Sounds good

Bats find their way around and also track down prey by emitting high frequency sound waves. When these hit a surface, the echo pattern returned to the bat can be used to build up a sonic image of the surroundings. Now, thanks to some clever engineering by Marc Holderied and his colleague Elizabeth Clare, we have a much clearer idea of how these animals use this information to hunt, and she spoke to Chris Smith about their discoveries...

Chris - So, how did you approach this then? What did you actually do to try and work out what the bats were doing because they obviously are quite good at doing this, otherwise they would all starve to death.

Elizabeth - Yes. Well, it's a very specialised group of bats that does this. What we did was, we took some moths that were dead and we put them on surfaces. So, some bark, some rock, some leaves, and some slate that's really, really smooth. We actually built almost an artificial bat that has a microphone and a emitter that puts out echolocation. We had that little machine scan the leaf or the bark or the stone with the moth on it. And then we recorded the echoes that came back to see what it was that they would actually see. And then using some new technology, we can turn that into a visual image. So, we actually get a picture of what the bat sees acoustically.

Chris - And what does the bat see on those different surfaces?

Elizabeth - Well, that was what was interesting. On some surfaces, it's really obvious there's something there. If you have something like slate or a leaf that's really, really smooth then you get basically a long, thin, smooth sound back that's the surface and suddenly, you get disruption which is where the prey is. If they look at something like bark which is really, really rough then there's a lot of disruptions all along and they might not notice that there's anything there because disruption is what the surface looks like - all these different holes, and channels, and valleys, and ridges. And so, what we found was that some surfaces are really, really hard to find to prey on and some surfaces are really easy - the smooth ones that are like mirrors make it very easy to find the prey item. And the second thing we found though is that the prey itself may be hard to see because it's unpredictable. It could be any size, it could be any shape, it might have its wings up, or its wings down. But what they probably are noticing is that when they scan that surface, you get smooth background, background, background, and then disruption, and then background, background, background. So, what they might actually be looking at is the missing background. If they see a disruption then they pay attention because there's something there disrupting the surface.

Chris - Does the observation of what the bats really do tally with what your model, your fake bat says they must be doing?

Elizabeth - In a way yes, so the bats that are really good at this, they hunt in patches of leaves and they'll go out every night to the same leaves and scan them around and around, and around. They have a little patch that they defend. And so, they should be really, really used to what it looks like to be in their patch. They know what the leaves look like. They have a search image for their patch. So, if something disrupts it, they're going to notice and it's a lot more effective than if they were just randomly hunting in the forest. And so, these bats probably have a very, very good sense of what their environment should look like and they're looking for disturbances in there. The second thing about them is that they hunt off leaves not rough surfaces. So, they're using the surfaces that we know should give them the strongest signals.

Chris - So, why haven't things that do have rough surfaces that would give this sort of acoustic masking, this camouflaging effect, why aren't they absolutely riddled with insects that have evolved to "know" that if they lived there, bats won't get them?

Elizabeth - Well, that's a big unknown whether the insects have any idea that there's a surface that gives them more protection than others. This is one of the new things we found that this is the potential for camouflage on rough surfaces.