Life and Wildlife along the Little Buffalo River

Three weeks ago, I noticed a well constructed (upstream) facing Beaver dam near the inlet end of the Boxley Mill Pond. They have been working on it at night continuously through January and February and make more progress each day. They have now constructed a “lodge” across the Mill Pond upstream form the dam. I did not notice how large it was last summer. This is one of my favorite spots for Bird photography all year long. The lodge may be approaching five (5) feet in total height, and the dam is perfectly built with an upstream “U” to prevent wash out. The dam now spans the entire mill pond (maybe 100 feet). Beavers do not hibernate, but store sticks and logs in a pile in their ponds, eating the under-bark. Some of the pile is generally above water and accumulates snow in the winter. This insulation of snow often keeps the water from freezing in and around the food pile, providing a location where beavers can breathe when outside their lodge. All the photos below are from the Boxley Mill Pond in January and February 2013. I did not notice that it had reached these proportions prior to this winter.

Dams

Beaver dams are created as a protection against predators, such as coyotes, 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

Contrary to popular belief, beavers actually 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 where the family actually lives.

Beaver houses are formed of the same materials as the dams, with little order or regularity of structure, and seldom contain more than four adult and six or eight young beavers. Some of the larger houses have one or more partitions, but these are only posts of the main building left by the builders to support the roof, for the apartments usually have no communication with each other except by water.

When the ice breaks up in spring beavers always leave their embankments and rove about until just before fall, when they return to their old habitations and lay in their winter stock of wood. They seldom begin to repair the houses until the frost sets in, and never finish the outer coating until the cold becomes severe. When they erect a new habitation they fell the wood early in summer, but seldom begin building until nearly the end of August.

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. 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.