North American Beaver

Lore & Background

Historically, beavers have been hunted for their fur, meat, and castoreum. Castoreum has been used in medicine, perfume, and food flavoring; beaver pelts have been a major driver of the fur trade. Before protections began in the 19th and early 20th centuries, overhunting had nearly exterminated both species. Their populations have since rebounded, and they are listed as species of least concern by the IUCN Red List of mammals. In human culture, the beaver symbolizes industriousness, especially in connection with construction; it is the national animal of Canada. The genus name Castor has its origin in the Greek word κάστωρ kastōr and translates as 'beaver'. German zoologist Heinrich Kuhl coined C. canadensis in 1820. Many scientists considered both names synonymous for one same species until the 1970s, when chromosomal evidence became available confirming both as separate where the Eurasian has 48 chromosomes, while the North American has 40. The difference in chromosome numbers prevents them from interbreeding. Twenty-five subspecies have been classified for C. canadensis.

In Their Own Story

The North American beaver (Castor canadensis) has 40 chromosomes, while the Eurasian beaver has 48—a difference that prevents them from interbreeding. Beavers hold territories and mark them using scent mounds made of mud, debris, and castoreum—a liquid substance excreted through the beaver's urethra-based castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors. Adult males and females live in monogamous pairs with their offspring. After their first year, the young help their parents repair dams and lodges; older siblings may also help raise newly born offspring. Beavers are the second-largest living rodents, after capybaras, weighing up to 50 kg (110 lb). They have a head–body length of 80–120 cm (31–47 in), with a 25–50 cm (10–20 in) tail, a shoulder height of 30–60 cm (12–24 in), and generally weigh 11–30 kg (24–66 lb). Their bodies are streamlined like marine mammals and their robust build allows them to pull heavy loads. A beaver coat has 12,000–23,000 hairs/cm2 (77,000–148,000 hairs/in2) and functions to keep the animal warm, to help it float in water, and to protect it against predators. Guard hairs are 5–6 cm (2.0–2.4 in) long and typically reddish brown, but can range from yellowish brown to nearly black. The underfur is 2–3 cm (0.8–1.2 in) long and dark gray. Beavers molt every summer.

Reader's Guide

Beavers can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges. Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams restrict water flow, forming ponds. Lodges, which are usually built in ponds, serve as shelters. Their infrastructure creates wetlands used by many other species. Beavers can swim at 8 km/h (5 mph); only their webbed hind feet are used to swim, while the front feet fold under the chest. On the surface, the hind limbs thrust one after the other; while underwater, they move at the same time. Beavers are awkward on land but can move quickly when they feel threatened. They can carry objects while walking on their hind legs. The beaver's distinctive tail has a conical, muscular, hairy base; the remaining two-thirds of the appendage is flat and scaly. The tail has multiple functions: it provides support for the animal when it is upright (such as when chewing down a tree), acts as a rudder when it is swimming, and stores fat for winter. It also has a countercurrent blood vessel system which allows the animal to lose heat in warm temperatures and retain heat in cold temperatures.

Did You Know?

• Beavers are the second-largest living rodents, after capybaras, weighing up to 50 kg (110 lb).
• The North American beaver has 40 chromosomes, while the Eurasian beaver has 48—preventing interbreeding.
• Beavers can swim at 8 km/h (5 mph), using only their webbed hind feet.
• A beaver coat has 12,000–23,000 hairs per square centimeter.
• Beavers mark territories using scent mounds made of mud, debris, and castoreum, a liquid excreted through their castor sacs.