Acoustic mirrors as sensory
traps for bats
Stefan Greif,1,2*† Sándor Zsebők,1†‡ Daniela Schmieder,1§ Björn M. Siemers1
Sensory traps pose a considerable and often fatal risk for animals, leading them to
misinterpret their environment. Bats predominantly rely on their echolocation system to
forage, orientate, and navigate. We found that bats can mistake smooth, vertical surfaces
as clear flight paths, repeatedly colliding with them, likely as a result of their acoustic
mirror properties. The probability of collision is influenced by the number of echolocation
calls and by the amount of time spent in front of the surface. The echolocation call analysis
corroborates that bats perceive smooth, vertical surfaces as open flyways. Reporting on
occurrences with different species in the wild, we argue that it is necessary to more closely
monitor potentially dangerous locations with acoustic mirror properties (such as glass
fronts) to assess the true frequency of fatalities around these sensory traps.
Anthropogenic changes to the environment, such as habitat alteration or interference with food resources, are often evidently detrimental to wild animals. Furthermore, ecologically novel cues are capable of misleading animals into responding maladaptively
to formerly reliable environmental cues (1–4).
Well-known examples are artificial light sources
attracting insects and birds at night (5) or smooth
human-made surfaces that aquatic insects mistake for bodies of water because of similar light
polarization patterns (6). To find, evaluate, and
mitigate such sensory traps requires consideration
of the sensory ecology of a particular animal (7, 8).
The primary sensory modality for most bats is
their echolocation system (9, 10). Bats use the
returning echoes of emitted calls to detect, classify,
and localize objects in their environment (11–13).
In a previous study, we showed that bats perceive any extended, smooth, horizontal surface as
a water body, resulting in drinking attempts. This
is attributable to the acoustic mirror properties
of smooth surfaces, which reflect calls away from
the bat except for a strong perpendicular echo
from below (9) (Fig. 1A). Several observations of bats
colliding with smooth vertical surfaces (such as
glass windows) suggest that bats have problems
recognizing them (14–16). This raises concerns
about the millions of artificial vertical smooth surfaces introduced in bat habitats and their hazard
potential for injuries. We predicted that these collisions are based on the acoustic mirror paradigm
and investigated the underlying sensory mechanism and possible occurrence in natural settings.
For our flight room experiments, we flew
greater mouse-eared bats (Myotis myotis) in a
continuous, rectangular flight tunnel (height 2.3 m,
(1.2 m × 2.0 m) was placed 1.2 m away from a
corner of the felt-covered tunnel, either horizon-
tally on the ground or vertically on the wall. The
bats’ flight behavior was recorded with two high-
speed cameras (100 fps) and their echolocation
calls with an ultrasound microphone (Fig. 1B) (17).
Eleven bats were presented with the horizontal
plate on the first night and the vertical plate on
the second night. The order was reversed for 10
other bats. A trial lasted between 5 and 15 min
with, on average, 20 passes by the plate. We
counted drinking attempts as well as collisions
with the plate, the ground, and the normal wall.
Of 21 individuals, 19 collided with the vertical
plate at least once (on average 22.8% of passes)
but never with the horizontal plate (Wilcoxon
matched-pair test, P < 0.001) nor any other parts
of the wall. Thirteen individuals made at least
one drinking attempt from the horizontal plate
(on average 13.0% of passes), but none from the
vertical plate (Wilcoxon matched-pair test, P =
0.002) (Fig. 2). After the experiments, all bats were
carefully examined and no injuries were found.
To understand the sensory basis of those collisions with the vertical plate, we conducted analysis of the flight and echolocation behavior in the
space immediately in front of the plate (“plate
zone,” limited by the plate’s perpendicular projection; Fig. 1B) for 25 bats when flying toward
the vertical plate. On the basis of our high-speed
recordings, we categorized the approach events
into three groups: (i) “near collision,” where bats
approached to within 25 cm of the plate (
body-to-plate distance) but did not touch it; (ii) “
collision with maneuver,” where bats collided with
the plate despite clear evasive maneuvers at the
last moment; and (iii) “collision without maneuver,” where bats collided without any noticeable
evasive action. We measured the time and counted
echolocation calls from entering the plate zone
until reaching the closest point to the plate (either
collision or turning point). We further calculated
the bat’s flight speed, the three-dimensional angle
between its flight trajectory and the plate, and
its distance to the plate when it entered the
plate zone. The 78 events of approaching the
plate (3.1 ± 1.8 events per individual, mean ± SD)
consisted of 25 “near collision” events, 13 “collision
with maneuver” events, and 40 “collision without
maneuver” events (movie S1). We found that
for “collision without maneuver” approaches,
bats produced fewer calls, spent less time in front
1Sensory Ecology Group, Max Planck Institute for
Ornithology, 82319 Seewiesen, Germany. 2Department of
Zoology, Tel Aviv University, Tel Aviv 6997801, Israel.
*Corresponding author. Email: firstname.lastname@example.org
†These authors contributed equally to this work.
‡Present address: Behaviour Ecology Research Group, Eötvös
Loránd University, H-1117 Budapest, Hungary.
§Present address: Institute of Ecology and Evolution, University of
Bern, 3012 Bern, Switzerland.
Fig. 1. Experimental setup. (A) Schematic of sound propagation at a smooth, vertical surface
(top view). For a bat within the red-dashed “plate zone,” sound impinging at an oblique angle is
reflected away while only the perpendicularly impinging sound is reflected back. (B) Flight tunnel
setup depicting the vertical situation. The smooth metal plate is shown in gray on the wall; the
dashed lines represent the plate zone. In the horizontal situation, the smooth plate was lying on the
floor of the plate zone (fig. S1).