Echolocation – A Biological Sonar
Navigating around in the dark to find food items, avoid obstacles or locate others would be a difficult task for us, but bats could do it with their eyes closed! It must be noted, though, bats do not close their eyes when navigating the darkness and they are not blind (contrary to popular belief). Their vision is actually pretty acute and is well adapted to low light settings. Despite this, it’s their ability to echolocate that really allows bats to fly with such high accuracy in the dark of night.
What is echolocation?
Echolocation is a high frequency system that involves emitting sound waves and listening to echoes to locate objects in space. Bat echolocation works similarly to sonar, but the sound waves are transmitted through air, rather than water. Indeed, sonar was actually developed by scientists who had studied echolocation! The length of time it takes for a sound to echo back to the bat indicates how far away an object is. Bats can also determine the size, shape and direction of movement of objects using echolocation. For instance, larger objects will reflect more of the sound wave than smaller objects, and as such produce a greater echo. All of this information is then compiled into a sound ‘map’ of the environment and allows bats to detect the precise location of mobile insect prey within this environment.
Bats produce this sound with their larynx (voice box), and most vesper bats (Vespertilionidae) emit the sound or ‘call’ from their mouth. Horseshoe bats (Rhinolophidae), however, emit the sound through their nostrils – these bats have horseshoe-shaped structures around their nostrils that act as a megaphone!
Lesser Horseshoe bat (Rhinolophus hipposideros). Image © Steve Wadley.
Red Riding Hood: "Oh Granny, what big ears you've got!",
Wolf: "All the better to hear you with!"
…a line that can be related to bats! Bats’ ears are suited to the types of sounds they detect; those who listen out for the weaker echoes have bigger ears (“all the better to hear you with”!) and those who listen out for stronger echoes have smaller ears.
Identifying Bats by their Echolocation Calls
Bats have to produce very high energy calls to get the sounds to travel many metres through the air. As an example, a Pipistrelle bat’s call can be up to 120 decibels, which would be the equivalent volume of a chainsaw or smoke alarm being held next to your ear! However, as humans we are not able to hear these sounds using our ears alone because the frequencies of bat calls (usually 20kHz – 80kHz) are not within our hearing range (approx. 20Hz – 18kHz). As such, we need to use specific detectors that record the bat calls and convert the sounds into a frequency that we are able to hear. The data from the detectors are uploaded onto a computer programme (BatExplorer) that allows us to play the sound clips of the calls and also creates a visual display of the call.
The images below are examples of what the calls of two different bat species can look like when uploaded to the computer.
What a Soprano Pipistrelle echolocation call looks like on the computer. Image © Steve Wadley.
What a Lesser Horseshoe bat echolocation call looks like on the computer. Image © Steve Wadley.
On the detector, many bat calls sound like a series of ‘pew’ sounds of varying speeds and pitches, sort of similar to a bird chirping. However, there is a lot of variation and each species of bat has its own specific call within a specific frequency range, which can be distinguished from the call of another species. This is not always an easy task, though, as the calls of different species can sound very similar!
If you’d like to hear the echolocation calls of the different species of British bats for yourself, visit the Bat Conservation Trust website at: https://www.bats.org.uk/about-bats/what-are-bats/uk-bats. It’s an interesting listen!