The opossum, Didelphis virginiana (previously D. marsupialis virginiana) is one of the world's oldest living mammals, and the only marsupial on our continent. Marsupials are born before they are well developed, compared to other mammals, and continue their growth and development in a pouch on their mother's abdomen. Most members of the order Marsupialia are native to Australia and South America. Structurally, they have changed very little in millions of years; the opossum's relatives date back to the Cretaceous Period, 90 million years ago. However, the opposum didn't appear in North America until the Pleistocene Epoch, less than a million years ago.
"Opossum' is an Algonquin Indian name meaning "white animal." A creature without specialized body structure or food preference, the opossum thrives in many settings. It is found throughout Pennsylvania, where it is classified as a furbearer.
Mature opossums are 24-40 inches long, including a 10-12 inch tail. They weigh 4-12 pounds. Males are larger and heavier than females, and the average adult is about the size of a large house cat.
An opossum has a long, pointed snout with abundant teeth (50, the most of any North American mammal), small, dark eyes, and rounded, bare ears. The tapering tail is naked and scaly, like that of a rat. The feet have five toes, each with a claw except the first toe of each hind foot, which is long and capable of grasping, like a thumb.
The long, coarse body fur is light gray; outer hairs may be tipped yellow-brown. Legs and feet are dark brown or black. Males, females, and immatures are colored alike, although fur and skin color may vary in different geographic areas.
Opossums walk with an ungainly shuffle, averaging .07 m.p.h.; their running speed is a little over 4 m.p.h. Excellent climbers, they ascend hand over hand, using their prehensile tails for gripping and balancing. They are good but slow swimmers.
An opossum's brain is small and of primitive structure. Senses of smell and touch are well developed, but hearing is not especially keen and eyesight is weak. When walking, an opossum sniffs the air occasionally, and stops and stands on its hind feel to look around. Although normally silent, it may growl, hiss, or click its teeth when annoyed.
If an opossum is threatened and cannot climb a tree or hide in rocks or brush, it may crouch and defend itsself - or, more likely, feign death.
When feigning death, also called "playing possum," an individual lies limp and motionless, usually on its side. Its eyes and mouth remain open, its tongue protrudes, its forefeet clench, and its breathing becomes shallow. This state may last from a few minutes to several hours. Feigning may help an opossum survive attack, because some predators ignore dead prey. Opossums also exude a musky odor which may repel some enemies. Wildlife biologists have yet to determine whether feigning death is deliberate (a behavior evolved for survival) or involuntary (perhaps caused by nervous paralysis).