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Little brown myotis. Photo: Cori Lausen
By Cori Lausen
How well do you see in the dark? Many of us might struggle to see a moose on a moonless night, let alone a mosquito. But some bats have a nifty trick – they use their ears to locate their bug prey.
It’s not that bats can’t see -- many have excellent full-colour vision – but most don’t depend on their eyes to navigate. The cute dog-like flying foxes of Australia and Africa do rely on their eyes to find food, but luckily for them, their food doesn’t move – they eat fruit. But for the rest of the bats (“Micro-Chiroptera”), the challenge is dealing with prey that moves and this requires far more sophisticated ways of searching for food. Enter echolocation. Bats can use returning echoes from sounds they generate out of their mouth or noses to detect objects as fine as a human hair in total darkness.
How many bats are there and what species are where? This is increasingly important information to know, because North American bats are being devastated by a deadly fungal disease called white nose syndrome (WNS).
In eastern North America, WNS has reduced bat populations by an estimated 90% or more. By studying bats in British Columbia and Alberta, where the fungus has not yet arrived, we hope to help bats survive when the fungus inevitably shows up there. Understanding how bats echolocate, and then recording them appropriately, is fundamental to that effort.
Bat brains map echolocation echoes in a way that lets bats hone-in on insects or avoid obstacles. Far from our cartoon characterizations, bat ears come in many different shapes and sizes with folds and wrinkles that are thought to enhance their hearing. Some bats, particularly ones that can snatch prey off of the ground or leaves, have huge ears to capture both echoes and the soft sounds generated by their prey – like the flutter of a moth’s wings while it perches on a leaf or the scratch of a beetle’s feet as it scurries on the ground. Most others rely on smaller ears that are adept at listening for echoes but not necessarily to the sounds that their prey generate.
Now the one problem with this system is that sound waves need to bounce off of an object to generate an echo. And that means the length of the sound wave has to match the size of the object so that the sound is blocked and bounces back to the bat. Insects are small, so the sound’s wavelengths must be small. These short wavelengths result in high frequency sounds, and most bats produce such high frequency sounds that human ears can’t hear it – hence it is called ultrasound.
Ultrasound doesn’t actually travel very far in air, so bats have to really belt their echolocation calls out in order to have enough sound range to avoid flying into an object before they detect it or to find a tiny insect out in front of them. This results in another problem. The sounds bats make can be absolutely deafening – the equivalent of holding a shrieking smoke detector up to your ear. Fortunately, most bats use frequencies that are above the human range of hearing, and we rarely have bats held up close to our ears where the sound intensity would be damaging. But wait a minute -- bats generate their really loud sounds right next to their own ears, so how do they not deafen themselves?
Bats use their tiny middle ear muscles to essentially “close their ears” while they are sending out sound waves. Of course, to hear the reflected sound waves, bats must quickly re-open their ears. Bats can do this ten times per second. And interestingly, some bat prey have also developed the ability to hear bats’ sounds and take evasive action, setting up a relentless battle of who hears who first. Jiggle your keys (gives offs a lot of ultrasound!) near a streetlight at night and watch to see if moths drop from the sky. If they do, you succeeded in fooling these unique eared insects into thinking they were about to be attacked by a bat.
Bats also tailor their sound to what they are finding. They may use only a small range of relatively lower frequencies while searching for insects, but switch immediately to higher frequencies and more repetitive chirps to discover size, distance and speed of movement to narrow in on a target. Bats end up using different sounds for capturing prey, identifying obstacles and communicating with each other.
The way that different species use different sound frequencies in certain situations, can give us a useful way to identify species flying around unseen in the night sky. Using acoustic detectors, we can listen in on ultrasonic bat calls and then use the acoustic patterns we record to analyze what species of bats are in the neighbourhood. But it is tricky. The variation in the calls that one individual can make is tremendous, making the study of bat sounds challenging, considering how many species of bats and individuals are echolocating in the night sky at any one point in time.
Different species do often have different types of calls that can be used to differentiate them. But different species don’t always have different calls. Similar-size bats in similar environments feeding on similar-size prey will have very similar echolocation calls, making it extremely tricky for biologists to use recordings of echolocation to differentiate them. Small bats that hunt small insects amongst highly vegetated areas like forests generally use higher frequency sounds. Bigger bats that hunt larger prey out in the open generally use lower frequency sounds. But big bats can be found foraging in forests, and small bats can be found foraging in open areas and they typically change their calls to adjust for the presence or absence of obstacles in their flight paths. This makes for a challenging combination of sounds when one tries to match species to echolocation call.
Because it is difficult to differentiate all species using sound, sometimes we must capture bats instead in finely woven nets. These nets are very thin and hard enough for bats to “see” with sound that they often don’t have enough time to turn back once they do finally pick up on their presence. After assessing bats captured in our nets, we quickly release them to go about their business.
Deploying bat detector in a cave. Photo: Hobson
Listening for bats is an excellent way for us to fill in the many gaps in our understanding of which bats are present in different habitats, where they are roosting and to follow trends in population. Sound is an important tool for the North American Bat Monitoring Program, which records bats across the continent each summer and is designed, in part, to serve as an early warning system for bat declines as well as giving us a better picture of bat abundance and distributions.
We need bats, who are voracious insect eaters, to eat pest insects like mosquitos, spruce budworm, corn earworm, and alfalfa weevil. Bats’ echolocation abilities inspired the development of radar and sonar, and today their ability to battle viruses may help humanity further.
Our work with bat sounds gives us important insights into their behavior and ecology that is virtually impossible to study otherwise. So we will continue to crawl through caves, trek through forests and climb up mountains to listen to our night-flying friends.
If you want to hear some bat sounds for yourself, listen to this audio interview with Dr. Lausen.
Placing bat detector in a cave. Photo: Hobson
Photo credits: Banner | Lila Tauzer © WCS Canada