A beaver with wet fur walks through dense vegetation and shrubby plants in a natural wetland habitat.

A muskrat swimming in murky water, viewed from above, showing its brown fur, rounded head, and whiskers as it moves through the shallow water.

North American river otter facing the camera, with left paw lifted up.

Biodiversity: Mammals

Beaver

The North American beaver, nature's ecosystem engineer, is found in wetland areas of the United States and Canada, and also helps to create them.

It is the second-largest living rodent after the capybara, typically weighing 45-60 pounds, but up to 100 pounds. It has a stout body with a large head, long incisors that never stop growing, hand-like front feet, webbed back feet, and wide, flat, and hairless tails.

This keystone species may be most known for its behavior of building dams and lodges along waterways. A beaver creates these structures primarily for safety purposes, as it is vulnerable to predators on land, but a highly adapted swimmer. A beaver family's territory typically ranges about one-half mile in length, and a series of ponds will be created through damming over time across a beaver's territory. These structures lead to the formation of ponds and channels beavers can navigate while they search for food.

Beaver ponds create wetlands which are among the most biologically productive ecosystems in the world. They increase plant, bird, and wildlife diversity, improve water quality, and raise salmon and trout populations. This one species supports thousands of species, including humankind.

Historically, the beaver has been hunted for its fur, meat, and castoreum -- a chemical beavers secrete from special sacs near their tail that is used to produce medicine, perfume, and flavor food. In the 19th and early 20th centuries, beavers were overhunted and nearly exterminated. Since, the beaver has been reintroduced across its range and beaver populations have recovered to healthy levels. While the beaver can be a nuisance animal to landowners because of its tree felling and damming behaviors, its value as a living species is recognized today.

A dark-furred porcupine eating evergreen branches while standing in snow next to a coniferous tree trunk.

A small furry mammal with brown fur digging in soil and dirt, with a white tag attached to its back, surrounded by dried grass and twigs.

A blurry image of a beaver in a wood box.

A tangled pile of driftwood and branches lies in shallow water surrounded by green vegetation and reeds along a riverbank or wetland area.

Muskrat

The muskrat is a semi-aquatic mammal found in wetland areas. It is native to North America, but was introduced in South America and Eurasia by humans in the early 1900s as a valuable fur resource. It is now considered an invasive species outside of North America.

The muskrat, 1-4 pounds and 8-10 inches in length without its tail, is appropriately named based off its similarity in appearance to a rat. It also produces a strong musk during mating season it uses to mark its territories. Like the otter, it has thick, protective fur to keep it warm in cold water, short legs with webbed feet, and a long, flat, and hairless tail used to propel it while swimming. This tail makes muskrat tracks easily identifiable, as when it walk on land, its tail drag along the ground, leaving a long line between its footprints.

The muskrat plays an important role in certain ecological systems, as its eating and denning habits create the ideal flat nesting areas for certain birds.

With a high reproductive rate, it also provides an important food resource for many other species, including various larger mammals, birds, fish, and reptiles.

Though, as primarily a herbivore, the muskrat is capable of decimating vegetation in an area when living with dense enough population numbers. The muskrat may also be a vector of disease and source of property damage for landowners.

A muskrat with wet, dark fur grazes on grass and vegetation near muddy ground, displaying its characteristic round body and scaly tail. — Muskrats are semiaquatic, spending time in the water and on land.

Uprooted tree with exposed root ball partially submerged in shallow water, surrounded by marsh grasses and dead vegetation along a wetland shoreline. — Muskrats often live in burrows with underwater entrances like otters, but may also build above water dens for shelter

River Otter

While elusive, the playful North American river otter is a semi-aquatic mammal found in any water habitat of North America. River otters dwell in dens made from abandoned burrows along these water habitats, often with entrances found underwater. If you see a trail of broken or flattened grasses from the bank of Cypress Creek into the water, there's a good chance that is where an otter slid down the bank to enter the creek.

The river otter, 3-4 feet in length and 11-30 lbs as an adult, is well adapted to the water habitats in which it resides. It has thick, protective fur to keep it warm in cold water. Its long, flexible body and flattened head make it streamline and nimble. Its short legs with webbed feet and muscular tail propel it through the water swiftly. The otter also can close its nostrils when diving into the water and hold its breath for up to 8 minutes.

The otter provides valuable services to water bodies its inhabits, like Cypress Creek. As an apex predator, it keep all of its prey species populations balanced. As an indicator species, its presence tells us our water quality is relatively good, and its absence warns us pollution may be affecting the cleanliness of our water. Historically, the otter has been valued as a furbearer, as well.

The North American river otter was hunted to near extinction for its fur in the 19th and 20th centuries, and is still hunted in some places. It was extirpated from portions of its range, but efforts to restore river otter populations are helping the species to recover. Water population and habitat destruction are still threats to the species.