Castor canadensis, Kuhl, 1820

Don E. Wilson, Thomas E. Lacher, Jr & Russell A. Mittermeier, 2016, Castoridae, Handbook of the Mammals of the World – Volume 6 Lagomorphs and Rodents I, Barcelona: Lynx Edicions, pp. 150-168 : 167-168

2.

North American Beaver

Castor canadensis

French: Castor du Canada / German: Kanadischer Biber / Spanish: Castor americano

Other common names: American Beaver, Canadian Beaver

Taxonomy. Castor canadensis Kuhl, 1820 ,

“Ad fretum Hudsoni, [Hudson Strait or Bay],” Canada .

Twenty-four subspecies have been described and used in the past, but extreme exploitation and mixing of populations during reintroductions seem to make these meaningless. More genetic analysis of both mtDNA and nDNA will require a reevaluation of the concept of geographic subspecies based on morphological differences. There is currently no genetic evidence to support subspecies in the North American species. Monotypic.

Distribution. North America, from the Mackenzie River Delta, Northwest Territories, Canada, S throughout Canada and USA, excluding peninsular Florida and arid parts of the SW, to N Mexico (Baja California to Tamaulipas). North American Beavers have been introduced into Europe (Finland, Russia, Central Europe), Russian Far East (Kamchatka and Sakhalin Island), and Tierra del Fuego, Argentina.

Descriptive notes. Head—body 800-900 mm, tail 200-300 mm; weight 15-20 kg (rarely, 30-40 kg). The North American Beavers is the second largest rodent species in the world (tied with the Eurasian Beaver, C. fiber ) and the largest North American rodent. Sexes are externally similar, with no sexual dimorphism. North American Beavers are stocky, robust rodents with relatively massive skeletons. Fronts of incisors are reddishorange, caused by iron concentration for strength. Coat color ranges from light brown or blonde to darker reddish-brown to almost black. Reddish-brown color, called Algonquin, is most common; very dark to blackish fur is rare in natural populations. Fur consists of long, course guard hairs and soft, dense underfur. Underfur was processed into felt for the European hat industry that flourished from the late 16™ century to the mid-19" century. Other physical adaptations are associated with their aquatic habits. North American Beavers have small forelimbs and hands, with powerful digging claws, and completely webbed hindfeet. Ears and nose can close like valves; eyes have a membrane that covers them when diving. Tail is prominent and dorsal-ventrally flattened, oval-shaped, and brownish black in color. Tail is scaly in appearance, with sparse fine hairs; it is darker and hairs more prominent in young individuals.

Habitat. Associated with all types of freshwater wetlands from large rivers to small streams, lakes, and ponds and along wetland systems associated with agricultural land such as irrigation canals. North American Beavers are most closely associated with wetlands that contain species in the family Salicaceae (e.g., willows, Salix spp. , and aspens and poplars, both Populus spp. ), but they are also found in areas that lack these species. They are found in ponds and lakes that contain waterlilies ( Nuphar and Nymphaea , Nymphaeaceae ), which might be related to sodium availability. North American Beavers occur from lowland areas to mountainous regions up to tree line (elevations of 2000-2500 m). In mountain areas, stream gradient limits colonization. They can tolerate human activity and are found living on ponds and large rivers in cities (e.g. Chicago and Washington D.C.) and on rivers in more historically industrial areas (e.g. Martinez, California).

Food and Feeding. North American Beavers eat a variety of riparian woody and herbaceous plants. They are generalist herbivores but can be highly specialization in their food selection, usually relying on 2-5 species for the bulk of their diets regardless of plant diversity. They prefer willows and aspen but also eat many other deciduous species including maples (Acer spp., Aceraceae ); alders ( Alnus spp. ) and birches ( Betula spp. , both Betulaceae ); oaks ( Quercus spp. , Fagaceae ); black walnut ( Juglans nigra, Juglandaceae ); and a variety of fruit bearing trees. They might seasonally eat evergreens such as pines ( Pinus spp. ), firs ( Abies spp. ), and hemlocks (75uga spp., all Pinaceae ). They eat herbaceous plants seasonally, taking more in spring and summer than in other seasons. North American Beavers, like Eurasian Beavers, take trees that are much larger than they are as food items, but they primarily eat branches, bark with associated cambium layer, and leaves. They have associated gut micro-biota that helps in partial cellulose digestion. Cutting of tree and woody vegetation is most pronounced in autumn but can occur during all seasons. Across their distribution, North American Beavers build food caches of branches from woody species in autumn that they eat in winter when plant productivity is low and mobility is restricted by snow and ice.

Breeding. North American Beavers breed once a year, and females come into estrus in December—May, with a peak in January-February. Estrusis 12-24 hours, and if fertilization does not occur, females might come into estrus again. Gestation is 100-110 days, and young are born in spring or early summer. Littersize is 1-6 young but averages 2—4. Males and females become reproductively active at 2-3 years of age. North American Beavers are socially monogamous, if not genetically monogamous, and form pair bonds that last for years. Rarely, multiple pregnant females have been reported in one family group, usually associated with high densities or populations under extreme exploitation. Copulation occurs in the water in a ventral-ventral position or in a lodge or burrow. North American Beavers live 8-15 years in the wild—relatively long for a rodent. Survival is slightly lower for young individuals but generally similar for all age classes.

Activity patterns. North American Beavers are nocturnal, becoming active in the evening (17:00-19:00 h) and remaining active until morning (06:00-08:00 h). They might return to their daytime rest site (lodge or burrow) many times during an activity period. When active, they patrol their territory and dams, swim, feed, and groom. Interactions outside of the rest site with other family members are observed, although they are not common. Most often interactions are among adults and young individuals, especially young-of-the-year. North American Beavers are relatively inactive in winter, and in extreme northern parts of their distribution, they might exhibit a type of winter torpor with little or no movement, although thisis still highly speculative. Over much of their northerly distribution, mobility is restricted by snow and ice in winter. In autumn, they are active cutting woody vegetation, building a food cache, and putting mud and branches on their lodges for insulation and protection.

Movements, Home range and Social organization. North American Beaverslive in family groups (a mated pair of adults, young-of-the-year, and yearlings from the previous year), often called a colony. They are territorial and mark territorial boundaries with scent mounds. They move throughouttheir territories and might even move between territories during spring and summer. Movements tend to be only a few hundred meters. Two-year-old beavers generally disperse from their natal areas in spring, summer, or autumn. Dispersal movements are 5-10 km, but they might be longer depending on availability of quality habitat and density of the population. There is some indication that females disperse farther than males, but this might not be consistent for all populations. Home ranges are usually represented as linear lengths of shoreline occupied and are 0-5-1-5 km. North American Beavers tend to forage within 30 m of the shoreline although longer forays of 100 m or more are observed, usually for preferred foods like aspen. Adults are dominant to young individuals within a family, and no consistent dominance of one sex over the other within an age group has been consistently documented. Young individuals most often initiate social interactions. There is evidence that beavers recognize anal gland secretions and castoreum scents of their family members and nearest neighbors more than less familiar individuals and react more strongly to unfamiliar scent. Evidence is clearer for this in the Eurasian Beaver, but given their morphological and behavioral similarities,it should also be true for the North American Beaver.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The North American Beaver currently has a stable population across its distribution. It was the target of the European fur trade from the late 16" century to the mid-19" century. In many areas of the USA, populations were extirpated and regionally extinct by the end of the 19" century or beginning of the 20" century. In Canada, populations remained more stable but were still depleted from historical numbers. Overall population of the North American Beaver is estimated to have been 60-400 million before the arrival of Europeans. In the early 17" century, 50,000-80,000 beaver skins/year were exported from the eastern USA, which led to rapid decline in the population and fueled westward expansion of European interests in North America. Exploration of western North America was primarily the result of the search for fur, particularly beaver fur. By the mid-19"™ century in North America, up to 260,000 km? of wetlands were drained and converted to agriculture and other uses, which reduced available beaver habitat. Populations of North American Beavers are now stable or growing, and reintroduction efforts and a reduction in trapping pressure have allowed them to recolonize much,if not all, of their historical distribution in North America. Current population estimate in North America is 6-12 million beavers. In North America, beavers were primarily hunted and trapped for their fur, although castor sacs and castoreum were used for medicinal purposes in the past. Numerous place names that refer to beavers across North America and that the beaver is the one of the national animals of Canada attest to its importance to humans in North America. Increasing beaver populations across North America has increased the number of conflicts with humans. Dam construction and digging activities by beavers can damage human structures and influence land-use patterns. Loss of timber and agricultural land is occurring in areas where for centuries beavers were absent from the landscape. Impacts of increasing and spreading beaver populations on agriculture, other land uses, human structures, managed forests, roadways, road culverts, and railway beds, and how humans will learn to live with these impacts, are continuing concerns. Nevertheless, the successful return of the North American Beaver and its ability to increase and enhance wetland function represent a conservation success story. Human activity can still negatively impact the North American Beaver, especially because human use of water sources might not be compatible with sustainable beaver populations. Increased interest in use of fur on a global scale, primarily in China, might return beaver fur to a valuable economic resource in the future.

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Figures

Distribution. North America, from the Mackenzie River Delta, Northwest Territories, Canada, S throughout Canada and USA, excluding peninsular Florida and arid parts of the SW, to N Mexico (Baja California to Tamaulipas). North American Beavers have been introduced into Europe (Finland, Russia, Central Europe), Russian Far East (Kamchatka and Sakhalin Island), and Tierra del Fuego, Argentina.