The opposum is the only marsupial native to North America.
By Constance Casey
Opossums, animals that can eat almost anything, are increasingly joining their fellow omnivores at the suburban garbage can buffet. Opossums are shy, shambling creatures—the opposite of aggressive—but an opossum on its nocturnal foraging rounds often elicits a scream of disgust or fear. People tend to see them as ugly and vicious because their tails are bare and their narrow snouts are full of sharp teeth.
An opossum’s tail is ratlike, bald, and scaly. But to think of them as large rats or naked-tailed squirrels is wrong. Opossums, commonly known as possums, are not rodents; they are marsupials. The fact is that they’re North America’s only pouch-bearing animals. (Marsupium is “pouch” in Latin.) The world’s most famous marsupials, kangaroos and koalas, are Australian and cute. (Possums Down Under are in a different family, the Phalangeridae, and have appealing, furry tails.)
The western hemisphere has about 100 opossum species, all in the family Didelphidae. Of these, only one, the Virginia opossum (Didelphis virginiana), lives in North America. When the land bridge linking South and North America formed three million years ago, some opossums trudged north. By the 1600s the creature that would be the Virginia opossum had traveled up to the Virginia colony. There, Captain John Smith of Jamestown observed, “an opassum hath an head like a Swine and a taile like a Rat….” (He derived the name “opassum” from an Algonquin word.) Smith correctly observed that the animal carried its young in a pouch.
The only slightly swinelike opossum head bears 50 pointed teeth. The animal’s first defensive move is to show these little daggers, leading some to see the animal as ferocious. If cornered, an opossum will hiss in warning, then defend itself, but not very strongly. Faced with an extreme threat, the animal feigns death—the famous “playing possum”—a state that can last several hours. Lying on its side, mouth open and drooling but fully conscious, the opossum can suffer and survive a severe mauling. Opossum skeletons often reveal healed bone fractures other mammals of similar size could not have survived. It’s a lifesaving, if painful, maneuver, but it doesn’t work against cars.
In fact we most often see opossums squashed on the road. We rarely see the creatures at their best. Because they’re nocturnal, we encounter living opossums in the daytime when they’re sluggish.
Until recently, when possums did surprisingly well on memory tests, it was presumed they were very dumb. The Virginia opossum skull is small and looks primitive, with many of the features of early fossil mammals; an opossum-like animal toddled around the feet of dinosaurs.
In one insulting experiment, researchers filled the cranial cavities of different mammals with dried beans. The cat brain held 15 dried beans, the raccoon 150, and the opossum’s brain space five. We human beings, proud of our large brains, assume that the larger the brain size, the more intelligent the animal. Not necessarily, and it’s always tricky trying to define intelligence in animals.
Small though it is, the opossum brain is very good at finding food. The animals have an acute sense of smell, indicated by large olfactory lobes. When tested for the ability to remember which of four runways led to a food box, opossums scored better than cats, chicks, dogs, goats, pigs, rabbits, rats, and a turtle.
Tests less pleasant for opossums than tracking down food have revealed a couple of unique qualities that make them good neighbors.
In one, researchers at the Cary Institute of Ecosystem Studies in Millbrook, New York, introduced dozens and dozens of black-legged ticks (Ixodes scapularis, formerly called deer ticks) into the cages of six species of forest animals, including opossums. This was not gratuitous torture; the scientists were trying to find out the part different animals play in the spread of Lyme disease, as black-legged ticks carry the Lyme spirochete.
The ticks dined contentedly on birds, chipmunks, and squirrels. Blood filled, they dropped off, satisfied and alive. The ticks on the opossums were not so lucky—only 3 percent fed successfully. Opossums turn out to be meticulous groomers. They spend a lot of their waking hours washing up, front paws to the face like cats, or using a hind foot as a comb. Researchers found that 97 percent of the ticks on opossums were groomed away and consumed.
In yet another research project that was not much fun for the animals, caged opossums were injected with snake venom. The lab opossums fortunately didn’t suffer, because they have developed resistance to the venom of pit vipers, including copperheads, rattlesnakes, and cottonmouths. In parts of the United States such snakes, a nice piece of protein, are an important part of the opossum diet. Most pit viper venom is hemolytic; it breaks down red blood cells, causing massive hemorrhaging. A protein in opossum blood, with the elegant name von Willebrand Factor, has evolved to allow the animal to neutralize the effect.
Opossums have turned ticks into a snack and marshaled the genetic changes necessary to protect against snake venom, but they do not age well. For their size, opossums, which are relatively defenseless, are one of the shortest-lived animals. An opossum is lucky to live two years, while the well-armored porcupine can live 20. But the short life is not only because they’re vulnerable to predators. In their second year opossums experience a steep decline, becoming weak and thin, and they develop arthritis and cataracts. We human beings manage, up to a point, to repair the DNA damage that occurs as the years add up. Opossums seem to lack that capability, which makes them an interesting model for researchers studying the diseases of human aging.
Life is very tough for opossums at the end and also at the beginning. Despite their brief individual lives, opossums are considered a successful species because of their high reproductive rate. They become sexually mature in less than a year, and females have two or three litters a year.
Life for a newborn opossum is fraught. The mother opossum can carry up to 50 embryos in her divided womb (Didelphis is the Greek for two wombs), more usually 20. After only 12 or 13 days of gestation the young are about the size of a raisin. Blind and deaf, the tiny newborns climb the few inches from the birth canal to the mother’s pouch, where there are 13 teats. That unlucky number leads to a deadly game of musical chairs; more young are born than can be raised. The newborns’ back legs are undeveloped, but their forepaws are equipped with grasping claws that will drop away after successful attachment. The fittest, or luckiest, spend about two months in the pouch and then venture out, remaining near their mother to nurse. This is the most photogenic moment for opossums, as the young hang on to their mother’s back as she forages. Of those that make it past weaning, fewer than 20 percent live beyond their first year, falling prey to dogs, cats, owls, snakes, foxes, or coyotes. Human activity is also hard on the animals; opossum hunting (then eating the catch with sweet potatoes) has long been popular from the Ozarks to the Everglades. Southerners, longing for possum stew, imported and released the animals into California and Oregon.
Some human beings have plenty of sympathy, even love, for these marsupials with difficult lives. The Opossum Society of the United States educates interested persons in how to care for an orphan opossum. In captivity opossums can live eight years, and they can learn to use a litter box.
The most frequently asked question for these opossum advocates is, “Help! There’s an opossum in my yard, what should I do?” The answer is, “Nothing.” Unless there’s an irresistible and constant food source, the opossum will move along. Moving along is likely to mean moving north. Though in cold regions opossums suffer from frostbite of the ears, some have been seen in New England and southern Ontario where they are drawn to pet food, garbage, and the warmth emanating from houses. Homeowners should do nothing but consider themselves lucky to catch sight of North America’s only marsupial.
Constance Casey, a former gardener for the New York City Parks Department, is a contributing editor to LAM.
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