Bats can help us devise better gadgets for people with visual impairment

First of all, its calls must be strong enough to hit the target and actually form an echo, which returns to base. But high-frequency sounds die out quickly and only work when the bat is within 10-12 m of its target — and this is for the larger bats (it’s much less for the little ones). The calls spread out like the beam of a flashlight and some species emit two such beams — one straight ahead, scanning for targets and the other sweeping low (so it can get an idea of its altitude). But yes, the calls are loud — 138 decibels in the case of the big brown bat, as loud as a jet engine.

This can be deafening — for the bat itself — so it shuts its middle ear while emitting the call and opens it in time to catch the returning echo. Even as it gets closer to its target, it ensures that the returning echo remains at the same volume and doesn’t get intolerably loud.

The calls are emitted in a stream of pulses, like stroboscopic lights, so the bat can keep track of its fast-moving and fleeing prey. The frequency of the pulses may be so high that (like the frames in a film) they meld into a continuous stream. But to prevent outgoing and incoming calls from bungling into each other and causing confusion, the bat has to match every outgoing call with its incoming echo and it does this by emitting extremely short calls. And also by ensuring that an outgoing call is only emitted once it has received the echo of the previous call, so there is no overlapping.

The time it takes for an echo to return tells the bat how far the prey is: The quicker the returning echo, the closer the prey. But the bat must also know what the prey is likely to be. Its ears and brain are so finely tuned that they can figure this out by the (infinitesimal) difference in the time it takes the returning echoes bouncing off various parts of say a moth, thus painting a kind of ‘sound picture’ of the insect. A pulse hitting a wing may take a tiny fraction of a second less or more to return to the bat than one, say, bouncing off its body. To further hone the picture, pulses are emitted at different frequencies — the low-frequency ones bounce off the larger parts of the prey, while the higher-frequency ones fill in the details of the sound picture. So now it knows how far away its prey is and also what it looks like and how and in which direction it is moving.

All this is happening while both the bat and its prospective prey are flying at top speed and you can imagine the rate at which the bat has to update its information and react accordingly. But first, it has to find its target and it does this by emitting long loud pulses in a narrow-frequency band. If it receives an encouraging echo it switches to a broader frequency so that it can gauge the distance and discern the form of its victim, calling more frequently to receive updates on its position.

A small target against a large background can also cause problems because the echoes from the background are louder and snuff out those from the target. The common big-eared bat, from Canada, the US and Mexico, has cracked this problem ingeniously: it positions itself in front of its target and hovers up and down before it, gathering echoes and information from various angles — and kind of etching out its victim in bas relief as it were, from the background.

Bats often fly together in groups and this can cause confusion if they are all sending out echo-locating beeps at the same time (like everyone blowing their horns). So, some, like the big brown bats, direct their calls away from one another or simply keep quiet when other bats call. But this can only work when there are a few bats flying around — not when there are millions streaming out of a cave. Scientists are still trying to figure out how echolocation works in such situations. Perhaps bats don’t use it now and rely more on other senses and memories.

If bats can do it so can we: many blind people are learning to echo-locate much the way bats do, emitting clicks and picking up the echoes as they make their way unerringly through streets and marketplaces and even ride bicycles. Of course, bats are way ahead in the game, but then they had a head start.