Beavers make up a very small family (2 modern species in one genus, Castor), but its members are conspicuous members of forest communities in the north temperate zone, and their ecological impact is considerable. They also played a pivotal role in the European exploration and settling of many parts of North America. Modern beavers are found in North America, northern Europe, and northern Asia.
Beavers are semiaquatic. Like many mammals that spend much of their lives in the water, beavers are large; an adult may weight over 30 kg. Water is a more efficient conductor of heat than air, and large size confers a favorable surface area/mass ratio that helps reduce the rate of heat loss. Another characteristic of aquatic and semiaquatic mammals is that they are often well insulated; beavers accomplish insulation by a pelage that consists of long overfur (guard hairs) and dense underfur. Their fine coats have made them a primary target of fur trappers. Beavers have large webbed hind feet and a moderately long but highly flattened tail, which is used for propulsion in the water. Their eyes are protected during swimming by a nictating membrane, and their nostrils and ears can be closed by special muscles.
Beavers have a strongly built skull with a relatively flat profile. They are sciuromorphous and sciurognathus. Their jaws are powerful, and the jugal, from which part of the masseteric musculature arises, is broad and strong. Their skulls are also notable because the external auditory meatus is connected to the bulla by a long bony tube, which projects downward and outward from the bulla. The dental formula is 1/1, 0/0, 1/1, 3/3 = 20. The incisors are heavy; cheekteeth are hypsodont but not evergrowing; and the occlusal surface of the cheekteeth has numerous transverse folds, which provide cutting edges for slicing and tearing food.
Another odd characteristic of beavers is that their epiglottis lies above the soft palate, within the narial pasage. Air flows directly from the nares to the trachea without entering the mouth cavity. The back of the tongue is raised and can be fitted tightly against the palate, blocking the passage of water from the mouth. These modifications allow a beaver to open its mouth to gnaw or carry branches while underwater.
Beavers are extremely important in the forest ecosystems of the north temperate zone for two reasons. They feed on bark and leaves, often felling large trees to obtain food. They prefer willows, maples, aspens, and poplars, and by preferentially chosing those species, they can have an important impact on the tree species composition of their communities. Second, beavers modify the streams and lakes they occupy by building dams, digging canals, and undermining banks with burrows. The dams, which can be many meters in length and up to 2 m high, are constructed by interlacing branches and sealing with mud. Extensive areas are often flooded, creating habitat for some kinds of wildlife but often dramatically altering the nature of the dammed streams. Elaborate systems of canals are often dug to connect streams or flooded areas with feeding areas.
Beavers live in family groups. Their territories are marked by scent mounds. In lakes or quiet waters they build "lodges" of piled sticks and mud. These have an underwater entrance that leads to a nest chamber above the water level. Lodges may be used for several years, with successive generations of beavers adding to the lodge until it reaches very large size. Beavers also burrow into the banks of streams or rivers and nest within the burrows; this is often the strategy used in waters that are too swift or deep to dam. They are skillful swimmers, capable of traveling up to 750 m under water and remaining submerged for 4 or 5 minutes. When they become aware of danger, swimming beavers often loudly slap the water with their tail as they dive, startling the predator and alerting family members of potential danger.
The geologic record of the family extends to the Oligocene. A giant beaver, Castoroides, lived in North America during the Pleistocene and was about the size of a black bear (but its brain was not much larger than that of the modern beaver!). It probably was a grazer that did not cut trees or feed much on bark. Another extinct member of the family, Palaeocastor, made peculiar spiraling burrows known as "Devil's corkscrews."
References and literature cited:
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
McLaughlin, C. A. 1984. Protrogomorph, sciuromorph, castorimorph, myomorph (geomyoid, anomaluroid, pedetoid, and ctenodactyloid) rodents. Pp. 267-288 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
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.
uses smells or other chemicals to communicate
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.
having the capacity to move from one place to another.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate