Which animal is famous for building dams?

Beavers dam streams to raise the water level, because deep water lets them swim to their lodge and reach food without walking about in the open where wolves are. The dam is not the home - the lodge is, with an underwater entrance that freezes over into a locked front door in winter.

The trigger appears to be the sound of running water. Beavers will pile sticks and mud against a loudspeaker playing a recording of a trickle, on dry land, which suggests the construction of one of the more impressive structures in nature runs off a fairly simple rule.

The effect on a landscape is enormous, and North America has spent a century relearning it. Before the fur trade there were perhaps hundreds of millions of beavers, and the continent's small rivers were chains of ponds and wet meadows rather than single fast channels. Reintroducing them raises water tables, slows floods downstream, filters sediment and creates wetland. Their teeth are orange because the enamel is reinforced with iron, which makes them hard enough to cut hardwood and means they self-sharpen as they wear.