BEAVERS
Beavers are large, semiaquatic rodents. The second-largest living rodents, after capybaras, they weigh up to 50 kilograms (110 pounds), reside in the Northern Hemisphere and belong to the genus Castor and the family Castoridae. There are two existing species: the Eurasian beavers (Castor fiber) and North American beavers (Castor canadensis) and. Beavers arguably have more impact on their environments than any animal in the way they dam up rivers and streams and transform forests into ponds and marshes. They have 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.[Source: Phil Myers, Animal Diversity Web (ADW); Wikipedia] |=|]
Beavers 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. David Ferry wrote in The Atlantic: Your standard beaver comes with webbed hind feet and a scaly, flat tail (perfect for maneuvering underwater and for balancing on land), as well as grasping paws and a pair of buckteeth. It is further equipped with nose valves that close underwater and eye membranes that act as swim goggles. Adult beavers weigh about 40 pounds and are appealing in the way squirrels are: they’re cute, but they’re still rodents. From a historical perspective, the beaver’s most fateful feature is its coat, a combination of thick underfur and longer guard hair that allows the animal to thrive in widely varying climes. It is rugged enough to let the beavers of northern Canada linger beneath the surface of frozen ponds, but light enough to help keep the animal cool during hot New Mexico summers. Unfortunately for beavers, these qualities also make for very fine hats. [Source: David Ferry, The Atlantic, June 2012]
Beavers are herbivores — consuming tree bark, aquatic plants, grasses and sedges — and spend much of their lives in the water — in a number of freshwater habitats, such as rivers, streams, lakes and ponds. Water is a more efficient conductor of heat than air, and relatively large size of beavers confers a favorable surface area/mass ratio that helps reduce the rate of heat loss. As an adaption for life in the water, semiaquatic mammals are often well insulated. To this end beavers have long overfur (guard hairs) and dense underfur. Their fine coats were sought after by fur trappers, which is how beavers played a role in the expansion of Europeans in North America. 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 store up food during the summer and autumn and spend all winter sealed in their homes with the entries under the ice. They impact forest ecosystems by feeding on bark and leaves, often felling large trees to obtain food, and modify the streams and lakes they occupy by building dams, digging canals, and undermining banks with burrows. The dams, which can be several meters in length and up to two meters high, are made of interlacing branches and sealed with mud. The dams often cause extensive flooding dramatically altering the landscape, creating habitat for some kinds of wildlife but damaging the habitats for other kinds of wildlife. Elaborate systems of canals are often dug to connect streams or flooded areas with feeding areas. |=|
The geologic record of the Castoridae family extends to the Oligocene Period (33 million to 23.9 million years ago). A giant beaver, Castoroides, lived in North America during the Pleistocene Period (2.6 million to 11,700 years ago). It was the size of a modern 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 usually spiraling burrows known as "Devil's corkscrews." |=|
See Separate Article: BEAVERS, ECOSYSTEMS, DAMS AND HUMANS factsanddetails.com
Eurasian Beavers and American Beavers
The North American beaver and the Eurasian beaver were confirmed to be separate species in the 1970s. The two beaver species are about same size and both build dams and mainly differ in skull and tail shape and fur color. Both are semi-aquatic and inhabit freshwater systems, including lakes, ponds, rivers, and streams, usually in forested areas but also in marshes and swamps. Permanent access to water is necessary.
Eurasian beavers (Castor fiber) are also known as European beavers. They once populated all of Europe and Asia. However due to overhunting for fur and castoreum, a chemical from their castor sacs, and habitat loss, populations fell nearly to extinction. By the 19th century most countries in Europe and Asia had no remaining beavers. By the 20th century, by some estimates, only 1,300 beavers remained in the wild. Management efforts and reintroductions have resulted in the limited comeback of Eurasian beavers in some places. Significant populations now exist in France, Germany, Poland, southern Scandinavia, and central Russia. However, populations are small, fragmented and scattered throughout this area. The preferred tree species for Eurasian beavers are willows, aspen, birch, and alder. They can live 10 to 17 years in the wild but rarely live longer than seven to eight years. Their average lifespan in captivity is 25 years with unverified reports of them living up 35 years. [Source: Josh Holden, Animal Diversity Web (`ADW) |=|]
American Beavers (Castor canadensis) are found throughout all of North America except for the northern regions of Canada and the deserts of the southern United States and Mexico. They are a fair amount of them. One of the earliest accounts of beaver natural history was written by Samuel Hearne in the late 1700s. Their lifespan in the wild is typically 10 to 20 years. [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
North American beavers and Eurasian beavers are believed to have diverged from a common ancestor around 7 to 8 million years ago. A study by National Institutes of Health (NIH) estimates the divergence at over 7 million years ago. Another source suggests around 7.5 million years ago, with the ancestors of the North American beaver crossing the Bering Land Bridge from Asia to North America. It is remarkable that the divergence took place so long ago as the two species today are very similar in size, appearance and lifestyle.
Beaver Habitats
Beavers prefer slow-moving or still, deep water and can alter their habitat if necessary to create these conditions. Water quality is not as important as water access, food availability, and depth of water. Beavers live in lodges, of which there are three types: 1) those built on islands, 2) those built on the banks of ponds, and 3) those built on the shores of lakes. The island lodge consists of a central chamber, with its floor slightly above the water level, and with two entrances. One entrance opens up into the center of the hut floor, while the other is a more abrupt descent into the water. [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
Beavers do not hibernate. David Attenborough wrote: “They have found a way of maintain a supply of food throughout the winter, even though the woods around them are deep in snow and their lake is ice-bound. They use the lake as a larder... They feed on bark and tissues that lie just beneath it, together with rushes, leaves and sedges, which they in and around the lake. [Source: “Life of Mammals” by David Attenborough]
“As they swim, propelled with their webbed feet, all you are likely to see of them is their head just above the water and a V-shaped ripple spreading across the surface of their lake. If they catch sight of you, they make take fright and smack the surface of the water with their broad, scaly tail, making a loud sound that warns other members of their family of the danger you present. Then they dive. They can easily stay submerged for minutes and are said to be able to do so for almost a quarter of an hour. But it is much longer than that before their heads reappear on the surface of the lake a stey will be safe inside their lodge, having swum across to it and entered.
Beaver lodges are oven-shaped dwellings made of sticks, grass, and moss, woven together and plastered with mud. Over the years, repairs, extensions, and elaborations increase their size. The room inside may measure 2.4 meters (8 feet) wide and up to one meter (3 feet) high. The floor is blanketed with bark, grass, and wood chips. Pond lodges are built either a short way back from the edge of the bank, or partly hanging over it, with the front wall built up from the bottom of the pond. Lake lodges are built on the shelving shores of lakes. One of main reasons beavers dam streams with logs, branches, mud, and stones is to make sure there is adequate water depth surrounding the lodge. [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
Beaver Characteristics
Beavers are primarily aquatic animals, and the largest rodents in North America, Europe and Asia. They have a waterproof, rich, glossy, reddish brown or blackish brown coat. The underhairs are much finer than the outer, protective, guard-hairs. The ears are short and round. Beaver hind legs are longer than their front legs, making the rear end higher than the front end while walking. Beaver skulls and teeth are disproportionately large. They need strong teeth and powerful jaw muscles to cut through hard woods like maple and oak. Their upper incisors are bright orange in color and are at least five millimeters wide and 20-25 millimeters long. These teeth grow throughout the animal's lifetime and are necessity to survival. Sexual Dimorphism (differences between males and females) is not present: Both sexes are roughly equal in size and look similar.
Beavers have closable nostrils, closable ears, and transparent eye membranes for life in the water. Their tail is broad, flat, and covered in large blackish scales. Anal and castor glands are found in both male and female beavers at the base of the tail which are used to mark their territory. The anal and castor glands in American beavers are as large as 8.6 to 5.6 centimeters (3.4 by 2.2 inches) for the castors, and 7.5 by 3.5 centimeters (three by one inch) for the anal glands. Secretions from these glands give the beavers their odd odor. |=|
American beavers range in weight from 13 to 32 kilograms (28.6 to 70.5 pounds) and have a head and body length ranging from 90 to 117 centimeters (35.4 to 46 inches).Eurasian beavers range in weight from 13 to 35 kilograms (28.6 to 77 pounds) and have a head and body length ranging from 73 to 135 centimeters (28.74 to 53.15 inches). Eurasian beavers have two layers of fur: a a soft dense undercoat that is dark grayish in color and an outer layer of longer, stiff reddish brown guard hairs. Fur color tends to be darker in northern populations. [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
Josh Holden wrote in Animal Diversity Web Eurasian beavers have two castoreum glands located next to the anus. These glands produce a pungent, sweet smelling chemical called castoreum and is used to mark territories. The muzzle is blunt and the legs are short. The tail is naked, black with scales, broad, oval, and flattened horizontally. The feet are dark brown to black and each have five digits. The rear feet are webbed and the inside two toes have a split nail used for grooming. The tail is narrower and the skull smaller than those of North American beavers, American beavers. Inside the mouth, beavers have a skin fold that allows them to gnaw on branches under water without getting water in their mouths. They have two, large incisor teeth with hard, orange-colored enamel on the anterior surface. Sexes are alike, although females may tend to be larger. [Source:Josh Holden, Animal Diversity Web (`ADW) |=|]
Beavers have a strongly built, rodent-style skull with a relatively flat profile. Their jaws are powerful, and their cheekbones, on which their jaw muscles are attached, are 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.[Source: Phil Myers, Animal Diversity Web (ADW)]
One unusual characteristic of beavers is that their epiglottis (the flap of cartilage behind the root of the tongue that depresses during swallowing to cover the opening of the windpipe) lies above the soft palate part towards the back of the roof of the mouth. Air flows directly from the nostrils to the trachea (windpipe) 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. |=|
Beaver Food and Eating Behavior
Beavers are herbivores (eat plants or plants parts) and also recognized as folivores (eat leaves), lignivores (eat wood) and coprophage (consume feces). Among the plant foods they eat are leaves, roots, tubers, wood, bark, stems, fruit and flowers. They also eat dung and store and cache food. They feed primarily on woody vegetation in the winter months because it stays preserved in water but prefer herbaceous plant foods over woody vegetation when it is available. Beavers do not have cellulases, an enzyme used to break down cellulose. They consume feces to obtain caecal microbes during reingestion which help break down cellulose that can be absorbed after reingestion. [Source: Josh Holden, Animal Diversity Web (`ADW) |=|]
Eurasian beavers prefer willow, maple, birch, aspen and poplar trees with diameters less than 10 centimeters. By preferentially feeding on these species, they can impact the tree species composition of their communities. These food items are stored in the water during the fall months in quantities large enough to supply food for the entire colony or family until the ice melts in the spring. During summer months beavers feed heavily on aquatic vegetation, shoots, twigs, bark, leaves, buds, and roots. In agricultural areas beavers may consume crops. Food caches consist of woody vegetation, such as willow and aspen branches.
American Beavers eat bark and cambium (the softer growing tissue under the bark of trees). Their favorite tree species include willow, maple, poplar, beech, birch, alder, and aspen. Like their European counterparts they also eat water vegetation, as well as buds, and roots. Cellulose, which usually can not be digested by mammals, is a major component of their diet. Beavers have microorganisms in their cecum (a sac between the large and small intestine) that digest this material. In zoos, beavers are fed yams, lettuce, carrots and "rodent chow." [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
Beavers have orangish teeth. This is because, whereas other rodents have magnesium in their tooth enamel, beavers have iron. The iron makes the teeth stronger against mechanical stress, and makes them more resistant to acid. Researchers are looking into these traits for ways to strengthening human teeth. [Source: Ada McVean, McGill University, June 13, 2018]
Beaver Behavior
Beavers live in family groups and mark their territories with scent mounds. They are skillful swimmers, capable of traveling up to 750 meters under water and remaining submerged for four or five minutes. Beavers are primarily active at night. They are only occasionally seen during the day, usually around dusk. In the summer, beavers may good distances from their homes to find food. If they find a good source, they build canals to the food source as a way to float the food back to their lodges. Logs and twigs are often stored underwater for winter feeding. When beavers 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. This, however, is not always effective, as older beavers often ignore the warning slaps of younger members of the colony. |=|[Source: Phil Myers, Animal Diversity Web (ADW)]
Beavers are natatorial (equipped for swimming), nocturnal (active at night), motile (move around as opposed to being stationary), sedentary (remain in the same area), territorial (defend an area within the home range), social (associates with others of its species; forms social groups), and have dominance hierarchies (ranking systems or pecking orders among members of a long-term social group, where dominance status affects access to resources or mates). Eurasian beaver home range size varies by available food, watershed size, colony size, and time of year. During the winter months territory size drops to an area that can be patrolled daily with one trip under water, since there is ice cover. During the warm months territory size can extend from one to five kilometers along a shoreline. [Source: Josh Holden, Animal Diversity Web (`ADW) |=|
American beavers usually live in family groups of up to eight related individuals called colonies. The younger siblings stay with their parents for up to two years, helping with infant care, food collection, and dam building. Beaver families are territorial and defend against other families. Territory marking is done by making mud piles around the edges of a territory, and then by depositing anal and castoral secretions on these piles.
Eurasian beavers live in colonies of up to 12. Rebecca Anderson wrote in Animal Diversity Web: These colonies consist of only one dominant, monogamous breeding pair. They are active throughout the year, hardly ever coming above the ice surface during the winter months in their northernmost regions. For this reason beavers spend the autumn season building food caches in the water to last them through the winter. [Source: Rebecca Anderson, Animal Diversity Web(ADW) |=|]
Eurasian beavers are very territorial and mark their territory with castoreum deposited on scent mounds built on the shore. They bring mud and vegetation up from the bottom, holding it tight to their chest with their forelegs and pushing themselves up the bank with their hind legs until they have a mound. The beaver then apply castoreum to the mud pile creating a scent mound. Beavers act very aggressively towards an unknown scent mound in their territory often hissing at it and slapping the water with the tail and resurfacing right away. Most often they will create a scent mound next to it or on top of it.
Eurasian beavers must groom themselves constantly to maintain the water repellency of their fur. They use the split toe nails on their rear toes to comb oils from their oil glands into their guard hairs. This makes the outer layer of fur waterproof and the inner layer never gets wet. Without these oils, beavers would become wet to the skin and not be able to spend as much time in the water or withstand cold water temperatures.
Beaver Lodges and Yearly Cycle
In ponds, lakes or quiet waters beavers behind their dams build "lodges" out 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. Eurasian beavers dens are usually burrows in the bank of a river or pond. In locations where the bank is not suitable, they construct lodges away from shore out of sticks and mud. In the lodges, beavers live in colonies of up to 12. [Source: Phil Myers, Animal Diversity Web (ADW)]
David Attenborough wrote: “At one side or, even better, on a small hillock that the rising waters of the lake have turned into an island, they build their lodge. They begin by excavating a tunnels that starts underwater and open near the margins of the lake. Then they heap mud, boulders and branches on top of the land entrance. As they add more, they hollow out the inside of the mound until they have created a large dome-shaped chamber, roofed with rushes, with a tunnel in its floor that leads down to the lake. Now the owners are very safe indeed, for they can get out of the water without being seem [Source: “Life of Mammals” by David Attenborough]
“As autumn approaches, the beavers continue to chop down the surrounding woodland and feed on them. Some, however, they place in the lake. There they submerge them. These are their winter rations. Storing them in near-freezing water will keep them relatively fresh and green, As winter approaches, the temperature if the water falls until eventually, ice spread across the surface, Snow blankets the forest. The mud, rushes and branches forming the roof of th beaver’s lodge freezes sold and it becomes hard like concrete,
“The aspen and willow around the lake are now leafless but some of their branches, with leaves attached, are lying in the cold store on the lake bottom. The beavers, unseen by the outside world, leave their lodge and swim down to collect fresh supples. The days get shorter and shorter, It is always pitched black in the lodge and very little light filters through the roof of ice that covers their underwater larder. Nevertheless, they maintain a daily rhythm , just as they did in summer, alternating activity with sleep. As winter progresses, that daylight that cues this rhythm becomes dimmer and shorter. Eventually, the beavers become largely impervious to the diurnal cycle of light and darke=ness outside. They work to their own clocks. By mid-winter they may remain continuously active for as long as twenty hours at a stretch before they reckon that the time has come for a sleep. So beavers may have a year which has fewer days in it than that of nearly all other mammals.
Sometimes smaller animals like muskrats, which are smaller than beavers but have a similar lifestyle and usually lie in river-ban dens of smaller versions of beaver lodges,. move into the beaver lodge. The muskrats may pilfer some of the beavers winter supplies, In the darkness it seems like the beaver are unaware of their presence.
Beaver Senses, Communication and Predators
Beavers sense and communicate with vision, sound and chemicals usually detected by smelling. They leave scent marks produced by special glands and placed so others can smell and taste them. Their broad, flat, scaly tail is used to send warnings signal when it is slapped against the water. Tail slapping is used when they are frightened or upset. Beavers also use postures and vocalizations. Vocalizations include whining calls, whistling, and hissing. Beavers also call out to others, making a low, groaning sound. [Source: Rebecca Anderson, Animal Diversity Web (ADW) |=|]
Beavers have a pair of anal scent glands, called castors, which secrete a musk-like substance called castoreum. This is used mainly for marking territories. For Eurasian beavers castoreum is not only used as a way to mark territory, but the scents produced by their oil glands also serve as way to distinguish between individuals, males and females, and family members and non-family members. |=|
Predators of beavers include humans, gray wolves, brown bears and red foxes. American beavers are also preyed upon by wolverines, Canada lynxes, northern river otters, black bears and fishers (Martes pennanti). Young beavers are most vulnerable. An adult beaver's size is a deterrent to most predators, Lodges and burrows make beavers mostly inaccessible to predators. Eurasian beavers use a “tail slap” when they are frightened, which is a warning to all other beavers that something is near. Beavers slap the water surface with the tail as they dive under water and out of harms way. In response, all beavers in the area will do the same. Eurasian beavers will also avoid food items that have the odors of predators on them. [Source: Josh Holden, Animal Diversity Web (`ADW) |=|]
Beaver Mating and Reproduction
Beavers are monogamous (have one mate at a time), but if one mate dies, the other will "remarry", or seek out a new mate. They are also cooperative breeders (helpers provide assistance in raising young that are not their own) and engage in seasonal breeding and breed once a year. Mating takes place during the winter season, usually in January or February. The average gestation period for American beavers is three months. The number of offspring ranges from one to four, with the average number of offspring being 3.5. The gestation period for Eurasian beavers is 60 to 128 days and they can have up to six young, but one to three is more common. [Source: Rebecca Anderson, Josh Holden, Animal Diversity Web (ADW) |=|]
Male and female beavers are sexually mature at about three years of age. Eurasian beavers live is small family groups consisting of one breeding adult female and male, young of the year, yearlings, and sub-adults. Under favorable conditions, beavers produce their first litters when they become sexually mature. In preparation for birth females prepare a soft bed within the lodge. [Source: Josh Holden, Animal Diversity Web (`ADW) |=|]
Beavers mate between January and March in cold climates, and in late November or December in the south. Usually only one adult pair breeds per colony. Family members cooperate to care for the young of the primary pair. Eurasian beavers live is small family groups consisting of one breeding adult female and male, young of the year, yearlings, and sub-adults. |=|
Eurasian beaver females come into estrus between January and February, but sometimes warm winter weather can result in a breeding season as early as December. If a mature female is not impregnated the first time she will come into estrus two to four times again throughout the season. Copulation takes place in the water most of the time but, in some cases, takes place in the lodge. The male approaches a female floating in the water from the side, copulation may last anywhere from 30 seconds to three minutes. Most copulations occur at night.
Beaver Offspring and Parenting
Beaver young are called kits. They are precocial. This means they are relatively well-developed when born. Both parents and family helpers are involved in the raising of offspring. Pre-weaning provisioning is done by females and protecting is done by males and females. Pre-independence provisioning and protecting are done by males and females. There is an extended period of juvenile learning. Weaning can take place after as little as two weeks. The average weaning age is six weeks and the age in which they become independent ranges from 1.5 to two years. On average males and females reach sexual or reproductive maturity between 1.5 to three years. [Source: Rebecca Anderson, Josh Holden, Animal Diversity Web (ADW) |=|]
Beavers usually give birth in their lodge between April and July. In preparation for birth females prepare a soft bed within the lodge. They use their flat tails as birthing mats.and lick each kit clean, and nurse them after birth. When they are born young are fully furred, have open eyes, and can swim within 24 hours. At birth kits are usually around 38 centimeters long, including their tails, are weigh from 250 to 600 grams and can be red, brown, or almost black. After several days they are also able to dive out of the lodge with their parents to explore the surrounding area. They remain in the lodge for a month, afterwards leaving for longer periods of time to swim and take in solid foods. The young are skittish outside the lodge and are never far from an adult.
After the young are weaned, sub-adults in the colony help feed them by bringing small twigs and soft bark to them until they are about three months old. Beavers are driven away from their colonies usually around their second year of life, right before a new litter is born. The dominant female decides when it is time for the young to travel outside the den for the first time and when the young need to disperse. Dispersed subadults then make a colony of their own, usually several kilometers away.
Image Sources: Wikimedia Commons
Text Sources: Animal Diversity Web animaldiversity.org ; National Geographic, Live Science, Natural History magazine, CNTO (China National Tourism Administration) David Attenborough books, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Discover magazine, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Wikipedia, The Guardian, Top Secret Animal Attack Files website and various books and other publications.
Last updated May 2025