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American beaver

Castor canadensis

What do they look like?

Beavers are the largest rodents in North America, and they spend most of their time in the water. To protect themselves from the cold and wetness they have waterproof reddish brown or blackish brown hair. They have small, round, brown ears, and powerful back legs for swimming. A beaver's front legs are not as large or as strong as its back legs. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996; Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

Beaver skulls and teeth are very big. The two front teeth are orange colored, and they can be up to 5 mm wide and between 20 and 25 mm long. These teeth grow throughout the animal's life, and they are used for cutting wood. Without these teeth beavers could not cut down or eat trees and wood. Beavers also have see-through eye lids, and closable nostrils and ears for swimming underwater. (Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

Beavers also have anal and castor glands, which they use to mark their territory. These glands are located beneath the tail. A beaver's tail is broad, flat, and covered with large black scales. (Frazier, 1996; Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    13 to 32 kg
    28.63 to 70.48 lb
  • Range length
    900 to 1170 mm
    35.43 to 46.06 in

Where do they live?

Beavers are found throughout all of North America except for the northern regions of Canada and the deserts of the southern United States and Mexico. (Frazier, 1996)

What kind of habitat do they need?

Beavers live in lodges. They will either live in one built on an island, one built on the shore of a pond, or one built on a lakeshore. The lodge has one main room with its floor above water. There are two entrances to the lodge. (Encarta, 2004)

The lodge is oven-shaped, and is made of sticks, grass, moss, and mud. The inside room may be 8 feet wide and up to 3 ft high. Over the years beavers add more sticks and mud. This makes the lodge larger. The floor of the lodge is covered in bark, grass, and wood chips. (Encarta, 2004)

The pond lodge is built either a short way back from the edge of the bank, or partly hanging over it, with the front wall built up from the bottom of the pond. The lake lodge is built on the shelving shores of lakes. To ensure adequate water depth surrounding the lodge, beavers dam streams with logs, branches, mud, and stones. (Encarta, 2004; "Castor Canadensis", 2000)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

How do they reproduce?

Beavers are monogamous, but if one mate dies, the other will "remarry", or seek out a new mate. Beavers are driven away from their colonies usually around their second year of life, right before a new litter is born. They then make a colony of their own, usually several kilometers away, and they first breed around their third years of life, give or take a year depending on the quality of the environmtnt. ("Castor Canadensis", 2000)

Female beavers are sexually mature when they are about 3 years old. They give birth to one litter each year, usually between April and July. Baby beavers develop inside their mother for about 3 months. Baby beavers are called kits. When they are born they already have all of the fur and have their eyes open. (Frazier, 1996; Frazier, 1996; Hall and Kelson, 1959)

At birth kits are usually around 38 cm long including their tales. They tend to weigh from 250 to 600 grams and can be red, brown, or almost black. They remain in the lodge for a month, afterwards leaving for longer periods of time to swim and take in solid foods. Most beavers are weaned within two weeks, although it can take up to 90 days. The young usually stay with their parents for 2 years and then leave to make their own homes. (Frazier, 1996; Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

  • How often does reproduction occur?
    Beavers breed once a year.
  • Breeding season
    Mating takes place during the winter season, usually in January or February.
  • Range number of offspring
    1 to 4
  • Average number of offspring
  • Average gestation period
    3 months
  • Average gestation period
    128 days
  • Average weaning age
    2 weeks
  • Average time to independence
    2 years
  • Average age at sexual or reproductive maturity (female)
    3 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    639 days
  • Average age at sexual or reproductive maturity (male)
    3 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    639 days

Both male and female beavers will care for their young for up to two years. To prepare for new kits, a mother beaver will make a soft bed on the floor of the lodge. She will then use her tail to help deliver her babies. She licks each kit clean and nurses it. Both mother and father beavers will protect their young from predators, catch food for them, and teach them how to catch food on their own. (Encarta, 2004; Frazier, 1996)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • male
      • female
    • protecting
      • male
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
    • protecting
      • male
      • female
  • pre-independence
    • provisioning
      • male
      • female
    • protecting
      • male
      • female

How long do they live?

Under favorable conditions, beavers will produce their first litters at two or three years of age. The average lifespan of a beaver in the wild is 10 to 20 years. While its size saves it from most predators, a beaver's lifespan can be cut short by predators, most commonly humans, wolves, and coyotes. Parasites and disease also play a factor in mortality. (Whitaker and Hamilton, 1998; Frazier, 1996; Whitaker and Hamilton, 1998)

  • Typical lifespan
    Status: wild
    10 to 20 years

How do they behave?

Beavers usually live in groups with their relatives called colonies. These groups can have up to 8 beavers in them. Younger siblings stay with their parents for up to two years, helping with infant care, food collection, and dam building. Beaver families don't interact with other families, and they defend against eachother. To warn eachother of danger, beavers slap their tails against the water, making a loud noise. Beavers also mark their territories with piles of mud and gland secretions. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996; Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

Beavers are primarily nocturnal. They are only occasionally seen during the day, usually around dusk. Beavers travel good distances from their homes to find food. If they find a good source, they build canals to the food source as a way to float the food back to their lodges. Logs and twigs are often stored underwater for winter feeding. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996; Hall and Kelson, 1959)

Beavers build dams to slow down the flow of water in streams and rivers and then build stable lodges for shelter. The dams are engineered according to the speed of the water; in slow water the dam is built straight, but in fast water the dam is built with a curve in it. This provides stability so that the dam will not be washed away. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996; Hall and Kelson, 1959)

How do they communicate with each other?

Beavers have a pair of anal scent glands, called castors, which secrete a musk-like substance called castoreum. This is used mainly for marking territories. The broad, flat, scaly tail is about 25 cm (about 10 in) long and serves as a warning signal when slapped against the water. Beavers also call out to others, making a low, groaning sound. (Encarta, 2004; "The Beaver", 2002)

What do they eat?

Beavers eat bark and cambium, the soft wood underneath bark. Their favorites include maple, willow, beech, birch, alder, and aspen trees. Beavers also eat plants that live in the water, including root and flowers. When beavers live in zoos, they are fed yams, lettuce, carrots, and "rodent chow". (Frazier, 1996; ; )

  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems

What eats them and how do they avoid being eaten?

Young beavers are very vulnerable, and are threatened by bears, wolves, wolverines, lynx, fishers and otters. An adult beaver's size is a deterrent to most predators, and though natural predators pose a very real danger to kits, man has proven to be, by far, the most dangerous predator to beavers. Killing beavers for their pelts, disrupting them through a change in habitat, and slowly poisoning them through pollution, which is known to infect wounds, all have lead to the threat which man poses on beavers. ("Castor Canadensis", 2000)

What roles do they have in the ecosystem?

Beaver dams create ponds that provide homes for many other animals. They also raise the level of the water in the area and prevent damage from flood waters. (Frazier, 1996; )

Do they cause problems?

Although beavers are beneficial to the environment, they can destroy it also. Dams slow the flow of water in fast streams, changing the flora and fauna and sometimes creating silting. They may flood low-lying areas, sometimes causing extensive loss of timber. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998)

How do they interact with us?

Beaver fur has been a significant trade item for the last century, creating large amounts of money for merchants.

Beavers are incredibly beneficial to the environment. They are instrumental in creating habitats for many aquatic organisms, maintaining the water table at an appropriate level and controlling flooding and erosion, all by building dams. See the Sevilleta Long-Term Eocological Research Project (LTER)/ RKM and KVP-- University of New Mexico account on the web at for a more detailed explanation of the benefits of beavers in the environment. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998)

  • Ways that people benefit from these animals:
  • body parts are source of valuable material

Are they endangered?

The conservation status differs with respect to source, but there have been significant threats to the survival of the beaver. Beavers have been hunted and trapped extensively in the past and by about 1900, the animals were almost gone in many of their original habitats. Pollution and habitat loss have also affected the survival of the beaver. In the last century, however, beavers have been successfully reintroduced to many of their former habitats. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996)

Some more information...

One of the earliest accounts of beaver natural history was written by Samuel Hearne in the late 1700s. His journal entry on beavers is online at:


Tanya Dewey (editor), Animal Diversity Web.

Rebecca Anderson (author), University of Michigan-Ann Arbor.


Frazier, Janice. 1996. Pittsburgh Zoo. Castor canadensis (On-line). Available (1 August 2002)

Hall, E. Raymond Ph.D, Kelson, Keith R. Ph.D. 1959. The Mammals of North America. vol. 2. The Ronald Press Company. NY.

Sevilleta Long-Term Ecological Research Project (LTER)/ RKM and KVP. University of New Mexico. 1995. Castor canadensis.

Sevilleta LTER. 1998 "Data : Species : Mammal : American Beaver - Castor canadiensis" (On-line). Available (1 August 2002)

Toronto Zoo. 2000. "Castor Canadensis" (On-line). Accessed February 15, 2004 at

1998. "Data: Species: Mammal: American Beaver- Castor canadiensis" (On-line). Sevilleta LTER Data. Accessed August 01, 2002 at

Beavers: Wetlands and Wildlife. 2002. "The Beaver" (On-line). Accessed February 09, 2004 at

Encarta, 2004. Beaver. Microsoft Encarta Online Encyclopedia. Accessed February 09, 2004 at

Frazier, J. 1996. "Castor canadensis" (On-line). Pittsburgh Zoo. Accessed August 01, 2002 at

Hall, E., K. Kelson. 1959. The Mammals of North America, vol. 2. New York: The Ronald Press Company.

Whitaker, J., W. Hamilton. 1998. Mammals of the Eastern United States. New York: Cornell University Press, Sage House.

University of Michigan Museum of ZoologyNational Science Foundation

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Anderson, R. 2002. "Castor canadensis" (On-line), Animal Diversity Web. Accessed May 24, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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