North American Beaver
Beavers are famously busy, and they turn their talents to reengineering the landscape as few other animals can. When sites are available, beavers burrow in the banks of rivers and lakes. But they also transform less suitable habitats by building dams. Felling and gnawing trees with their strong teeth and powerful jaws, they create massive log, branch, and mud structures to block streams and turn fields and forests into the large ponds that beavers love.
Domelike beaver homes, called lodges, are also constructed of branches and mud. They are often strategically located in the middle of ponds and can only be reached by underwater entrances. These dwellings are home to extended families of monogamous parents, young kits, and the yearlings born the previous spring.
Beavers are among the largest of rodents. They are herbivores and prefer to eat leaves, bark, twigs, roots, and aquatic plants. These large rodents move with an ungainly waddle on land but are graceful in the water, where they use their large, webbed rear feet like swimming fins, and their paddle-shaped tails like rudders. These attributes allow beavers to swim at speeds of up to five miles (eight kilometers) an hour. They can remain underwater for 15 minutes without surfacing, and have a set of transparent eyelids that function much like goggles. Their fur is naturally oily and waterproof.
There are two species of beavers, which are found in the forests of North America, Europe, and Asia. These animals are active all winter, swimming and foraging in their ponds even when a layer of ice covers the surface.
Dams[]
Beaver dams are created as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously. Beavers may create a series of dams along a river.
Lodges[]
A Beaver's Dam
The ponds created by well-maintained dams help isolate the beavers' homes, which are called lodges. These are created from severed branches and mud. The beavers cover their lodges late each autumn with fresh mud, which freezes when frosts arrive. The mud becomes almost as hard as stone, thereby preventing wolves and wolverines from penetrating the lodge.
The lodge has underwater entrances, which makes entry nearly impossible for any other animal, although muskrats have been seen living inside beaver lodges with the beavers who made them. Only a small amount of the lodge is actually used as a living area. Beavers dig out their dens with underwater entrances after they finish building the dams and lodge structures. There are typically two dens within the lodge, one for drying off after exiting the water and another, drier one, in which the family lives.
Beaver lodges are constructed with the same materials as the dams, with little order or regularity of structure. They seldom house more than four adults and six or eight juveniles. Some larger lodges have one or more partitions, but these are only posts of the main building left by the builders to support the roof. Usually, the dens have no connection with each other except by water.
When the ice breaks up in spring, beavers usually leave their lodges and roam until just before autumn, when they return to their old lodges and gather their winter stock of wood. They seldom begin to repair the lodges until the frost sets in, and rarely finish the outer coating until the cold becomes severe. When they erect a new lodge, they fell the wood early in summer but seldom begin building until nearly the end of August.
Social Behavior[]
Family Life[]
The basic units of beaver social organization are families consisting of an adult male and adult female in a monogamous pair and their kits and yearlings. Beaver families can have as many as ten members in addition to the monogamous pair. Groups this size or close to this size build more lodges to live in while smaller families usually need only one. However, large families in the Northern Hemisphere have been recorded living in one lodge. Beaver pairs mate for life; however, if a beaver's mate dies, it will partner with another one. Extra-pair copulations also occur. In addition to being monogamous, both the male and female take part in raising offspring. They also both mark and defend the territory and build and repair the dam and lodge. When young are born, they spend their first month in the lodge and their mother is the primary caretaker while their father maintains the territory. In the time after they leave the lodge for the first time, yearlings will help their parents build food caches in the fall and repair dams and lodges. Still, adults do the majority of the work and young beavers help their parents for reasons based on natural selection rather than kin selection. They are dependent on them for food and for learning life skills. Young beavers spend most of their time playing but also copy their parents' behavior. However while copying behavior helps imprint life skills in young beavers it is not necessarily immediately beneficial for parents as the young beaver do not perform the tasks as well as the parents.
Older offspring, which are around two years old, may also live in families and help their parents. In addition to helping build food caches and repairing the dam, two-year-olds will also help in feeding, grooming and guarding younger offspring. Beavers also practice alloparental care, in which an older sibling may take over the parenting duties if the original parents die or are otherwise separated from them. This behavior is common and is seen in many other animal species such as the elephant and fathead minnow. While these helping two-year-olds help increase the chance of survival for younger offspring, they are not essential for the family and two-year-olds only stay and help their families if there is a shortage of resources in times of food shortage, high population density, or drought. When beavers leave their natal territories, they usually do not settle far. Beavers can recognize their kin by detecting differences in anal gland secretion composition using their keen sense of smell. Related beavers share more features in their anal gland secretion profile than unrelated beavers. Being able to recognize kin is important for beaver social behavior and it causes more tolerant behavior among neighboring beavers.
Territories and Spacing[]
Beavers maintain and defend territories, which are areas for feeding, nesting and mating. They invest much energy in their territories, building their dams and becoming familiar with the area. Beavers mark their territories by constructing scent mounts made of mud, debris and castoreum, a urine based substance excreted through the beavers castor sacs between the pelvis and base of the tail. These scent mounts are established on the border of the territory. Once a beaver detects another scent in its territory, finding the intruder takes priority, even over food. Because they invest so much energy in their territories, beavers are intolerant of intruders and the holder of the territory is more likely to escalate an aggressive encounter. These encounters are often violent. To avoid such situations, a beaver marks its territory with as many scent mounds as possible, signaling to intruders that the territory holder has enough energy to maintain its territory and is thus able to put up a good defense. As such, territories with more scent mounts are avoided more often than ones with fewer mounts. Scent marking increases in August during the dispersal of yearlings, in an attempt to prevent them from intruding on territories. Beavers also exhibit a behavior known as the "dear enemy effect". A territory-holding beaver will investigate and become familiar with the scents of its neighbors. As such they respond less aggressively to intrusions by their territorial neighbours than those made by non-territorial floaters or "strangers".
Occurrences of beaver aggression, however, have been reported. In April 2013, an angler, near Minkovichi in the Brest region of Belarus, died after being bitten twice on the leg by a wild Eurasian beaver.
Gallery[]
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