The world of bats: echolocation

by Claire Wordley

 

Bats have big, sensitive ears to hear soft echoes; this helps them sense where they are going even in the dark

You can put bats into a pitch black room full of objects, and they’ll fly around without ever crashing into anything. Let some flies or mosquitoes loose in the room, and insect-eating bats will happily hunt their tiny prey on the wing – in total darkness. For us humans, who depend so heavily on light to find our way, this ability seems little short of magical. But how is it done?

The answer is echolocation. Bats make calls and listen for the echoes bouncing back, then interpret the echoes to find out what is around them. This ability lets them rule the night, as they don’t rely on light to get about. Some blind people have learnt to echolocate by clicking their fingers or tongues as they walk and use the sound produced to judge what is around – try it and see if you can learn to do this!

Like a ripple

In order to understand how echolocation works, you need to think about sound. As a bat calls it sends out a ‘wave’ of sound, compressing the air molecules closer together. This wave travels outwards like a ripple from a pebble dropped in water, and then waves ripple back towards the bat if the sound hits an object. The bat can use these returning sound waves to judge where an insect is. As bats call, they make lots of sound waves – if the number of waves produced per second is very high, we describe the sound as ‘high frequency’.

Most bats use very high frequency sound for their calls – so high we can’t hear it – to echolocate. Some bats can make 200 calls per second! Why do they use such high sounds? Well, not many other animals hear or use very high frequency sound so most predators and prey can’t hear the calls. This lets bats sneak up on insects unnoticed. Also, high frequency sound is very good for finding small things close by – like insects – where low frequency sound is better for travelling a long way, like an elephant’s deep rumble. So high frequency is the best for bats hunting for dinner!

Look at the Figure 1 to know how bats echolocate. Sound waves produced by a bat bounces off an insect and back to the bat’s ears. (The image is reproduced from ‘Bats: From Evolution to Conservation’ with permission from Prof. John Altringham.)

Big, sensitive ears

Bats have evolved many features to help them echolocate. To start with they have very sensitive ears to pick up the quiet echoes of their calls. All calls get weaker as they travel away from the source – it’s harder to hear your mum calling you to wash the dishes when you’re two blocks away than if you are in the kitchen with her – and the high frequency sounds that bats use get quieter more quickly than low frequency sounds.

Many bats have very big ears to catch these soft echoes. Bats have to call very loudly to make sure they can hear the echoes coming back, and they have to direct that sound carefully. So some bats have facial features to direct sound more precisely – these ‘noseleaves’ bounce the sound into a narrow beam in front of the bat. These can look very strange, as you can see from the picture.

The ‘noseleaf’on the face of Rhinolophuslepidus from India is highly evolved to channel sound into a narrow beam in front of the bat. The large ears pick up quiet echoes.

Far from being hair-tangling pests, bats have orientation systems better than expensive military technologies. Indeed, tech specialists are studying bat echolocation to try and improve military sonar, especially for the detection of underwater mines. The shape shifting vampires of folklore may be entertaining, but echolocation is a real superpower.

This article appeared in the Hindu in School on 9 May 2012.