New World Opossums: Didelphimorphia

The word "opossum," commonly used to refer to all species within the family Didelphidae, is derived from an Algonquian Indian word for the Virginia opossum, the only living marsupial species north of the U.S.-Mexico border. "Possum," without the first "O," refers to certain Old World marsupials in Australia and New Guinea.

Didelphidae are tiny to medium-sized animals, most tending toward the smaller end of the size spectrum. Males are larger than females. In most species, the tail is about the same length as the combined head-and-body length, or longer, scaly and only lightly furred, and is prehensile (able to grasp) to varying degrees among species. In the smallest species, adult head and body length runs 3.3 to 7.2 inches (8.5 to 18.5 centimeters) and tail length is 3.5 to 10 inches (9 to 25 centimeters). In the largest species, adult head and body length runs 13 to 19.5 inches (32.5 to 50 centimeters) and tail length is 10 to 21 inches
(25.5 to 53.5 centimeters). Adult weight in the larger species is usually between 4.5 and 12 pounds (2 and 5.5 kilograms).

The limbs of Didelphidae are short, except for the yapok (or water opossum), whose hind legs are a little longer than the forelegs. All four feet bear five digits and the hallux (HAL-lux; big toe) is opposable. All digits are clawed, except for some species in which the hallux lacks a claw. The muzzle (mouth area) is long and pointed, and the ears are prominent. The canine teeth are long and large.

The fur may be fine and velvety, thick and woolly, or somewhat coarse and stiff. Pelt colors, combinations, and patterns vary widely among genera (JEN-uh-rah) and species. The brown four-eyed opossum and the gray four-eyed opossum owe their common names to a colored spot of fur above each eye. In some species, there are dark brown or black patches around the eyes.

In most Didelphidae species, the back and sides of the body are dark, the underparts lighter. Upperparts may be gray, dark brown or reddish brown, the underparts white or yellowish. The thick-tailed opossum has an elaborate coloration that varies among individuals. The upper body fur may be yellow, yellow-brown, or dark brown, while the underparts are reddish-brown, light brown, or dark brown. The fur may have an unusual purple tinge. The face may show vague markings. The body shape of this species is also unusual, tending toward a long, low-slung, weasel-like form, with short but strong legs.