Wild Things: Opossum (Didelphis virginiana)
The Opossum, or Virginia Opossum, is the only marsupial mammal in North America. Marsupials are endemic to Australia and migrated to South America. The Opossum evolved from marsupial ancestors in South America and subsequently expanded its range to North America during the Cenozoic geological era some 65 million years ago, when the land bridge formed connecting South and North America.
The range of the Opossum in North America covers most of Mexico and up into the central and eastern United States. A population along the west coast states has grown from introductions there. The northern limit of the range, east and west, is at about the U.s.canadian border and probably represents a winter temperature regime that the Opossum cannot survive.
Like all marsupials, the Opossum is a pouched mammal. Reproduction is different from placental mammals in that marsupials give birth to live young at a very early stage of development. The embryo instinctively crawls from the birth canal up the belly of the mother using claws present at birth, until it reaches the pouch, or marsupium. It crawls inside and attaches to a milk producing teat located there, where the young will stay and grow in the warmth and protection of the mother's abdominal pouch. After several months the young are weaned and leave the pouch, where they may ride on the back of the mother for a time as she forages. The marsupium provides a similar growth chamber to the womb in placental mammals. Opossums are nocturnal and seek shelter in natural cavities and burrows of other animals. They don't excavate their own dens. Opossums have a short life span for a medium-sized mammal, just one to two years typically in the wild, or up to four or more years in captivity.
The Opossum is found in wooded, brushy habitats near water. It readily climbs trees and with its prehensile tail and opposable first digit on both foreand hind legs, is able to move around easily among tree limbs. In diet, Opossums are quite omnivorous, taking anything from insects, to amphibians, reptiles, rodents, birds and eggs, and even carrion. They consume the bones of vertebrate prey in order to acquire needed calcium. They are also able to effectively groom themselves, eating large numbers of ticks that may attach to their bodies.
The main distribution of the Opossum in the U.S. covers the eastern and Midwest states, ending where the Great Plains becomes arid, at about 20 inches of annual precipitation or less. But the species is occasionally found in New Mexico. Earliest records date back to 1928 in the southeast corner of the state. The most recent publication on Opossum occurrence in the state is a 1995 paper in the Southwestern Naturalist.
Opossums are known for assuming a defensive posture in which they appear to be dead. This unusual behavior has been found to be an involuntary physiological response to stress or danger, like a fainting spell in humans. The animal lies with open mouth and bared teeth, and may even be handled without eliciting a response. The individual in my photos appeared to be at least partially assuming this state of “playing dead.” Perhaps it had not entirely “fainted” and lost consciousness at the time I observed it.
There is evidence that the Opossum is expanding its distribution to the north as climate warming mitigates winter temperature extremes in the northern portions of its range. That same climate heating effect may also serve to contract the range limit of the species eastward along its western edge as precipitation decreases there and drought conditions persist.
James Taulman is a semi-retired research wildlife biologist who worked with the U.S.F.S. research branch and taught zoology, ecology, and more in several university positions. Search for him on Youtube to see wildlife videos.