American beaver (Castor canadensis)
|Size||Max total length: 120 cm (2)|
|Weight||16 - 31.5 kg (2)|
Classified as Least Concern (LC) on the IUCN Red List (1).
North America’s largest rodent, the American beaver, exhibits a wide range of physical adaptations to its largely aquatic lifestyle (2) (3) (4). The heavily muscled body is shaped more like a marine mammal than like other terrestrial mammals, while the hind-feet are webbed for swimming (2) (3) (5). Furthermore, the characteristically flattened, scaly tail provides steering and propulsion, particularly when swimming fast or diving (2) (5). While the colouration of the coarse outer-fur varies from chestnut to almost black, the dense underfur is typically dark grey and maintains body warmth even in freezing waters (2) (3). The ears and nose are equipped with valve-like flaps that can be closed underwater, while the small eyes have a protective transparent eyelid (nictitating membrane) (3) (6). Owing to the need for a strong foundation for the prominent tree-felling incisors, the beaver has an exceptionally thick and heavy skull and jaw (2). Large claws on the short forefeet provide dexterity with handling food and also facilitate digging (2) (4). As many as 24 subspecies of the American beaver are recognised, but reintroductions have blurred their geographic boundaries and resulted in genetic mixing (2) (6).
The natural range of the American beaver extends throughout much of North America, with the exception of peninsular Florida, the southwestern deserts, and the Arctic tundra (1) (6). Outside of its original range, the American beaver has been introduced to Argentina, Finland, and far eastern Russia (1) (2) (6).
Occurs near streams, ponds and lakes, where suitable food and building resources are available (1) (6).
The beaver is renowned for the unique ability to fell relatively large trees with its robust front teeth. Not only does this give it greater access to food in the form of leaves, twigs and bark, but it also provides the raw materials needed for damming waterways and building its lodge (3) (6). The dam provides an area of still, open water where the lodge can be conveniently constructed and protected from terrestrial predators (7). The lodge is essentially a pile of mud and sticks, which usually has one or more underwater entrances, with an inner chamber that rises above the surface of the water (3) (6).
Secretive and nocturnal, the beaver typically rises at sundown and returns to the lodge at sunrise. During spring and summer, non-woody plants form the bulk of this herbivore’s diet but over autumn and winter, tree and shrubs, such as aspen and willow, are increasingly consumed (5) (6). During the autumn, populations in the north often build a cache of branches and logs underwater near the lodge, to be eaten over the winter (6).
The social life of the beaver is built around the colony, which typically consists of an adult pair and two to six young (5) (6). Longterm monogamy is unusual among mammals but beaver pairs generally remain together until one adult dies. Mating takes place once a year, usually around January and February, with three to four kits (young beavers) being born in late spring, following a gestation period of around 107 days (2) (3) (6). The young kits are weaned within two months, but require a long period (up to two years) within the family in order to learn the varied skills that are essential for independent life (2) (5).
The American beaver and the high demand for its luxuriant pelt, is often cited as being the key stimulus for the westward expansion of European settlement across North America (5) (6) (7). During the 19th century in particular, the species was hunted so intensively that only scattered and greatly reduced populations remained by the early 1900s (7). Fortunately, following subsequent re-establishment programs and the enforcement of hunting regulations, the beaver made a tremendous comeback and is once again abundant throughout much of its historical range and beyond (1) (2) (5) (7). Although there are no longer thought to be any major threats to the beaver, it is still commonly killed in areas where beaver activities, such as tree-felling and dam building, conflict with human activities (1) (2) (5).
Owing to the stability of its widespread population, as well as its occurrence in several protected areas, the American beaver is currently classified as Least Concern on the IUCN Red List (1). Although demand for pelts has decreased substantially (8), hunting and trapping of beavers continues to be regulated at the national level (1).
To find out more about the American beaver, see:
- Baker, B.W. and Hill, E.P. (2003) Beaver (Castor canadensis). In Feldhamer, G.A., Thompson, B.C. and Chapman, J.A. (Eds) Wild Mammals of North America: Biology, Management, and Conservation. Second Edition. The Johns Hopkins University Press, Baltimore, Maryland, USA.
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- Gestation: the state of being pregnant; the period from conception to birth.
- Monogamy: having only one mate during a breeding season, or throughout the breeding life of a pair.
IUCN Red List (May, 2009)
- Baker, B.W. and Hill, E.P. (2003) Beaver (Castor canadensis). In: Feldhamer, G.A., Thompson, B.C. and Chapman, J.A. (Eds) Wild Mammals of North America: Biology, Management, and Conservation. Second Edition. The Johns Hopkins University Press, Baltimore, Maryland, USA.
- Burnie, D. (2001) Animal. Dorling Kindersley, London.
Smithsonian National Museum of Natural History (August, 2009)
- Macdonald, D. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford.
- Jenkins, S.H. and Busher, P.E. (1979) Castor canadensis. Mammalian species, 120: 1 - 8.
- Nowak, R.M. (1999) Walker's Mammals of the World. Johns Hopkins University Press, Baltimore, Maryland.
Global Invasive Species Database (August, 2009)