Do echolocating bats use the acoustic flow field?

Abstract

In transfer flight, CF bats continuously emit signals consisting of a long CF and a short terminal FM (tFM) component. So far, the CF component is only discussed in the context of the perception of fluttering insects. However, changes in the relative movement between the bat and the targets it passes create systematic changes in the amplitude and frequency of the echoes. These changes represent an acoustic flow field containing spatial information. To establish whether CF bats evaluate flow-field information, the echolocation and flight behavior of Pteronotus parnellii and Rhinolophus ferrumequinum was studied when flying on known flight paths in the field. In undisturbed flight, signals with a prominent CF and a weak tFM component were emitted (CF mode), well suited for analyzing the available flow-field information. An obstacle introduced directly into the flight path caused an evasive flight maneuver and a distinct change in echolocation behavior indicated mainly by more prominent tFM components allowing a better target localization. New data suggest that bats avoid obstacles positioned aside of the flight path without leaving the CF mode. It is assumed that information encoded in the CF component (possibly flow-field information) is sufficient to guide the bat on known flight paths and around newly appearing lateral targets.