The beaver is the second largest rodent after the South American capybara (Hydrochoerus hydrochaeris). Beavers belong to the family Castoridae in the suborder Sciuromorpha of the order Rodentia. They are more closely related to squirrels and marmots than to mouselike rodents (Muridae). Beavers split from their closest living relatives 90–100 million years ago.¹ For expediency, clever politicians have maneuvered the beaver into strange taxonomic neighborhoods. In 1760, the College of Physicians and the Faculty of Divinity in Paris permitted beaver meat on fasting days because the beaver’s scaly tail classified it as a fish and not a mammal! By...

The weight of an adult North American beaver ranges between 40 and 50 lb (~18–23 kg). Recorded beaver weights are 96 lb (43.6 kg) in 1960 in Missouri¹ and about 110 lb (50 kg) in 1921 at the Iron River in Wisconsin.² The body including the tail reaches about 48 inches (1.2 m) in length. The tail itself is about 16–17 inches (~40 cm) long, about 6–7 inches (16 cm) wide, and ¾ inch (1.9 cm) thick. In pairs of Eurasian beavers, the female is almost always larger than the male.³

Most distinctive for the beaver, the...

The beaver’s body and how it functions can be understood as a compromise of life on land and life in the water. The basic mammalian design has evolved into a superb amphibious, semiaquatic animal. Most peculiarly, the tail is dorsoventrally flattened. Although helpful in diving, aquatic life does not mandate this; the muskrat, with a laterally compressed tail, occupies the same habitat and even forages inside beaver lodges.

The body of the beaver is drop-shaped. Other aquatic animals, such as fish, penguins, seals, or whales, share this result of convergent evolution. In addition to having valvular ears and nose, and...

Beavers work very hard. They modify their habitat extensively. How much energy does their proverbial and incessant activity require, and how do they meet these requirements? Some of the earlier measurements and estimates of food consumption by beaver were done between 1938 and 1962.^1-6 Beavers’ choices of plant species to eat are covered in chapter 9, on food selection.

Daily consumption of bark, twigs, and tree leaves ranged from 1.5 to 2.2 lb (0.7–1.0 kg) for one beaver per day⁶ to 4.5 lb (2 kg).^2-4 It should be noted that in addition to bark, beavers eat much herbaceous and...

The basic social unit of beaver society is the family. It consists of the parents, the young of the year, and yearlings. Two-year-olds may or may not be present. They usually leave or are expelled when or before a new litter is born. On average, there are 2 kits of the year and 2 yearlings, so that a typical family on a reasonably good site numbers about 6 members, provided there is no trapping or other disturbance to the colony. Family size typically ranges from a kitless pair (or even a temporarily single adult) to about 10 members. Of 46...

To this day, beavers keep dredging up mud from the bottoms of their ponds, and not only for their dams and lodges. Like most mammals, beavers frequently communicate with one another by chemical signals. The most conspicuous sign of scent marking is the scent mound (Fig. 6.1). From pond sediment, the beaver builds a mud pile on which it places its territorial marks. This is unique among mammals. How does the beaver construct a scent mound? First the beaver dives to the bottom of the pond, dredges up a batch of mud with its hands, and holds it against its...

Beavers build dams to impound water along streams. They may create one pond or several. We found as many as 18 consecutive dams in one colony, each containing a pond with a different water level. Counting tiny dams across branches of parted streams, there may even be around 40 dams on one site. On the other hand, some bank lodges along large, unchanging streams or on banks of lakes are used for a long time without any dam building.

The impoundment keeps the lodge entrances under water and permits the floating of logs and branches, diving to safety, and travel...

When a beaver is active and when it rests depends largely on three factors: exposure to the natural light cycle, air temperature, and season. Beavers stay in their lodge during the daytime, from about 0800 to 2000 hours in the summer in northern latitudes. During the first part of the night they feed, and during the second half they construct dams and lodges.

Most surprising is the beaver’s ability to decouple its daily activity rhythm from the natural 24-hour light cycle. Western colonies in Alberta, Canada,¹ as well as eastern ones in Quebec,² have shown a free-running cycle of activity...

Beavers fell trees. They shock us by cutting down a specimen tree in our backyard, or make national news by destroying cherry trees at the Tidal Basin in Washington, D.C. At the very least, many of us have seen stumps left by beavers or pruned willows along a stream. Indeed, many professional studies of food habits have focused on stumps, severed tree trunks or limbs on the ground or in the water, and food caches, which are piles of branches from various trees that beavers conveniently place near their lodge for the winter. This woody material is literally hard evidence...

Beavers form permanent breeding pairs and are socially, but not necessarily genetically, monogamous. In North American beavers, five of nine litters showed signs of extra-pair matings. This was demonstrated by “genetic fingerprinting” using microsatellite analyses. Females mating with extra males may avoid inbreeding depression and secure additional resources, as young that have parents outside the colony often travel to these neighboring sites.¹ In the Eurasian beaver, females come into estrus from late December on. The peak occurs in mid-January. Warm-weather spells in winter can accelerate estrus. Mating occurs in the water, with the male approaching the floating female from the...

How many beavers can live in a given area? Because beavers hold territories that contain essential food and water resources, their population density in a given area is limited. Water is indispensable to beavers; therefore, the density of beavers is traditionally calculated as the number of colonies along a unit length of stream and the numbers of beavers in each colony. Suitable habitats can accommodate up to 1.2 colonies/km of stream (1.8/mile).^1–5 Nevertheless, others calculate the population density as number of colonies per area.

In Canada, beaver densities of untrapped populations in the western mixed forests of the national...

Young beavers have to leave their family at one point. Beavers as a relatively long-lived species produce 3–4 newborns every year. Were the young to stay with their parents and siblings, the colony would grow huge in a few years and soon outstrip its food resources. More importantly, grown-up offspring must find mates to start reproducing themselves. Mating with close relatives would often result in disastrous genetic defects, a phenomenon called inbreeding depression. For these reasons, beavers have to disperse from their native family.

Once beavers set out to disperse, they lose all benefits their home affords, including food...

Before Europeans arrived in North America, beavers used to range across the entire United States (except Florida and a few desert areas) and a large part of Canada (except the Arctic tundra). Today they dwell from the subarctic to the Rio Grande, which separates the United States from Mexico. Beavers colonize elevations as high as the timberline in Colorado.¹ In the eastern United States we found beavers in extreme places. At the base of Mt. Marcy (to Indians Tahawus, the “Cloud Splitter”), the highest Adirondack mountain, lies Lake Tear of the Clouds, which spawns the mighty Hudson River. The lake...

Without mortality at young stages, beaver populations would grow rapidly to enormous numbers, as pointed out in chapter 11. The total annual mortality of beavers of all age classes was 30% in one population in Newfoundland,¹ and 27% in another study in Newfoundland.² During the first 6 months of life, as many as 52% may perish.² In the first 2–3 years of life, mortality usually is highest, and between the ages of 5 and 9 years very few beavers die in unexploited populations.³ In most beaver populations the total mortality consists of two components: natural deaths from predators, accidents,...

Two human diseases linked to endoparasites of beavers have repeatedly made headlines: tularemia and Giardiasis. In addition to such endoparasites, beavers may be host to a number of helminth (worm) species. Tuberculosis epidemics have also been described for C. fiber in the wild and C. canadensis in captivity. Ectoparasites, on the other hand, include beetles and mites. Finally, tumors occur occasionally (Fig. 15.1). Thyroid cancer in captive beavers was discussed in chapter 9, on food selection.

Tularemia is caused by the bacterium Pasteurella tularensis, named after Tulare County in California. The bacteria are found in blood, all organs, body fluids...

Beavers are “ecosystem engineers.” They profoundly affect their ecosystem by damming up water and removing trees. Needless to say, the stored water and raised water table can be important for many plants and animals, especially during droughts. Furthermore, the water flow pattern is altered, reducing erosion. Larger areas are wetted, and there is more sediment accumulation. The beaver dam creates a “hydraulic head,” which raises the water level at the creek bank and moves water around the dam both above ground and as groundwater flow.¹ The water downstream of the dam is only slightly warmer than the overall running stream...

No other wild animal has shaped history as much as the beaver. It has lured fur trappers and traders more and more deeply into the northern wilderness for two centuries, from the mid-1600s to the late 1850s. The fur trade painted the map of North America’s interior and paved the way for European settlement, the founding of empires, and the destruction of indigenous cultures. After trapping out the tributaries of the St. Lawrence River, Indian and half-caste fur trappers and traders pushed westward and northward, eventually exploiting the Rocky Mountains and reaching the Pacific Ocean. The beaver trade’s best-known and...

Beavers of both species have been transplanted many times and in many parts of the world, both to reintroduce them where they had become extinct, and to introduce them as “exotics” to new areas. At the present time, especially the Eurasian beaver is rapidly expanding both its populations and its overall range by artificial and natural recolonization.

The purpose of most transplants has been to replenish depleted stocks or replace extirpated populations of the same species in parts of its former range. In northeastern North America beaver populations had been critically reduced or even extirpated in large areas at the...

Beavers can modify the landscape in dramatic ways that are often unwelcome to humans. “Nuisance beavers” attract attention by the media. In the spring of 1999, beavers appeared at the tidal basin in the nation’s capital and cut down several ornamental cherry trees. They had to be trapped alive and transferred to an undisclosed destination. In the other North American capital, Ottawa, beavers have felled cottonwood trees along the scenic Ottawa River Parkway, used for recreational walking.¹

One day in the summer of 1986 a local conservation officer in Delaware County, New York, received a call from then U.S. Senator...

The beaver, untiring dam builder, can and should be our important ally in wetland restoration. For millennia, beavers have created and maintained wetlands that stored water and kept the water table high. Their leaky dams evened out the flow of streams. For instance, a side canyon (Butler Wash) of the San Juan River in southern Utah is said to never flood, thanks to beaver dams.¹ The two branches of the Satsop River in Washington State differ in their water flow after rainstorms and snow thaws: The East Fork runs through more flat ground and less elevation gradient. It has more...

Many urban people ask the question, Should we humans manage beaver populations? The short answer is that we have no choice. Where beavers are rare, reintroductions and the conservation of beavers and the restoration of their habitat are called for. Burgeoning beaver populations, on the other hand, require proactive management to prevent adverse consequences such as flooding and damage to trees. In either case, conservation of functioning wetlands constitutes the main task. All too often, beavers cause some damage to unprepared property owners, and immediate action is called for. Instead of having to put out fires, we should plan ahead...