Although bats use their eyes to see during the day, they hunt in darkness. How do they navigate without light?
The word ‘echolocation’ says it all – bats locate objects in their environment, using sound echoes. People often describe echolocation as bat ‘sonar’. But, though it might seem a very alien mode of experiencing the world, echolocation isn’t really that different to how we humans see – it’s just an alternative method of using waves to sense the world.
When we humans navigate through the world, we rely heavily on our eyes and our sense of sight, so we can move around without bumping into everything. All we are doing is using our eyes to collect light waves, which bounce off objects in our surroundings. While we see with light waves, the waves bats use in echolocation are sound waves. So how does it all work?
When they’re flying around, bats call with their voicebox, tongue, or nose. We can’t hear these sounds, because they’re ultrasonic – that is, above the range of sounds we humans can hear. These calls bounce off objects around them, making echoes. The echoes travel back to the bat, whose ears detect them. When it hears the echoes, the bat can judge its location relative to the object which the echo bounced off. It can work out if it’s near, or far away, and it works for both static and moving objects. This is the key to bats’ survival – with echolocation, not only can bats avoid flying straight into walls, but they can home in on an insect flying through the darkness. Bats need to create very loud calls in order for the echoes to stay audible. In fact, they would deafen themselves with their calls, were it not for their handy ‘send/receive’ mechanism. The muscles in the bat’s middle ear contract as the bat calls, ‘disconnecting’ their ears so the noise doesn’t deafen them. The middle ear muscles then relax, which ‘reconnects’ the ear, of course, in order to hear the echo.
Though we can’t hear bats with our range of hearing, we can hear their noises when fed through a device called a bat detector. These devices can indicate what species of bat is in the vicinity, as different kinds of bats use different frequency bands. Some bats use FM (frequency modulated) calls, whereas others use calls of only a single frequency (constant frequency, or CF.) Even more surprisingly, some bats which employ CF sounds – like the greater horseshoe bat – have evolved to compensate for the Doppler effect. (A familiar topic in school physics lessons, the Doppler effect describes the phenomenon when wave frequencies change, if their source, or when the observer, is moving. This is why ambulance sirens shift their pitch as they move towards or away from you. They sound higher-pitched as they speed towards you, as the sound waves from the siren ‘bunch up’ and shorten in frequency. The siren sounds lower pitched as the ambulance speeds off into the distance.) This effect could pose a problem for bats, as the echoes bouncing off a moving target might shift the soundwaves below or above the hearing range of the bat’s ears. To prevent this, these bats adjust their calls in such a way that their echoes are all a single, constant frequency. To complicate things further for bats – how do they avoid being ‘jammed’, or confused when they hear the echoes of other bats? Scientists still don’t know exactly how they tell their own echo apart from the tangled soundscape of other echoes. Some experiments have observed that bats increase the number and intensity of their calls in order to hear themselves amid other bats’ noise. Recently, some researchers have suggested that bats just ‘recognise’ their own voice.
Bat echolocation is just one astounding example of evolution. It demonstrates how natural selection can give rise to animals armed with complex, unusual, and highly precise survival tools. (And bats aren’t the only animal to use echolocation – dolphins and whales also make use of it, as Ellie’s blog post explores here! When very different animals from different families have evolved the same characteristic, it’s known as convergent evolution – a remarkable phenomenon.) The US scientists Donald Griffin and Robert Galambos discovered bat auditory echolocation in the 1940s – and it was Griffin who coined the word ‘echolocation’. As I mentioned above, bat echolocation is very similar to the technological detection and navigation system known as sonar. When Griffin and Galambos revealed their discovery, many biologists saw this as a far-fetched, impossible idea. This is because sonar (along with radar) was, at the time, a cutting-edge military technology, which was seen as a pinnacle of human technical achievement. So, many scientists simply couldn’t accept that bats have evolved such a system naturally – thousands of years before the emergence of humans as a species.
For further exploration of this topic, see Richard Dawkins’ excellent book on evolution, The Blind Watchmaker, which has an intriguing chapter on how echolocation works, set in the context of evolutionary biology.