These river rodents are industrious workers and gifted engineers, but have some problematic instincts. These instincts make them very easy to manipulate with just a bit of sound equipment.
In the 1960s, a biologist called Lars Wilsson took an interest in beavers. He studied some in the wild, but his most famous research involved capturing a few beavers and raising them, in successive generations, in all kinds of different habitats. At first he just did things like putting them in large plexiglass terrariums which allowed him to watch their behavior inside their shelters.
Then he started raising the young separately from their parents to see if their dam-making skills were innate or learned. When he set the young beavers loose in running water, he saw that they built dams using the same techniques as their parents. Dam-building, then, is a natural behavior. But not all beavers build dams. When Wilsson set up animals in still-water habitats or places where the water moved slowly enough to be silent, they burrowed into mud and never displayed any interest in building anything. But if he put a speaker nearby, and played the sound of water running over rocks, the beaver would start making things, even though those things typically weren’t dams. Whatever they made, they would always build over the speaker. If Wilsson buried the speaker in a concrete wall, the beavers stacked mud and sticks against it. If he put it on a flat plain, they covered the thing. If he put a clear but silent outflow pipe through their burrows, allowing them to see where the water was leaking, and then played the sound of running water somewhere else, the beavers would cover the speaker, but be unable to figure out how to plug the plainly visible leak.
The fact that beavers aren’t that bright about identifying the real source of leaks might save their lives. When beaver dams cause problems for people by making rivers overflow their banks and damage property, it’s not necessary to knock down the dam or kill the beavers. A special outflow pipe can be put right through the dam, fitted with special filters to damp down on noise and prevent the beaver from damming it, and the beavers will leave it alone. If there’s no way to block the sound of water, some people cut notches in the dam near the pipe, and the beavers will instinctively fix the notch near the sound instead of fixing the actual leak.