People often say that someone who cannot see well is "as blind as a bat". In…

People often say that someone who cannot see well is "as blind as a bat". In fact, the expression is unfair. All bats can see, and some see rather well. But many species have developed another, remarkable way of finding their way in the dark: they listen to echoes.

The process is called echolocation. As a bat flies, it produces a series of very high-pitched calls, most of them far too high for human ears to hear. When these sounds hit an object, such as a tree, a wall or a flying insect, they bounce back. By listening to the returning echoes, the bat can work out where the object is, how big it is and even which direction it is moving in. Some bats can detect objects as thin as a human hair.

When a bat is simply flying from one place to another, it may call only a few times a second. As it closes in on an insect, however, the calls come faster and faster, reaching well over a hundred a second in the final moments before it catches its prey. Scientists who record these sounds call this final rapid series the "feeding buzz".

Not all bats rely on echolocation to the same degree. Many large fruit-eating bats, often called flying foxes, have big eyes and an excellent sense of smell, and they use these, rather than sound, to find ripe fruit. Altogether there are more than 1,400 species of bat, which makes them one of the largest groups of mammals on Earth, and they live on every continent except Antarctica.

Their lifestyles are as varied as their senses. Most species eat insects, and a single colony can eat enormous numbers of them each night, including many that damage farmers' crops. Others feed on fruit or the sweet liquid inside flowers, and in doing so they carry pollen from plant to plant. In parts of Southeast Asia, for instance, bats are among the main pollinators of durian trees.

Despite their usefulness, bats face serious threats. The loss of forests and caves, disease and, in some places, wind farms have all reduced their numbers. Conservation groups argue that protecting bats is not only good for nature but also makes economic sense, since the services they provide to farmers would be very expensive to replace.