In the wild, most nutria live less than 3 years. Nutria that are captive may live 15 to 20 years. Annual mortality of nutria is between 60% and 80%. Leading causes of mortality include predation, disease, parasitism, fluctuations in water levels, quality of habitat, traffic, and extreme weather.
Predators of nutria include humans, alligators, garfish, bald eagles, other birds of prey, turtles, cottonmouths, and several mammalian predators. Population densities of nutria vary. In Louisiana, densities of about 18 animals per acre have been found in floating freshwater marshes in fall. In Oregon, densities of 56 individuals per acre have been observed in freshwater marshes in summer.
Nutria breed in all seasons throughout most of their range. Peaks in reproduction occur in late winter, early summer, and mid-autumn, and may be regulated by prevailing weather conditions. Under optimal conditions, nutria reach sexual maturity at 5 months of age. Female nutria are polyestrous. Non-pregnant females cycle into estrus every 2 to 4 weeks. Estrous is maintained for 1 to 4 days in most females. Sexually mature males can breed at any time as sperm is produced throughout the year. The gestation period for nutria is about 130 days. A postpartum estrus occurs within 48 hours after birth and most females probably breed during that time. Litters average 4 to 5 young, with a range of 1 to 13. Sizes of litters generally are smaller during winter in suboptimal habitats and for young females. Females often abort or assimilate embryos in response to adverse conditions. Young are precocial and are born fully-furred and active. Young weigh approximately 8 ounces at birth and can swim and eat vegetation shortly thereafter. Young normally suckle for 7 to 8 weeks until they are weaned. Females typically produce 3 litters per year.
In summer, nutria live on the ground in dense vegetation and use burrows at other times of the year. Burrows may be those abandoned by animals such as armadillos, beavers, and muskrats, or they may be dug by nutria themselves. Underground burrows are used by individuals or multi-generational family groups.
Entrances to burrows usually are located in the vegetated banks of natural and human-made waterways, especially when there is a slope greater than 45o. The layout of burrows ranges from a simple, short tunnel with a single entrance to complex systems with several tunnels and entrances at different levels. Tunnels usually are 4 to 6 feet long, though lengths up to 150 feet have been recorded. Compartments within the tunnel system are used for resting, feeding, and escape from predators and the weather. Compartments vary in size from ledges that are 1 foot in width to large family chambers up to 3 feet in width. The floors of the chambers are above the water line and may be covered with plant debris that is shaped into crude nests.
Nutria often build flattened, circular platforms of vegetation in shallow water. Constructed of coarse emergent vegetation, the platforms are used for feeding, loafing, grooming, birthing, escape, and often are misidentified as muskrat houses. Initially, platforms may be relatively low and inconspicuous, but as vegetation accumulates the platforms may attain a height of 3 feet.
Nutria tend to be crepuscular and nocturnal, with the start and end of activity periods coinciding with sunset and sunrise. The peak activity period occurs near midnight. When food is abundant, nutria rest and groom during the day and feed at night. When food is limited, feeding during the day increases, especially in areas that are free from frequent disturbance. Nutria can be very active in urban areas, especially when people feed them.
Nutria generally occupy small home ranges throughout their lives. In Louisiana, the home range of nutria is about 32 acres. Daily movements are less than 600 feet, although some individuals travel much farther. Nutria move farthest from their home range in winter due to an increased demand for food. Adults usually move farther than young. Seasonal migrations of nutria may occur. Nutria that live in agricultural areas emigrate from marshes and swamps when crops are planted and leave after the crops are harvested.
Nutria have poor eyesight and sense danger primarily by hearing. They occasionally test the air for scent. Although they appear to be clumsy on land, they can move with speed when disturbed. When frightened, nutria head for the nearest body of water, dive in with a splash, and either swim underwater to protective cover or stay submerged near the bottom for several minutes. When cornered or captured, nutria are aggressive and can inflict serious injuries to pets and humans through biting and scratching.
Nutria adapt to a wide variety of environmental conditions and persist in areas that are unsuitable for many species. Farm ponds, freshwater impoundments, drainage canals with spoil banks, rivers, bayous, freshwater and brackish marshes, swamps, and other types of wetlands provide habitat for nutria. The habitat of nutria usually is a semi-aquatic environment that occurs at the boundary between land and water. This zone usually has an abundance of emergent aquatic vegetation, small trees, shrubs, and may be interspersed with small clumps and hillocks of high ground. In the US, all large populations of nutria are in coastal areas. Freshwater marshes and rivers are the preferred habitat.
Nutria are almost entirely herbivorous and eat animals, mostly insects, only incidentally when they feed on plants. Freshwater mussels and crustaceans are eaten occasionally in some parts of their range. Nutria eat approximately 25% of their body weight daily.
Important food plants in the US include cordgrasses, bulrushes, spikerushes, chafflower, pickerelweeds, cattails, arrowheads, and flatsedges. The succulent, basal portions of plants are preferred, but nutria also eat entire plants or several parts of a plant. Roots, rhizomes, and tubers are important during winter, when the bark of trees such as black willow and bald-cypress also may be eaten. Nutria also eat crops and lawn grasses found adjacent to aquatic habitats.
Nutria can excavate soil and handle very small food items with their dexterous forepaws. Food is eaten in shallow water, on feeding platforms constructed from vegetation, on floating stations that are supported by logs, on decaying mats of vegetation, and on other debris. In some areas, the tops of muskrat houses and beaver lodges also may be used for feeding platforms.