Foraging in the dark is challenging, but not when you're equipped with echolocation. Plucking insects from the open air is simple for bats. But it's much trickier hunting at the forest edge. Detecting an insect amongst the barrage of reflections from the surroundings seems almost impossible. So how do echolocating bats locate tasty treats on the forest floor? Björn Siemers from the Max Plank Institute of Ornithology explains that some bats tune their acute hearing to the tiny rustling sounds made by insects. But how much of an effect does the material that an insect is clambering over have on the tell-tale sound it makes? And what could an approaching bat learn about its victim from its rustling? Siemers and his students, Holger Goerlitz and Stefan Greif, decided to measure sound volumes as insects scuttled across various natural surfaces to see how the landscape affects their acoustic trail(p. 2799).
Starting out in Germany, Siemers and Greif decided to measure the sounds made by insects as they wandered over three different surfaces; a beech forest floor, a freshly mown meadow and newly ploughed earth. But the team needed to make their sensitive recordings in a completely silent environment, so they excavated 50 cm square chunks of each surface and transported them back to a soundproof room in the University of Tübingen to record the noises made by wandering carabid beetles. Equipped with exquisitely sensitive recording equipment, Greif waited patiently for the beetles to go about their everyday business, recording their tiny footsteps as they walked over each surface when dry and damp.
Analysing the recordings, Siemers found that the beetles were much nosier ambling through the beech leaf litter than the meadow or bare earth. And when he compared the sound generated by the dry surfaces with that from the same surfaces when damp, the volume doubled across all surfaces. The team also found that the rustling became significantly louder as the beetles walked faster.
But what effect did the beetles' size have on their rustling volumes?Siemers needed to find insects with a wide range of sizes and knew that the Madagascan rainforest is home to some of the most diverse populations of insects on the planet. Collecting beetles and cockroaches ranging in size from a few tens of milligrams up to 10 g, Siemers and Goerlitz recorded the sounds generated by the animals as they walked across dry leaf litter, bark or sand and found that the larger beetles made louder rustling noises. Also, the volume increase was more significant for larger creatures on noisy leaf litter than sand, with relatively small increases in the insects' size generating significantly larger sound volumes.
So what does all this mean for a ravenous bat hunting for a snack? Siemers explains that given the way sounds fade as you move further from their source,a beetle clambering over dry leaf litter could be heard eight times further away than another ambling over dry soil. He also suspects that an approaching bat could distinguish between a millipede and a six-legged beetle, but probably couldn't differentiate between a spider and a beetle. And if the bat knew a little about the nature of the surface beneath the insect, it might even be able to estimate its size, all crucial information for helping a bat to decide whether it's worth snatching that snack.