New World Opossums: Didelphimorphia

Physical characteristics: Unlike most of the New World opossums, the yapok, or water opossum, is specialized for an aquatic lifestyle. It is the only living aquatic marsupial species. In general terms, the yapok can be thought of as a sort of marsupial otter. The name "yapok" is derived from the Oyapock River in northern South America.

Adult head and body length runs 10.5 to 16 inches (27 to 40 centimeters); tail length, 12 to 17 inches (31 to 43 centimeters); and adult body weight, 1.3 to 1.7 pounds (0.6 to 0.8 kilograms). The animal is covered with short, dense, water-repellent fur, unique among the Didelphidae. The sides and upper body are black, with three pairs of The water opossum is the only marsupial that lives in the water. It has water-repellent fur and webbed hind feet. (Illustration by Jonathan Higgins. Reproduced by permission.) prominent, grayish bands that run vertically from the light gray underbelly almost to the spine. The head is blunter and wider than is common among Didelphidae species. The upper part of the head, including the eye area, is black. A dark gay bar runs the length of the snout from the nostrils to the crown. The lower part of the head is grayish. A prominent white stripe runs from above each eye to the ear. The eyes are large and black. The prominent, nearly furless ears are oval in shape. Conspicuous tufts of long, white or gray whiskers are mounted on each side of the head near the nostrils and over the eyes.

The hindfeet are webbed and the yapok uses them as its main propulsion organs when swimming. The hallux (first toe), usually shorter than the other toes in mammals, is elongated in the yapok, making the foot shape symmetrical and thus able to push more efficiently against the water. The forefeet are not webbed, and have elongated, furless fingers with reduced claws, which are furnished with a well-developed tactile sense.

Among the yapok's many peculiarities is that both females and males carry well-developed pouches that open toward the rear. The female uses a muscle to close her pouch when carrying young, which can survive without oxygen for several-minute intervals. The male uses his pouch to hold and protect the scrotum, drawing it up and into the pouch when he swims.

Geographic range: Yapoks are found in Central and South America, from southern Mexico and Belize through all of Central America, and into Colombia, Venezuela, the Guianas, Ecuador, Peru, Paraguay, Brazil, and northern Argentina.

Habitat: Yapoks live along streams, rivers, and lakes in tropical and subtropical rainforests of Central and South America, from sea level to 6,000 feet (1,830 meters) or more above sea level.

Diet: The yapok eats crayfish, shrimp, fish, and some water plants.

Behavior and reproduction: Females are polyestrous, meaning that they come into heat and become receptive to mating more than once a year. A breeding pair stays together for several days, the male following and circling the female until actual mating. A typical litter contains one to five young.

Yapok young have the fastest rate of development among all the Didelphidae species. After about forty days in the pouch, the young have grown body fur, pigmentation and the various markings, and opened their eyes. At about fifty days, the young begin to let go of the nipples and leave the pouch, but continue to suckle and stay with the mother, sometimes riding on her back.

Individual water opossums are solitary and hostile toward others of their species, except during mating times. An individual hunts and forages in freshwater streams, between rest periods, throughout the night. During the day, the animal rests in a temporary ground nest that it constructs from leaves and grass in a shady area. Close by is a more permanent underground burrow, which the yapok excavates in the stream bank, with its entrance a few inches above the water line. The entrance tunnel is about 2 feet (0.6 meters) long, and leads to a den lined with leaves or grasses. Individuals use their prehensile tails to carry nesting materials.

A yapok fishes and forages underwater, propelling itself with alternate strokes of its powerful hind legs and webbed feet. The animal shuts its eyes and ears and depends partly on its whiskers to detect motion, while its fingers, acutely sensitive to touch, are used to contact, check the texture of, and grasp prey.

The longest known lifespan for a captive yapok is three years.

Water opossums and people: Water opossums, confined to forests and riversides by their specialized lifestyles, are no threat or bother to humanity. Humans hunt them for their waterproof pelts, to be made into garments and accessories.

Conservation status: The yapok is listed as Near Threatened, not currently threatened, by the IUCN. ∎