Reputation
Beavers are industrious beasts. They gnaw through trees, but eat fish. Their have cool tails that they use to propel them through the water.
Reality
Beavers are busy indeed, but not eating fish: they are strict vegetarians. Their tails are good for many things, but not propulsion. Beavers do not harm fish. You’re thinking of an otter ripping into a struggling trout.
It is understandable perhaps that many people are under the illusion that beavers eat fish. They do, after all, spend a lot of time in rivers. But give it a moment’s thought, particularly with those massive incisors in mind, and it’s obvious that beavers are herbivores, feeding off woody plants like poplar, aspen, willow and birch.
The incisors have a structure that means they are self-sharpening, the outer surface coated in enamel and the inner surface coated in dentine. “The softer dentine erodes faster than the hard enamel, maintaining a sharp cutting edge as the beaver gnaws,” says Frank Rosell, an ecologist at the Telemark University College in Norway and the author of The Eurasian Beaver.
To compensate for the continual erosion, a beaver’s incisors grow at the fairly nifty speed of almost half a centimeter a month. If a beaver loses a tooth, however, this can spell disaster. In 1998, Rosell and a colleague reported on a Eurasian beaver with a wonky lower incisor, a tooth that appears to have grown – uneroded – for some three years. “The tooth had grown at an angle in towards the beaver’s left eye,” they wrote.
But when everything works as it should, and the top and bottom teeth are keeping each other sharp, a beaver is a formidable gnawing force. This is in large part down to its jaw muscles, giving it a far larger bite force for its body size than most other rodents.
Beavers will take on some seriously big trees. One of the largest on record was an aspen in Telemark, Noway that was about 20 m tall and over a metre in diameter, says Rosell. Several animals might work on the same tree at different times, he says. “It can take years.”
A beaver’s tail is another trait to be marveled at. But it’s not really used to propel the beaver through the water. “Beavers swimming underwater use only their webbed hind feet for propulsion,” according to zoologists figuring out the energy requirements of beavers swimming underwater. Rather, the beaver’s tail works more like a rudder, steadying and steering the animal towards its lodge.
This is not all. “Tails of obese individuals appear swollen,” noted one zoologist, who went on to describe a dramatic fluctuation in the tail’s fat content over the year, from around 50% during the winter months to just 15% in the summer. This suggests that the tail also acts as “a fat storage depot”. This variation is likely to have important consequences for the beaver’s body temperature, the tail helping to offload excess body heat when it’s warm. As a general rule, beavers are quiet animals, but they can use their tails as an instrument, slapping it on the surface of the water or the ground to indicate danger.
During the Pleistocene, there were a bunch of giant beavers, the largest species of the Castorioides genus roughly the height and weight of a tall man. Today, there are just two surviving species: the North American and the Eurasian beaver. Although these species are thought to have been going their separate ways for around 7.5 million years, they are still remarkably similar to look at. A simple DNA assay can distinguish one from the other. But if you don’t have any genetic tools to hand, there is another way.
Beavers go in for chemical communication. Big-time. In fact, there was once a lively trade in castoreum, a niffy cocktail produced by a set of specialised “castor sacs” at the base of the beaver’s tail that they use to mark out their territory. In addition, there’s an anal gland secretion that varies in colour and viscosity depending on the sex and species of the palpated beaver. Armed with these pertinent details and a vial of anal secretion, it’s possible to identify the sex and species with 100% accuracy, says Rosell
Beavers go in for chemical communication. Big-time. In fact, there was once a lively trade in castoreum, a niffy cocktail produced by a set of specialised “castor sacs” at the base of the beaver’s tail that they use to mark out their territory. In addition, there’s an anal gland secretion that varies in colour and viscosity depending on the sex and species of the palpated beaver. Armed with these pertinent details and a vial of anal secretion, it’s possible to identify the sex and species with 100% accuracy, says Rosell.
This article was first published appeared on BBC Earth in 2015.