by Sandy Sohns

The Virginia opossum is much maligned, and has a reputation as being a repulsive, aggressive, dirty, garbage-eating pest that should be avoided or killed. Sadly, it is misunderstood and is unappreciated for its contribution to the environment, public health, and science.

The Virginia Opossum (Didelphis virginiana) is one of the oldest surviving mammals on the planet, and the only marsupial found in North and Central America, Mexico, and Canada. 

In 1608 in Jamestown, Virginia, John Smith first observed and described the opossum as the size of a cat with the tail of a rat and the head of a pig.

Males are usually larger than females. They weigh from 4-15 pounds and are 2-3 feet long, including the tail. Opossums have hairless black or white ears, black eyes, a triangular shaped head with a long white face, a pink nose, and mostly dark gray fur. They have 50 teeth, the most of any marsupial. Their front toes have a span of 180 degrees and their opposable great toes on their hind feet act as thumbs and help them grasp and climb. Their scaly mostly hairless prehensile tail also helps with climbing, provides balance, as well as carrying leaves, grass, twigs, or other material for nesting.

Opossums are generalists, adapting to a wide variety of habitats such as deciduous forests, open woods, marshes, streams, and urban and suburban areas. They move from an area when water and food are not available. They don’t hibernate but they do slow down in cold temperatures; their hairless ears, nose, and tail are susceptible to frostbite. Opossums are not destructive in creating habitat: they don’t dig holes or build burrows. Rather, they’re opportunistic in selecting living arrangements made by other animals or seeking shelter in garages or under sheds. Opossums are not territorial but can be confrontational during mating season or if encountering individuals in their current habitat.

Opossums are nocturnal, solitary, independent, and do not initiate an attack on animals larger than themselves including humans. If confronted by a perceived threat, its offense, which is really bluffing, is to open its mouth showing its teeth, drool, growl, hiss, belch, scream, or screech to scare away whatever is frightening them.

If stress increases, their defense—which is an involuntary physiological response—is to collapse. This is what is often called “playing ‘possum.” Their heart rate and respiration decreases, eyes are open or slightly closed, mouth is open, drooling is evident, defecation and urination can occur and if that isn’t enough to ward off whatever is bothering them, they can excrete a foul-smelling green liquid from their paracloacal glands near the base of the tail. This coma-like state can last from a few minutes to 4 or 6 hours.

They are meticulous groomers to keep their fur clean and dry. And it is estimated that they can consume upwards of 5,000 ticks per year. Females are especially fastidious before and after giving birth.

Opossums are omnivores and eat whatever is available including fruit, nuts, insects, frogs, rodents, grass, pet food, garbage, and carrion. They are referred to as “nature’s sanitation engineer” or “nature’s cleanup crew.” Their keen sense of smell and ability to remember where to find food is second only to humans.

They have a low body temperature which is unsuitable for the rabies virus and they are resistant to venomous snake bites due to a naturally occurring protein in their blood, the Lethal Toxic Neutralizing Factor (LTNF) which binds and neutralizes the venoms. This is scientifically important and the NIH’s National Center for Biotechnology Information has found LTNF to be a potent antidote for animal, plant, and bacterial toxins including scorpion and honeybee stings, plant-derived ricin, and botulism toxins.

Opossums have a short life span of 1-2 years. Motor vehicle accidents, hunting and trapping, disease and parasites, exposure and starvation all contribute to their brief existence. While their shy demeanor and proclivity for being killed by cars during their nocturnal food hunts implies decreased intelligence, they outperformed rats and cats in maze testing.

Their reproductive cycle is the shortest of any mammal, 12-13 days, with usually 2 litters a year between December and June. Females may bear as many as 25 babies (also called joeys like their distant kangaroo cousins) but the average survival rate is only 7-9 young. The reasons for this high mortality are many: the embryonic newborns are light pink, blind, about 1/2 inch long, and weigh 0.006 of an ounce. While they have deciduous claws on their front feet to climb up into the pouch, once there, they need to locate one of only 13 teats—some of which will not be viable. Those who make it will remain in the pouch for about 10 weeks, then gradually begin to leave and return to the pouch, and finally be completely weened by about 13 weeks. They stay with the mother for another 3 to 4 months, becoming stronger and independent. Around 7 or 8 months they become sexually mature, and then the mating season begins again.

The Virginia opossum is indeed a remarkable animal with many distinctive characteristics that is worthy of respect and protection.

References

Krause, William J. and Krause, Winifred A. The Opossum: Its Amazing Story, Research Gate, January 2005. www.researchgate.net/publication/265347494_The_Opossum_Its_Amazing_Story 

“Virginia Opossum Didelphis Virginiana,” Nature Works,  nhpbs.org/natureworks/opossum.htm 

Kirchner, Jane. “Opossums: Unsung Heroes in the Fight Against Ticks and Lyme Disease,” National Wildlife Federation. June 13, 2017, blog.nwf.org/2017/06/opossums-unsung-heroes-in-the-fight-against-ticks-and-lyme-disease/ 

Debczak Michelle.“13 Facts About Opossums.” Mental Floss. June 8, 2018, www.mentalfloss.com/article/544902/facts-about-opossums. 

“Creature Feature: Opossums Are Nature’s Pest Control,” The Buzz, March 1, 2019, www.reconnectwithnature.org/news-events/the-buzz/opossum-creature-feature. 

by Rosemary Jann

Although we may not be aware of it, we live in a region of borderlands. Of course, our area is politically shaped by the explicit borders of the District, Maryland, and Virginia. But the diversity of our plant and animal life derives in part from the fact that the southern limit for many northern species overlaps here with the northern limit for many southern species [https://www.dcr.virginia.gov/natural-heritage/natural-communities/document/ncoverviewphys-veg.pdf, p.23]. We also straddle a third significant geographic borderland, which is responsible for some of the most dramatic features of our region, like the Great Falls of the Potomac depicted below.

This border is known as the Atlantic Seaboard Fall Line, formed by the points on eastward-flowing rivers at which navigation becomes impossible because of rapids and waterfalls. This line runs down the mid-Atlantic, as seen in the diagram below, and has shaped the development of this area since its earliest history. The barriers to upstream navigation hindered inland migration by Europeans and spawned cities to their east to facilitate the transfer of goods and people: Georgetown and Alexandria in our area, but also Fredericksburg, Richmond, and Petersburg. Today, Interstate 95 roughly parallels the Fall Line in the mid-Atlantic region.

The Fall Line forms along the border between two physiographic provinces that meet in our area. A physiographic province is a geographic region with distinctive soils, topography, and vegetation. The Piedmont province is underlain by hard, crystalline bedrock. In the Coastal Plain, bedrock is deeply covered by softer sedimentary soils. As rivers flowing to the Atlantic over resistant bedrock meet the Coastal Plain, they begin to move more quickly and to cut down through those softer soils, creating waterfalls and rapids as the river descends to sea level [http://www.virginiaplaces.org/regions/fallshape.html]. 

The Great Falls of the Potomac, where the river drops 77 feet in less than a mile [https://www.nps.gov/grfa/learn/nature/naturalfeaturesandecosystems.htm], is the Fall Line’s most dramatic local manifestation. However, the rapids that originally blocked upstream navigation for European immigrants are actually located at Little Falls, slightly north of the Chain Bridge [https://en.wikipedia.org/wiki/Little_Falls_(Potomac_River)]. Over time, the energy of the Potomac has continued to erode the bedrock it flows over, causing the more dramatic waterfalls to migrate westward over millions of years, leaving Great Falls today approximately 14 miles upstream from Washington, DC.

That fact helps underscore the point that it is more accurate to talk about a Fall Zone than a Fall Line. Rather than a knife-edge transition from bedrock to sedimentary soils, the harder rocks of the Piedmont intrude into the Coastal Plain in irregular outcroppings over an area approximately 10 miles wide, creating a ragged boundary with patches of bedrock upthrust into Coastal Plain soils. We can witness this irregular border at Theodore Roosevelt Island, described by the National Park Service as the last bedrock island in the Potomac as it flows eastward to the Chesapeake Bay:  “The island thus marks the Fall Line with bedrock exposures on the northern shoreline (Piedmont) and swamp and tidal marshes on the southern shoreline (Atlantic Coastal Plain)” [http://npshistory.com/publications/gwmp/nrr-2009-128.pdf, p. 14]. Both are illustrated in the photographs below.

When we add the plant diversity that we gain from having vegetation characteristic of both these physiographic provinces in our area, we can fully appreciate the richness of our Fall Zone borderland: it offers a living map to our geological history, serves as a reminder of how our cultural history has been shaped by natural forces, and helps enrich the biodiversity we enjoy as residents of northern Virginia.