An aquatic marsupial - a member of the opossum family - with black and grey waterproof fur, webbed hind feet and a backward-pointing pouch in both sexes.

Life span
Up to 3 years in captivity.

Statistics
27-40cm with a 31-43cm long tail.

Physical description
A small opossum with dense, waterproof fur in a grey and black marbled pattern. The muzzle and around the eyes is always black and there is a grey stripe above the eyes. The long tail is furred at the base and dark for most of its length, with a pale tip. The ears are rounded and naked. The hind legs are quite powerful and the hind feet webbed. The front feet are not webbed and have long, sensitive fingers. Both sexes have a backwards-facing pouch with strong muscles to close it and (in females) make it waterproof. Males draw their scrotum up into the pouch when swimming or running fast, to stop it getting caught in vegetation.

Distribution
From southernmost Mexico through Central America, Colombia, Venezuela, Guianas, Ecuador, Peru, Paraguay, eastern Brazil, and North-eastern Argentina.

Habitat
Tropical rivers and streams.

Diet
Crustaceans, fish and amphibians.

Behaviour
Yapoks are solitary and fight when they meet. They are mainly active at night, spending the day in a nest lined with vegetation or an underground nest dug into the river bank. During the night they show several bursts of activity, swimming underwater using their hind feet for propulsion and feeling for crustaceans with their front feet. They also collect vegetation for their nest, carrying it in a bundle rolled up in their tail.

Reproduction
Births occur in December and January and litters of 1-5 naked youngsters are born in the nest. By 22 days they are beginning to show fur and by 40 days their eyes are open and their bodies protrude from the pouch. At 48 days they detach from the nipples but still nurse and sleep with the mother.

History
Fossils dating back to the Pliocene have been found in Argentina.