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Communication Breakdown?

We know that the superpower of bats is their biosonar system, where they "hear" their surroundings. But does this make them good listeners, too? My latest research, done with colleagues at the Technical University of Munich, has been accepted for publication!

Almost all bats live (and sometimes even hunt) in groups. Like for other mammals, communication is a key feature in their behaviour, and since they spend most of their lives in darkness, vocal communication plays a crucial role. The bat Phyllostomus discolor, for instance, has a large repertoire of complex communication sounds and is even considered a vocal learner. At the same time, bats are masters of precise perception of tiny time differences, because with their biosonar system, they measure the distance to objects by the so-called echo delay. This delay is the time that passes between the outgoing echolocation call and the incoming echo that is bouncing back from objects around the bat. Echo delay depends on the speed of sound and as a rule of thumb, an echo delay of 1 millisecond corresponds to a distance between bat and object of 17 cm. In short, bats can deal with very small time scales.

My colleagues Stephen Hörpel, Uwe Firzlaff, Lutz Wiegrebe and I therefore hypothesized that the mechanisms for temporal precision that have been shaped by the requirements of biosonar might also serve as a pre-adaptation for high resilience to temporal distortion in communication calls. What do we mean by distortion? To distort the communication signal, (for example a bat's aggression call) we apply a more or less complex manipulation. This could be a time-reversal -playing the signal backwards- or a scrambling of the signal's temporal structure while leaving the frequencies intact. These kinds of manipulation are also often used to study human speech perception. To explore the limits of resilience to distortion, we need to put a number on the levels of distortion. This is done by breaking the signal down into short windows and then doing the manipulations inside these windows. If you are a bat and very good at handling short time differences, shouldn't you also be good at recognising a communication signal even if the recognisable parts are very very short?

Was our hypothesis correct? Yes and no. We did find low thresholds for the detection of spectro-temporal distortions, both on the level of the animal itself (I did) and on the level of the animal's auditory cortex (Stephen did). This translates to the high distortion-tolerance that we had expected. But when we started looking for the mechanism shaped by biosonar that we thought would be the basis for this, we discovered something surprising... We teamed up with Jonas Reijniers and Herbert Peremans from Antwerp University and modelled the auditory periphery. Turns out, common, garden-variety mechanisms that are built into the middle and inner ear of mammals are already enough to achieve this high resilience, no specialised cortex areas are needed for it! What does this mean for the bat? From their high distortion-tolerance they probably gain the advantage that they can recognise each others communication even in the middle of loud and noisy bat caves or when flying around with many many other bats. It means they are good at listening!

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