Frequency modulation patterns in the echolocation signals of two vespertilionid bats

In this study we measure and classify frequency modulation patterns in echolocation signals of two species of bats. By using the derivative of an exponential model fitted to pulses emitted by Pipistrellus pipistrellus and Myotis myotis, we show that the modulation functions differ fundamentally between the two species and also vary within each species. This variation makes it unlikely that pulse design and the concomitant modulation pattern can be explained by a single common principle as previously suggested.