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General Biology, Reproduction and Behavior

Beavers are active for approximately 12 hours each night except on the coldest winter nights. The phrase “busy as a beaver” is appropriate. It is uncommon, however, to see beavers during daylight hours, particularly in large reservoirs.

Beavers are generally monogamous; copulation may take place either in the water or in the lodge or bank den.

After a gestation period of about 128 days, the female beaver generally gives birth to 3 or 4 kittens between March and June, and nurses them for 6 weeks to 3 months. The kittens are born fully furred with their eyes partially opened and incisors erupted through the gums. They generally become sexually mature by the age of 1 1/2.

Beaver communicate b y vocalizations, posture, tail slapping, and scent posts or mud mounds placed around the bank and dam. The beaver’s castor glands secrete a substance that is deposited on mud mounds to mark territorial boundaries. These scent posts are found more frequently at certain seasons, but are found year round in active ponds. 

Beavers have a relatively long life span, with individuals known to have lived to 21 years. Most, however, do not live beyond 10 years. The beaver is unparalleled at dam building and can build dams on fast moving streams as well as slow moving ones. They also build lodges and bank dens, depending on the available habitat. All lodges and bank dens have at least two entrances and may have four or more. The lodge or bank den is used primarily for raising young, sleeping, and food storage during severe weather.

The size and species of trees the beaver cuts is highly variable, from a 1 inch diameter at breast height softwood to a 6 foot diameter at breast height hardwood. in some areas beavers usually cut down trees up to 10 inches and merely girdle or partially cut larger ones, although they often cut down much larger trees. Some beavers seem to like to girdle large pines and sweetgums. They like the gum or storax that seeps out of the girdles area of sweetgum and other species.

An important factor about beavers is their territoriality. A colony generally consists of four to eight related beavers, who resist additions or outsiders to the colony or the pond. Young beavers are commonly displaced from the colony shortly after they become sexually mature, at about 2 years old. 

They often move to another area to begin a new pond and colony. However, some become solitary hermits inhabiting old abandoned ponds or farm ponds if available. 

Beavers have only a few natural predators aside from humans, including coyotes, bobcats, river otters and mink, who prey on young kittens. In other areas, bears, mountain lions, wolves and wolverines may prey on beavers. Beavers are hosts for several ectoparasites and internal parasites including nematodes, trematodes and coccidians. Giardia lamblia is a pathogenic intestinal parasite transmitted by beavers, which has caused human health problems in water supply systems. The Centers for Disease Control has recorded at least 41 outbreaks of waterborne Giardiasis, affecting more than 15,000 people. 

Damage and Damage Identification

The habitat modification by beavers, caused primarily by dam building, is often beneficial to fish, furbearers, reptiles, amphibians, waterfowl and shorebirds. However, when this modification comes in conflict with human objectives, the impact of damage may far outweigh the benefits. 

Most of the damage caused by beavers is a result of dam building, bank burrowing, tree cutting, or flooding. Some southeastern states where beaver damage is extensive have estimated the cost at $3 million to $5 million annually for timber loss, crop losses, roads, dwellings and flooded property, and other damage. In some states, tracts of bottomland hardwood timber up to several thousand acres in size may be lost because of beaver. Some unusual cases observed include state highways flooded because of beaver ponds, reservoir dams destroyed by bank den burros collapsing, and train derailments caused by continued flooding and burrowing. Housing developments have been threatened by beaver dam flooding, and thousands of acres of cropland and young pine plantations have been flooded by beaver dams. Road ditches, drain pipes, and culverts have been stopped up so badly that they had to be dynamited out and replaced. Some bridges have been destroyed because of beaver dam building activity. In addition, beavers threaten human healthy by contaminating water supplies with Giardia.

Identifying beaver damage generally is not difficult. Signs include dams, dammed up culverts, bridges, or drain pipes resulting in flooded lands, timber, roads and crops; cut down or girdled trees and crops, lodges and burrows in ponds, reservoir levees, and dams. In large watersheds, it may be difficult to locate bank dens. However, the limbs, cuttings, and debris around such areas as well as dams along tributaries usually help pinpoint the area. 

Information is from Prevention and Control of Wildlife Damage-Cooperative Extension University of Nebraska-Great Plains Agricultural Council Wildlife Committee-United States Department of Agriculture Animal and Plant Health Inspection Service Animal Damage Control