The nutria () or coypu () (Myocastor coypus) is an herbivorous, semiaquatic rodent native to South America.
Classified for a long time as the only member of the family Myocastoridae, Myocastor has since been included within Echimyidae, the family of the spiny rats.
The nutria lives in burrows alongside stretches of water and feeds mainly on aquatic and riparian vegetation. In its native range it forms part of South American wetland food webs and can contribute to the dispersal of aquatic plants and spores.
Native to subtropical and temperate South America, it was introduced to North America, Europe and Asia, primarily by fur farmers. Although it is still hunted and trapped for its fur in some regions, introduced populations can damage wetland vegetation and infrastructure through feeding and burrowing, and the species is considered invasive in several countries. Some populations can carry some zoonotic pathogens, including pathogenic Leptospira, although their epidemiological significance is scarce and region-specific.
Etymology The genus name Myocastor derives from the two Ancient Greek words () 'rat, mouse', and () 'beaver'. Therefore, the name Myocastor literally means 'mouse-beaver'.
Two names are commonly used in English for Myocastor coypus. The name nutria (from the Spanish word nutria 'otter') is generally used in North America, Asia, and throughout countries of the former Soviet Union; however, in most Spanish-speaking countries, the word nutria refers primarily to the otter. To avoid this ambiguity, the name coypu or coipo (derived from Mapudungun) is used in South America, Britain and other parts of Europe. In France, the nutria is known as a ragondin. In Dutch, it is known as beverrat 'beaver-rat'. In German, it is known as Nutria, Biberratte 'beaver-rat', or Sumpfbiber 'swamp-beaver'. In Italy, the popular name is, as in North America and Asia, nutria, but it is also called castorino 'little beaver', by which its fur is known in Italy. In Swedish, the animal is known as sumpbäver 'marsh/swamp-beaver'. In Brazil, the animal is known as ratão-do-banhado 'big swamp-rat', nútria, or caxingui (the last from Tupi).
Taxonomy
The nutria was first described by Juan Ignacio Molina in 1782 as Mus coypus, a member of the mouse genus. The genus Myocastor was assigned in 1792 by Robert Kerr. Geoffroy Saint-Hilaire, independently of Kerr, named the species Myopotamus coypus, and it is occasionally referred to by this name.
Four subspecies are generally recognized: M. c. bonariensis: northern Argentina, Bolivia, Paraguay, Uruguay, southern Brazil (RS, SC, PR, and SP) M. c. coypus: central Chile, Bolivia M. c. melanops: Chiloé Island M. c. santacruzae: Patagonia
M. c. bonariensis, the subspecies present in the northernmost (subtropical) part of the nutria's range, is believed to be the type of nutria most commonly introduced to other continents.
Phylogeny Comparison of DNA and protein sequences showed that the genus Myocastor is the sister group to the genus Callistomys (painted tree-rats). In turn, these two taxa share evolutionary affinities with other Myocastorini genera: Proechimys and Hoplomys (armored rats) on the one hand, and Thrichomys on the other hand.
Appearance
The nutria somewhat resembles a very large rat, or a beaver with a small, long and skinny hairless tail. Adults are typically in weight, and in body length, with a tail. It is possible for nutrias to weigh up to , although adults usually average . Nutrias have three sets of fur. The guard hairs on the outer coat are three inches long. They have coarse, darkish brown midlayer fur with soft dense grey under fur, also called the nutria. Three distinguishing features are a white patch on the muzzle, webbed hind feet, and large, bright orange-yellow incisors. They have approximately 20 teeth with four large incisors that grow during the entirety of their lives. The orange discoloration is due to pigment staining from the mineral iron in the tooth enamel. Nutrias have prominent four inch long whiskers on each side of their muzzle or cheek area. The mammary glands and teats of female nutrias are high on her flanks, to allow their young to feed while the female is in the water. There is no visible distinction between male and female nutria. Both are similar in coloring and weight.
A nutria is often mistaken for a muskrat (Ondatra zibethicus), another widely dispersed, semiaquatic rodent that occupies the same wetland habitats. The muskrat, however, is smaller and more tolerant of cold climates, and has a laterally flattened tail it uses to assist in swimming, whereas the tail of a nutria is round. It can also be mistaken for a small beaver, as beavers and nutrias have very similar anatomies and habitats. However, beavers' tails are flat and paddle-like, as opposed to the round tails of nutria.
Life history
Nutrias can live up to six years in captivity, but individuals rarely live past three years old in the wild. According to one study, 80% of nutrias die within the first year, and less than 15% of a wild population is over three years old. A nutria is considered to have reached old age at four years of age. Male nutrias reach sexual maturity as early as four months, and females as early as three months; however, both can have a prolonged adolescence, up to the age of nine months. Once a female is pregnant, gestation lasts 130 days, and she may give birth to as few as one or as many as 13 offspring. The average nutria reproduction is four pups. Female nutrias will mate within two days after giving birth. The years of reproduction cycle by litter size. Year one might be large, year two litter size will be smaller and year three the litter size will be another larger size. Females can only produce six litters in her life, rarely seven litters. A female on average will have two litters a year.
Nutrias generally line nursery nests with grasses and soft reeds. Baby nutrias are precocial, born fully furred and with open eyes; they can eat vegetation and swim with their parents within hours of birth. A female nutria can become pregnant again the day after she gives birth to her young. If timed properly, a female can become pregnant three times within a year. Newborn nutrias nurse for seven to eight weeks, after which they leave their mothers. Nutrias have been known to be territorial and aggressive when caught or cornered. They will bite and attack humans and dogs when threatened. Nutrias are mainly crepuscular or nocturnal, with most activity occurring around dusk and sunset with highest activity around midnight. When food is scarce, nutrias will forage during the day. When food is plentiful, nutrias will rest and groom during the day.
Native ecological role Within its native range in southern South America, the coypu participates in a variety of ecological interactions characteristic of wetland ecosystems. Studies conducted in these environments describe it primarily as a large semiaquatic herbivore, a consumer of aquatic and riparian vegetation, a disperser of plant propagules, and a prey species for native predators.
One documented ecological function is the dispersal of plant propagules through endozoochory. A 2024 study conducted at the Taim Ramsar site in southern Brazil recovered intact seeds, aquatic plant propagules, and fern spores from the feces of coypus and capybaras. The study estimated that an individual coypu dispersed approximately 691 propagules per day, including about 566 fern spores. Coypus dispersed proportionally more aquatic-plant propagules than capybaras, and the authors concluded that the two large rodents perform important and complementary roles as plant dispersers in Neotropical wetlands.
Herbivory by coypus is also part of wetland trophic dynamics. In two wetlands in southern Brazil, researchers identified 24 plant species in the diet; aquatic plants accounted for 84% and 54% of the plant species consumed at the two study areas, respectively. Diet composition varied with habitat, environmental conditions, and food availability. At Taim, a separate study identified 49 plant species from 14 families in the coypu diet. Although dietary overlap with capybaras was high, the two rodents coexisted through partitioning of other resources, indicating that both species occupy overlapping but not identical ecological niches as large semiaquatic herbivores.
Studies in the Argentine Pampas have likewise found a close relationship between coypu foraging and riparian vegetation. Borgnia, Galante and Cassini found that the diet was dominated by monocotyledons associated with aquatic habitats. In the agricultural systems they studied, the authors concluded that coypus did not behave as an agricultural pest where even a narrow strip of natural riparian vegetation was retained. Experimental work in Buenos Aires Province also found that the coypu's preference for aquatic plants and tendency to forage near water were related to food availability and to the costs associated with moving away from aquatic refuge, consistent with its adaptation to a semiaquatic environment.
Coypus also form part of native vertebrate food webs. In fragmented forests of the Araucanía Region of southern Chile, Myocastor coypus was identified among the principal prey items of the cougar, together with southern pudu and European rabbit. In Villarrica National Park, coypus were also recorded in the diet of the culpeo, with myocastorids making a notable contribution when prey biomass was taken into account. These studies document the coypu's participation in both plant–herbivore interactions and predator–prey relationships within ecosystems in its native range.
Distribution
Native to subtropical and temperate South America, its range includes Chile, Argentina, Uruguay, Paraguay and the southern parts of Brazil and Bolivia. It has been introduced to North America, Europe and Asia, primarily by fur ranchers. The distribution of nutrias outside South America tends to contract or expand with successive cold or mild winters. During cold winters, nutrias often suffer frostbite on their tails, leading to infection or death. As a result, populations of nutria often contract and even become locally or regionally extinct as in the Scandinavian countries and such U.S. states as Idaho, Montana, and Nebraska during the 1980s.Carter, Jacoby and Billy P. Leonard (Spring, 2002.) "A Review of the Literature on the Worldwide Distribution, Spread of, and Efforts to Eradicate the Coypu (Myocastor coypus)" Wildlife Society Bulletin 30(1): 162–175. During mild winters, their ranges tend to expand northward. For example, in recent years, range expansions have been noted in Washington and Oregon, as well as Delaware.
Many nutrias have been introduced to Europe, and large populations of them were reported in 2020 across continental Europe. They also populated eastern areas of England, but were eradicated through wide-ranging trapping efforts in the 1980s.