By Mike Reiter, Polk County Naturalist
The opossum is a very unique animal that can occasionally be observed in both rural and urban settings. It is the only marsupial in Wisconsin. Marsupials are animals that give birth to their young which are very small. These small, fingernail-sized babies will then crawl up into a pouch on their mom’s underside and spend the next several months there being nursed and protected by her. Kangaroos from Australia are another example of a marsupial.
The opossum is a rather simple animal that moves about slowly looking for food. It has the most teeth of any mammal in Wisconsin (50) and will eat just about anything. At times when threatened with danger, they will exhibit a phenomenon known as “playing possum”. While it looks like the animal is “playing dead” it has in reality “passed out”, due to the constriction of the blood vessels that supply blood to the brain. The “Flight or Fight” mechanism due to the release of compounds from the adrenal gland, causes this condition. Once the threat has passed, the hormones are cleared and the blood flow returns. The opossum will revive and continue on its way. It is also a frequent casualty on our roadways as it has little fear of motor vehicles and will always come in second in these encounters. Sometimes these animals are referred to as “Wisconsin Road Bumps”.
In this area, the opossum is a rather new resident. When I was a youth, their range extended as far north as southern Wisconsin. Within the last 30 years possums have moved from their southern range and are now quite common. They really don’t belong this far north however. If one would examine an older opossum closely, one will note the frost-bit ear tips and shortened tail tip frozen off by our harsh winters. It may only take a few very cold winters in a row to send the newcomers packing to a warmer part of the state. Some folks have felt that the opossum made its way north in hay bales which were brought in from the south during drought periods. According to Jim Evrard, DNR Research Biologist, as published in “Duck Production and Harvest in St Croix and Polk Counties Wisconsin” (2002), “As a result of a recent range expansion from the south, the Virginia opossum (Didelphis virginianus) appeared late in the study and was first reported in a road kill in 1990.”
Keep a sharp eye out for the opossum and you might be lucky enough to see one out and about looking for a bit of food. Their small even spaced tracks in the sand or snow with the tail drag down between the paw prints point out the presence of this interesting creature.