American Beaver
(Castor canadensis) #63-168

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Physical characteristics and distribution

American Beaver Castor canadensis

Head and body length of C. canadensis is 600-800 mm with a tail length of 250-450 mm. Both male and female adults weights range between 12-25 kg., though some have been recorded to weigh as much as 40 kg. The pelage is unusually dense, and the thick underfur is overlaid with coarse guard hairs. Color ranges from glossy brown to yellowish brown above and brown to tawny below. C. canadensis has short legs and the feet have five digits with sharp claws. The hind feet are webbed, but the first and second toe are split. This is thought to aid the beaver with grooming. All four feet are black, as is the tail. Flattened horizontally, the broad, paddlelike tail is covered with large scales and mostly hairless.

Both sexes of C. canadensis have two pairs of large glands known as "castor glands" located in the area of the cloaca. These produce an odiferous secretion used to scent mark piles of stones or mud, thought to be signs for other beavers. In males, baculum is present, females have four pectoral mammae.

Within the massive skull of the American
Beaver, the incisors are strongly developed. The high-crowned cheek teeth have flat grinding surfaces and numerous fold, but do not grow throughout the animal's lifetime.

Being semiaqautic, C. canadensis is found primarily along streams and lakes with nearby growths of willow, aspen, poplar (cottonwood), birch and alder. In these habitats they build complex dams, lodges and canals. The foundation of the dam is made of mud and stones, with poles (butt ends facing upstream) and brush added on top. Mud, stones and wet plant material are used to plaster the structure, with the top rising above water level. The dam provides an area of deep, still water, where a lodge can be constructed. This habitat provides C. canadensis with a safe place from predators and a storage area for food to be floated without danger of it washing away. The lodge is constructed in the same manner as the dam, and is usually surrounded by the water that the dam has backed up. This dome shaped structure can extend more than 2 meters above the water's surface, having a diameter up to 12 meters at the base. There is a single internal chamber with a floor that is above the water level. This is covered by a thick bedding of dry vegetation. The walls of the lodge can be 1 meter thick at the base, but are generally much thinner at the top of the dome. C. canadensis enter the lodge via one of several underwater doorways which are built below the winter ice level. American
Beavers living near large rivers may build large terrestrial dens rather than lodges.

The third component to the American
Beaver's habitat is the canal. These are used to float logs to a pond and the dams may be used to hold up the water levels in these canals. Several trails can extend from these canals, over which beavers carry timbers to the water. These elaborate sequences of dams, lodges and canals are used by generations of North American beavers, and are repaired and used over many years.

C. canadensis uses its lower incisors to fell trees and then cut them into manageable sections which are pushed to the water. American
Beavers have also been observed walking on their hind find, carrying a bundle of twigs in their arms. They are good swimmers, able to store oxygen and remain submerged for 4-5 minutes, but the record is 15 minutes. There are various types of dives, one known as a "quiet dive" is used when the animal is undisturbed. A "fright dive" is used when the animal is alarmed. To warn others of danger, the American Beaver slaps its tail on the surface of the water as it dives.

American Beavers are nocturnal, and though they do not hibernate, in the northern part of their range, may only leave the lodge during the coldest winter months to visit their food cache. The diet consist of the bark, cambium, twigs, leaves and roots of deciduous trees such as alder, willow, birch, aspen, and on many parts of aquatic plants. They particularly like the young shoots of water lilies.

Social groups are known as colonies. These are comprised of 4-8 related individuals including a mated pair and the offspring from the previous two years. C. canadensis is probably monogamous, having only one breeding female per colony. Breeding occurs once each year, mating occurs in January or February and births occur from April to June. Estrous cycles last for about 2 weeks, but the females are receptive for only 10-12 hours. Gestation is about 100-110 days, producing an average of 2-4 young, but as many as 9 babies may be born. Fully furred babies weigh 230-630 grams at birth and their eyes are open. They are weaned at about three months, and take solid foods at just one week old. Sexual maturity of both males and females is reached at 1.5 - 2 years, and the young adults are usually forced out of the colony at this time. Life expectancy in the wild is as long as 24 years, and as many as 50 years in captivity.

C. canadensis is found in Alaska and Canada south of the Arctic Circle (including Vancouver and Newfoundland), most of the continental USA (absent from parts of the SW USA and from most of Florida), extending into N Mexico. Intoduced into Tierra del Fuego (South America) and Eurasia, including Finland, NW Russia. Poland, Germany, and Austria.

Description of the brain

Animal source and preparation
All specimens collected followed the same preparation and histological procedure.

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