Genus of mammal
Beavers (genus Castor) are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver ("Castor canadensis") and the Eurasian beaver ("C. fiber"). Beavers are the second-largest living rodents, after capybaras, weighing up to . They have stout bodies with large heads, long chisel-like incisors, brown or gray fur, hand-like front feet, webbed back feet, and tails that are flat and scaly. The two species differ in skull and tail shape and fur color. Beavers can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges.
Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams restrict water flow, and lodges serve as shelters. Their infrastructure creates wetlands used by many other species, and because of their effect on other organisms in the ecosystem, beavers are considered a keystone species. Adult males and females live in monogamous pairs with their offspring. After their first year, the young help their parents repair dams and lodges; older siblings may also help raise newly born offspring. Beavers hold territories and mark them using scent mounds made of mud, debris, and castoreum—a liquid substance excreted through the beaver's urethra-based castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors.
Historically, beavers have been hunted for their fur, meat, and castoreum. Castoreum has been used in medicine, perfume, and food flavoring; beaver pelts have been a major driver of the fur trade. Before protections began in the 19th and early 20th centuries, overhunting had nearly exterminated both species. Their populations have since rebounded, and they are listed as species of least concern by the IUCN Red List of mammals. In human culture, the beaver symbolizes industriousness, especially in connection with construction; it is the national animal of Canada.
Etymology.
The English word "beaver" comes from the Old English word or and is connected to the German word and the Dutch word . The ultimate origin of the word is an Indo-European root for 'brown'. Cognates of "beaver" is the source for several European placenames, including those of Beverley, Bièvres, Biberbach, Biebrich, Bibra, Bibern, Bibrka, Bobr, Bober, Bóbrka, Bjurholm, Bjurälven, and Bjurum. The genus name "Castor" has its origin in the Greek word and translates as 'beaver'.
Taxonomy.
Carl Linnaeus coined the genus name "Castor" in 1758 as well as the specific (species) epithet "fiber" for the Eurasian species. German zoologist Heinrich Kuhl coined "C. canadensis" in 1820, many scientists considered both names synonymous for one same species until the 1970s, when chromosomal evidence became available confirming both as separate where the Eurasian has 48 chromosomes, while the North American has 40.) The difference in chromosome numbers prevents them from interbreeding. Twenty-five subspecies have been classified for "C. canadensis", and nine have been classified for "C. fiber".
There are two extant species: the North American beaver ("Castor canadensis") and the Eurasian beaver ("C. fiber"). The Eurasian beaver is slightly longer and has a more lengthened skull, triangular nasal cavities (as opposed to the square ones of the North American species), a lighter fur color, and a narrower tail.
Evolution.
Beavers belong to the rodent suborder Castorimorpha, along with Heteromyidae (kangaroo rats and kangaroo mice), and the gophers. Modern beavers are the only extant members of the family Castoridae. They originated in North America in the late Eocene and colonized Eurasia via the Bering Land Bridge in the early Oligocene, coinciding with the "Grande Coupure", a time of significant changes in animal species around 33million years ago (myr).
The more basal castorids had several unique features: more complex occlusion between cheek teeth, parallel rows of upper teeth, premolars that were only slightly smaller than molars, the presence of a third set of premolars (P3), a hole in the stapes of the inner ear, a smooth palatine bone (with the palatine opening closer to the rear end of the bone), and a longer snout. More derived castorids have less complex occlusion, upper tooth rows that create a V-shape towards the back, larger second premolars compared to molars, absence of a third premolar set and stapes hole, a more grooved palatine (with the opening shifted towards the front), and reduced incisive foramen. Members of the subfamily "Palaeocastorinae" appeared in late-Oligocene North America. This group consisted primarily of smaller animals with relatively large front legs, a flattened skull, and a reduced tail—all features of a fossorial (burrowing) lifestyle.
In the early Miocene (about 24 mya), castorids evolved a semiaquatic lifestyle. Members of the subfamily Castoroidinae are considered to be a sister group to modern beavers, and included giants like "Castoroides" of North America and "Trogontherium" of Eurasia. "Castoroides" is estimated to have had a length of and a weight of . Fossils of one genus in Castoroidinae, "Dipoides", have been found near piles of chewed wood, though "Dipoides" appears to have been an inferior woodcutter compared to "Castor". Researchers suggest that modern beavers and Castoroidinae shared a bark-eating common ancestor. Dam and lodge-building likely developed from bark-eating, and allowed beavers to survive in the harsh winters of the subarctic. There is no conclusive evidence for this behavior occurring in non-"Castor" species.
The genus "Castor" likely originated in Eurasia. The earliest fossil remains appear to be "C. neglectus", found in Germany and dated 12–10 mya. Mitochondrial DNA studies place the common ancestor of the two living species at around 8 mya. The ancestors of the North American beaver would have crossed the Bering Land Bridge around 7.5 mya. "Castor" may have competed with members of Castoroidinae, which led to niche differentiation. The fossil species "C. praefiber" was likely an ancestor of the Eurasian beaver. "C. californicus" from the Early Pleistocene of North America was similar to but larger than the extant North American beaver.'
Characteristics.
Beavers are the second-largest living rodents, after capybaras. They have a head–body length of , with a tail, a shoulder height of , and generally weigh , but can be as heavy as . Males and females are almost identical externally. Their bodies are streamlined like marine mammals and their robust build allows them to pull heavy loads. A beaver coat has 12,000–23,000 hairs/cm2 (77,000–148,000 hairs/in2) and functions to keep the animal warm, to help it float in water, and to protect it against predators. Guard hairs are long and typically reddish brown, but can range from yellowish brown to nearly black. The underfur is long and dark gray. Beavers molt every summer.
Beavers have large skulls with powerful chewing muscles. They have four chisel-shaped incisors that continue to grow throughout their lives. The incisors are covered in a thick enamel that is colored orange or reddish-brown by iron compounds. The lower incisors have roots that are almost as long as the entire lower jaw. Beavers have one premolar and three molars on all four sides of the jaws, adding up to 20 teeth. The molars have meandering ridges for grinding woody material. The eyes, ears and nostrils are arranged so that they can remain above water while the rest of the body is submerged. The nostrils and ears have valves that close underwater, while nictitating membranes cover the eyes. To protect the larynx and trachea from water flow, the epiglottis is contained within the nasal cavity instead of the throat. In addition, the back of the tongue can rise and create a waterproof seal. A beaver's lips can close behind the incisors, preventing water from entering their mouths as they cut and bite onto things while submerged.
The beaver's front feet are dexterous, allowing them to grasp and manipulate objects and food, as well as dig. The hind feet are larger and have webbing between the toes, and the second innermost toe has a "double nail" used for grooming. Beavers can swim at ; only their webbed hind feet are used to swim, while the front feet fold under the chest. On the surface, the hind limbs thrust one after the other; while underwater, they move at the same time. Beavers are awkward on land but can move quickly when they feel threatened. They can carry objects while walking on their hind legs.
The beaver's distinctive tail has a conical, muscular, hairy base; the remaining two-thirds of the appendage is flat and scaly. The tail has multiple functions: it provides support for the animal when it is upright (such as when chewing down a tree), acts as a rudder when it is swimming, and stores fat for winter. It also has a countercurrent blood vessel system which allows the animal to lose heat in warm temperatures and retain heat in cold temperatures.
The beaver's sex organs are inside the body, and the male's penis has a cartilaginous baculum. They have only one opening, a cloaca, which is used for reproduction, scent-marking, defecation, and urination. The cloaca evolved secondarily, as most mammals have lost this feature, and may reduce the area vulnerable to infection in dirty water. The beaver's intestine is six times longer than its body, and the caecum is double the volume of its stomach. Microorganisms in the caecum allow them to process around 30 percent of the cellulose they eat. A beaver defecates in the water, leaving behind balls of sawdust. Female beavers have four mammary glands; these produce milk with 19 percent fat, a higher fat content than other rodents. Beavers have two pairs of glands: castor sacs, which are part of the urethra, and anal glands. The castor sacs secrete castoreum, a liquid substance used mainly for marking territory. Anal glands produce an oily substance which the beaver uses as a waterproof ointment for its coat. The substance plays a role in individual and family recognition. Anal secretions are darker in females than males among Eurasian beavers, while the reverse is true for the North American species.
Compared to many other rodents, a beaver's brain has a hypothalamus that is much smaller than the cerebrum; this indicates a relatively advanced brain with higher intelligence. The cerebellum is large, allowing the animal to move within a three-dimensional space (such as underwater) similar to tree-climbing squirrels. The neocortex is devoted mainly to touch and hearing. Touch is more advanced in the lips and hands than the whiskers and tail. Vision in the beaver is relatively poor; the beaver eye cannot see as well underwater as an otter. Beavers have a good sense of smell, which they use for detecting land predators and for inspecting scent marks, food, and other individuals.
Beavers can hold their breath for as long as 15 minutes but typically remain underwater for no more than five or six minutes. Dives typically last less than 30 seconds and are usually no more than deep. When diving, their heart rate decreases to 60 beats per minute, half its normal pace, and blood flow is directed more towards the brain. A beaver's body also has a high tolerance for carbon dioxide. When surfacing, the animal can replace 75 percent of the air in its lungs in one breath, compared to 15 percent for a human.
Distribution and status.
The IUCN Red List of mammals lists both beaver species as least concern. The North American beaver is widespread throughout most of the United States and Canada and can be found in northern Mexico. The species was introduced to Finland in 1937 (and then spread to northwestern Russia) and to Tierra del Fuego, Patagonia, in 1946. As of 2019[ [update]], the introduced population of North American beavers in Finland has been moving closer to the habitat of the Eurasian beaver. Historically, the North American beaver was trapped and nearly extirpated because its fur was highly sought after. Protections have allowed the beaver population on the continent to rebound to an estimated 6–12million by the late 20th century; still far lower than the originally estimated 60–400million North American beavers before the fur trade. The introduced population in Tierra del Fuego is estimated at 35,000–50,000 individuals as of 2016[ [update]].
The Eurasian beaver's range historically included much of Eurasia, but was decimated by hunting by the early 20th century. In Europe, beavers were reduced to fragmented populations, with combined population numbers being estimated at 1,200 individuals for the Rhône of France, the Elbe in Germany, southern Norway, the Neman river and Dnieper Basin