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Abstract:
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.