Do horseshoe bats complement their echolocation with listening for prey-generated sounds?

Echolocation is a highly sophisticated sensory system for actively probing light-deficient environments. However, due to the stroboscopic and directional emission of the calls and the strong attenuation of ultrasonic frequencies, the space that can be probed by biosonar is limited both temporally and spatially. We hypothesised that this limitation will favour the opportunistic use of additional information for prey detection, such as prey-generated rustling sounds, and that bats thus exploit a much wider range of environ- mental information than previously believed. We tested this hypothesis in greater horseshoe bats (Rhinolophus ferrumequinum), whose echolocation is specialised for the detection of fluttering insects, but spatially strongly limited due to high call frequencies. We predicted that bats will react to prey-generated rustling sounds by steering their sonar beam towards the position of the sound for further biosonar-based evaluation. To present prey rustling sounds and monitor bat echolocation, we developed a spherical three loudspeaker - eight microphone array. Each loudspeaker was placed next to one microphone and was symmetrically surrounded by three other microphones. Bats were trained to perch in the centre of the spherical array. Per trial, we played back a rustling sound of a moth fluttering in vegetation from one of the loudspeakers, or alternatively phase-randomized or amplitude-inverted versions of the recordings to control for temporal and spectral cues. We recorded the bat’s echolocation behaviour simultaneously with all eight microphones for offline analysis of relative call intensity and thus call direction