Beavers

(Castoridae)

Distribution

North America, Asia, and Europe except desert and tundra regions

Distribution

North America, Asia, and Europe except desert and tundra regions

Evolution and systematics

The Rodentia is a monophyletic mammalian order. Molecular evidence suggests a close association of Castoridae to the Geomyidae (pocket mice and pocket gophers). The two beaver species differ by eight chromosomes of Castor (40 for the North American and 48 for the Eurasian beaver). The genus Castor dates to the early Pliocene late Miocene boundary, separating from giant beavers (Castoroides in North America, and Trogontherium of Eurasia) about this time. The family Castoridae dates to the Oligocene and was once quite diverse. Today it contains two species, C. canadensis and C. fiber.

Physical characteristics

Beavers are the largest rodents in the Northern Hemisphere. Males and females are similar in size and cannot be distinguished by external appearance except near parturition and during lactation when females have four prominent pectoral mammae. Beavers have yellowish brown to black fur with long guard hairs and short underfur that aid in water shedding and insulation. Their flat tail serves as a fat storage area. The feet have five digits. The hind feet are webbed with a double claw on the second toe of each hind foot that is used for grooming. The front feet are useful for digging and food handling. The orangish incisors are open-rooted, long, curved, and continually growing while the cheek teeth are high crowned (dental formula is (I1/1 C0/0 P1/1 M3/3) X 2 = 20 teeth total). Both the ears and nostrils can be closed by valves when underwater. A skin-fold inside the mouth permits items to be carried by the teeth without water entering

A North American beaver (Castor canadensis) hauling willow onto a dam in Alaska, USA. (Photo by Animals Animals ©Johnny Johnson. Reproduced by permission.)

A North American beaver (Castor canadensis) den on the upper Mississippi River in Minnesota, USA. (Photo by Animals Animals ©C. C. Lockwood. Reproduced by permission.)

the throat. A nictitating membrane covers the eyes providing protecting and enhancing visual acuity underwater. Their eyesight is not great but smell and hearing are acute. The genus name refers to a pair of castor sacs that release a pungent, musky odor when urine passes through them and out the cloaca (called castoreum). Paired anal glands release secretion through separate anal papillae also in the cloaca. The secretion can distinguish male and female beavers of each species

A North American beaver (Castor canadensis) family inside their lodge. (Photo by Wolfgang Bayer. Bruce Coleman, Inc. Reproduced by permission.)

A young North American beaver (Castor canadensis) feeding on plants in the Grassmere Wildlife Park in Nashville, Tennesee, USA. (Photo by Byron Jorjorian. Bruce Coleman, Inc. Reproduced by permission.)

A North American beaver (Castor canadensis) eating willow on Vermillion Lakes, Banff National Park, Alberta Canada. (Photo by Animals Animals ©Patti Murray. Reproduced by permission.)

and even the two species apart. The reproductive organs are internal. Males have a baculum that is detectable upon palpation. They are hindgut fermenters with an enlarged caecum containing micobiota. They practice coprophagy and possess a cardiac gland in the stomach that secretes digestive chemicals.

Distribution

Beavers were once found throughout Europe, much of Asia, and North America. Hunting by humans for their pelts greatly decreased their range in previous centuries. Recent reintroduction programs successfully restored the beaver to Russia and Scandinavia and C. canadensis to Finland. A few pairs of North American beavers were imported some 50 years ago by the Argentinian government to start commercial fur farming, and when the venture failed, they were released in the environment. They have now proliferated to some 100,000 pairs that cause ecological mayhem and are now found in Chile as well.

Habitat

Beavers live in riparian habitats and are semi-aquatic. They build dams that can be quite extensive, reaching over 10 ft (3 m) in height and hundreds of feet (meters) long, although 65-98 ft (20-30 m) is common. If food is readily available, beavers are not impelled to create dams. Beavers may create multiple homes in their territory. The homes can be bank burrows, a bank den, or a wood lodge, each with several entrances.

Behavior

Beavers begin their active periods in the evening around 6:00 P.M. Adults spend much of their time traveling, foraging, and being in the lodge. Time budgets do not vary much with season. During ice cover, beavers minimize their energy expenditure by remaining close to their home. Beavers paddle with their webbed hind feet and use the tail as a rudder. The tail is slapped on the water to let an approaching intruder know they have been spotted and to warn family members. Beavers vocalize as well and kits are the most vocal with their cries discernible even through the walls of the lodge. Both sexes are involved in scent marking, but adult males mark most frequently. Castoreum and sometimes anal gland secretion provide a territorial specific odor to the marks. Resident adult beavers will investigate and often mark on top of the mark from nonresident beavers. They can distinguish scents of neighbors from strangers, and recognize their mate, family, kin, and even species. Territorial scent marking is most prevalent in the spring and early summer when beavers, especially two-year-olds, disperse. Body and tail scars are evidence of conflicts between individuals from different families. Scent marking, body postures, splashing of sticks, tail-slapping, tooth sharpening, agitated swimming, and even low growls are indicative of an aroused territory holder. Females tend to disperse further than males but distance can vary greatly from less than 0.6 mi (1 km) to tens of miles (kilometers).

Feeding ecology and diet

The foraging of beavers can have a dramatic impact on community trophic dynamics and forest composition. Beavers can remove stands of small trees and fell trees up to 46 in (117 cm) in diameter. This creates patches in the forest and allows understory vegetation to grow. Beavers are thought to remove a greater proportion of biomass per area foraging than any other herbivore. Beavers are generalist herbivores and central place foragers in that they carry food back to their home site. In northerly climates, beavers often create a cache of woody material near their lodge as a winter food supply.

A North American beaver (Castor canadensis) swimming underwater. (Photo by J. & D. Bartlett. Bruce Coleman, Inc. Reproduced by permission.)
A North American beaver (Castor canadensis) working on a dam. (Photo by J. T. Wright. Bruce Coleman, Inc. Reproduced by permission.)

Beavers generally prefer soft wood trees but in spring and summer consume a variety of herbaceous material and aquatic plants. The denuded branches are used for dam and lodge construction. Some tree species may be used primarily for building and rarely consumed.

Reproductive biology

Beavers lives in families (average three to eight) composed of an adult monogamous pair and offspring from two successive years. As population density increases, the number of beavers at a single site increases (maximum 18), apparently because viable habitat is not available. Mating takes place once a year from January to March. Beavers usually mate first in their third winter. Gestation is 100-110 days, and births generally occur in April through June. Kits weigh 0.4-1.5 lb (0.2-0.6 kg) at birth, and remain in the lodge for at least the first four weeks of life. Family members will bring forage into the lodge for the kits. Fertility is highest for beavers from 2.5 to seven years of age with one to nine kits born each year. Nursing ends after two to three months. Beavers are long-lived with record ages of 24 years in the wild but typical longevity is generally only seven to eight years.

Conservation status

Populations of each species once ranged in the tens of millions. While nowhere near their former numbers, beavers are recovering throughout their traditional range. If not hunted excessively, beavers should maintain viable populations. The Eurasian beaver is listed as Lower Risk/Near Threatened by the IUCN, and protected under Appendix 3 of the Convention on the Conservation of European Wildlife and Natural Habitats.

Significance to humans

Historically, beavers provided humans with excellent pelts that were used for clothes from hats to coats. This popularity lead to the extirpation of beavers throughout much of Eurasia and North America. Their meat was consumed and the castor sacs used extensively in the perfume industry. By the early twentieth century, beavers lost much of their consumptive economic value, probably saving them from extinction. Beavers are ecosystem engineers, greatly affecting water flow and the cycling of biogeochemicals. Beavers alter the landscape through their construction of

A North American beaver (Castor canadensis) fells a tree in Wyoming, USA. (Photo by Wolfgang Bayer. Bruce Coleman, Inc. Reproduced by permission.)

A North American beaver (Castor canadensis) emerges from its lodge in New Hampshire, USA. (Photo by Animals Animals ©Ted Levin. Reproduced by permission.)

dams, carving of canals, cutting of trees and burrowing into banks. Their actions increase wetland area and overall biological production. Invertebrates, amphibians, fish, waterfowl, and numerous other terrestrial species benefit by these alterations. Beaver activity increases the resistance of ecosystems to disturbance by stabilizing water flow and the water table. By filtering and retaining sediments, beaver dams improve water quality and reduce erosion. However, beavers may be given the label as a nuisance species. Valuable and ornamental timber may be girdled, cut, or flooded. Damming can flood roads, recreation areas, crops, or dwellings. Control is generally performed by harvesting beavers or pre-ventative maintenance. The Clemson beaver leveler is one device that prevents beavers from flooding surrounding land. Feeding damage can be reduced by chemical repellents, fencing, or by using wire around individual trees. Most negative impacts can be reduced through appropriate management. Local human attitudes and education have a huge impact on whether beavers are viewed as positive or negative components of the biological community.

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