Opossums: An Adaptive Radiation of New World Marsupials

Many people think of marsupials as Australian mammals, which get almost all the press attention. Most of the marsupials in nature documentaries are from Down Under: kangaroos, wallabies, wombats, koalas, bandicoots, sugar gliders, and so forth. But, despite Australia’s reputation as the marsupial homeland, marsupials first evolved in South America, where they’re still alive and well. Today, over 100 species of marsupials are found in the Americas, and most of them are opossums: members of the order Didelphimorphia. In fact, opossums inhabit almost every terrestrial biome from Patagonia to Canada. If you live in the eastern United States, the chances are good that there’s an opossum in your backyard right now.

Photo by Antoine Baglan

Opossums: An Adaptive Radiation of New World Marsupials is the first book ever written about opossums, which include some remarkable species. The water opossum, the world’s only semiaquatic marsupial, swims in rainforest streams and gropes for fish and crustaceans with enlarged, six-fingered hands. Lemur-like woolly opossums feed on fruit and visit flowers in the forest canopy, and tiny shrew-like opossums rustle through the leaf litter searching for insects and other invertebrate prey on the forest floor. Desert-adapted opossums that that store fat in their carrot-shaped tails live in some of the driest places on earth, and weasel-like carnivorous opossums slink through tropical and subtropical grasslands hunting mice. Our North American species, a Pleistocene immigrant of South American ancestry, has become a human commensal in the northern part of its geographic range. One species, the short-tailed opossum (Monodelphis domestica) is now a popular model organism for biomedical research.
 
“If there's a book you really want to read, but it hasn't been written yet, then you must write it.”
                                                                                                               –Whoopi Goldberg

Sharon and I began drafting chapters for this book almost ten years ago. We tried to write the book we wanted to read back when we first started working on opossums in the late 1990s. That book, however, would have been very different than the one we eventually wrote. For one thing, it would have been shorter. Opossum research has exploded in just the last two decades: nowadays, the opossum research community is a thriving international enterprise, and significant contributions are being made in the field and in the laboratory on both American continents. Much of this important work, however, has never been synthesized into a coherent account of opossum biology. To do so, our book reviews many topics that have never been summarized before, including chapters on physiology, behavior, habitat occupancy, parasites, predators, competitors, mutualists, and population biology.   

Photo by Antoine Baglan

Our theme concerns adaptive radiation—the evolution of ecological diversity among opossums and the range of adaptations that enable them to make a living in natural ecosystems. We explain what is known about opossum adaptations, but we also emphasize what is not. We know, for example, that some opossums eat venomous snakes and poisonous amphibians, but the metabolic costs and ecological advantages of toxin resistance have not been investigated. We know that the water opossum has unique sensory organs on its palms and fingers, but what those organs actually do is unknown. Remarkably, the adaptive significance of some relatively well-known opossum traits is still a mystery. One example is death feigning, commonly known as “playing opossum.” This behavior makes no sense from an evolutionary perspective: why fake your death when attacked by a predator? In fact, only the Virginia opossum is definitely known to “play possum,” and nobody really knows why. It’s one of many unsolved problems we identify, discuss, and encourage future researchers to investigate.

Photo by Antoine Baglan

One of the most enduring myths about marsupials is that they are evolutionary losers: hapless victims in a struggle for global dominance with placental mammals. To be sure, marsupial ancestors went extinct long ago in North America, Europe, and Asia, but so did the ancestors of many placental mammals. Impressively, however, opossums now thrive in one of the most placental-rich habitats on earth, the Amazonian rainforest, where they rank third in diversity after bats and rodents. And that opossum in your backyard? It’s living proof that marsupials are here to stay.

Order Opossums: An Adaptive Radiation of New World Marsupials at the following link: https://jhupbooks.press.jhu.edu/title/opossums

Robert S. Voss is a curator of mammals at the American Museum of Natural History and an adjunct professor at the Graduate Center of the City University of New York. He is the coauthor of Opossums: An Adaptive Radiation of New World Marsupials.

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