The muskrat and the nutria are both semi-aquatic rodents frequently encountered in North American waterways, leading to common confusion about their identity. Though they share a superficial resemblance and similar habitats, they represent two distinct species with differences in size, physical structure, and ecological impact. This guide clarifies the definitive characteristics that distinguish the native muskrat from the introduced nutria.
Clarifying Taxonomy and Origin
The two species belong to entirely different biological families despite their similar appearance and lifestyle. The muskrat (Ondatra zibethicus) is classified within the family Cricetidae, making it a relative of voles and lemmings. This species is native to North America, where it plays an established role in wetland ecosystems.
The nutria, or coypu (Myocastor coypus), comes from the family Echimyidae, meaning it is more closely related to South American spiny rats. Its natural range is in South America, but it was introduced to North America primarily for the fur farming industry. Subsequent escapes led to its designation as an invasive species in many regions outside of its native range.
Distinguishing Physical Features
A primary difference between the two rodents is their adult size. The muskrat is a medium-sized rodent, typically weighing between 1.5 and 4.5 pounds, with a body length of about 8 to 14 inches. The nutria is substantially larger, often reaching weights between 11 and 22 pounds and a total length of up to 37 inches.
The most definitive visual distinction is the tail structure. The muskrat possesses a long, thin tail that is vertically flattened, or compressed from side to side, which acts like a rudder while swimming. In contrast, the nutria has a long, round, and sparsely haired tail that closely resembles a rat’s tail.
Another easily observable trait is the color of the incisor teeth. The nutria has prominent, large incisors that are a bright, unmistakable yellow-orange to red-orange color due to iron deposits. While muskrats also have large incisors, they are typically less visible and are a paler yellow hue. Nutria also have unique adaptations, including mammary glands located high on the female’s flanks, which allows the young to nurse while the mother is floating.
Differences in Habitat and Ecology
Both animals are semi-aquatic herbivores, but their methods of constructing shelter vary significantly. Muskrats often build dome-shaped lodges made from piled-up aquatic vegetation in marshes and ponds. They may also burrow into banks with an entrance located underwater, but their burrows are generally less extensive than the nutria’s.
Nutria prefer to create deep, complex burrow systems within the banks of rivers, canals, and ditches rather than building above-ground lodges. These deep burrows can destabilize earthen structures such as levees, dikes, and roadbeds. Nutria are highly adaptable, thriving in a wider variety of freshwater and brackish habitats compared to the muskrat’s preference for stable marshes.
The dietary habits of the nutria create a more severe ecological impact. While muskrats feed on a variety of aquatic plants, the nutria’s appetite is voracious, consuming up to 25 percent of its body weight daily. Nutria often graze heavily on the roots and rhizomes of wetland plants, a behavior that kills the vegetation and causes an “eat-out.” This destruction removes the natural anchors for soil, accelerating coastal erosion and turning vegetated wetlands into open water environments.