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MIT News: Feature story

I’m a beaver.
You’re a beaver.
We are beavers all.

Amazing facts about MIT’s ingenious mascot.

For more than 20 million years, beavers have been, well, busy. They’ve been felling trees for that long, and building dams and lodges for at least the last few million years, earning a well-deserved reputation for industriousness and ingenuity. It seemed only fitting, then, that MIT saw fit to claim the beaver as its mascot in 1914. By 1921, The Tech reported that gray beaver hats had become “the distinguishing mark of an Institute man” at college gatherings. The toothy, mainly nocturnal rodent has appeared on every rendition of the MIT class ring—now lovingly called the brass rat—since it was introduced in 1929. 

Read on to learn more about Castor canadensis, the remarkable four-legged engineers.

Family life

The North American beaver is the largest rodent in the Northern Hemisphere, typically weighing in at 35 to 65 pounds. (Only the South American capybara weighs more.) They make their homes in ponds, rivers, streams, and wetlands throughout most of North America. 

long scene of beavers in a natural environment swimming, chewing wood and grooming
SUZI KEMP

They are one of the few species in the world that typically mate for life. Their offspring, known as kits, can swim within days of birth, but their childhoods are among the longest in the animal world. They generally live for two years with their parents, which both take part in raising them. It takes that long for the parents and older siblings to show them, by example, how to build dams and lodges, how to plan and dig channels, and how to select food, harvest it, and store it for the winter. It’s kind of like going to engineering school. Beavers then move on to form their own families, often building their own colonies. They typically live to age 10 or 12 in the wild. 

A well-planned diet

Beavers are vegetarians but with a twist. They favor the inner bark of certain tree species, including willow, poplar, aspen, birch, and maple, feasting on the cambium, the soft, sap-laden layer immediately under the outer bark. Conifers, however, are not considered a delicacy. Beavers eat them only rarely, and tend to fell them mainly for dam building and to encourage growth of things they’d rather eat. In summer they consume readily available grasses, leaves, herbs, fruit, and aquatic plants. To prepare for winter in cold climates, they create an underwater cache of sticks and logs they’ve gnawed from trees they’ve felled. First they assemble a floating raft of not-so-delicious branches above a deep part of their pond; then they stash their preferred branches beneath them. The pile absorbs water and sinks to the bottom, with the less-favored branches often freezing in the ice at the surface and acting as a protective covering that secures the more-desirable lower branches, which remain accessible below the ice. The cold water preserves the nutritional value of the branches. 

While humans can’t digest cellulose, beavers have a small sac between the large and small intestines containing microorganisms that ferment this material, helping them digest up to 30% of it.  

Achieving the perfect pelt

Forget mink, ermine, and sable. Of all fur-bearing animals, beavers have the coat that is rated the warmest. So it’s no surprise that European demand for hats made of warm, water-resistant, and durable beaver felt led to lucrative trapping and fur-trading ventures in North America. In the 17th and 18th centuries, as many as 200,000 North American beaver pelts were exported annually to Europe. (Fierce competition to monopolize the fur trade led to a series of so-called Beaver Wars between 1628 and the Treaty of Montreal in 1701: the Iroquois Confederation, backed by the Dutch and British, battled the Huron Confederation, backed by France.) These enterprises gave rise to many European settlements and trading centers in North America—and nearly wiped out the continent’s beaver population. 

On January 17, 1914, MIT President Richard Maclaurin accepted the Technology Club of New York’s proposal that the beaver—nature’s engineer—serve as MIT’s mascot. In 1977, TIM the beaver first showed up on campus to celebrate the 50th reunion of the Class of 1927.

Beavers evolved to have such cozy pelts for good reason: they don’t hibernate, so they need to stay warm and dry as they venture from their lodges into frigid waters to access their winter food cache. Maintaining such a pelt requires diligent grooming to remove debris and insects and prevent matting. A beaver rakes the fur of its face, head, and belly with its fingernails, and gets at some difficult-to-reach spots with its hind feet. The innermost toenails on those feet open and close over the toe, like a bird’s beak, and function as coarse-toothed combs. The second hind toe has a serrated horny growth between the true nail and the toe, which serves as a fine-toothed comb. Beavers waterproof their skin and coats with oil produced by a pair of glands located beneath their tails and use these custom toenail combs to work it into their fur.

Beavers also engage in social grooming, usually inside their lodge. A beaver’s body is so round and its limbs so short that it must rely on family members to groom its back. Mutual grooming also reinforces family bonds.

Getting around

In the animal kingdom, beavers, humans, bears, and the occasional ape are among the few mammals that can hold their bodies upright and walk for any distance. Like humans, beavers can carry things at the same time, and often walk up from a pond onto their dam holding a rock, branch, or clump of muddy clay with their front paws. Thanks to an opposing digit (the equivalent of the human pinky finger), they can grasp items with their front paws, whose long claws work well for digging. They also use their “hands” to carry their kits and to hold and rotate food as they gnaw it, similar to how humans eat corn on the cob. 

Beavers are excellent swimmers thanks to strong rear legs with wide, webbed feet. They also benefit from built-in swim goggles—an extra set of transparent eyelids called nictitating membranes that let them see well underwater and protect their eyes from any dangerous microorganisms in ponds and lakes. Beavers’ ears and nostrils can seal out water, and their large lungs let them stay underwater as long as 15 minutes. 

A tail of many talents

The shape of a beaver’s tail runs in the family: some are short and wide, others long and narrow. The tail offers an outstanding example of multifunctional engineering, allowing beavers to maintain stability, steer, communicate, store food, and more.

When a beaver walks upright, carrying a heavy load like a rock or tree limb, its tail slides behind on the ground, helping the animal maintain its balance. In the water, the tail serves as a rudder and helps a swimming beaver maneuver past obstructions.

Since sound travels well underwater, the beaver can signal an alarm to the colony by beating its tail on the water surface. And research suggests that beavers’ furless tails help regulate body temperature: keeping them in cool water on warm days allows the animals to avoid overheating. 

During the fall, the beaver stores a large amount of fat in its tail that it will use to supplement its winter rations. Woodsmen and frontiersmen living off the lean meat of wild game treasured cooked beaver tail as a source of fat in their own diet.

Nature’s engineers

In addition to having multifunctional tails, beavers come equipped with constantly replenished and self-sharpening cutting tools. Their trademark front incisors are rusty orange thanks to an iron-rich enamel coating that helps them withstand mechanical stress and makes them resistant to acid. And these teeth never stop growing. Beavers must regularly gnaw bark to keep them sharp and to avoid starvation. If they don’t keep their incisors short enough to close their mouths, they can’t grind food with their back molars. 

Beavers famously put these built-in tools to good use building dams to turn streams into ponds and swampy wetlands conducive to their way of life. (Like humans, they are on a very short list of creatures that alter their environment to suit their purposes.) The average beaver dam has a height differential slightly under five feet, but at least one dam has been found with a 16-foot difference between the water height behind the dam and in front of it. 

beaver in a box attached to a deployed parachute

After humans and dogs, beavers have the most parachute jumps on record. In 1948, 76 beavers parachuted into a remote area of Idaho as part of a relocation program. Using surplus World War II parachutes, they descended in boxes that automatically opened upon landing.

When beavers dam rivers and streams, they tend to choose relatively slow-flowing waterways and build straight across. Like human engineers, they build curved dams on more swiftly flowing streams to better withstand the pressure of the rushing water. Beavers place about 50% to 70% of the wood in their dams and let flowing water help with the rest: the remaining wood (mostly smaller sticks) gets trapped when it floats downstream. Although less visible, river rocks the animals carry one at a time typically make up a major portion of the dam. Beavers routinely inspect and maintain their dams, so they rarely break.

The world’s longest known beaver dam is visible from space and was discovered by a researcher viewing satellite images of a remote part of Wood Buffalo National Park in Alberta, Canada. Measuring 2,788 feet, it’s more than twice as long as the Hoover Dam. While human-built dams of that scale are constructed with massive, pollution-­spewing earth-moving equipment, the Wood Buffalo colony used just their teeth and powerful limbs—and the only pollution involved was emitted by the occasional “gassy” beaver. (Incidentally, while beavers do pass gas, they are incapable of burping.)

Beavers—which cut an average of 216 trees a year and can topple a young willow in six minutes—go to all this dam-building trouble to slow the flow of water so they can create ponds in which to situate their lodges. The pond must be deep enough to keep water from freezing all the way to the bottom, allowing them to swim to reach food sources—and get in and out of their lodges—even in winter. 

To construct a lodge in the middle of their pond, beavers amass sticks, rocks, and mud to create a platform about 10 centimeters above the water level, topping that with a mound of sticks, bark, and more mud. Then they dig tunnels from the mud below into the pile of sticks, and gnaw out a large, dry room above the waterline to create areas for feeding and resting. Often, they’ll have a lower room for eating, topped by a drier room for hanging out and sleeping. 

National Geographic was on hand to document beavers constructing both a dam and a lodge.

Sometimes beavers dig into the bank of a waterway to create a den, which can involve piling sticks on top and gnawing out a space from the inside. But siting a lodge so it’s surrounded by a moat of deeper water helps fend off large predators like bears, which have difficulty breaking in if their back feet don’t have purchase on solid ground. Whether in a bank or mid-pond, lodges are accessible only from underwater tunnels, which offers protection from land-based predators. Typically, they have at least two underwater entrances to ensure an escape route should aquatic predators like otters invade. 

Before winter sets in, beavers insulate their lodge walls—which can be a foot or two thick—by using their front paws to coat them with mud, which freezes into a hard, plaster-like shell. A hole at the top of the domed structure provides ventilation.  

Beavers appear to be able to make the connection between flooding inside their lodge and the height of water behind their dam. If the water gets too high, they will partially dismantle their dam to let water out and lower the level in the pond it contains. 

Beavers also dig canals and channels, which can be several hundred meters long. Since most of their predators are at a disadvantage in water, they use canals to travel safely to faraway food sources and logging sites as well as to float logs back to their pond for eating and building. 

As efficient creators and stewards of wetlands, beavers provide a hospitable ecosystem for dozens of other creatures, from insects, frogs, and turtles to owls, otters, great blue herons, and even moose and deer. What’s more, by harvesting undergrowth for their dams and creating ponds and bogs that raise the moisture content of the soil, beavers lessen the likelihood that forest fires will spread. As forest fires devastated Oregon in 2021, beaver wetlands remained green and lush, acting as natural firebreaks. On aerial images of the charred landscape, the beaver’s habitat stands out, a wide and verdant ribbon running through the blackened trees.

While not all property owners who live near beaver habitats appreciate the animals’ tree removal services, the pro-­beaver movement seems to be getting more organized. In November 2023, some 300 beaver restoration advocates from North America and Europe gathered in the Beaver State (Oregon) for the annual State of the Beaver conference. “Seventy-five percent of the artificial wetland restoration projects done in America over the past 30 years have failed,” conference cofounder Stanley Petrowski told the Daily Yonder. “But when beavers do it, they do it perfectly.” 

BeaverCon, held near Baltimore in June of 2022, and the Midwest Beaver Summit, held in Chicago and online in September 2023, attracted similar crowds of humans interested in promoting beaver welfare.

It is, in fact, possible to find ways to allow beavers to continue creating their watery habitats in ways that minimize damage to human infrastructure. For example, devices such as the Beaver Deceiver can be installed to prevent beavers from damming culverts, which often leads to flooding of roads. Skip Lisle, founder of Beaver Deceivers International of Grafton, Vermont, first developed the device in the 1990s to beaver-proof the Penobscot Nation’s 130 miles of roads in Maine. “In all likelihood, they are the first large landowner to completely beaver-proof their property nonlethally,” he says.

Living organic chemical factory

At the base of their tail, all beavers have two castor sacs that store castoreum, a complex, granular substance with a strong and long-lasting musky smell. It is made up of at least 24 different compounds, primarily derived from the barks of the various trees in the beaver’s diet. Beavers deposit castoreum atop foot-high mounds of mud, sticks, and grass to mark the edges of their territory. 

Humans have long valued castoreum. About 400 BCE, Hippocrates, a chronicler of natural cures, wrote of its wonderful medical properties. Around 77 CE, the Roman naturalist Pliny listed castoreum as a cure for headaches, constipation, and epilepsy. In the Middle Ages the list of maladies castoreum was said to cure expanded to include dysentery, worms, fleas, pleurisy, gout, rheumatism, insomnia, hysteria, memory loss, and liver problems. 

old black and white photo of William Miller in a canoe
Author William Miller ’51, SM ’52, reports that his foot once crashed through a beaver dam while he was dragging his canoe over it to get to the next lake in Jasper National Park in Alberta, Canada. About 100 feet away, a watching beaver immediately began to slap its tail on the pond surface. Having just unleashed a string of curses directed at the beavers, Bill assumed that the beaver was cursing at him. But he now suspects it was sending a warning signal to the other beavers—or possibly urging them to come quickly to repair the damage caused by the trespassing human oaf.
COURTESY OF WILLIAM MILLER

As it turns out, quite a few of the tree barks that beavers prefer contain compounds with known medicinal benefits. Phenols, for example, are often anti-­inflammatory and antiseptic and can have antiviral properties. They include salicylic acid (a precursor to aspirin), which can be found in the bark of willow, poplar, and alder trees—all beaver favorites. The beaver’s system functions as a natural pharmacy, extracting these compounds (among others) and secreting them in the form of castoreum. 

Humans have also used castoreum for several nonmedical applications, such as in high-end “leather note” perfumes including Shalimar, Givenchy III, and Chanel’s Antaeus. It is an ingredient in some bourbons and vodkas and has been used in Sweden to flavor “Bäverhojt” (literally, beaver shout) schnapps.

Today, most castoreum is harvested in a sterile environment by anesthetizing beavers and expressing the castor sacs near their tails. As a food additive, castoreum extract is “generally recognized as safe,” according to the FDA. But at close to $100 per pound, it’s used sparingly. The total annual US consumption of dried castoreum is around 300 pounds.

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