The Virginia Opossum - North America's Marsupial
The Underappreciated and Misunderstood Virginia Opossum
May 28, 2009
Five of these species are found in South and Central America, all the way to southern Mexico. The sixth, the Virginia Opossum, is the only species of opossum (and the only marsupial) north of the Rio Grande. This unique characteristic should confer a degree of appreciation for this animal. When Europeans settled North America they reported the northern range of the opossum ending in Pennsylvania. Now they are expanding northward into Canada. Many suggest climate change as a catalyst for their range expansion. Another possibility is the loss of higher level predators, which could release predation pressure from smaller prey animals, allowing their populations to expand. The synergistic effects of these environmental changes likely provides the opportunity for opossums to colonize new areas.
Behavior and Morphology of the Virginia Opossum
The opossum is a solitary species. It is an omnivore, and often scavenges its meals, which unfortunately contributes to its underappreciation. It is a valuable link in the ecosystem, as not only does it clean up the environment from carcasses and potential disease, but it is a food source for a variety of animals. It is about the size of a house cat. Opossums are agile climbers, and two adaptations greatly aid them: they have opposable digits, analagous to human thumbs, and prehensile tails to help with grasping and balancing. Many despise them as disease-carrying varmints, but opossums seem to be highly resistant to rabies when compared to other native mammals.
Virginia Opossum Reproduction
Didelphis means "two wombs", as opossums are not placental mammals but rather marsupials; they give birth to underdeveloped young at only 11-13 days after copulation. The babies weigh less than a fifth of a gram; for perspective, they are about the size of a bee, and an entire litter could fit in a teaspoon. They are naked and blind and helpless. Upon birth, they make their way into the mother's pouch instinctively where they begin to nurse. They finish developing here, much as placental babies do in the womb. They will remain attached to the mother's nipples for the first seven weeks of life, and will stay with her for about two more weeks before dispersing.
Defensive Adaptations of the Virginia Opossum
Opossums are some of the oldest living mammals residing in the new world. They have remained relatively unchanged for over fifty million years, giving them the title "living fossils". They are well adapted to many environments, allowing them to be so ubiquitous across their range.
Opossums have gained a reputation of being nasty and aggressive, due to their open-mouth display, hissing, and growling when they feel threatened. However, this is mostly an intimidation technique when the animal feels as if it is cornered and has no escape; it is actually quite frightened, and in the most extreme cases of terror the opossum may roll over and "play dead". This is thought to be an involuntary physiological response in which the animal actually enters a catatonic state which may last minutes to hours. As many predators will only eat live prey, it is a successful escape maneuver that has served the opossum well. They are actually shy, gentle animals who rely on stealth and nocturnal lifestyles to minimize human encounters.
Opossums and Humans
Virginia opossums are numerous today, and their status is likely attributed to the fact that they are generalists and have high reproductive rates. They tolerate human presence well, and even benefit by gaining shelter and food. They occupy broad ecological niches, and thus are not severely impacted by disturbance, which works to their advantage. Opossums mate between January and July, and have one or two litters a year consisting of usually five to nine young. Females are sexually mature between six to twelve months. However, many young die during the first year of their life, and the average lifespan is only eighteen months to two years.
One of the largest mortality factors is collision with vehicles, and sadly many of these cases are likely intentional. If people could appreciate our only native marsupial, perhaps opossums could be respected for the important ecological role they fill instead of persecuted.
References:
Newell, T. and R. Berg. 2003. "Didelphis virginiana" (On-line), Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Didelphis_virginiana.html
Sean Werle, Dept. of Entomology, UMASS Amherst. http://www.bio.umass.edu/biology/conn.river/opossum.html
Texas Parks and Wildlife Department: Virginia Opossum. http://www.tpwd.state.tx.us/huntwild/wild/species/oposum/
The Virginia Opossum in Southern Ontario. Ontario Ministry of Natural Resources. http://www.mnr.gov.on.ca/en/Business/FW/2ColumnSubPage/251071.html
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