Bats are renowned masters of using echolocation to navigate. They emit sound waves and listen to how they bounce off of objects. From this, bats can fly through even the densest of forests without a hitch. But, even with acoustic vision, it seems impossible that bats would be able to identify a tiny moth (otherwise known as dinner) amongst the foliage. So how can bats pinpoint such tiny prey?
To find out, a team of scientists made a device that mimics the way bats listen for obstacles. They created a custom-built acoustic tomography system, which consists of an ultrasound microphone that’s positioned to replicate a bat's ear. They then hooked it up to a machine that visualizes what the microphone picks up on, compiling it a bit like a CAT scan.
You can read their findings in eLIFE.
A moth on a smooth surface compared to a rough surface. Elizabeth Clare.
The team scanned a sample (like the pictures above) with multiple frequencies of sound. They then interpreted what the "ear" recorded and turned this into an image. They tested the ability of the ear to discern the presence of a dead moth on different surfaces, such as rock, bark, leaves, and very smooth slate. When looking at the sound waves the ear recorded, the team found that there was no clear insect-shaped signal popping out from the background. So how do bats identify stationary prey?
"Is it maybe not the prey itself that they're paying attention to, but missing surfaces?" Elizabeth Clare, co-author of the paper, asked IFLScience.
"If you're a bat, and you scan from one end of a leaf to the other, all you'll get is leaf leaf leaf leaf leaf. If there's something sitting on it, you won't necessarily see information that something is sitting on it. Instead, what you just get is missing leaf."
A is slate only, and the blue line represents the background signal of slate. B is slate plus moth, and the green line has a different signal, indicating a moth on the slate. Elizabeth Clare, Queen Mary University of London/Marc Holderied, University of Bristol.
The bats that use this technique tend to hunt in the same place every evening, and they will scan leaves over and over again. That means it's easier to spot when something is different: say an insect perched on a leaf.
This hypothesis fits well with bat behavior. Bats tend to look for insects on smooth, predictable surfaces like leaves as opposed to rough, uneven backgrounds.
From this logic, it makes sense for insects to camp out on rough surfaces to avoid detection. This logic has fascinating implications about the hidden world of acoustic camouflage. "It would be, I think, one of the first forms of non-visual camouflage," postulated Clare to IFLScience.
Only about a third of bats solely use echolocation; other bats supplement it with vision. The species Micronycteris microtis (right) was identified as a possible candidate that hunts exclusively with echolocation.
It's not just bats that use echolocation, either. Some birds, as well as underwater animals such as some dolphins and whales, use it to navigate. It is not certain if these animals use the scientists' technique to process images and spot prey, but Clare is interested to see if they do.
"The next step would be to go out and do the experiments with the live animals and give them objects on different kinds of surfaces," and see if they have an easier time with smoother surfaces.
Bottom image: The Micronycteris microtis bat. Elizabeth Clare.