Types of Beavers in Montana

The American Beaver (Castor canadensis) is Montana's sole native beaver species. All 517 documented iNaturalist observations in the state record this single species. The American Beaver is native to Montana and ranges throughout nearly every accessible freshwater ecosystem in the state, from high mountain streams to lowland rivers. No other Castor species lives wild in Montana.

American Beavers are the largest rodents in North America and can reach impressive sizes. Adult beavers typically weigh between 35 and 65 pounds, with large individuals occasionally exceeding 80 pounds. Body length ranges from 24 to 39 inches from nose to tail base, and their flat tail adds another 8 to 16 inches. Males are generally heavier than females. Their size alone distinguishes them from any other mammal in Montana.

American Beavers have a stocky, barrel-shaped body covered in dense, waterproof fur that is reddish-brown to dark brown on the back and sides, with lighter brown or grayish underparts. Their most distinctive features are their flat, paddle-like tail covered in dark scales, their small rounded ears, and their prominent orange-yellow incisor teeth that never stop growing. Their eyes are small and positioned high on the head, allowing them to see while mostly submerged.

The beaver's flat tail serves multiple purposes. It functions as a rudder for swimming, an organ for fat storage during winter months, a prop when the beaver stands upright on land, and a tool for slapping water as a warning signal when threatened. The tail is covered with leathery skin and dark brown scales rather than fur. In water, the tail helps with balance and direction; on land, it anchors the beaver when it rears up to fell trees or carry branches.

Individual American Beavers show variation in fur color that ranges from pale brown to nearly black, with lighter brown being most common in Montana. Younger beavers often appear lighter in color than older adults. Some individuals have distinctly reddish-brown coats while others appear almost black. Fur color is largely determined by genetics and individual variation rather than age or season, though fur appears darker and glossier when wet.

American Beavers have numerous adaptations that make them superbly aquatic. Their hind feet are webbed, their eyes have a transparent eyelid that allows underwater sight, and their ears and nostrils can close underwater to prevent water entry. Their fur is so dense that water barely reaches the skin. Their flat tail provides steering, and they can hold their breath for 15 to 20 minutes. Their front teeth are angled forward, perfect for gnawing, and they possess powerful jaw muscles.

American Beavers are herbivores that prefer aspen, birch, and willow trees and shrubs. In spring and summer, they also consume aquatic vegetation including water lilies, cattail roots and shoots, and clover. In Montana, seasonal availability determines diet: winter means stored wood from felled trees and roots; warmer months include fresh bark, twigs, and aquatic plants. Beavers do not eat fish despite popular misconception. They require about 1 pound of food daily, often feeding at dusk and throughout the night.

American Beavers are primarily nocturnal and crepuscular, meaning most active at dawn, dusk, and throughout the night. Daytime activity increases during spring and early summer breeding season (April through July) when beavers must gather more food and repair dam structures. Peak observation months in Montana are July, May, and June when daylight hours extend and beavers are out working. Winter activity continues at night but is reduced overall; beavers rely on stored food and remain sheltered more often.

Scientists recognize several subspecies of American Beaver across North America, each adapted to regional conditions. Montana's beavers belong to Castor canadensis, but the specific local populations reflect Rocky Mountain and Great Plains adaptation. From a field identification perspective, these distinctions are subtle and not visible in the field. All beavers you see in Montana belong to the same species and share the same basic appearance and behavior.

American Beavers prepare intensively for winter by constructing or reinforcing lodges, stockpiling branches underwater for food, and thickening their fur coat. Their body temperature stays around 99 degrees Fahrenheit even when water is frozen. They remain active under the ice, accessing stored wood for food and constructing tunnels within lodges lined with dry wood chips and bark. A beaver lodge can house 6 to 8 animals. Their waterproof fur and fat reserves sustain them through months without access to fresh vegetation.