A beaver pauses on top of its dam. (Courtesy Photo / Chuck Caldwell)

A beaver pauses on top of its dam. (Courtesy Photo / Chuck Caldwell)

On the Trails: All about beavers

Leave it to ‘em.

Beavers (Castor canadensis) are family-oriented. They live in monogamous pairs (unusual for mammals) with their offspring. There may be several kits of the year plus some one- or two-year olds. Mating occurs in winter, gestation lasts about three months, and kits are born furry and active. They may nurse mother’s milk for a month or so but start eating solid food at an early age. Both parents and the older offspring help take care of the kits, who soon start swimming with the others to find food and observe the repair of dams. Offspring stay with their parents for two or three years before dispersing to find a mate and start their own family. Some have been known to disperse for many miles, including in salt water.

Each family is territorial, claiming a space and defending it against other beavers. Intruding beavers may be treated aggressively, but scent mounds at the water’s edge are used to announce ownership and warn off strangers. The mounds are built of vegetative debris and mud, anointed with urine that is scented by two kinds of glands that secrete oily stuff into the urine just before it leaves the body. Every beaver has its own combination of scents, permitting identification of individuals. Scents on the mound may be renewed frequently, especially if strange beavers are known to be wandering nearby.

Each family typically has one lodge, although sometimes additional simple burrows are used (for example, the male of a pair may move out, temporarily, while the young are being born). Lodges are constructed out of a pile of sticks in the middle of a lake or pond or modified bank-burrows with added sticks on top and near the entrance. A lodge commonly has an underwater entrance leading up to a platform used for drying off and then up to a living chamber, often floored with soft, dry vegetation. The sides of a lodge are usually plastered with mud, inside and out, and there’s a vent at the top (where hoar frost may develop in winter). Lodges in northern regions are quite well insulated, capable of keeping the inside temperatures near freezing even when outside temperatures plunge to minus 30 or 40 degrees F.

A beaver hauls a load of ferns and lupine. (Courtesy Photo / Jos Bakker)

A beaver hauls a load of ferns and lupine. (Courtesy Photo / Jos Bakker)

Dams are built to create ponds deep enough that they don’t freeze to the bottom. They are built of sticks, mud and vegetation. All members of the family work on building and on repairing breaches in the dam. In flat country, dams can be many meters long; in steep canyons, they can be over five meters high. In addition, sometimes beavers dig canals from the home pond out into the surrounding wooded area; this facilitates dragging branches from the woods back to the pond.

In regions where ponds are ice-covered in winter, beavers make caches of branches for winter food. Caches may poke up above the ice but the animals have access to the branches under the ice, and they seldom do much wood-cutting in winter. Although they can use any kind of tree or shrub, willows and aspens or cottonwoods are favorites. They eat the twigs and inner bark, with the aid of special bacteria that break down cellulose (in wood). In summer, however, they depend a lot on softer, herbaceous vegetation.

Beavers are the largest rodent in North America (and second largest in the world). They commonly weigh up to 70 pounds but occasionally exceed a hundred pounds. They’re heaviest in fall, after putting on fat for the winter. The muscular body is capable of hauling large branches for many meters.

Beavers have a very specialized way of life, maintained by behavioral adaptations (building dams, etc.) with a variety of physical adaptations to suit. They can stay underwater for fifteen minutes, if they have to, swimming more than half a kilometer. Their lungs are very efficient, capable of exchanging 75% of their capacity (three or four times more than humans) with each breath. Oxygen storage capacity is not outstanding but they have a high tolerance for carbon dioxide. The mammalian diving reflex works well; when submerged, the heart rate slows and most blood is shunted to brain and heart.

The big incisor teeth grow constantly; chewing wears down the back side of the teeth but leaves a sharp edge where the iron-hardened (and orange colored) front side of the teeth endures. When biting into wood, they commonly anchor the top teeth and actively chew with the lower jaw, which is moved by a powerful muscle in the muzzle. They can close their lips behind the front teeth, keeping wood chips out of the throat. The jaw joint is high on the skull, allowing the hard-ridged molars to meet in parallel for grinding vegetation. Eyes and ears are also high on the skull, so a swimming beaver can sense the upper world; ears and nostrils can be closed while underwater, and a protective nictitating membrane slides over the eyes.

Toes on the big hind feet are webbed, giving good thrust for swimming. The front feet are quite dexterous, the fingers adept at handling small items; they also carry wads of mud and vegetation while a beaver is swimming or walking. The wide tail is a multi-purpose appendage: a rudder when swimming, a brace when standing to chew a tree trunk, a stabilizer when walking on hind feet, sometimes a cushion for sitting on cold or lumpy ground, and the well-known tool for slapping the water to give the alarm signal.

This beaver was working during the daytime, but usually beavers work at night or twilight. (Courtesy Photo / Chuck Caldwell)

This beaver was working during the daytime, but usually beavers work at night or twilight. (Courtesy Photo / Chuck Caldwell)

Much of a beaver’s diet consists of woody material that is hard to digest. Beavers (and some other herbivores) have a special sac (a caecum) at the junction of the small and large intestines that houses symbiotic bacteria that break down cellulose. Partially digested food enters the sac, is further digested by the bacteria, and is eventually excreted as a soft pellet that beavers re-ingest, getting the benefit of the break-down activity of the bacteria. Regular pellets of material that did not go into the caecum are excreted as firm lumps of mostly sawdust.

The double coat of fur has long guard hairs covering softer, fluffier fur; the underfur is good insulation, trapping air and keeping water from the skin. The outer fur is waterproofed by grooming the fur with oils from anal glands; toes on the hind foot are modified to improve combing the fur while spreading the oils. That luxurious fur led to rapacious trapping for at least two centuries, resulting in near-extinction of beavers in North America. Sometime in the 1900s, better sense began to prevail and beavers have been re-introduced to many areas in order to reap the many benefits of their activity. Of course, such activities also conflict with some human activities, requiring some compromises. But that’s another long story…

• Mary F. Willson is a retired professor of ecology. “On the Trails” appears every Wednesday in the Juneau Empire.

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