One reason that Virginia opossums have been able to move north is that they will eat just about anything. Photo by Cody Pope, Creative Commons.

“Roadrunner! Tofu muffin! Bala shark!” “Hey, look, two opossums!”  

Family game time over Christmas break had us shouting some odd combinations of words around the dining room table. While most were guesses for a variation on charades that we call Salad Bowl, the opossums were seen drinking from my parents’ water feature and cleaning up fallen seeds under the bird feeder. It took my 15-year-old nephew several minutes to help me spot their long, white faces peeking through the dark prairie grass.  

They may not be as cuddly as koalas, but North America’s only marsupials are surprisingly cute.   A couple of decades ago, during a bitterly cold winter in northeast Iowa, I remember cringing at the sight of a sad opossum with a black, shriveled, frostbitten tail. I felt sympathy and disgust at the same time. Since temperatures barely fell below freezing over this holiday season, these critters were looking significantly happier. That seems to be a trend.  

And you know, I’ve become more excited to see them, too. In recent years, scientists have calculated that opossums will eat up to 5,000 ticks per season. Since ticks carry Lyme disease, babesiosis, ehrlichiosis, anaplasmosis, and several other nasty ailments, having plenty of little tick-Hoovers in our native prairie seems like a great idea.  

The key is that opossums are fastidious groomers, and whenever they find a tick, they lick it off and swallow it. This takes care of more than 90% of the ticks that attempt to draw their blood meal from an opossum. According to one forest ecologist, opossums are net destroyers of ticks. In comparison, mice are super lazy about grooming and are some of the primary carriers of Lyme disease and feeders of ticks.  

Hardly a summer goes by where my parents don’t take at least once course of antibiotics for Lyme or another tick-borne disease. Without opossums moving into our woods, it could be worse.

And it truly is moving in. The pre-settlement range of Virginia opossums (their official name) was focused on the South. They only went as far north as southern Illinois and Missouri, and didn’t extend all the way east or west. Since 1900, and especially in the past 20 years, opossums have moved all the way up the Atlantic coast to Maine, north to Canada, and west to Colorado. Originally, it was the clearing of dense forests that facilitated their northward march. Currently, warmer winters seem to be the key factor in their expanding range, and their occurrence seems to be restricted by temperature and snow depth. Opossums now inhabit all of Iowa and Wisconsin, and most of Minnesota. They were even introduced to the west coast as a food source during the Great Depression. (Recipes containing opossum can be found in early editions of The Joy of Cooking!)  

Like cardinals who have followed bird feeders northward, opossums seem to have followed trash cans. At the northern edge of their range, they’re better able to find food and shelter around human habitation.  

From warm beginnings in South America—waddling beneath the dinosaurs—opossums traipsed northward across the newly-formed Isthmus of Panama to North America, two million years ago. In order to spread even farther north, opossums have had to adapt.  

In ecology, there are three “rules” that describe how animals vary from the southern reaches of their range to their northern limits. Bergman’s Rule states that animals will have larger body sizes in colder climates. Larger bodies store more fuel for the hungry days of winter, and they lose less heat. Allen’s Rule talks about the benefits of shorter extremities when staying warm is a struggle. Shorter ears, for instance, are easier to keep from freezing. Gloger’s Rule describes the trend of less pigmentation at northern latitudes.  

These rules apply to mammals, but not many scientists have looked at them in relation to marsupial mammals. As it turns out, opossums fit the patterns nicely. Northern opossums have bigger bodies, shorter ears and tails, and thicker fur. You can easily imagine how those traits are advantageous in cold places.   Northern opossums also have paler skin. The brown melanins in skin contribute to the immune system’s ability to ward off bacteria and fungi, which are more common in warm, wet climates. Up north, pathogens are less common, and the pigments are less necessary.  

One of opossums’ greatest advantages for expanding into new areas is simply their un-picky eating habits. They’ll eat trash, grubs, cockroaches, rats, mice, slugs, dead stuff, rotting fruit, and even venomous snakes. Opossums’ opportunistic diet couldn’t be more different than the beef-stick-and-cookie cuisine that the younger members of our family seem to thrive on. But the variety is similar to the random words entered in our Salad Bowl game. Roadrunner, tofu muffin, bala shark…I bet an opossum would eat them all!  

Emily’s second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.  

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new Curiosity Center kids’ exhibit and Pollinator Power annual exhibit are now open! Call us at 715-798-3890 or email emily@cablemuseum.org.