Beavers

(Castoridae)

Class Mammalia

Order Rodentia

Suborder Sciurognathi

Family Castoridae

Thumbnail description
Beavers are large, dark brown rodents with a broad, flat mostly hairless tail; they have a compact body design with webbed hind feet, dexterous front feet and long incisors

Size
31–58 in (80–140 cm); tail one-third of length; 33–75 lb (15–33 kg) with records over 100 lb (45 kg)

Number of genera, species
1 genus; 2 species

Habitat
Freshwater wetlands, rivers, streams, lakes, and ponds

Conservation status
Lower Risk/Near Threatened: 1 species

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) × 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

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

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.

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

dams, carving of canals, cutting of trees and burrowing into banks. Their actions increase wetland area and overall biological production. Invertebrates, amphibians, fish, water-fowl, 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 preventative 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.

Species accounts

List of Species

Eurasian beaver

Castor fiber

taxonomy

Castor fiber Linnaeus, 1758, Sweden.

other common names

English: European beaver.

physical characteristics

Weight 33–75 lb (15–35 kg). Yellowish brown to black fur and a flat tail. Has longer nasal bones, larger and more massive skull and smaller tail than the North American beaver. The anal gland secretion is a thick paste of a grayish color in females but oily with a whitish or pale straw color in males. It is brown and viscous in male but it is whitish or light yellow and runny in female North American beavers. Eurasian beavers are significantly more resistant to tularemia.

distribution

Formerly distributed continuously across Eurasia from the British Isles to eastern Siberia. Presently, established throughout Europe except for Iberia, Italy, and the southern Balkans. Present also in China, Mongolia, and Khabarovsk.

habitat

Common to freshwater wetlands, rivers, streams, lakes, ponds, and even bogs.

behavior

Nocturnal, but more diurnal (especially at dusk) if undisturbed. Territorial throughout the year. Tends to live in burrows and is less likely to construct dams than the North American beaver. Mark tufts of grass, rocks, and logs, as well as directly onto the ground.

feeding ecology and diet

In spring and summer, particularly aquatic plants, and a wide range of grasses, forbs, ferns, shrubs, leaves and twigs, crops, bushes and trees in fall, and twigs and bark in winter. Tree preferences are aspen, poplar, and willow.

reproductive biology

The Eurasian beaver matures later and has smaller litters (average 2–3 kits; maximum 7) than the North American beaver. Monogamous.

conservation status

Overhunting reduced Eurasian beaver populations to about 1,200 animals, in eight isolated populations around the end of the nineteenth century. Protection, natural spread, and reintroductions led to a powerful recovery in both range and populations during the twentieth century, which continues at a rapid pace despite considerable loss and degradation of habitat. The minimum population estimate is 593,000 in 2002. The Eurasian beaver is protected under Appendix 3 of the Convention on the Conservation of European Wildlife and Natural Habitats and is listed as Lower Risk/Near Threatened by the IUCN.

significance to humans

In Eurasia nearly all beaver-human conflicts are caused by beavers feeding on cultivated plants, or dam building. They are

keystone species in wetland habitats, and humans increasingly acknowledge their environmental contributions. Hunting and dead-trapping are allowed in the Nordic countries, Russia, and Baltic countries. Presently, beaver safaris are offered to observe beavers in Norway, Sweden, Poland, and France.

North American beaver

Castor canadensis

taxonomy

Castor canadensis Kuhl, 1820, Hudson Bay, Canada.

other common names

English: Canadian beaver.

physical characteristics

Weight 33–75 lb (15–35 kg). Yellowish brown to black fur and a flat tail.

distribution

Historically located throughout the continental United States, the subarctic of Canada to the tundra and northern Mexico, excluding desert regions and southern Florida. Presently, beavers are found in most of these areas but in lower numbers. Successful introductions in Finland, Russia, and Argentina.

habitat

Nocturnal, but more diurnal (especially at dusk) if undisturbed. Territorial throughout the year. Tends to live in burrows and is less likely to construct dams than the North American beaver. Mark tufts of grass, rocks, and logs, as well as directly onto the ground.

behavior

Alters the landscape more with extensive dams than the Eurasian beaver. Scent marking occurs almost exclusively by creating mud mounds. Otherwise, behavior is similar to the Eurasian beaver.

feeding ecology and diet

Strict herbivores, prefer the cambium and leaves of soft wooded trees like aspen (Populus). However, incorporate a wide range of food items into their diet such as aquatic plants, herbs, grass, shrubs, conifers, and deciduous hardwoods.

reproductive biology

As for the Eurasian beaver but mature earlier and has larger litters (average 3–4 kits; maximum 12). Monogamous.

conservation status

In the late twentieth century, population estimates were 6–12 million, probably less than 10% of historical levels. Beavers have no special status although in some regions it is illegal to tamper with their dams.

significance to humans

In past centuries, the Canadian beaver was an important animal for the First Nations and is part of Native American myths and folklore. The Apaches endowed beavers with the magic powers of the medicine men and they are also part of the folklore of European settlers. Much of the exploration of North America was also a result of the search for beavers, highly prized because of the value of their pelts. In the latter half of the twentieth century, the poor economic value of beavers has assisted the recovery of many populations. Beavers provide enormous ecological benefits as well as recreational and aesthetic value. They probably affect their immediate environment as much as humans. The greatest environmental change results from their dam-building activities. The damming of streams raises the level of the water table. Several tree species cannot survive in waterlogged soil and their death allows the spread of species that are adapted to permanently wet soil. These include for example, willows, cottonwoods, and alders. Beaver ponds are a favorable habitat for many forms of life: insects lay eggs in them, fish feed on the insect larvae, and muskrats, mink, shore-birds, and fish thrive. There are over 50 species of animals that live in beaver ponds.

Resources

Books

Busher, P. E., and R. M. Dzieciolowshi. Beaver Protection, Management and Utilization in Europe and North America. New York: Kluwer Academic Plenum Publishers, 1999.

Conley, V. A. The War Against the Beavers. Minneapolis: University of Minnesota Press, 2003.

Novak, M., J. Baker, M. Obbard, and B. Mallock, eds. Wildlife Furbearer Management and Conservation in North America. Toronto: Ashton-Potter Limited, Ontario Ministry of Natural Resources, 1987.

Owl, Grey. Tales of An Empty Cabin. Toronto: Key Porter Books, 1999.

Periodicals

Halley, D., and F. Rosell. "The Beaver's Reconquest of Eurasia: Status, Population Development and Management of a Conservation Success." Mammal Review 32 (2002): 153–178.

Wilsson, L. "Observations and Experiments on the Ethology of the European Beaver (Castor fiber)." Viltrevy 8 (1971): 115–266.

Organizations

Arkansas Soil and Water Conservation Commission, Beaver Conservation Program. 101 East Capitol Avenue, Suite 350, Little Rock, Arkansas 72201 USA. Phone: (501) 682-3953. Fax: (501) 682-3991. Web site: <http://www.state.ar.us/aswcc/page6.html/>

Bruce A. Schulte, PhD

Frank Rosell, PhD