Chesapeake Bay Nutria Eradication Project
Northeast Region


Nutria Biology and Identification

Nutria are highly prolific and breed all year. Reproductive peaks occur in late winter, early summer, and mid-autumn. Reproduction and survival may be influenced by extreme weather conditions. Nutria reach sexual maturity at four to six months. Sexually mature male nutria can breed throughout the year. Females are pregnant from 128 to 130 days, and are ready to breed within forty-eight hours after giving birth. Litters average four to five young; however, nutria can have up to thirteen young per litter and may have three litters per year. Young are born fully furred and active, weighing 8 oz. at birth. They can swim and eat vegetation shortly thereafter, still feeding on mother's milk for up to eight weeks. Within five days of life, nutria can survive away from the mother.

As an example of their proliferation: in 1938, twenty individual nutria were introduced into Louisiana and within twenty years, the nutria population exceed 20 million animals. By 1962, the nutria had replaced the native muskrat as the leading furbearer in Louisiana.

With no natural predators to help control population growth, nutria populations in Maryland have grown rapidly. Population estimates on the inhabited 10,000 acres of Blackwater National Wildlife Refuge have grown from about 250 animals in 1968 to as many as 50,000 animals by the mid 1990s. At Tudor Farms, a 6,000-acre private wildlife management area adjacent to the refuge, population estimates between 1995 and 1998 were estimated at 17,000 to 24,000 animals. Ecologists believe that random commercial trapping has been unable to decrease the nutria population in these areas over the past decade because harvest rates have remained between 5,000 to 10,000 nutria on each area every year.

Nutria share habitat with beaver, muskrat, groundhog and otter and are frequently mistaken for one of these species. However, several characteristics can aid in the correct identification.


Nutria. USFWS photo.
Photo of a nutria. USFWS pphoto.

Nutria have short legs and a robust, highly arched body that is approximately 24 inches long. Their round tail is from 13 to 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, respectively.

The dense grayish underfur is overlaid by long, glossy guard hairs that vary in color from dark brown to yellowish brown. The 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 than 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 yel-low-orange to orange-red on their outer surfaces.

In addition to having webbed hind feet, nutria have several other adaptations to a semiaquatic life. The eyes, ears, and nostrils of nutria are set high on their heads. Additionally, the nostrils and mouth have valves that seal out water while swimming, diving, or feeding underwater. The teats of the female are located high on the sides, which allows the young to suckle while in the water. When pursued, nutria can swim long distances under water and see well enough to evade capture.



Muskrat. Photo by Terry Spivey, USDA
Muskrat. Photo by Terry Spivey, USDA

Muskrat fur is soft and velvety to touch, composed of an inner layer of soft, short fur protected by a layer of long, glossy guard hair. A muskrat will be 16 to 25 inches in total length with a tail 7 to 12 inches long. The tail is a distinctive identifier of this species because it is rat like, but flattened from side to side, rather than round. An adult weighs about 3 to 4 pounds.Young muskrats reproduce after 10 to 12 months.


Groundhog. Photo by EIC, CC
Photo of a groundhog. Credit, EIC

The groundhog typically measures 16 to 26 inches long, including a 6-inch tail and weighing 4 to 9 lbs. In areas with fewer natural predators and large amounts of food, groundhogs can grow to 30 in and 31 lbs. Groundhogs are well adapted for digging, with short but powerful limbs and curved, thick claws.

Groundhogs are excellent burrowers. They hydrate through eating leafy plants rather than from a natural water source.
One litter is produced annually, usually containing 2–6 young, who are weaned and ready to seek their own dens at five to six weeks of age.


Beaver. Photo by Tom Smylie, USFWS
Photograph of a beaver by Tom Smylie, USFWS

Beaver, the largest North American rodent, can weigh up to 100 pounds but this is unusual. The usual weights for beaver are from 35 to 68 pounds. Typical total length for this species is from 39 to 47 inches, and the large flat tail varies from 10 to 13 inches long and from 3.5 to 8 inches wide.

The large flat tail easily identifies this species. Other identifying characteristics are the hind feet which have four webbed toes and a fifth free toe that supports an articulating split nail used for grooming. The coat consists of two layers; a coarse outer layer of guard hair, often yellowish to reddish in color, and a fine dense layer of underfur.


Otters. Photo by Alan Pennington, CC
Photo of 2 otters. Credit, Alan Pennington through Creative Commons

Otters are one of 13 species of semiaquatic mammals, noted for their playful behavior, that belong to the weasel family. Sea otters have round heads, small eyes, and visible ears. The slender body has short legs, a strong neck, and a long flattened tail and webbed feet that help propel it through water. Retractable claws on a sea otter’s front paws allow the sea otter to grab food. The average length of an adult female is 4 feet and average weight is 60 lbs. At birth, sea otters weigh approximately 5 lbs and are 10 inches in length.

Species Nutria Silhouette of a nutria Muskrat Silhouette of a muskrat Beaver
Silhouette of a beaver
Groundhog Silhouette of a groundhog Otter Silhouette of an otter
Weight 10-20 lbs. 2-5 lbs. 40+ lbs. 12-15 lbs. 10-30 lbs.
Length 30 inches, including 10-inch tail 20 inches, including 9 inch tail 50 inches, including 12 inch tail 20 inches, including tail 34-60 inches
Front feet Five toes, only four show up in tracks Five toes   Four toes visible  
Hind feet Partially webbed No webbing between toes. Fine stiff hairs along margin of toes to aid in swimming. Fully webbed feet No webbing Fully webbed feet
Tracks Photo        
Tail Heavy, scaly, ratlike tail sparsely covered in bristly hairs Thin ribbon-like scaly tail thinly covered with fine black hairs. Large oval-shaped and flattened scaly tail. No hairs present Short, stubby tail, heavily furred in bushy hairs Thick, muscular tapered tail covered in fine, dense fur.
Tail cross-section Circular/round Taller than wide, sharp ridge on top and bottom Flattened Round/furry  
Distinguishing features Long white whiskers and large, orange teeth

Serpentine tail movement when swimming Slaps tail on water surface Hibernate during winter Slender, elongated body
Scat (feces) Large 2-3 inch scats resembling tootsie roll. May float in water. Juvenille scat is smaller Smaller, kidney bean shaped scats, often deposited in small piles on logs and other structures Large loose piles of sawdust-like scat usually deposited in water   Loose tar-like scat comprised of fish scales

Creates beds of cut vegetation. Sometimes digs volley ball-sized bank dens at water's edge.


Creates hut out of mounds of vegetation and mud. Entrance to den is underwater. Also digs softball-sized bank dens. Creates large lodges out of branches and mud. Entrance also under water. Also digs basketball-sized bank dens. Uses dams to create ponds. Digs holes and tunnels in upland habitats. Loose dirt piled in front of holes  




























Last updated: October 19, 2011