Praising Opossum

Virginia opossums (Didelphis virginiana) aren’t particularly attractive animals. They have pointy snouts and beady eyes; long, grizzled, gray fur; and a long, naked tail. When threatened, they gape widely and hiss to display their large canine teeth. They are beloved by few, and perhaps because of this, most people do not appreciate how unusual Virginia opossums really are.

A mother Virginia opossum (Didelphis virginiana) with several young riding on her back. The tail of an opossum is prehensile and can be used like an extra limb to hang from a branch. Photo by Kipp Teague CC BY-NC-SA 2.0.

Virginia opossums are marsupials, and that distinguishes them from every other mammal native to the United States, all of which are placentals. In case you aren’t familiar with the difference between a marsupial and a placental, they represent the two main groups of mammals alive today. Their ancestors diverged from one another at least 125 million years ago, and they evolved fundamentally different reproductive strategies. In placental mammals, the offspring spend a relatively long time inside the mother’s uterus and are comparatively well-developed at birth (though there is a great deal of variation in this regard). In marsupial mammals, babies are born after spending only a short time in the uterus; when they emerge, only their arms and mouth are well-developed. Each baby crawls to one of its mother’s nipples – often located within a pouch or fold of skin – attaches to it, and remains there until it reaches a stage of development comparable to that of many newborn placentals.

There are advantages and disadvantages to both reproductive strategies, but in recent geological history, marsupials have not fared as well as placentals. Both types of mammals were widespread during the Mesozoic Era (the Age of Dinosaurs), and this was also true for the first 20 million years of the Cenozoic Era (the Age of Mammals), when tropical to subtropical conditions prevailed throughout the globe. As global climates became cooler and more seasonal, initially around 30 million years ago and again around 15 million years ago, marsupials went extinct in many parts of the world, surviving only on Southern Hemisphere islands and continents such as Australia, New Guinea, Tasmania, and South America.

This brief history of marsupial biogeography highlights a common misconception about the Virginia opossum: that it is a “living fossil” that has been hanging around North America (so to speak!) since the days of Tyrannosaurus and Triceratops. In fact, marsupials went extinct in North America by about 15 million years ago; it is only within the past million years that the Virginia opossum and its relatives have been able to repopulate the continent. This was made possible by the development of a land connection between North and South America, the Isthmus of Panama, which facilitated the Great American Biotic Interchange. The Virginia opossum has continued its long, slow march to the north since then and now has one of the northernmost distributions of any South American mammal group that participated in the GABI.

Close-up view of the palate of a Virginia opossum (Didelphis virginiana). The triangular molars at the back (toward the left) are similar to those present in some of the earliest mammals, and for this reason, opossums are commonly thought to be “living fossils” (which they are not). They have simply retained so-called “tribosphenic” teeth because they are very effective for an omnivore like an opossum that eats both plants and animals. Note also the five pairs of tiny upper incisors to the right, which are characteristic of opossums. From the collections of the Cleveland Museum of Natural History (CMNH 21719). Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

The idea that the Virginia opossum is a “living fossil” is also incorrect from another aspect of its evolutionary history: the opossum family (Didelphidae) represents a recent branch of the marsupial evolutionary tree rather than an ancient one. It is true that opossum teeth are broadly similar to those of very early mammals, and for that reason they are often used as an example of the ancestral or “primitive” mammal dentition, but the oldest unequivocal opossum fossils are a mere 20 million years old. Molecular data suggest that the family may have evolved somewhat earlier, 30 to 40 million years ago, but this is still long after the end-Cretaceous mass extinction 66 million years ago. Ancient marsupial fossils that were once thought to be close relatives of modern opossums, including those found in North America, are now thought to represent separate branches of the marsupial evolutionary tree.

One “fun fact” that is true about the Virginia opossum is that it has more teeth than any other mammal in the United States: 50 in total (as opposed to, for example, 42 in a coyote, 32 in a deer or human, and a mere 16 in a cotton rat). This is mostly because it has more incisor teeth than any placental mammal: five pairs of uppers and four pairs of lowers rather than three pairs above and below in placentals. However, all opossums have 50 teeth, not just the Virginia opossum, so it is not quite accurate to say that it has more teeth than any other mammal in North America; several other types of opossums can be found in Mexico.

Illustrations of nearly a dozen New World marsupials, most of which are opossums (family Didelphidae). The exceptions are the top two species in the plate on the right: the left one (a) is the monito del monte (order Microbiotheria), and the right one (b) is a shrew “opossum” (order Paucituberculata), both of which are only distantly related to true opossums. Left plate from Eisenberg (1989), right plate from Redford and Eisenberg (1992).

That leads me to one last aspect of the Virginia opossum that is important to highlight: it is only one of well over 100 species of opossums that inhabit the Americas today. The vast majority of these opossums live in tropical forests, are active only at night, and are only a fraction of the size of the Virginia opossum, which is the largest of them all. Therefore, you aren’t likely to see most of these other types of opossums unless you are specifically looking for them, and even then, it is a challenge. For that reason, few people are aware of how diverse opossums are and the important roles they play in many ecosystems. For example, the yapok is semi-aquatic and feeds on fish and aquatic invertebrates in Central and South America, whereas many types of wooly opossums are tree-dwelling fruit-eaters that live throughout the Amazon Basin and in other tropical forests. Our omnivorous, opportunistic Virginia opossum is an outlier not only among other temperate mammals, it is also an outlier among opossums.

Just the other night I happened to catch a glimpse of an animal shuffling across our patio, and it turned out to be a Virginia opossum. It wasn’t the first one I’d seen in the wild, and I’m certain it won’t be the last. But will I never tire of receiving a visit from this distinctive marsupial, which is conquering eastern North America one back yard at a time.

References and further reading:

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