Nutria and muskrats are often mistakenly grouped together due to their shared semi-aquatic existence and dark, furry appearance in marsh environments. The nutria (Myocastor coypus) is a large rodent from South America, while the muskrat (Ondatra zibethicus) is a smaller rodent native to North America. They are distinct species with different origins and classifications.
How to Tell Them Apart: Physical Traits
The most immediate and noticeable difference between the two species is their size, as a fully grown nutria can be four to five times heavier than a muskrat. An adult muskrat typically weighs between 2 and 5 pounds, while an adult nutria is a large rodent that can weigh up to 20 pounds. When observed in the water, the tail provides the clearest point of distinction between the two semi-aquatic rodents.
The muskrat possesses a thin, scaly, and vertically flattened tail, which acts as a rudder and often undulates visibly as the animal swims. In contrast, the nutria has a long, round, and sparsely haired tail that is more rat-like and remains mostly still during swimming. The nutria also has large, protruding incisor teeth, which are famously colored a bright orange or reddish-orange due to iron compounds in the enamel.
Both animals have adaptations for swimming, but their foot structure differs significantly. The muskrat has partially webbed hind feet. The nutria, however, has fully webbed hind feet, with four of its five toes interconnected by a membrane, making it a highly efficient swimmer.
North American Native vs. Global Invader
The muskrat is native to North America, where it is found across most of the continental United States and Canada. It is classified within the family Cricetidae, which includes a variety of New World rodents. Its population is historically stable, having long coexisted with other native wildlife.
The nutria is native to the temperate and subtropical regions of South America, spanning from southern Brazil down to Argentina and Chile. Nutria were first introduced to the United States in the late 19th and early 20th centuries, primarily to establish a fur farming industry. When the fur market collapsed, many animals were intentionally or accidentally released into the wild, establishing feral populations in the Gulf Coast and Pacific Northwest.
This introduction has resulted in the nutria being classified as an invasive species in many parts of the U.S. Because they lack natural population controls outside of their native range, nutria populations have expanded rapidly, leading to widespread environmental damage. This status is the primary reason for the extensive management and eradication efforts undertaken in states like Louisiana and California.
Divergent Diets and Burrowing Habits
While both species inhabit aquatic areas, their interactions with the environment differ in their foraging and den-building behaviors. Muskrats construct dome-shaped “lodges” from vegetation and mud in shallow water, or they will dig simple burrows into banks, with the entrance typically submerged.
Nutria create extensive, complex burrow systems that tunnel deep into the banks of waterways and levees. These large, interconnected tunnels can undermine infrastructure, causing severe bank erosion and breaches in flood control structures. Unlike muskrats, nutria do not typically build large, protective lodges, but they will create flattened feeding and resting platforms by trampling down marsh plants.
The difference in their diets also contributes to ecological impacts. Muskrats are herbivores that feed on a variety of aquatic vegetation, but they will occasionally consume small animal matter. Nutria are strictly herbivores, capable of consuming up to 25% of their body weight in vegetation daily. Nutria prefer the roots, rhizomes, and tubers of marsh plants, and their habit of digging up and consuming the entire root system causes large areas of marsh to be denuded, converting healthy marshland into open water.