How bats and whales build pictures of the world out of echoes
In experiments by J. A. Simmons, bats relying on sweeping calls could tell apart two targets placed less than half a millimetre apart, using nothing but returning sound. Toothed whales, shrews and a few cave-dwelling birds also practise this living sonar, called echolocation, to navigate and hunt in darkness or murky water.
The principle is simple. An animal emits a call and times how long the echo takes to come back, which gives distance. Because its two ears sit slightly apart, the echo reaches each at a fractionally different moment and loudness, revealing direction. From those cues a bat can judge not only where an object is but how large it is and what sort of creature it might be. Unlike ship sonar with many narrow beams and receivers, the animal version manages with one transmitter and two ears.
Calls come in two basic flavours. A frequency-modulated call sweeps downward through a range of pitches, giving superb precision at short range, ideal for weaving through branches, though its energy spreads thin and fades quickly. A constant-frequency tone holds one pitch for up to 100 milliseconds, reaching further and letting the bat read Doppler shifts, so it can sense a target's speed and even the flutter of an insect's wings. Horseshoe bats hunt this way, lowering their own pitch to keep echoes in the band their ears hear best.
Timing forces trade-offs. A bat can listen for only one call's echoes at a time. Searching with gaps of 100 milliseconds between calls, it can detect objects up to about 17 metres off; in the final lunge, calling every 5 milliseconds, its range shrinks to 85 centimetres but updates arrive far faster. Loudness varies from 60 to 140 decibels, and some bats quieten mid-call to avoid deafening themselves. Moth-hunting whispering bats keep calls soft so their hearing prey stays unaware, and the spotted bat uses a pitch of 12.7 kilohertz that moths cannot hear.
Unravelling all this took centuries. Lazzaro Spallanzani showed in the 18th century that bats navigate by some sense beyond sight, and Louis Jurine pinned it on hearing in 1798. Donald Griffin, who with Robert Galambos first demonstrated echolocation in bats, had coined the term by 1944; whale sonar was properly described in 1956.
Source: Animal echolocation
