Myocastoridaecoypus and nutria

The single Recent species in this family was originally restricted to central and southern South America, but it has been introduced widely in the United States and Europe.

Nutria are large rodents, weighing up to 10 kg. They have a robust body that shows many indications of their aquatic lifestyle. The ears and eyes are small. The webbed hind feet are large and have 5 toes. Four toes are found on the forefeet, which lack webbing. All toes have well-developed claws. The tail is long, scantily furred, and rounded. It is not compressed as in beavers or muskrats.

Like many aquatic mammals, nutria have thick, soft underfur overlain with long and coarse guard hairs. Their color is dark dorsally and whitish yellow on the underside.

The cranium of a nutria is massively built, with well-developed ridges (including a sagittal crest) and a deep rostrum. Nutrias are hystricomorphous, and their infraorbital foramina lack a distinct groove for the passage of nerves to the rostrum. The zygomatic arches are heavy and broad, but the jugal does not contact the lacrimal. On the ventral surface of the skull, the auditory bullae are small but the paroccipital processes are unusually long. The lower jaws are strongly hystricognathous. The coronoid process is reduced to little more than a knob.

Nutrias have the following dental formula: 1/1, 0/0, 1/1, 3/3 = 20. The molars are hypsodont, flat-crowned, and rooted. The upper molars have two labial and two lingual folds , while the lowers have one labial and three lingual. The incisors are massive and chisel-like, deeply orange-yellow in color.

Nutria are primarily herbivorous, feeding on vegetation on land and in shallow water, but they also consume some invertebrates. They are excellent swimmers, capable of staying underwater for up to around 5 minutes. They prefer slow-moving streams, lakes, and brackish or freshwater marshes. They are good diggers, constructing burrows in river banks. Nutria often live in groups of up to 10-15 individuals, usually made up of parents and offspring.

The fur of these animals has been the basis of a considerable industry, first in their native South America, and more recently in the United States. Nutria have been introduced many times to North America and Europe, both intentionally and accidentally as a result of escapes from fur farms. They can be tremendously destructive to both wild vegetation and to agriculture.

References and literature cited:

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

Macdonald, David. 1984. The encyclopedia of mammals. Facts on File Publications, New York.

Nowak, R. M. and J. L. Paradiso. 1983. Walker's mammals of the world. The Johns Hopkins University Press, Baltimore and London, pp 803-810.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

Wilson, D. E. and D. M. Reeder (eds.). 1993. Mammal species of the world: A taxonomic and geographic reference, 2nd ed.. Smithsonian Institution Press, Washington and London.

Woods, C. A. 1984. Hystricognath rodents. Pp. 389-446 in Anderson, S. and J. K. Jones, Jr. (eds.). Orders and familes of mammals of the world. John Wiley and Sons, New York.


Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.


bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


having the capacity to move from one place to another.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate