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​Castor canadensis - American Beaver

Skull Pictures:            
Anterior  Ventral
Dorsal     Mandible



Beavers are the largest rodents in North American On average beavers are 24-40 inches (60-102 cm) long and weigh 24-60 lbs. (12-27 kg).  They have small legs and a long, chunky body with a small surface area to volume ratio. They are very well adapted to their semi-aquatic life style (Long 2000). Beavers maintain an average body temperature of 36.3˚C most of the year with it dropping slightly lower in the winter.  Beavers build fat reserves in the fall and slow lose weight through the winter (Dock and MacArthur 1992).

Beavers have a long, broad, flattened tail which is mostly hairless, with few scattered hairs present.  The hardened, thick, dark, leathery skin that covers the tail appears scaly. An adult tail is normally 10-18 inches (25-45 cm) long, 4-5 inches (10-13 cm) wide and one half to one inch (1.3-2.5 cm) thick.  The furred portion of the body extends roughly a quarter of the length of the tail.  There are some muscles and ligaments in the tail, mostly concentrated in the furred portion.  The center of the tail is made of flattened caudal vertebrae.  The rest of the tail is made of fat, which acts a storage unit and can be utilized for energy during times of food storage.  Beavers will also sit on their tail for support while grooming and cutting wood (Long 2000).

Like all rodents, beavers have a set of incisors that continually grow during their lifetime.  The enamel on the front face of incisors is yellow/orange in color. The inner face of the incisors is softer then the outer face, therefore wearing more quickly keeping the teeth sharp. There is an absence of canines creating a gap between the prominent incisors and the premolars.  The molars are designed for grinding of food, with many ridges that are perpendicular to the tooth row (Long 2000). The dental formula is 1/1, 0/0, 1/1, 3/3=20 (Kurta 1995).

To assist in swimming beavers have webbed hind feet. On the hind feet the second nail is split, forming a grooming claw that can be used to clean dirt and other debris out of their fur (Long 2000).  The front feet are dexterous, able to grip and manipulate objects, dig and groom (Muller-Schwarze and Sun 2003).

To stay warm in cold water beavers have waterproof, thick, brown fur. Their fur consists of two layers, the short, fine undercoat and the thicker, longer outer-coat.  The outer hair mats the undercoat trapping air to help keep the beaver warm.  The hair is also covered with oil produced in glands near the tail.  This oil helps to create the smooth, streamline, and waterproof hair (Long 2000). Fur also serves as a lifejacket, helping it to stay afloat, and also serves as protection against teeth and claws of predators.  The fur is very dense with 12,000-23,000 hairs/cm² (Muller-Schwarze and Sun 2003).

In the water beavers need ways to protect both their eyes and ears.  Their eyes have a third transparent “eye lid” called a nictitating membrane. Their eyesight is very poor and near-sighted.  Beavers rely heavily on their senses of smell and hearing.  Beavers may have small ears, but they have a large capability to detect sound.  They have an auditory cavity that is much larger then humans. Large ear organs help them to detect small sounds in both air and water.  Since water is a better conductor then air, beavers can hear much better underwater.  To help keep water out of the ears, beavers have a valve that covers the ear canal.   Smelling is probably the most important sense for beavers.  They use their noses to find food, detect predators, and detect colony members (Long 2000).


Current distribution- Beavers are found throughout North America from Mexico to Canada, including the entire state of Wisconsin. They do not occur in Florida, north of the Arctic Circle and in most of Mexico. Current population estimates for North America range from 6 to 12 million. They are always found near water, namely lakes, rivers, and streams. (Long 2000; Muller-Schwarze and Sun 2003; Himes 2002).

Historical Distribution- Historically beavers were found throughout North America, with populations estimating 60 to 400 million.   When North America began to be colonized, beavers began to be hunted.  Native Americans traded beaver pelts for settlers’ goods.  The fur trade was responsible for westward expansion, the settlement of many cities including St. Louis and Albany, and many historians claim beaver contributed more then any other animal to the development of North America.  Beavers were used for many things including clothing, men’s felt hats, medicinal uses, perfume bases, and food.  They were so desired that they were hunted and trapped to near extinction. (Himes 2002)

Fossil Record- Beaver fossils are found throughout the world and there are many species recognized in the evolution of today’s beavers.  The earliest beaver fossils in North America are from the early Oligocene to the Holocene.  During the Tertiary beaver diversity was at its greatest.  Notable amounts of fossils are found throughout the Great Lakes Region.  It is believed that the genus Castor originated in Eurasia and migrated to North America during the Pliocene. The largest beaver was the Giant beaver, Castoroides ohioensis, which is estimated to be as large as a black bear or 5 to 6 times larger then beavers presently (Muller-Schwarze and Sun 2003).


Beavers form permanent, monogamous breeding pairs that continue between breeding seasons.  Copulation takes place in the water mainly from December to January.  The gestation period on average lasts 107 days.  In May to July kits are born in litters of 3 to 4.  Reproduction normally takes place only in the adult pair of a colony and most females do not reproduce until age two. In populations with heavy trapping, females may breed at a younger age. At age seven, 90% of females will reproduce and will continue to do so until death. There are no visible reproductive organs. Both sexes look identical externally.

Development of kits is relatively rapid.  Beavers are born fully furred and teethed weighing about 340-630g.  With in several hours they will have full use of their eyes and show defensive behaviors. They may also enter the water on their first day of life.  Beaver young are nidicolous, they are not completely dependent on parental care, but are not completely independent. Young are able to swim by 9 days old.  By 3 weeks young are able to groom themselves and are eating mainly vegetation.  Young are normally fully weaned by week 10, even thought lactation may continue. When weighing 3-4 kg, kits begin to leave the lodge to explore and feed.  A yearling weighs 11-12 kg.

If a member of a pair dies it will often be replaced with a 2-year old disperser.  The average pairing lasts 2.5-3.1 years.  The high replacement rate may be explained by the age discrepancies of repairing.  On average a beaver lives to 10-12 years and up 19 years in captivity (Muller-Schwarze and Sun 2003).

Ecology and Behavior:

Lodge and dam building- Beavers live in lodges or burrows.  Burrows can be used in faster moving water, but most beavers live in lodges.  Lodges can extend several feet out of the water and are made of sticks, mud, leaf materials, and sod.  There are normally two underwater entrances and a main room, which may be 6-8 ft. diameter and is completely above water.  Often, beavers will have a main lodge or the nursing lodge and one to several alternative lodges. The second lodge site is used heavily after the kits are born, at varying water levels, and in the summer months. During the winter and cold seasons the lodges maintain a warmer temperature inside than outside (in one study the lowest temperature inside was 0˚ C while out side it reached –6˚ C).  The lodges are well ventilated with no seasonal variation in CO2 or O2 (Dock and MacArthur 1993).

The site selection for the lodge is influenced by many factors, which include population levels, territoriality, and habitat quality.  Environmental factors that influence the site are percent canopy cover, slope of the riverbank, and water depth.  Sites with greater slope and depth are normally chosen.  The site of the lodge also tends to have more canopy cover then surrounding areas and is not harvested for building or food (Dieter and Mccabe 1989).

Beaver dams can have a major effect on an ecosystem.  Beavers are second only to humans in alterations made to their habitat.  Beavers create a habitat that is ideal for their semi-aquatic life, by damming up fast moving streams.  There is a preference in the usage of alder stems with a diameter or 1.5 to 3.5 cm (Barnes and Mallik 1996).  Some alterations that are caused by dams include: decreased current velocity, increased wetted surface area, increased water depth, increases amount of open area, enhancement or degradation of conditions for fish, favorable conditions for some wildlife and unfavorable for others, change in water invertebrate, increased plankton production, increased organic matter, carbon, nitrogen, and other nutrients, and increased trapping of sediments increasing clarity down stream (Hammerson 1994).  These effects are often temporary, since beavers often move to a different area after several years.  When dams break, the soil is often rich and high in nutrients, which is perfect for many plants to colonize.

Feeding- Beavers feed on trees.  They will eat many parts of trees, but mostly the outer bark and cambium.  They will also eat some aquatic plants.  Since they consume large amounts of cellulose, they have a specialized digestive tract.  They have very long intestines, using both the caecum and proximal colon for hind-gut fermentation, and a card​iogastric gland in the stomach.  Also to further aid in digestion the also are coprophagic.  Beavers will ingest their feces to gain the undigested nutrients (Vispo and Hume 1995).

Beavers create a food cache each fall.  The food cache is a stock of branches and twigs stored under the water near the lodge for a winter food supply. These are compiled from September to November.  Beavers preferred tree species are notably aspens and willows, with other tree species eaten and used based on relative availability.  In different studies beaver will cache many different species including aspens, willows, alders, witch hazel, red maple, sugar maple, red oak, white birch, yellow birch, and white pine.  Caches do not contain the amount of calories needed for the entire colony.  Any unused branches will often be used in construction in the spring. Summer diets include grasses, forbs and aquatic vegetation (Busher 1995; Muller-Schwarze and Sun 2003).

Diving- Beaver dives normally last 5-6 minutes, but they can stay under the water for a maximum of 15 minutes.  When diving blood is signaled to move to vital organs that could suffer from oxygen shortages.  The heart rate and metabolism will also drop on a dive.  The average resting heart rate of a beaver is 100 beats/minute and can drop to 50 beats/minute while diving (Muller-Schwarze and Sun 2003).

Mound Building- Beavers hold territories of 10-75 acres.  Scent mounds built from pond sediment and are marked with castoreum form these territories. Castoreum is a urine based excretion that is stored in castor sacs.  The adult male primarily does the scent marking, but other family members will also take part in this activity.  Beavers will periodically remark the mounds.  They are also able to distinguish between family and non-family members as well as neighbors and non-neighbors.  The main mound building time is in late spring, early summer, during the time a dispersal of young. Reasons for marking scent on a mound include elevation of the point of odor release, intensify odor with moist substrate, and protection from flooding. (Schulte 1998; Muller-Schwarze and Sun 2003)

Activity time- The normal activity pattern is a crepuscular-nocturnal active period and a diurnal resting period.  This pattern becomes much more irregular in the winter (Dock and MacArthur 1993).

Roles- Beavers live in colonies that consist of the adult male and female, their yearlings and kits.  Offspring tend to stay with their parents until they are 2 and help with taking care of the young, building, food caching, and scent mound work.  The male and female also allocate their time between many different roles or jobs.  Both males and females spend about the same amount of time traveling to get food, but females spend more time on interactions, feeding and on food caching.  Males on the other hand spend more time on lodge work and alarm displays.  There is a shared responsibility in the care of young with both males and females (Buech 1995).

Beavers produce many vocalizations and unintended noises.  Hissing often takes place in response to strange scents, for defense, and towards intruders.  It is also believed that beavers make sounds to express pleasure, excitement, and satisfaction.  When cutting wood a gnawing sound can be heard, possibly making their presence known to predators.  Young can be heard whining in the lodges.  This sounds much like a baby crying and is believed that it is made to solicit food from others (Muller-Schwarze and Sun 2003).

Parasites- Beavers are hosts to two parasites that are potentially dangerous to humans. Tularmia is a bacteria that infects beaver, often causing death.  Humans can contract these bacteria by handling carcass, consumption of infected or even from mosquito bites by mosquitoes that had fed on infected beavers. There was an outbreak in the Necedah National Wildlife refugee in 1981 and 1982. Beaver mortality was substantial and the area in Central Wisconsin had to be closed for recreation in those years.

Beavers also carry Giardia. An infection of this protozoan is often called “beaver fever.”  Giardia is contracted by drinking infected water.  Beavers often concentrate Giardia into an area, but the main source is untreated human wastes. Symptoms include diarrhea, abdominal cramps, gassiness, and weight loss.

Other parasites of beavers include, several species of each trematodes, halminths and nematodes, also beetles and mites. (Muller-Schwarze and Sun 2003)


Sub-species - There are 23 recognized subspecies of Castor canadensis, all varying slightly: C.c. acadicus, C.c. baileyi, C.c. beluqae, C.c. caecator, C.c. carolinensis, C.c. concisor, C.c. duchesnei, C.c. frondator, C.c. idoneus, C.c. labradorensis, C.c. leucodontus, C.c. mexicanus, C.c. michiganensis, C.c. missouriensis, C.c. pallidus, C.c. phaeus, C.c. repentinus, C.c. rostralis, C.c. sagittatus, C.c. shastensis, C.c. subauratus, C.c. taylori, C.c. texensis (MSW 1993)

Literature Cited:

Barnes, D. M. and A. Mallik.  1996.  “Use of woody plants in construction of beaver dams in northern Ontario.” Canadian Journal of Zoology 74:  1781-1786.

Buech, R.  1995.  “Sex differences in behavior of beavers living in near-boreal lake habitat.” Canadian Journal of Zoology 73: 2133-2143.

Busher, P.  1996.  “Food caching behavior of beaver (Castor canadensis): Selection and use of woody species.”  American Midland Naturalist 135: 343-348.

Dieter, C. D., and T. R. Mccabe.  1989.  “Factors influencing beaver lodge site selection on a prairie river.” American Midland Naturalist 122: 408-411.

Dyck, A.P., and R. Mac Arthur.  1992.  “Seasonal patterns of body temperature and activity in free-ranging beaver (Castor canadensis).” Canadian Journal of Zoology 70: 1668-1672.

______.  1993.  “Seasonal variation in the microclimate and gas compostition of beaver lodges in a boreal environment.”  Journal of Mammalogy 74: 180-188.

Hammerson, G.A.  1994.  “Beaver (Castor canadensis): Ecosystem alterations, management, and monitoring.”  Natural Area Journal 14: 44-57.

Himes, A.  “At the zoo- Busy Beavers.”  2002.  ZooGoer.  Smithsonian National Zoological Park. <>.  Assessed 2 December 2003.

Kurta, A.  1995.  Mammals of the Great Lakes Region. Ann Arbor, Michigan:  The University of Michigan Press.

Long, K.  2000.  Beavers:  A Wildlife Handbook Boulder, Colorado: Johnson Books.

“MSW Synonym List.”  1993.  Mammal Species of the World.  Smithsonian Institution.  <>.  Assessed 30 October 2003.

Muller-Schwarze, D. and L. Sun.  2003.  The Beaver: Natural History of a Wetlands Engineer. Ithaca, New York : Cornell University Press.

Schulte, B.A.  1998.  “Scent marking and responses to male castor fluid by beavers.” Journal of Mammalogy 79: 191-203

VispoLong, C. and I. Home.  1995.  “The digestive tract and digestive function in North American porcupine and beaver.”  Canadian Journal of Zoology 73: 967-974.

Reference written by Melissa Weber, Biol 378: Edited by Chris Yahnke.  Page last updated 4-19-04.

©1993- University of Wisconsin-Stevens Point