The Nutria Rat (Myocastor coypus,) of the order rodentia, is a large, semi-aquatic rodent native to southern South America. It has been described as “a cross between a beaver and a New York sewer rat.”1 Now found on 4 continents, and recognized as an invasive wildlife species in the United States (present in 22 states,) measures are being taken to control populations and manage the devastation this species is wreaking on coastal ecosystems.
Nutria rats are large, robust rodents that are well equipped for an aquatic life. They are sometimes mistaken for beavers or muskrats due to similar physical traits, but are approximately 1/3 the size of beavers and larger than muskrats. Their small ears, eyes, and nostrils are situated high on their large heads and remain above water during swimming.2 Valves located in their mouths and nostrils prevent water intake when submerged. Nutria rats have a total of 20 teeth, including 4 large yellow/orange incisors that protrude from the front of their mouths. Their white whiskers are 3” to 5” long. Glands located at the corners of the mouth produce oils that are combed through the fur to provide waterproofing.3 Nutria rats have partially webbed back feet (all back toes are connected by webbing except one that is used much like a thumb.) The front feet, which are smaller and not webbed, have 4 toes that are similar and one toe that is smaller and functions similar to a thumb.4 Nutria rats have stout bodies that appear hump-backed when not in the water. They reach a mature weight of 15 lbs. to 22 lbs. (males tending to be larger,) and a mature length of 27” to 41” (including the tail, which tends to make-up over half of the overall length.) Their tails are thick, round and scaly, with sparse bristly hairs. When swimming, the tail glides smoothly through the water behind the body. Because of a lack of insulation on their tails, Nutria rats living in northern climates tend to loose portions of their tails to frostbite during hard winters.5
Their coats are made of a coarse outer layer comprised of yellow to dark brown shaggy hairs and a lush undercoat (called nutria,) that is soft and grey. It is the undercoat that is sought by trappers. Their muzzles and chins are white. Although uncommon, albino Nutria rats have been observed in the wild.
Nutria rats are strong swimmers and spend the majority of their time in the water (since they are more agile in the water than on land.) They see well underwater and can stay submerged for up to five minutes.
Nutria rats are shy and nocturnal so they are not often seen in the wild. However, they are social within their species and live in large colonies.
On average, Nutria rats live for 8 to 10 years.
Nutria rats have adapted to a variety of habitats but always maintain a close proximity to fresh or brackish water. They may be found along riverbanks, lakeshores or coastlands, in wetlands, farm ponds, drainage systems, canals, bayous, swamps, marshes, overgrown lots, or even in cities beneath buildings. They build burrows near water in low vegetation or root systems. The inner chambers of their dens are lined with grasses and located above the water line but are accessed by entrances that are up to 24” in diameter and 12” to 24” below the surface of the water. In marshes, Nutria rats build flat platforms of dead vegetation for feeding, birthing, or grooming purposes.6
Within their habitats, Nutria rats are vulnerable to alligators, large snakes, birds of prey, and turtles.7
Nutria rats are only native to southern South America. However, because of both accidental and intentional introduction, they can now be found in Canada and 22 of the United States (most highly concentrated along the Gulf Coast, but also found in eastern and northwestern coastal regions,) Europe, and Asia. Louisiana has an estimated population of 5 million Nutria rats.8 The fur of this species was once popular in the fur trade and Nutria rats were domesticated around the world on fur farms (the first recorded domestication in the U.S. was in1889.) Wild populations that were started by farm escapees exploded in size when the nutria fur trade collapsed (during the 1940’s in the U.S.,) forcing farmers to release their rats into the wild because they could no longer afford to keep them. Wild populations were intentionally expanded by wildlife agencies and private companies who attempted to use the rats to control invasive or noxious weeds.9
Nutria rats are omnivorous surface eaters who often over-harvest their dietary staple, aquatic vegetation (including the roots, stems, leaves, and bark.) They do not eat vegetation in the water, but rather swim with it or carry it back to their platforms to feed. Their diets also include small animals, such as snails and mussels. Nutria rats are capable of consuming 25% of their own body weight every day.10 Their eating habits have proven detrimental to coastal ecosystems, destroying vegetation that is important in the prevention of soil erosion. They are also troublesome to farmers, devouring crops such as rice, corn, alfalfa, wheat, oats, barley, sugarcane, peanuts, and vegetables.11
Nutria rats are prolific reproducers. Typically, a male will live in the same burrow as several females and breed throughout the year. Each female has, on average, 2 to 3 litters a year, each consisting of 1 to 11 young (but usually 4 to 6.) It has been observed that litter sizes are cyclical, with a large litter of 4 to 6 young followed by a small littler of 2 to 4, and so on.12 Gestation lasts slightly longer than 4 months and the young reach sexual maturity in 4 to 8 months, leaving their mother after only 1 to 2 months. Females’ teats are situated high on their backs so the young may feed while she is swimming. Females are ready to breed within days of birthing a litter.13
Notes of Interest
Common names of the Nutria rat include coypu, coypu rat, swamp beaver, and nutria.14
The aquatic diet of the Nutria rat is partially to blame for an increased rate of coastal erosion. In Louisiana, with a population estimated to be around 5 million, the nutria rat is responsible for a rate of erosion of 40 square miles per year. This species damages and destroys vegetation that is necessary for anchoring soil, turning marshes and wetland environments into open water. Additionally, Nutria rats have been known to undermine levees, dams, buildings and roadbeds. Nutria rats have also negatively affected agricultural resources and are responsible for disease transmission. They may carry tuberculosis, septicemia, blood flukes, tapeworms, liver flukes, and a type of nematode that causes “nutria itch,” a rash that is spread through water contaminated by feces and urine.
Federal, state, and local governments are working closely with wildlife agencies to manage Nutria rat populations and protect native vegetation and resources.15
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