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Nature Notes: Beaver

Ed Robinson
October 1, 2018

At a snail’s pace, I belly-crawled down the bank toward the beaver pond. Pushing a fresh aspen branch in front of me, I screened my approach as much as possible. The big male eyed me from the water, clearly uncertain how to respond to this unusual visit. Nearly breathless with anticipation, I felt some of its uncertainty. Never would I have dreamed of being this close to a wild beaver, but I was uncertain whether it would accept the branch. It was an encounter I will not forget.

The place was Utah, deep in the mountains where we were camped for a week of elk hunting. Having filled my tag on opening day, I spent the morning improving our camp and getting organized for dinner while the others were hunting. I then decided to take a walk. I soon came upon a small beaver pond, and as I slowly raised my eyes over the dam I saw a pair of beavers busily wedging aspen branches in the mud for their winter food cache.

beaver sits on snowy ground

photo by Anna39, iStock

The female soon spotted me and dove with a slap of its tail, but the larger male chose to back away to the far side of the pond, so it could gauge the threat. The female resurfaced in a few minutes and swam in circles, murmuring quietly to its mate, probably looking for reassurance. Gradually they seemed to understand I meant them no harm, so I was able to rest there quietly for three hours that first day, watching them work. On the second day, I took the opportunity to offer the aspen branch at close range and was stunned when the male finally swam over and pulled my offering into the water.

It’s unlikely that I will ever have a similar experience with these amazing animals, but the event served to further reinforce the great respect I had for the industrious beaver. It is no exaggeration to state that beavers played a huge role in the exploration and settling of the United States, and that they were nearly eliminated in the process. It is also true that the beaver is one of the most influential animals in nature, with the ability to change landscapes and to improve wildlife habitat in a way that no creature other than man is able to accomplish. Hopefully, you will agree with this assessment as you learn more about them.

There are fossil records going back seven million years to document the amazing longevity of the North American beaver. It is known scientifically as Castor Canadensis, being first catalogued by early explorers in Canada, who learned about the beaver from their Native American guides. The genus name “castor” refers to large scent glands located at the base of the beaver’s tail that produce an oily substance called castoreum, which the beaver uses to waterproof its fur and for scent marking. The animal is almost universally called a beaver, even though there is another, much smaller rodent living in the western Cascade and Olympic Mountain ranges called the mountain beaver (Aplodontia rufa). Since the two are only distant relatives, I will use the shorthand name for the animal we know here in Maine.

Our beavers are the largest rodents in North America, and second in the world to the South American capybara. Adult beavers average around 45 pounds, but very old specimens have been recorded up to 110 pounds. They run up to three feet in length, plus a tail that can exceed 12 inches. The beaver is semiaquatic, capable of walking and working on land, but far more adept in the water, where it has few equals. With large webbed hind feet and a powerful paddleshaped tail, the beaver has been clocked at underwater speeds up to 34 mph. Its ears and nose close when the beaver dives underwater and a special membrane covers its eyes to allow vision. If it chooses, an adult beaver can slow its heart rate to remain submerged up to 15 minutes, quite handy if it is performing underwater chores or swimming from danger. Those four large front teeth, incisors, continue to grow throughout a beaver’s life and are self-sharpening for efficient cutting of wood. When threatened, beavers will make hissing or growling noises, and may lunge forward with those razor-sharp teeth.

In rivers or existing ponds and lakes with sufficient water, beavers will dig dens along the shoreline, generally adding a pile of logs and sticks to protect the entrances. But we are most familiar with beavers and their amazing ability to build lodges and the dams that create their own secure ponds. These dams need to have enough water depth so that even the coldest winter weather cannot freeze the pond to the bottom. If the beaver miscalculates in building the dam, or the water level falls in midwinter for any reason, the beavers may perish.

A beaver lodge begins life as a pile of tree limbs, often using a stump or downed tree as a base. By gradually adding more limbs woven together, further secured with weeds, stones and mud, and leaving an air hole near the top, the beaver creates a home with one or two underwater entrances. Inside will be one or two warm and secure chambers above the water level so the beavers can rest and care for newborns. A family group might contain three or four adults and several juveniles. Most predators are unable to attack the beavers, thanks to hard mud on the roof of the lodge, along with snow and ice in winter. Sometimes large bears manage to break through, killing the beavers or forcing them into the water with no refuge.

The dam is critical for holding back moving water, either a slow-moving slough or a stream with a modest pitch. The beaver uses its mouth and small, clawed front feet to move sticks, rocks and mud into place, making an amazing number of trips in the process. As the water begins to accumulate behind the dam, the beavers will add new material to raise the water level. They are highly attuned to the sound of running water, and any break in the dam will trigger an immediate repair mission. In a test of this trait, scientists placed a tape recorder in a field near a beaver pond, playing the sounds of a stream. Despite the recorder being on dry ground, the beavers covered it with branches and mud.

It is common for beavers to exhaust the food supply around their dam and then move to a new location, sometimes walking miles in a dangerous pilgrimage. If there is sufficient food available, beavers will gradually widen and raise a dam in suitable locations, flooding more and more land upstream, and making it easier to reach new trees. In other sites, they simply build additional dams up- and downstream of the main pond. Many years ago, on a jet over northern Canada, I spotted a large dam far below the plane, guessing the dam’s length at several hundred yards. In 2007, scientists spotted a dam on satellite photographs of Northern Alberta that measured nearly 1,000 yards long, twice as long as the Hoover Dam. Of course, this dam building can bring beavers into conflict with humans, and while the beavers generally lose such battles, they don’t give up easily.

Night is the period of most activity for beavers, providing them safety from some winged predators, such as bald eagles. Their eyesight is rather weak, but they have superb hearing and sense of smell to detect danger. When on land, beavers are slow and ungainly, but they are often forced to emerge to gnaw through the trunks of trees to fell them, hopefully close to or in the water. Around their ponds, beavers excavate channels that allow them to swim into shallow water, to float building materials, and to provide escape routes if they are attacked by coyotes, wolves, lynx, bobcats, mountain lions or bears. While beavers are amazingly skilled at cutting down even large diameter trees with only their large front teeth, it is hazardous work, and I have seen photos of beaver skulls lodged in fallen trees.

Beavers live almost exclusively on a diet of twigs, inner bark, new shoots and leaves of their favored trees. While their digestive systems are evolved to convert all the essential nutrients from only the aspen tree, beavers will also feed upon softwoods like alder, willow, birch and pine or hardwoods like ash, beech, maple and black cherry. In the spring, they will feed upon sedges, water lilies and cattails. They prefer new growth as opposed to mature trees, and in the autumn, beavers will create large piles of branches stuck in the mud around their lodge to guarantee winter food supplies with minimal expenditure of energy.

A female matures at three years and has a short estrous period of a day or less, with mating often done in winter dens between December and April. Unlike most rodents, beavers will remain with a mate for many years, both of them involved in raising a family. Gestation occurs over four months, with two to six kits born. The young beavers may live with their parents up to two years, learning the skills needed to survive on their own, before they are pushed out of the family group to find a new territory. In areas with adequate water and food supplies, beavers have been known to live up to 20 years.

To survive in a harsh climate, the beaver is protected by a heavy pelt covered in long, brown outer hairs and a dense layer of fine inner hair, with thick layers of fat underneath. That dark, lustrous pelt is what drew so many European adventurers and trappers to push deep into the interior of North America. The hides were in great demand in England, used to make hats and other clothes, while the castoreum has long been used in perfumes and food additives. Native Americans had also harvested small numbers of beaver for food and hides, but the white trappers knew no restraint, only the lure of gold. Tens of thousands of beaver pelts were sold each year in the 1600s and 1700s.

The trappers took great risks pushing further and further into an untamed wilderness and suffered all manner of hardships to collect large bundles of beaver pelts for trading or selling to buyers like the Hudson Bay Company. As beaver numbers dropped in the eastern US and Canada, the trappers kept moving farther west until they reached California in the early 1800s. While the trappers led the opening of the west to future settlement, by the 1840s beavers were nearly eliminated from their vast former range. Fortunately, the population has rebounded to an estimated 15 million today, far below the estimated original population of 100 million to 200 million. Beavers are considered fur bearers, and as such, there are legal restrictions and open seasons for harvesting them. With fur coats having fallen out of favor, there is far less trapping of beavers today. The most recent Maine statistics I found showed fewer than 1,000 active trappers of any species in the state, and 12,600 beavers tagged in 2015. Nuisance beavers may only be removed with permission of the Department of Inland Fisheries & Wildlife.

Earlier I mentioned the vital role that beavers play in our natural world. Many scientific studies have documented the long-term impact that beaver dams and ponds can have on flora and fauna around them. The extensive cutting of trees by beavers creates new openings in thick forests, allowing more sunlight to penetrate and triggering regeneration. Beaver dams play a major role in controlling the flow of rivers in arid regions, slowly releasing water over time and allowing streams to maintain more flow during the summer months, sustaining groundwater levels along with fish and other creatures that otherwise would die. The dams collect sediment and sequester pollutants that may break down while resting on the floor of the pond.

Beaver ponds are major habitats for waterfowl, trout and salmon. A study of Wyoming waterways showed that rivers with beaver activity held up to 75 times more ducks and geese than rivers without beavers. The grasses, forbs and shrubs that flourish around beaver ponds are perfect habitat for waterfowl breeding, and for the insects that provide nourishment. Standing dead snags in beaver ponds provide excellent nesting habitat for woodpeckers. The cavities hammered out by the woodpeckers then become homes for wood ducks, flickers and owls. Herons, kingfishers and bitterns work the beaver ponds in their search for small fish and crustaceans. Populations of brook trout, rainbows and salmon also benefit from beaver activity, both because of improved flows of clean water, and the deep standing water behind the dams while the fish are migrating or holding over the winter.

While beavers may cause trouble in settled areas when they block drainage culverts and flood highways, our environment is far better off with a healthy beaver population. The cumulative beneficial impact of beavers upon river systems is such that states like Utah are actively reestablishing beavers in dozens of new streams every year. In the state of Washington, studies have shown that beaver dams in the upper reaches of the state’s major rivers can have a significant impact on holding billions of gallons of spring run-off and flood waters from major rainstorms, reducing the need for hugely expensive concrete dams and all of the ecological damage they cause.

If you have the opportunity to spend some time watching beavers and the wildlife around their homes, I hope you will do so. They are quiet, retiring creatures by nature, but they are very diligent in caring for their dams and lodges, thus the term “busy as a beaver.” Once valued mostly for their luxuriant pelts, there are many other reasons to respect these unique animals. This species has long had an outsized impact upon our country and our environment. Long live the beaver!

If you like Ed Robinson’s writing, check out his two Nature Notes books! Click here for more information.