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-color vision — but most don't depend on their eyes to navigate. For many bats, the challenge they must overcome to survive is detecting prey that move in the dark.
Enter echolocation. Many bats can use returning echoes to detect objects as fine as a human hair in total darkness. Bat brains map the echoes in a way that lets them home in on insects or avoid obstacles. Bats use of echolocation can help us protect them.
These cryptic creatures flit around at night and hide by day, making it difficult to monitor them by sight.
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.
Some bats are loud, some less so; some species prefer to feed amongst trees, others over water. 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 moth wings. Most others rely on smaller ears that are adept at listening for echoes but not necessarily to the sounds that their prey generate.
The one problem with this system is that sound waves need to bounce off an object to generate an echo. 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. Most bats produce such high-frequency sounds that human ears can't hear them — hence, it is called ultrasound.
Ultrasound doesn't actually travel very far in air, though, so most 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 in front of them. It's sort of like headlights on a car — bright lights are needed to drive fast. Faster bats must be loud and have their sound travel far.
This results in another problem. The sounds bats make can be absolutely deafening to bats themselves — the equivalent of holding a shrieking smoke detector up to your ear if the sound were within human hearing range. Bats generate these really loud sounds right next to their own ears so how do they not deafen themselves?
Bats use their middle ear muscles to essentially "close their ears" while they are sending out sound waves. Of course, to hear the reflected sound waves, they must quickly re-open their ears. Bats can do this 10 times per second. 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.
Bats also tailor their sound to what they are finding. They may use only a small range of relatively lower frequencies while se arching for insects, then switch to higher frequencies to discover size, distance and speed of movement to narrow in on a target.
The way that different species use different sound frequencies can help us identify species flying around unseen in the night sky. Using acoustic detectors, we can listen in on ultrasonic bat calls and then analyze the acoustic patterns to figure out what species of bats are in the neighborhood. But the variation in the calls that one individual can make is tremendous, making the study of bat sounds challenging.
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. This monitoring is critical at a time when bats are facing many challenges like the steady spread of WNS. We're keeping our ears open to better understand how we can help these fascinating creatures. What we don't want to be listening to is silence.
Cori Lausen is Associate Conservation Scientist with Wildlife Conservation Society (WCS) Canada, where she leads the Western Bat Program.