairless, ratty opossum tail, for one thing. And then there's the creature's distressing habit of ending up its life as a messy road kill.

"Many people think opossums are stupid because so many are hit by cars," says Donna Holmes, rising once again to her charges' defense. "But there are reasons for that. They're terribly nearsighted. They're very numerous, so there are a lot of opossums to be killed. And they themselves are cruising around looking for road kills to eat," which, she points out, tends to increase the odds that they will soon join their meal.

To make matters worse, the animal instinctively freezes or feigns death when danger threatens, a tactic that seems to inhibit a predator's kill instinct. Unfortunately for the opossum, a car is not a typical predator. But as likely as not, that dead opossum in the road has already left behind a crowd of offspring to pass on its genetic legacy. One small misstep for an opossum; another biological victory for opossumkind.

Doug Stewart writes about science, from fossil footprints and Canada geese to robot soccer at MIT He has gained new appreciation for the opossums wandering around his Massachusetts home.

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