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Castor canadensisAmerican beaver

By Rebecca Anderson

Geographic Range

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)

Habitat

Beavers live in lodges, of which there are three types: those built on islands, those built on the banks of ponds, and those built on the shores of lakes. The island lodge consists of a central chamber, with its floor slightly above the water level, and with two entrances. One entrance opens up into the center of the hut floor, while the other is a more abrupt descent into the water. (Encarta, 2004)

The lodge, itself, is an oven-shaped house of sticks, grass, and moss, woven together and plastered with mud. Over the years, repair and elaboration leads to an increase in hut size. The room inside may measure 2.4 m (8 ft) wide and up to 1 m (3 ft) high. The floor is blanketed with 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

Physical Description

Beavers are primarily aquatic animals, and the largest rodents in North America. They have a waterproof, rich, glossy, reddish brown or blackish brown coat. The underhairs are much finer than the outer, protective, guard-hairs. The ears are short, round, and dark brown in coloration. A beaver's hind legs are longer than its front legs, thus making the rear end to be higher than the front end while walking. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998; Frazier, 1996; Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

Beaver skulls and teeth are disproportionately large. This is crucial for cutting through hard woods like maple and oak. Most noteably, the upper incisors, bright orange in color, are at least 5 mm wide and 20-25 mm long. These teeth grow throughout the animal's lifetime and are a necessity to survival, just as the animal's closable nostrils, closable ears, and transparent eye membranes are for aquatic existence. (Hall and Kelson, 1959; Whitaker and Hamilton, 1998)

Also notable are the anal and castor glands, found in both male and female beavers. Both sets of glands lie at the base of the tail, which is possibly the most defining characteristic of the beaver. It is broad, flat, and covered in large blackish scales. The anal and castor glands have been recorded as large as 3.4 by 2.2 inches for the castors, and 3.0 by 1 inch for the anal glands. Secretions from these glands are used in scent-marking, and give the beaver its odd odor. (Frazier, 1996; 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

Reproduction

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)

Male and female beavers are sexually mature at about 3 years of age. They mate between January and March in cold climates, and in late November or December in the south. Beavers give birth to one litter of kits per year, usually between April and June. The gestation period is about 3 months, or 105-107 days. During this time, the young develop inside the female's body. When they are born they are fully furred,have open eyes, and can swim within 24 hours. After several days they are also able to dive out of the lodge with their parents to explore the surrounding area. (Frazier, 1996; Hall and Kelson, 1959)

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)

  • Breeding interval
    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
    3.5
    AnAge
  • Average gestation period
    3 months
  • Average gestation period
    128 days
    AnAge
  • 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
    AnAge
  • Average age at sexual or reproductive maturity (male)
    3 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    639 days
    AnAge

Parental care begins before birth, and continues for 1-2 years until the young have reached the stage of independence. In preparation for birth females will prepare a soft bed within the lodge. She then will use her flat tail as a sort of birthing mat. She will lick each kit clean, and nurse it. Both mother and father beaver play a part in providing food for the young and protecting them from predators. (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

Lifespan/Longevity

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

Behavior

Beavers usually live in family groups of up to 8 related individuals called colonies. The younger siblings stay with their parents for up to 2 years, helping with infant care, food collection, and dam building. Beaver families are territorial and defend against other families. One method is territory marking. This is done by making mud piles around the edges of a territory, and then by depositing anal and castoral secretions on these piles. Beavers will also warn others of danger by slapping their tails against the water, creating a powerful noise. This, however, is not always effective, as olders beavers will often ignore the warning slaps of younger members of the colony. ("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)

Communication and Perception

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)

Food Habits

Beavers eat bark and cambium (the softer growing tissue under the bark of trees). Their favorites include willow, maple, poplar, beech, birch, alder, and aspen trees. They also eat water vegetation, as well as buds, and roots. Cellulose, which usually can not be digested by mammals, is a major component of their diet. Beavers have microorganisms in their cecum (a sac between the large and small intestine) that digest this material. In zoos, beavers are fed yams, lettuce, carrots and "rodent chow." (Frazier, 1996; ; )

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

Predation

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)

Ecosystem Roles

Beavers maintain wetlands that can slow the flow of floodwaters. They prevent erosion, and they raise the water table, which acts as a purifying system for the water. This happens because silt occurs upstream from dams, and toxins are then broken down. As ponds grow from water backed up by the damn, pond weeds and lilies take over. After beavers leave their homes, the dams decay, and meadows appears. (Frazier, 1996; )

Economic Importance for Humans: Positive

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 http://sevilleta.unm.edu/animal/mammal/beaver.html for a more detailed explanation of the benefits of beavers in the environment. ("Data: Species: Mammal: American Beaver- Castor canadiensis", 1998)

  • Positive Impacts
  • body parts are source of valuable material

Economic Importance for Humans: Negative

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)

Conservation Status

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)

Other Comments

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: http://web.idirect.com/~hland/sh/an020.htm.

Contributors

Tanya Dewey (editor), Animal Diversity Web.

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

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

freshwater

mainly lives in water that is not salty.

herbivore

An animal that eats mainly plants or parts of plants.

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Frazier, Janice. 1996. Pittsburgh Zoo. Castor canadensis (On-line). Available http://zoo.pgh.pa.us/wildlife_search_animal.asp?categoryname=Mammals&animal=12 (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 http://sevilleta.unm.edu/data/species/mammal/sevilleta/profile/american-beaver.html (1 August 2002)

Toronto Zoo. 2000. "Castor Canadensis" (On-line). Accessed February 15, 2004 at http://www.csh.rit.edu/~snell/beaver.html.

1998. "Data: Species: Mammal: American Beaver- Castor canadiensis" (On-line). Sevilleta LTER Data. Accessed August 01, 2002 at http://sevilleta.unm.edu/data/species/mammal/sevilleta/profile/american-beaver.html.

Beavers: Wetlands and Wildlife. 2002. "The Beaver" (On-line). Accessed February 09, 2004 at http://www.beaversww.org/beaver.html.

Encarta, 2004. Beaver. Microsoft Encarta Online Encyclopedia. Accessed February 09, 2004 at http://encarta.msn.com/encyclopedia_761575721/Beaver.html.

Frazier, J. 1996. "Castor canadensis" (On-line). Pittsburgh Zoo. Accessed August 01, 2002 at http://zoo.pgh.pa.us/wildlife_search_animal.asp?categoryname=Mammals&animal=12.

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.

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