The nutria is a large, dark colored, semiaquatic rodent that is native to southern South America. At first glance, a casual observer may misidentify a nutria as either a beaver or a muskrat, especially when its swimming. Nutria are members of the family Myocastoridae. They have short legs and a robust, highly arched body that is approximately 24 inches long. Their round tail is 13-16 inches long and scantily haired. Males are slightly larger than females; the average weight for each is about 12 pounds. Males and females may grow to 20 pounds and 18 pounds. The dense grayish underfur is overlaid by long, glossy guard hairs that vary in color from dark brown to yellowish brown. They forepaws have four well developed and clawed toes and one vestigial toe. Four of the five clawed toes on the hind foot are interconnected by webbing; the fifth outer toe is free. The hind legs are much larger that the forelegs. When moving on land, a nutria may drag its chest and appear to hunch its back. Like beavers, nutria have large incisors that are yellow orange to orange red on their outer surfaces. They eyes, ears and nostrils of nutria are set high on their heads.
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General Biology, Reproduction and Behavior
In the wild, most nutria probably live less than 3 years; captive animals, however, may live 15 to 20 years. Predation, disease and parasitism, water level fluctuations, habitat quality, highway traffic, and weather extremes affect mortality. Predators of nutria include humans, alligators, garfish, bald eagles and other birds of prey, turtles, snakes and several carnivorous mammals.
In summer, nutria live on the ground in dense vegetation, but at other times of the year they use burrows. Burrows may be those abandoned by other animals such as armadillos, beavers, and muskrats. Underground burrows are used by individuals or multigenerational family groups.
Burrow entrances are usually located in the vegetated banks of natural and human made waterways. Burrows range from a simple, short tunnel with one entrance to complex systems with several tunnels and entrances at different levels. Tunnels are usually 4 to 6 feet long. Compartments within the tunnel system are used for resting, feeding, escape from predators and the weather. These vary in size, from small ledges that are only 1 foot across to large family chambers that measure 3 feet across. The floors of these chambers are above the water line and may be covered with plant debris discarded during feeding and shaped into crude nests.
Nutria breed in all seasons throughout most of their range, and sexually active individuals are present every month of the year. Reproductive peaks occur in late inter, early summer, and mid autumn and may be regulated by prevailing weather conditions.
Nutria reach sexual maturity at 4 months of age. Female nutria are polyestrous and nonpregnant 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 because sperm is produced throughout the year.
The gestation period for nutria ranges from 130-132 days. A postpartum estrus occurs within 48 hours after birth and most females probably breed again during that time.
Litters average 4 to 5 young, with a ranged of 1 to 13. Litter sizes are generally smaller during winter, in suboptimal habitats, and for young females. Females often abort of assimilate embryos in response to adverse environmental conditions.
Young are precocial and are born fully furred and active. They weight 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.
Nutria tend to be crepuscular and nocturnal, with the start and end of activity periods coinciding with sunset and sunrise. Peak activity occurs near midnight. When food is abundant, nutria rest and groom during the day and feed at night. When food is limited, daytime feeding increases, especially in wetlands free from frequent disturbance.
Nutria have relatively 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 surprising speed when disturbed. When frightened, nutria head for the nearest water, dive in with a splash, and either swim underwater to protective cover or stay submerged near the bottom for several minutes. When corned or captured, nutria are aggressive and can inflict serious injury to pest and humans by biting and scratching.
Damage and Damage Identification
Most nutria damage is from feeding or burrowing. The numerous natural and human made waterways are used extensively for travel by nutria. Burrowing is the most commonly reported damage caused by nutria. Nutria are notorious in Louisiana and Texas for undermining and breaking through water retaining levees in flooded fields used to product rice and crawfish. Additionally, nutria burrows sometimes weaken flood control levees that protect low lying areas.
Nutria sometimes burrow into the Styrofoam flotation under boat docks and wharves, causing these structures to lean and sink. They may burrow under buildings, which may lead to uneven settling or failure of the foundations. Burrows can weaken roadbeds, stream banks, dams and dikes which may collapse when the soil is saturated by rain or high water or when subjected to the weight of heavy objects on the surface. Rain and wave action can wash out and enlarge collapsed burrows and compound the damage.
Nutria girdle fruit, nut and shade trees and ornamental shrubs. They also dig up lawns and gold courses when feeding on the tender roots and shoots of sod grasses. Gnawing damage to wooden structures is common. Nutria also gnaw on Styrofoam floats used to mark the location of traps in commercial crawfish ponds.
Nutria can be infected with several pathogens and parasites that can be transmitted to humans, livestock and pets. The role of nutria, however, in the spread of diseases such as equine encephalomyelitis, leptospirosis, hemorrhagic septicemia, paratyphoid and salmonellosis is not well documented. They may also host a number of parasites, including the nematodes and blood flukes that cause swimmers itch. The threat of disease may be an important consideration in some situations, such as when livestock drink from water contaminated by nutria feces and urine.
The ranges of nutria, beavers and muskrats overlap in many areas and damage caused by each m ay be similar in appearance. Therefore, careful examination of sign left at the damage site is necessary to identify the responsible species.