Reputation: Beavers are industrious beasts. They gnaw through trees, but eat fish. They 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 that so many people are under the illusion that beavers eat fish. They do, after all, spend a lot of time in rivers.

But pause for a moment and imagine those massive incisors. 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 tooth had grown at an angle in towards the beaver's left eye

The outer surface is 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 of the Telemark University College in Porsgrunn, Norway, author of The Eurasian Beaver.

To compensate for the continual erosion, a beaver's incisors grow at the fairly nifty speed of almost 0.5cm a month. However, if a beaver loses a tooth it can spell disaster.

In 1998, Rosell and a colleague described a Eurasian beaver with a wonky lower incisor. The tooth seems to have grown, un-eroded, for around 3 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 powerful jaw muscles, which give it a far larger bite force for its body size than most other rodents.

The beaver's tail works more like a rudder, steadying and steering the animal

Armed with their teeth, beavers will take on some seriously big trees.

One of the largest such trees on record was an aspen in Telemark, Norway that was about 20m tall and over 1m 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 a 1997 study of beaver swimming.

Instead, the beaver's tail works more like a rudder, steadying and steering the animal towards its lodge. But that is not all it does.

"Tails of obese individuals appear swollen," noted one zoologist. He 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 is "a fat storage depot".

The largest Castoroides species were roughly the height and weight of a tall man

Beavers also seem to use their tails to regulate their body temperature. The tail is not well insulated, so when a beaver is too hot it can offload excess body heat through its tail.

Finally, beavers can use their tails as percussion instruments, slapping them on the surface of the water or the ground to indicate danger.

Despite their ingenious abilities, today there are only two species of beaver: the North American and the Eurasian.

It wasn't always so. During the Pleistocene period, there were even giant beavers: the largest Castoroides species were roughly the height and weight of a tall man.

The two surviving beaver species are thought to have been going their separate ways for around 7.5 million years. Yet they are still remarkably similar to look at.

The truth about animals

Why our popular perceptions of so many species are wrong

A simple DNA test 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. There was once a lively trade in castoreum: a whiffy cocktail produced by specialised "castor sacs" at the base of the beaver's tail, which they use to mark out their territory. There is also 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 secretions, it's possible to identify a beaver's sex and species with 100% accuracy, says Rosell.