The Coypu (Myocastor coypus) or Nutria is a large, crepuscular, semiaquatic rodent native to South America, but now also present in Europe, Asia, and North America. In most regions the coypu is considered a pest. However in eastern Europe and Central Asia it is still valued for its fur. It is the only species in the genus Myocastor, and sometimes given its own family: Myocastoridae. The coypu has been introduced from South America to every continent except Australia and Antarctica. Africa is the only continent where the introduced coypu never became fully established. They are adapted to subtropical to mild temperate climate.
The coypu somewhat resembles a very large rat in appearance. Adults are typically 5 to 9 kg weight, and 40-60 cm in body length, with a 30-45 cm tail. They can also be identified by their bright orange-yellow incisor teeth (unlike rats, which have brownish yellow incisors). The nipples of female coypu are on their back. This allows their young to feed while the female is in the water.
Coypu can also be mistaken for another widely dispersed semi-aquatic rodent that occupies the same wetland habitats, the muskrat. However, the muskrat is smaller, more tolerant of cold climates, and has a dorso/ventrally flattened tail that it uses to assist in swimming. The tail of a coypu is round.
They were imported to many parts of the world due to their highly desired fur, known in the fur trade as nutria, but have since become pests in many areas, destroying aquatic vegetation, irrigation systems, eroding river banks and displacing native animals. Nutria damage in Louisiana is so severe that a bounty program (http://nutria.com/control_program) is in effect to aid in controlling the animal.
In addition to direct environmental damage, coypu are the host for a nematode parasite (Strongyloides myopotami) that can infect the skin of humans. When this happens the condition is called "nutria itch".
There are two common names used in the English language literature for Myocastor coypus. The Amerindian word "coypu" is generally used in Europe and Latin America. This use is probably meant to avoid confusion with, the term "nutria," which is Spanish for carnivores commonly called "otters" (Lutra sp.) in British and American English. In North America and Asia, the term "nutria" is the more commonly used one for Myocastor coypus.
There are several subspecies of coypu in their native range which includes parts of Argentina, Bolivia, Brazil, Paraguay and Uruguay. While as many as six different subspecies have been named generally there are only four recognized. They are: Myocastor coypus coypus, M. coypus bonarieusis, M. coypus melanups, and M. coypus santacruzae. Most literature indicates the coypu subspecies introduced around the world was M. coypus bonariensis. This subspecies is from the more northern (subtropical) part of their range. Local extinction in their native range due to overharvest led to the development of coypu fur farms in the late 1800s and early 1900s, The first farms were in Argentina and then later in Europe, North America, and Asia. In general, these farms have generally not been successful long term investments and farmed coypu often are released or escape as operations become unprofitable.
The distribution of coypu tends to expand and contract with successive cold or mild winters. During cold winters coypu often suffer frost bite on their tails leading to infection or death. Populations of coypu under these circumstances often contract and even become locally or regionally extinct (as in the Scandinavian countries during the 1980s). During mild winters their ranges tend to expand northward.