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Beaver

Scientific Name: Castor canadensis

Description

BeaverNew York State's official mammal, the beaver is unmistakable due to its large body size (26-65 pounds, 25-35 inches) and broad flattened tail (9-10 inches long, 6 inches wide), not to mention the characteristically altered habitat in which it resides. Characteristics unique to the beaver include a nictitating membrane, or a secondary internal, opaque inner eyelid, valvular ears and nose, and lips that close behind the incisors, thereby allowing a beaver to gnaw underwater. Large incisors are continuously growing and are kept at a manageable length by the gnawing action beaver use to procure food. Their rich, dense fur ranges in color from yellow-brown to almost black. Webbed hind feet serve to compliment the extraordinary swimming ability of the beaver, and the second hind toe has a split nail, purportedly for grooming purposes.

Although the beaver's tail is flat, primarily to aid in swimming and to navigate aquatic habitats, it serves other purposes as well. The tissue beneath the scaly outer layer, which is actually compressed, aggregated hairs, is highly vascularized at its base, and serves as a thermoregulatory tool. Blood is shunted from the surface of the tail, thereby minimizing heat loss. It also serves as a means for fat storage.

Habitat

Beaver rarely leave the water for any extended duration of time and can be found inhabiting wooded streams, the margins of lakes, ponds, and reservoirs, swamps and marshes, and many other sources of year-round water. Ideally, waterways will be of low gradient with an abundance of aspen, willow or alder, and a diversity of other woody and herbaceous vegetation.

Food and Feeding

The beaver's diet consists almost entirely of cellulose in the form of woody plant material. Woody plants are nearly indigestible to mammals, so digestion is aided by microorganisms inhabiting the small intestine. Beavers eat the leaves, bark and twigs of trees such as aspen, willow and red maple and a variety of herbaceous plants. During summer months, their dietary preferences may shift to aquatic vegetation including water lilies and rhizomes from shoreline ferns.

As winter draws near, beaver may collect and store or cache food items underwater near the entrance to their lodge in what is termed a 'raft' or 'feedpile' for use throughout the winter. This food store is imperative for survival when thick ice prevents access to fresh food during New York's long winters.

Behavior

Reproduction in beaver leads to the formation of their basic social unit, the family or colony. Beaver mate for life, but if one member of a pair dies, the remaining member will readily accept a new mate. These colonies usually consist of the parents, the present year's offspring, and often times there will be a representative from the previous year's litter. Infrequently an 'extra' adult will be found within a colony. As a colony grows, members may undoubtedly expand their breeding and foraging territories, building dams and lodges to support shelter requirements and expand home ranges. A typical number of beaver per colony is between four and six, but up to a dozen is possible.

Home ranges or colonial territories are established and passively defended by means of scent-mounding, where an adult beaver piles up muddy debris and mark the top with castoreum, which is washed out of the castor glands with urine. Active defense includes potentially violent encounters, as many beaver have been shown to bear the scars of territorial disputes, but such encounters seldom end in death.

Other forms of communication include vocalizations, postures and tail-slapping. Tail-slapping is thought to be a means for one beaver in a colony to warn other beaver of a potential threat. Another function may be to frighten would-be predators away.

Ecology

Like most rodents, beaver construct an elaborate den or lodge with multiple entrances. Beaver differ from other rodents not only in size, but the fact that beaver alter their surroundings to suit their needs. Possessing the unique ability to fell trees, they use this talent to not only get food, but also as a source of construction material for their lodges and the watertight dams they build to impound the area that they intend to inhabit. Cued to begin construction at the sound and motion of running water, beaver impound an area not only for a place to live and rear their young, but it also adds protection from certain predators.

Depending upon the type of habitat they colonize, they may create a home in a stream bank, or a lodge out in open water. These have two or more underwater entrances and the 'living quarters' of their lodge will be above the level of the surrounding water, and in the wintertime will maintain a temperature significantly higher than that of the surrounding air.

Dubbed "nature's finest wetland engineer", beaver activity has both disruptive and beneficial impacts on any body of water they colonize. Beaver impound a variety of wetland types and streams with different forest types and gradients. This colonization converts the existing habitat to open water and provides a valuable resource for a variety of furbearer and waterfowl species. However, it can eliminate the existing natural diversity of certain groups of organisms such as reptiles and some fish species that may require cooler water than is provided by a beaver impoundment. Subsequent inundations of surrounding landscapes may impede or halt natural succession or destroy actual forest stands of trees not adapted for prolonged submersion of their roots.

As their dam-building activity can cause widespread flooding of forest and agricultural lands, nuisance complaints regarding beaver activity are widespread across New York State. Flooding not only has the potential to change the ecology of a colonized area, it can also cause hazards to human habitation with the plugging of culverts and flooding of roads, railroad tracks, economically important agricultural lands and other general property damage concerns.

Reproduction

Beaver reproduce once a year and form lifelong breeding pairs. Breeding occurs in January or February and young are born in May or June after an average gestation time of 107 days. Litter size may range from 2 to 7 kits. The number of offspring may be attributed to the quantity and quality of available food and habitat in any given year.

Kits are fully furred, teeth are erupted, eyes are open at birth and they may enter the water within a few hours following birth. For the first few weeks of life a kit's fur is not water-repellent, as their anal glands, which beaver use to 'grease up' their fur, are not functional until the third or fourth week of life.

Predators, Parasites, and Diseases

Beaver are more susceptible to predation by animals such as coyote, fisher, bear and bobcat when traveling extended distances from water in search of food. Kits may fall prey to mink, otter and fox and great-horned owl. Studies in the Adirondack Park suggest that beaver are heavily relied upon by coyotes, and rank second to white-tailed deer as a preferred food source. Humans are one of the major sources of beaver mortality through trapping and automobile collisions.

Beaver harbor a variety of parasites, both internal and external. Internally, one would expect to find different varieties of nematode, a form of intestinal roundworm, in different parts of the intestinal tracts. There are also cestodes or tapeworms that inhabit large portions of the small intestine, and apparently cause little to no harm to the host. Giardia lambdia, a two-celled flagellate that inhabits parts of the intestines and causes "beaver fever" in humans, is known to be found in close association with the beaver, but has not been identified as the definitive host of this rather uncomfortable parasite.

In terms of external or ectoparasites, there are multiple species of mite that dwell in their dense fur. There can be as many as 10 species of mite living on a single beaver, and each species is specialized for life in a distinct part of the animal's body. For instance, the mites living around the head are not the same species of mite that one would find in the hindquarters.


More about Beaver:

  • Watchable Wildlife: Beaver - Information about the appearance, habits and habitat preference of the beaver. Where to go to view beavers in the wild.