For most of us, the word “marsupial” brings to mind kangaroos and koalas—animals we might describe as cute, or even charismatic. But those adjectives probably are applied less often to our one local marsupial, the Virginia opossum, Dildelphis virginiana. With their black-eyed pointy face, rat-like hairless tail, and toothy grin (their 50 teeth are the most of any North American mammal), opossums don’t so readily endear themselves to humans. And yet, opossums are a fascinating and beneficial (they eat ticks!) member of our natural community.
Marsupials are a subgroup of mammals, characterized by having young that are born at an early stage of development and then move to the mother’s pouch (“marsupium”) for continued nourishment. This contrasts with the great majority of mammals—the placentals—which develop more fully in the uterus, supported by nutrition from the placenta.
Today, marsupials are predominantly animals of the southern world. The best known are Australian, but South America is home to about 100 mostly small, opossum-like species. Surprisingly, though, the fossil record indicates that marsupials first evolved in the north. They entered South America from North America about 65 million years ago, after which the continents drifted apart. North American marsupials subsequently went extinct. It wasn’t until about 3 million years ago that the Panama land bridge formed and allowed some marsupials to extend their ranges northward through Central America. Only the Virginia opossum has made it beyond southern Mexico.
Despite that successful range expansion, Virginia opossums retain a number of marsupial characteristics that suit them poorly for cold winters. Like other mammals, opossums are endotherms, meaning that they generate internal heat from metabolism to establish a warm body temperature. However, those features are “turned down” slightly in marsupials. Opossums have rates of metabolism about 30% lower than similar-sized placental mammals like cats or skunks. Associated with that, body temperature of marsupials is slightly lower than in placentals, about 34oC (93oF) for opossums rather than 37oC (98.6oF).
The low metabolic rate means that opossums have a harder time staying warm in winter. Unfortunately (for them), they also lack several of the strategies that might offset that challenge. Many small placental mammals deal with winter’s cold temperatures and low food availability by hibernating. That state of suspended animation, with its greatly reduced metabolism and body temperature, requires much less energy expenditure. Hibernators can survive for weeks or months by burning stored fat, sometimes supplemented by food that was stashed away ahead of time.
Opossums do none of those things. Although some small marsupials hibernate, Virginia opossums do not. Nor do opossums store substantial body fat or food. Whereas a woodchuck, which does hibernate, might add three-quarters of its body mass as fat during autumn, opossums add very little. Similarly, while you might see squirrels (which do not hibernate) avidly collecting nuts as the weather cools, opossums do not store food. Moreover, opossum fur is not very dense or insulating, and that does not improve from summer to winter. And they are awkward! With short legs and a waddling sort of gait, the energy cost for opossums to walk is unusually high.
All of this adds up to an animal that is challenged to balance its energy needs during cold weather. As a result, opossums basically are home-bound on days that are too snowy or that stay below freezing; they find a sheltered spot and curl up to reduce heat loss. But, with their limited fat reserves, opossums still require something like 35 days of feeding during winter to make it through.
Even if they do survive, opossums may suffer from the cold. When temperatures drop, they prioritize warming their vital organs. Blood flow to the extremities is restricted, and it’s not uncommon for the hairless ears and tail to suffer frostbite. Many northern opossums run a little ragged at the edges!
This susceptibility to cold suggests that winter conditions limit Virginia opossums’ northward expansion. And yet, that range continues to expand. Opossums have been in Ohio for at least a few hundred years—they show up in trapping records—but they became established in southern Canada only in recent decades. At least three factors contribute to that expansion.
The first factor is that cold temperatures act as an agent of natural selection. Those individuals better able to tolerate cold are more likely to survive winter and to pass along their traits to the next generation. As a result, more northerly opossums now have larger body size, and shorter tails and ears, than their southern relatives. Those patterns are consistent with trends seen in mammals generally, known as Allen’s rule (shorter appendages in cold-adapted mammals) and Bergmann’s rule (larger body size).
The second factor promoting northward expansion of opossums is climate change. As northern winters warm, the number of days suitable for foraging increases. So, opossums progressively further north are able to maintain adequate energy balance through winter.
Finally, opossums benefit from patterns of land use by humans.
Opossums are true omnivores—they eat insects, fruits, carrion, trash, cat food, whatever—and human habitation provides both food and shelter. Conversion of forests to agricultural land probably promoted historical range expansion, and urban/suburban environments today may host denser populations of opossums than natural habitat.
Opossums may not possess classic good looks and charisma, but they are amusing and tenacious critters. If you happen to find one in your garage this winter, know that it’s just looking for a bit of warmth—and maybe some food. It’s cold out there!
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.