Nature’s Architects

A beaver nibbling on a plant
Image Source: yrjö jyske

Beavers are some of the greatest construction workers in the mammalian world. They are dam-builders, lumberjacks, engineers, architects, and transport planners all rolled into one. They have, perhaps, a bigger deliberate impact on their surroundings than any other animal besides humans, and they can easily reconfigure their environment to suit their needs.

Beavers are also extremely beneficial to the landscape around them. Their behaviour, as we will soon find out, can reduce the effects of flooding, improve water quality, create microhabitats for many species of plant and animal, and increase biodiversity. It turns out they can be powerful natural allies – a relatively low-cost solution to our many land- and water management problems.

But first, let’s look at a beaver’s body and find out how they are so good at what they do.

Beaver Biology

The beaver is the second-largest rodent in the world (after the capybara), reaching up to 30 kg in weight. There are two species: the North American beaver, which is native to Canada and the USA; and the Eurasian beaver, which lives in Scandinavia, Central and Eastern Europe, and parts of Central Asia. Though both are similar in size and appearance, the Eurasian beaver has a larger, squarer head and a longer, narrower muzzle than its American cousin [1].

Like the capybara, the beaver is amphibious: its webbed hind feet, flat tail, fur-lined waterproof lips, and closeable ears and nostrils are all adaptations to its semi-aquatic lifestyle. It even has transparent eyelids that act as goggles.

The beaver’s tail is particularly important to the animal. Not only does it act like a camel’s hump, storing fat for leaner times, but it also functions as a rudder when swimming, and helps prop the animal up when it is sitting or standing. It even serves as an alarm call: when danger approaches, a beaver slaps its tail loudly on the water to warn its companions. (Contrary to popular belief, however, a beaver’s tail is not used as a builder’s trowel or to pack mud.)

A beaver eating something in the water
Beavers never eat fish, despite what some people believe. In fact, they are strict vegetarians. Their diet consists of aquatic plants and grass, along with the bark and leaves of trees.
Image Source: Jetpendragon

Like other rodents, beavers have massive, ever-growing, self-sharpening front teeth. Human teeth have roots that come to a definite endpoint. For us, the cells that deposit enamel die once our teeth have developed. But rodent teeth are open-rooted – and that’s just as well because beavers use them so frequently as they gnaw through trees to reach the soft layer beneath the outer bark that they are quickly worn down.

The huge incisors are bright orange at the front because they contain extra enamel and iron; this reinforces them and gives them additional strength. The back of the teeth is made of dentine, the softer material normally found inside teeth. As the top incisors grind over the lower ones, the dentine is worn away more quickly and this exposes the blade of enamel at the front, keeping a sharp chisel edge.

Dam Builders

The ideal home for a beaver family is a mud-and-stick constructed lodge in the middle of a lake, where they can feel safe from predators and raise their young. This requires still, deep water. If there is no lake or pond in a beaver family’s territory, they simply create one themselves.

First, they dam up the river in which they live. Using their powerful jaws and iron-coated incisors, they cut through waterside trees to fell them and then use these trunks to create the initial framework of the dam, which is aligned in the direction of the water’s flow. The dam is reinforced with smaller branches, rocks and mud to keep it stable and sealed. The beavers work methodically to prevent leaks, creating a sort of organic reinforced concrete. Fifty tonnes of material may be required to fully stem the flow of water down the river. The largest beaver dam, discovered in 2007 in Canada’s Wood Buffalo National Park, is 850 metres in length – that’s over twice the size of the Hoover Dam and big enough to be seen from space. It is thought that successive generations of beavers have been building it since the 1970s.

A large dam constructed by beavers
The beaver’s extraordinary feats of engineering are mostly achieved instinctively. The urge to stop the flow of water is so deeply ingrained that even a tape recording of a babbling brook will encourage a beaver to start heaping sticks on top of a loudspeaker, despite being nowhere near any actual water.
Image Source: Juliux

As the water level behind the dam rises, it creates a large lake that spills out into the surrounding woodland. The beavers may have exhausted the supply of suitable trees nearby, but that’s no problem now – they build canals radiating out from the lake so that they can fell trees further afield and then simply float them back to the dam.

Next, the beavers begin construction of the lodge, which will provide shelter through the winter. Though it may look like a pile of twigs, branches and mud in the middle of the lake, it is actually a wonderfully crafted home with underwater passageways that all lead to an inside sleeping chamber that is above the water level. Because the roof is completely sealed apart from an air vent at the top, the beavers inside are protected from almost all predators – they’re essentially living in a sturdy castle surrounded by a defensive moat.

Even after the lodge has been completed, there is still work to be done. The dam must be constantly maintained. If there is heavy rain, spillways may need to be enlarged to allow the floodwater to escape before the dam bursts. And when the rain stops, these notoriously industrious rodents will have to build up the dam once more to prevent the level of the lake from falling so low that the lodge is exposed to ground predators.

During the warm summer, beavers labour fastidiously to prepare for the colder months, working up to 12 hours every day to build their accommodation and find plenty of food. But as winter approaches, they start to relax. The phrase ‘busy as a beaver’ is no longer apt, for the family now spend most of their time resting within their snug home. Warm air rising out of the vent helps to melt snow that accumulates on top of the lodge.

Even if the surface of the lake freezes over, the beavers can still find enough vegetation to eat. This is because, back in autumn, the forward-thinking rodents submerged some of the plant material that they collected at the bottom of the lake. These now act as their winter rations, for the near-freezing water has acted like a refrigerator, keeping them relatively fresh and green. Because of the underwater passageways leading out of the lodge, the beavers can move around freely beneath the ice, holding their breath for several minutes at a time as they gather food from their winter larder to take back into their home.

A beaver walking across the surface of a frozen lake
Beavers don’t hibernate – their food stores keep them going throughout the winter months.
Image Source: Jan Erik Engan from Pixabay

However, the lodge isn’t completely safe. It is hot and humid during the summer, smelly and pitch-black all year round, and infested with all manner of parasites and other invertebrates. There are also occasional aquatic trespassers, such as water snakes or otters. Otters, though cute in our eyes, can enter a lodge and kill young beavers if they get the chance.

Beaver Fever

Beavers were once hunted extensively by humans, both for their meat and their fur. They were allowed to be eaten by Catholics even on days when meat was banned, such as Lent, owing to the fact that they were conveniently classified by the Catholic Church as fish – rather like barnacle geese. Their fur, meanwhile, was heavily sought because it is very dense (about 37,000 hairs per square centimetre) and very, very soft. During the 18th and 19th centuries, beaver fur was called the ‘soft gold’ of the New World due to the amount of money that European markets would pay for it, to make fashionable hats, coats and belts. In fact, the very first multi-millionaire of the United States, John Jacob Astor, owed much of his wealth and success to the beaver pelt trade.

A beaver pelt
The beaver’s skin became an official unit of currency in the New World. In fact, the demand for beaver pelts drove the expansion of settlement into the western wilds of America.
Image Source: Leslie Seaton

Beavers were also hunted for a musky, oily anal secretion called castoreum, which they use to mark their territory and waterproof their fur. Castoreum is rich in a compound called salicylic acid, which is derived from the tree bark that beavers eat, particularly willow trees. Willow bark extract was the original source of aspirin, so castoreum was much sought-after by ancient and medieval physicians for its purported medicinal properties. In Europe, it was believed that castoreum was produced in the beaver’s testicles and was said to cure almost everything, including headaches, fever, toothache, flatulence, and women’s gynaecological afflictions. The ancient Romans even burnt this oily brown secretion in lamps to supposedly induce abortions.

Many bestiaries of old claimed that the beaver knew the worth of its own testicles. It was said that when pursued by huntsmen and unable to escape, a beaver would bare its enormous teeth and set to work castrating itself, surrendering its testicles to its attackers and thereby saving its life. Some even claimed that if a castrated beaver was chased in future, it would run to an elevated spot and lift up its legs to show the hunter that the object of his pursuit was no longer there.

In this 13th century manuscript depicting a beaver hunt, the (somewhat odd-looking) beaver can be seen castrating itself.

The beaver’s alleged act of self-castration was, of course, complete nonsense. For a start, unlike most mammals, beaver testicles don’t bob around between the legs – they’re actually hidden inside the body. While this may have validated the idea at the time that the animal had been castrated in some way, this anatomical design does, of course, mean that a beaver couldn’t chew off his own testicles even if he wanted to. (Castoreum, as it later turned out, wasn’t produced by the testicles at all but in a pair of organs near the anus, which connect to internal castor sacs. Both males and females produce it to mark their territories.)

This myth may have originated from a misreading of ancient Egyptian hieroglyphs, which depicted a beaver chewing off his own testicles as a representation of the punishment for adultery among humans in their society. This depiction was later picked up by Aesop, who publicised the beaver’s tale in his popular fables. The belief was, in turn, absorbed into early Greek and Roman scientific literature and, spread by early naturalists such as Pliny the Elder, was uncritically presented as fact for hundreds of years [2].

‘When pursued, the beaver runs for some distance, but when he sees he cannot escape, he will bite off his own testicles and throw them to the hunter, and thus escape death.’


The days of using castoreum to cure almost every ailment under the sun have long passed, but, perhaps surprisingly, we still sometimes use it. It pops up now and again as an essential ingredient in a number of classic perfumes to suggest the smell of leather. And, for over 80 years, it has even been used to add vanilla flavour to a variety of desserts, including ice cream, gelatins and fruit beverages. Due to the difficulty and expense in obtaining castoreum (the beaver needs to be anaesthetised and then have its anal glands ‘milked’), it’s not used for this purpose very often today – and when it is, manufacturers tend to simply list it as a ‘natural vanilla flavour’. I heavily suspect this might be because if it was listed as being the anal secretions of a beaver, it might put some consumers off.

Eco Engineers

We may not use (aka exploit) beavers like we once did, but that doesn’t mean we don’t still need them. They have a huge, and usually positive, impact on their surroundings. Increasingly extreme rainfall events in recent years have led to frequent flooding in many towns and cities, but it is believed that beaver dams can slow down the flow of water and therefore reduce the scale of these floods. In fact, beaver dams appear to be more effective than human-made concrete dams in trapping water and slowly releasing it. In addition, new research has confirmed the long-held suspicion that beaver activity can help stop forest fires – as they dam watercourses, they create areas of wet forest that are spared from the flames.

By buffering the flow of water and nutrients through the landscape, beaver dams also help keep aquatic systems healthy. They filter water as it passes through them, purifying it in the process and improving its quality. The cleaner water habitats that this creates increases the populations of insects, fish, amphibians and aquatic plants. In other words, beavers are great for boosting biodiversity.

A dam across a river constructed by beavers
Beavers and their activities can transform their environment into carbon-sequestering wetlands where a wider diversity of bird and aquatic life can flourish.
Image Source: Gillfoto

It’s clear that beavers can influence entire ecosystems on a major scale. And the potential of these huge rodents as valuable habitat restorers is recognised by scientists on both sides of the Atlantic. To accompany this article, our next British Wildlife of the Week will be looking at the long-overdue reintroduction of beavers into British rivers, and how their activities can help our mismanaged landscape and waterways.

[1] What seems to be a third member of the family, the mountain beaver, is not, despite its name, a true beaver. The only living member of its genus, the mountain beaver is considered a ‘living fossil’ and is the most primitive rodent alive today. It has difficulty regulating its body temperature and is unable to conserve either body moisture or fat. It is restricted to moist temperate rainforests along the Pacific coast of North America. Interestingly, in the fur of the mountain beaver lives the world’s largest living species of flea.

[2] The fact that the words castoreum and castration are very similar no doubt helped perpetuate the myth. However, the Latin word for beaver, castor (and by extension, castoreum), doesn’t share the same root as castration at all.


  • Jason Woodcock

    With a background in conservation and animal behaviour studies, Jason's passion lies in the natural world. He adores all things nature and enjoys nothing more than spotting rare and interesting species out in the wild. He has also worked in a zoo and knows plenty about keeping the animals inside our homes healthy and happy, too.

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