North American Beaver

Castor canadensis

Population, Conservation Status, Threats:

Considered least concern by the IUCN, with populations appearing stable. In the 1800s, beavers were trapped and hunted in huge numbers for their fur, greatly depleting their populations. Having now reestablished themselves, they are still vulnerable to degradation of waterways and loss of suitable habitat, as well as diseases like tularemia.

Physical Description:

Beavers are the largest rodent in North America. They are dark brown, with a broad head, round body, large nose and small ears. Their paws are hairless and webbed, the front paws being fairly agile and capable of holding and manipulating objects, and the tail is wide and flat, covered with thick, leathery skin devoid of fur, and used as a paddle when swimming.

Overall North American Beaver range from Montana Field Guide. Beavers are found throughout most of the United States and Canada, and even into the northernmost parts of Mexico.

Habitat:

Beavers are found in riparian zones across North America, and are dependent on rivers, ponds, and lakes for habitat. In the US, they are absent only from the Sierra Nevada mountains and southern California, having likely been extirpated in the 1800s.

Behavior:

Beavers are unique in their role as ecosystem engineers, building extensive watertight dams from sticks and mud, turning fast-flowing streams and rivers into wide, shallow ponds and marshland. It has become increasingly clear that this dam-building behavior has numerous benefits to the environment, by reducing erosion, creating watering holes for large animals, and habitat for aquatic and semiaquatic lifeforms. In addition to altering flow of water through their habitats, beavers affect tree composition by felling large trees and other woody vegetation. While beavers build dams in order to create suitable habitat for themselves, it is a common misconception that they also live in these dams. Beaver lodges are separate structures, built on islands in ponds and in marshes regardless of if they have been first dammed by beavers, as secure dens. These lodges can be quite large, up to 6 by 12 feet, with one or more entrances accessible only from underwater. Beavers are primarily nocturnal, and very social. Family groups consist of a mated pair, the current year's kits, and the adult yearlings from the previous litter. When threatened, beavers will slap their wide tail on the surface of the water to alert other beavers to danger. They will also scent mark their territory, creating scent mounds of mud and sticks that they mark with urine and castor, an anal gland secretion unique to the beaver.

Diet:

Beavers feed on leaves, aquatic plants, roots, and of course, tree bark. They prefer hardwoods like aspen and poplar, with willow being a favorite due to its long growing season and highly nutritious nature. During the winter, they feed on stored branches they cache on the bottom of their ponds - the cold water acts a natural refrigerator and preserves their food throughout the cold season.

Reproduction:

Beavers appear to mate for life. They breed in the winter, and an average of three young are birthed in the spring. Kits are born precocial and are able to swim within hours of birth. Young stay with their family for two years before leaving to form their own families. Females first reproduce at 2-3 years of age.

Associated Species:

Muskrats and otters are often mistaken for beavers, as they share habitat and a semiaquatic lifestyle. In higher elevations, moose and beaver occupy very similar habitat, both having a taste for aquatic vegetation and willow.

Illustration by Willow Sedam

Sources: https://www.nwf.org/Educational-Resources/Wildlife-Guide/Mammals/American-Beaver, https://nationalzoo.si.edu/animals/beaver, https://www.fs.usda.gov/detail/npnht/learningcenter/nature-science/?cid=fseprd878831, https://www.sierraforestlegacy.org/FC_SierraNevadaWildlifeRisk/NorthAmericanBeaver.php

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