Intensity control during target approach in echolocating bats; stereotypical sensori-motor behaviour in Daubenton's bats, Myotis daubentonii
School of Biological Sciences, University of Bristol, Woodland Road, BS8 1UG Bristol, UK
* Author for correspondence at present address: Department of Animal Physiology, Zoological Institute, University of Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany (e-mail): arjan.boonman{at}uni-tuebingen.de)
Accepted 11 June 2002
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: echolocation, sonar, compensation, feedback, target strength, Daubenton's bat, Myotis daubentonii
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bats are good model systems for studying the interaction between sensory
and motor systems because they use echoes from calls generated by their motor
systems as sensory information. Bats cannot perceive echo information about
their environment without sending motor commands to emit a call. By recording
the echolocation calls emitted by the bat, the investigator can monitor
precisely when and how bats update their sensory information. Bat echolocation
is highly adaptable and is strongly dependent on the surroundings of the bat
as well as the task that it faces
(Neuweiler, 1990;
Fenton, 1995
;
Schnitzler and Kalko,
2001
).
In this paper, the extent to which echolocation behaviour is stereotypical
or flexible is assessed in a bat responding to sensory information from a
target. Daubenton's bat (Myotis daubentonii) hunts in its natural
habitat for small insects by trawling and aerial hawking
(Jones and Rayner, 1988;
Kalko and Schnitzler, 1989
;
Vaughan, 1997
). Because of the
agility of its natural prey, Myotis daubentonii must have a short
response time, but to catch its prey also requires good accuracy. Here we have
studied the adaptation of signal intensity in echolocation behaviour during
prey capture.
Technical difficulties mean that the reduction of emitted signal intensity
in bats during target approach (intensity compensation) has only been studied
in three species: Pteronotus parnellii, Noctilio leporinus and
Eptesicus fuscus (Kobler et al.,
1985; Hartley et al.,
1989
; Hartley,
1992b
). None of these studies tested the flexibility of the bat to
adjust absolute emitted signal intensity, according to target size.
We assessed the flexibility of intensity compensation in Myotis daubentonii by offering the animals three target types (small sphere, big sphere or mealworm) that differed in dimensions and thus target strength. Our goal was to determine whether intensity compensation is independent of target type, or is tightly coupled to sensory input. Our hypothesis was that the intensity emitted by the bat becomes higher at small (weakly reflecting) targets, and lower at big (strongly reflecting) targets.
We also tried to determine whether emitted intensity of echolocation is
under closed-loop or open-loop (with no, or little, sensory feedback) control.
In greater horseshoe bats Rhinolophus ferrumequinum, accuracy in
emitted frequency to compensate for Doppler effect, matching a simulated
target speed, depends on how well the bat can hear the echo-frequency while
emitting the same pulse (Schuller,
1977). Greater overlap of the echo with the emission means a
higher degree of feedback, and greater accuracy in compensating the emitted
frequency for Doppler shift in the echo. This type of echolocation is
therefore under closed-loop, sensorimotor control, because its motor
production relies on sensory feedback.
In our experiments we assessed the degree of feedback control by measuring the flexibility in intensity compensation with changing sensory information. Our hypothesis was that if the intensity pattern is variable, the bats should show a reduced variability when plotted against distance as opposed to time. A corroboration of this hypothesis means that the pattern of intensity compensation is not a fixed motor programme, but relies on feedback of sensory information, and is therefore flexible.
We measured the differences in emitted intensity between attacks on targets with different reflective properties. Then we tried to find evidence that emitted intensity is controlled by feedback of sensory information (echoes) from the target, during (within) target approach. Answers to these questions will reveal more about the role of intensity perception in guiding the approach flight of echolocating bats.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We recorded the attack sequences of each bat until it had learned to reject
the non-mealworm targets. To eliminate the use of vision by the bats
(Hope and Bhatnagar, 1979), we
used a custom-made red filter (<10% transmission at <624nm) to reduce
the ambient light to a maximum level at the prey of 6.02x1017
photons m-2s-1.
Sphere target strengths
The frequency of echolocation pulses was between 20-100 kHz
(Fig. 1). The 3 mm sphere is
therefore in the Rayleigh scattering region, meaning that the reflection
strength is strongly frequency-dependent. The 9.5mm sphere is partly in the
region of geometric scattering where reflection strength depends little on
frequency (Stanton, 1990). To
calculate the absolute pressure of the echoes received by the bat, we first
calculated the target strength (TS) of the two spheres, assuming both
to lie in the region of geometric scattering, from Urick
(1983
):
![]() | (1) |
|
Experimental setup
During each attack by the bat on a target, we recorded the emitted
echolocation calls while using multiple-flash stereo photogrammetry
(Rayner and Aldridge, 1985;
Norman et al., 1998
) to
register that bat's positions over the duration of the attack. This technique
allowed a detailed reconstruction of the bat's flight path by inferring depth
from the disparity between two photographs, taken from two parallel camera
positions 0.249 m apart, enabling a reconstruction of the x, y and
z coordinates of a bat in real space
(Rayner and Aldridge, 1985
).
During the bat's approach to the target, eight flashes, each of 250 µs,
were fired at a rate of 15 Hz. Two Nikon FM2 cameras with opened shutters,
f-stop 16, 55 mm lenses, and loaded with ISO 400 film, recorded the bat's
position at each flash, while at the occurrence of each flash a spike was
recorded on a second channel of a Racal 4DS instrumentation recorder, along
with the received echolocation calls.
Using a magnifier, the film was later projected onto a sheet of A4 paper on
which was marked the bat's head. Images were subsequently digitised on a
SummaSketch III (Summagraphics) digitising tablet. The x, y and
z coordinates of the bat's head at each position and the target were
calculated using a custom-made programme written by J. M. V. Rayner. The bats'
trajectory was between 1.20 and 2 m in front of each camera. At 1.55 m from
the plane of the cameras, a cube with a mean side length of 14.92±0.18
cm could be reconstructed from stereo-pairs to have a side length of
14.67±0.13 cm (N=12 sides). The accuracy and rationale of the
photogrammetry, resection and differentiation algorithms are discussed in
detail by Spedding et al.
(1984) and Rayner and Aldridge
(1985
).
From the coordinates of the bat's consecutive positions we calculated the bat's speed relative to the microphone, using a Matlab routine. We used the flight speed data to align the sequence of pulses correctly with the flash spikes (see below).
When to open the shutters of the two cameras and trigger the flashguns was determined visually, helped by using a bat detector (Batbox III, Stag Electronics) set to a quiet level, to determine when the bat had taken off.
During the bat's approach to the target, we recorded the emitted echolocation calls by using a Larson Davis 2520, 1/4 inch microphone, with the grid (7 mm diameter) on, positioned 0.60 m behind the target, suspended on a thin string. Without the grid, the frequency response of the microphone was ±2 dB at 1-100 kHz. The difference in frequency response over the relevant frequency range with the grid on was measured and compensated for using an FIR filter in Matlab. The placement of the grid protected the delicate membrane as the bats frequently collided with the microphone. The microphone type was chosen and positioned to introduce as little clutter as possible behind the target while keeping directional sensitivity low.
The directionality of a comparable microphone (1/4 inch; Brüel &
Kjær) with grid is still very broad at 80 kHz (-6 dB beamwidth of
110°). Flying Daubenton's bats also show a low directionality (-6dB
beamwidth of 72-80°, ±40 kHz) in the emission pattern of
echolocation calls (Holderied,
2001), so neither position, flight direction nor head-aim of the
bat should strongly influence the intensity measurements in our
experiment.
The microphone signal was amplified using a Larson Davis 2200C amplifier and fed to a Racal 4DS Instrumentation recorder, operating at a tape speed of 0.762 m s-1. Before each session the system was calibrated using a constant test tone of 1 kHz at 114 dB sound pressure level (SPL), produced by a Dawe Instruments D-1411E calibrator. All bat calls recorded subsequently were related to the measured voltage, resulting from the test tone. For analysis, the tape was replayed at a speed 16 times slower than the recording speed, while the sequence of interest was digitised at a rate of 50 kHz and a bit-depth of 12 bits, using a National Instruments BNC 2110 soundcard. The effective sampling frequency was therefore 800 kHz. Two separate sequences belonging to a single attack, one consisting of the pulses emitted by the bat, and the other consisting of spikes denoting the occurrence of each flash, were stored as data files. All further analysis was performed by using Matlab v5.3, with Signal Processing, Data Acquisition and Statistics Toolboxes.
Reconstruction of pulse parameters
Each pulse arriving at the microphone was reduced in intensity because of
spherical spreading and atmospheric attenuation, and Doppler-compressed at
emission due to the bat's flight speed. Moreover, each pulse registered at the
tape was recorded later than it was produced, because of the travelling time
from bat to microphone, which in turn depended on the distance from the
microphone when the pulse was emitted.
The SPL at emission, at the target and at reception was reconstructed for a
point 10 cm in front of the bat's mouth by applying the laws of spherical
spreading, using frequency-dependent filtering to mimic the effects of
atmospheric attenuation (Bazley,
1976).
The SPL peak (in dB) was defined as the maximum voltage (not peak to peak)
of the reconstructed pulse relative to the reference voltage, as in:
![]() | (2) |
![]() | (3) |
The Doppler compression of the pulses received by the microphone was corrected by Doppler expanding the sequence of received echoes by a factor: c/(c-v), where v is the speed of the bat (m s-1). In the Doppler expansion, v was varied over the sequence, depending on the linearly interpolated bat's flight speed, which was obtained by means of multiple-flash stereo photogrammetry (see above). The technique used in Doppler-expanding the echo sequence can be compared with resampling the data at a sampling rate dependent on the bat's flight speed, but below the original sampling rate. The Doppler expansion of the sound sequence corrects the compression of the intervals between, and of the pulses themselves.
Because the x-axis of the sound sequence is scaled in time and not in delay units, the Doppler expansion does not correct the distance flown by the bat between pulse emission and reception by the microphone. The positional coordinates obtained with the photogrammetry programme are the coordinates of the bat, after having flown over a distance corresponding to the time it took for the pulse to arrive at the microphone. The further the bat is from the microphone, the longer it takes for the emitted pulse to reach the microphone and the longer the distance flown by the bat. The resulting error depends on distance and flight speed. The coordinates of the bat at pulse emission were recovered by Doppler-expanding the bat's distance to the microphone according to: Demission=Dreceptionc/(c-v), where Demission is distance to microphone at emission and Dreception is distance to microphone at reception. This Doppler expansion was applied as a time shift: [Dreception/(c-v)](Dreception/c), to each pulse in the sequence.
The latter Doppler expansion is independent of the expansion of the intervals between the pulses and of the pulses themselves, which is unrelated to microphone distance. With both corrective measures taken to align the sequence of pulses and the sequence of spikes, each representing a `flash' and its resulting spatial coordinates of the bat, the distance from bat to target is known from the moment at which the bat starts to emit a pulse.
We used a linear interpolation to estimate the distance between bat and
target at each sample point. A more formal mathematical description of the
methods used in reconstructing the emitted pulse at the distance of emission
can be found in Hartley et al.
(1989).
Statistical analysis was performed with Systat 10 (Systat, Chicago, Illinois, USA).
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Echolocation calls emitted by the bats 2-0.5 m from the target consisted of
a first harmonic sweeping from 100 kHz to 32 kHz in 1.5-2 ms
(Fig. 1). Because of the indoor
conditions, the lowest frequency may have been slightly higher, and the pulse
duration shorter far away from the target, than in outdoor conditions
(Kalko and Schnitzler, 1989).
But echolocation behaviour 0-1 m from the target showed no differences from
echolocation behaviour in the wild (Kalko
and Schnitzler, 1989
).
The four bats reduced the emitted intensity of their calls by 2.7-5.8 dB per halving of bat-to-target distance (Table 1). On average, the peak SPL was reduced by 3.9 dB and the rms SPL by 5.2 dB, per halving of target distance. For comparison with previous studies the intensity compensation behaviour was quantified from 0 to 70 cm from the target, using a logarithmic model. In all individuals, the main portion of the intensity reduction (±15 dB) took place between 0 and 30 cm from the target, showing a fairly linear decrease with distance (e.g. Fig. 3).
|
|
Intensity compensation occurred with all three target types. Disregarding atmospheric attenuation, the increase in peak and rms SPL received by the bat while halving its distance to the target could be calculated by subtracting the peak or rms SPL values listed in Table 1 from 12 dB. This means that, on average, the peak SPL in the bat's ears when approaching the target increased by 8.1 dB per halving of distance and rms SPL by 6.8 dB per halving of distance. Calculated received SPLs, which include the effects of atmospheric attenuation, showed an average increase per halving of distance of 8.4 dB for peak and 7 dB for rms SPL.
Compensation of absolute intensity
When the 9.5 mm sphere was replaced by the 3 mm sphere
(Table 2), three bats increased
the intensity (over all distances) of their emissions by only 4 dB.
|
The values displayed in Tables 2 and 3 are the constant terms of the log2 function describing the intensity compensation between 0 and 70 cm from bat to target, which serve well for mutual comparisons.
|
Fig. 2 shows the actual emitted and received peak SPLs by bat 1 at different distances from the target, with a slope of -6.9 dB per doubling of distance and an intercept for peak SPL of 41.7 dB. Table 3 shows that despite the small compensation in overall intensity, the echoes returning from the small sphere are still 17-18 dB weaker than the echoes returning from the big sphere at any distance. The bats therefore lower their emitted intensity over time to reduce the increase in received intensity from 12 dB to 7-8 dB per halving of distance, but they fail to alter the emitted intensity to compensate for a lowered target strength, even when the target strength is lowered by at least 17 dB.
|
Control of emitted intensity during target approach
On average, bats first reduced the intensity of their echolocation calls at
0.54±0.22 m (mean ± S.D.). This distance was measured as the
breakpoint between a logarithmic slope and a constant ceiling, obtained with a
piecewise nonlinear regression analysis, calculated per individual. The
compensation distance did not differ between peak and rms SPL (ANOVA,
F1,22=0.973, P=0.335), but differed significantly
between individuals, thus: 0.79 m, 0.63 m, 0.43 m and 0.30 m (ANOVA,
F3,20=27.157, P<0.001). Target type had no
influence on the distance at which the bat started to decrease emitted
intensity (ANOVA, F2,21=0.321, P=0.729). The
distance at which intensity was first reduced is therefore independent of the
intensity received by the bat. Thus, the distance at which intensity was first
reduced (0.54 m; minimum echo intensity, 30 dB at peak SPL) appears to be a
reactionrather than a detection-distance.
The hypothesis that emitted intensity is under closed-loop control from sensory information acquired during target approach predicts that changes in the execution of motor commands (intensity over time) should match the bat's consecutive positions relative to the target (intensity over distance). In other words, if feedback is used, emitted intensity over time should show more variability than the same intensity values plotted against distance. This is logical because a bat having access to information about its present distance to a target can adjust the emitted intensity better relative to the target than a bat lacking any updates on its position. For each target type and individual bat (N=12) we found that the fit of the regression line describing the decrease of intensity over the last 0.20 m to the target had mean squares residuals of 9.3 when plotted against distance and 12.6 when plotted over time. This difference is significant when tested in an ANOVA, as repeated measures within individual bats (ANOVA, F1,8=8.448, P<0.05) and within target type (ANOVA, F1,9=6.306, P<0.05). The mean-squares residuals were not different between individuals (ANOVA, F3,8=1.201, P=0.370) or target type (ANOVA, F2,9=1.429, P=0.289).
From the difference in the mean squares residuals between the data plotted against distance and against time, we conclude that the emitted intensity by bats shows less variability over distance than over time (see bat 4 in Fig. 3). The two attack sequences on a small sphere are not typical of all attacks, but illustrate the hypothesis that bats use received sensory information to `scale' emitted intensity to target distance.
Since the reduction in variability of intensity over distance, relative to the same intensity over time, was not apparent in all attacks, the problem was assessed in a different way. Flights with extreme rates of intensity reduction over time should result in less extreme rates of intensity reduction over distance. 13 of 50 flights showed extreme intensity reduction rates over time, so we plotted them against the reduction rates over distance of the same flights as y-values. If the bat adjusted its emitted intensity over time to its distance to the target, a slope in intensity reduction that deviates strongly from the average slope (5.2 dB rms SPL per halving of distance) over time, should be closer to the average slope when plotted against distance.
Plotting the two rates of the 13 flights against each other resulted in a best fit of `distance' rate (dB m-1)=0.914 x `time' rate (dB s-1) + 10.9. Both rates are expressed as the percentage deviation from the average rate of reduction over distance and time, respectively. The fitted regression line shows that deviation from the average fit along distance is 91.4% as high as the deviation from the average fit along time. This means a reduction of 8.6% in the variability of the slope around the average intensity reduction of the same data when plotted against distance, instead of time.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The absolute intensity emitted by Myotis daubentonii when flying 1
m from the target in this study is equal to the intensity emitted by other
Myotis bats while flying indoors, but measured by using a different
method (Waters and Jones,
1995). However, the measured absolute intensity of 103-105 dB rms
SPL (at 10 cm from the mouth), referred to above, is somewhat lower than the
intensity used by Myotis daubentonii while flying outdoors (103-113
dB rms SPL; Rydell et al.,
1999
). Measured from a distance of 70 cm to the target (to allow
direct comparison with previous studies), the emitted peak SPL is reduced by
about 4 dB per halving of distance, and the rms SPL by about 5 dB per halving
of distance. It is not surprising that the intensity compensation expressed as
rms SPL is higher than when expressed as peak SPL, since the peak of the
emitted waveforms is expected to increase with decreasing pulse duration,
close to the target.
A surprising result of this study, however, is that the target strength does not influence the emitted intensity by the bat. Table 2 shows that the emitted absolute intensity by the bat remained the same, even when the target strength was reduced by 17-18 dB. Weak echoes, do not apparently trigger the bat to call louder when approaching a target. The distance at which the bats started to reduce emitted intensity was also independent of echo strength, since it was independent of target type. One explanation is that the bats either waited for a preferred distance after the detection of the target to start intensity compensation, or they started to compensate intensity according to an expectation of the target's position. In either case, the received absolute intensity would have no influence on the initiation of intensity reduction by the bat.
The intensity compensation during target approach also stayed the same (4-5 dB per halving of distance) for each target type, as would be expected, as spreading losses will be similar between targets. Intensity compensation in Myotis daubentonii therefore appears to be a stereotyped behaviour, since the emitted absolute intensity is not related to the size of objects in the bat's environment.
The values of intensity compensation found in this study are somewhat lower
than those found in previous studies, which reported a decrease in emitted
peak SPL of 6 dB per halving of distance in both Noctilio leporinus
and Eptesicus fuscus (Hartley et
al., 1989; Hartley,
1992b
). In our experiments, we took care to introduce as little
background clutter as possible, using a small microphone positioned 0.60 m
behind the target, whereas in the study by Hartley et al.
(1989
), a radar apparatus was
positioned 0.28 m behind the target. However, if the bats in the study by
Hartley et al. also compensated their emitted intensity for the echoes
reflecting off the radar apparatus, whereas the bats in this study only
compensated for the target, we would expect our bats to compensate more
strongly than the bats in the study by Hartley et al.
(1989
). In fact, the opposite
trend was found. We therefore assume that the difference in magnitude of the
intensity compensation between the bats in this study and those in Hartley's
study truly reflects a behavioural difference between the different species
used in the two studies.
Feedback
One of the main questions we sought to answer is whether intensity
compensation in bats is a fixed, pre-programmed behaviour, or whether emitted
intensity is precisely adjusted to the sensory conditions at any instant. The
results in this paper show that any possible information that a bat receives
about the reflection strength of a target is not relayed to adjust the
intensity of its emissions, indicating a lack of feedback. Our first
hypothesis, stating that the intensity emitted by the bat becomes higher at
weakly reflecting targets, and lower at strongly reflecting targets, is
therefore rejected. Changing echo intensity information of a target
between attacks is not used by the bat to adjust its emitted call
intensity.
This study also tests the hypothesis that a bat uses feedback information from target intensity or distance to adjust emitted intensity over time within target approach. If a bat adjusted the emitted intensity over time to its changing distance from a target, a plot of intensity reduction over time may look chaotic, but the same pattern should precisely fit a specific decrease of emitted intensity along distance. However, if the emitted intensity pattern is a fixed behaviour, independent of sensory feedback, the fit of the decreasing intensity should be equally tight whether plotted against time or distance. Our results do show a better fit in the regression line between intensity against distance than against time, over the last 20 cm of target approach. However, the differences in variability between time and distance were often small and could have been caused by a more non-linear relationship with intensity over time than over distance. Fig. 4 shows that, also in terms of slope, a deviant rate (from the average rate) of intensity compensation against time is only slightly less deviant (9 %) when expressed as intensity compensation against distance.
|
The data therefore provide some evidence that the emitted intensity could
be fine-tuned to fit the target distance by using sensory feedback during
target approach. However, the improvements achieved by controlling emitted
intensity to match target distance are marginal, and the variation in emitted
intensity during each flight appears considerable. A reason for this could be
the bat's wing movements, which may influence the intensity produced during
the last 20 cm of target approach caused by the physiological coupling of wing
beats with pulse emission (for a review, see
Wong and Waters, 2001).
Because of the inaccuracy of intensity compensation, it seems unlikely that
the bat uses feedback information from each target echo to control each
subsequent emission. Furthermore, the fact that the bat achieves similar
accuracy in intensity compensation for mealworm targets as for the spheres
shows that feedback information, at least on reflected intensity, is unlikely
to be used. This is because mealworms, as opposed to spheres, have a strong
aspect-dependent target strength that will introduce extra variability in
returned intensity on top of the received intensity, depending on
distance.
In conclusion, most of the variation found in intensity compensation seems to be present without any apparent reason. It could be caused partly by the changing head aim of the bat, or by its wing beats influencing emitted intensity. We found no conclusive evidence in this study that emitted intensity during target approach by echolocating bats is not a fixed, pre-programmed behaviour.
Reasons for intensity compensation
During pulse emission, some bats contract the middle ear muscle in the ear
to attenuate the sensitivity of the ear
(Henson, 1965;
Suga and Jen, 1975
). The
sensitivity of the hearing system is quickly regained after emission at a
certain rate. This system is called automatic gain control (AGC). The
attenuation applied by AGC is necessary to protect the hearing system from
over-stimulation due to loud sound SPL levels during the emission. Suga and
Jen (1975
) found that this
contraction leads to a maximum attenuation of 20-30 dB, with the middle ear
muscle starting to relax after pulse emission. The relaxation period during
which sensitivity of the hearing system returns to normal lasts for
approximately 5 ms. This means that when the echo returns within 5 ms after
the emission (distance to target <0.86 m), the echo will be attenuated,
depending on its delay. Kick and Simmons
(1984
) and Simmons et al.
(1992
) measured the increase
in hearing sensitivity per doubling of distance (or delay) in Eptesicus
fuscus to be 11-12 dB. However, Hartley
(1992a
) found AGC to result in
an increase in hearing sensitivity of 6-7 dB per doubling of target distance
in both Eptesicus fuscus and Noctilio leporinus.
Hartley (1992b) suggested
that the phenomenon of intensity reduction may serve to make a bat perceive
each echo during target approach as equally loud. If the 11-12 dB recovery
rate of the AGC system in Eptesicus fuscus reported by Simmons et al.
(1992
) is correct, the bat
should reduce emitted intensity during target approach by 0-1 dB per halving
of distance. Hartley's (1992a
)
data suggest a reduction of 5-6 dB per halving of distance, which he reported
behaviourally (Hartley 1992b
).
Patheiger (1998
) found a
recovery slope of the AGC system of 3.6-5.7 dB per doubling of distance around
0.90 m from the target and 7-7.3 dB per doubling of distance around 0.60 m
from the target in Eptesicus fuscus, indicating that the increase in
sensitivity depends on the distance to the target, with the quickest recovery
at a close distance to the target. If Patheiger's data are also valid for
Daubenton's bat, the combined effects of AGC and the measured intensity
compensation of 4 dB per halving of distance would lead to a constant
sensation level of all received echoes during target approach.
An important question is why bats would need to have a constant sensation
level to all returning echoes. At present, we are unable to answer this
question. This paper only provides evidence to disregard the importance of a
constant cochlear stimulation level as a possible reason for bats to exhibit
intensity compensation. In our study, differences in the reflected absolute
intensity received from different target types only had a marginal (maximum 4
dB) influence on the emitted absolute intensity
(Table 2). At the same time,
the bats would steer themselves to any of the targets in the same way and
capture them equally well. These results show that Daubenton's bats can
process echoes over a dynamic range of at least 35 dB. This figure is derived
from a conservative estimate of the weakest echoes the bat can detect from the
small sphere, 30 dB at peak SPL at 54 cm (the average reaction distance; see
Results) from the target, and the average intensity it will receive from the
big sphere at 10 cm (65 dB at peak SPL). A cochlear dynamic range of at least
35 dB would certainly be typical of mammals, whose hearing systems are mostly
sensitive from 0-10 dB SPL to more than 60 dB SPL (>50 dB dynamic range)
(Fay, 1994). Most of the
investigated bats respond to acoustic stimuli between 0 and at least 70 dB SPL
(Moss and Schnitzler, 1995
).
Therefore, we conclude that the hearing system of bats should be able to
process all returning echoes, even without the bat performing any intensity
compensation. Our conclusion is therefore that Daubenton's bats hardly adjust
emitted intensity to target strength, but do reduce intensity when approaching
a target for reasons other than optimising the stimulation level of the
cochlea.
Conclusions
We found no strong indications that echolocating Myotis
daubentonii adjust emitted intensity continuously to the changing sensory
information during target approach. Presentation of targets with different
target strengths between trials did not significantly affect the emitted
intensity. Our data suggest that intensity compensation may be a fixed,
open-loop behaviour under little or no control once initiated. Thus, during
target approach, Myotis daubentonii will be unlikely to use received
intensity as an important guiding parameter to control its flight because it
is quite variable, and based on a fixed programme, rather than a physical
parameter changing instantaneously with the bat's movements. Our results have
important consequences for treating intensity as a possible `acoustic flow'
parameter in studies of acoustic scene analysis in bats
(Lee et al., 1992;
Müller and Schnitzler,
1999
; Moss and Surlykke,
2001
).
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Au, W. W. L. (1993). The Sonar of Dolphins. New York: Springer.
Bazley, E. N. (1976). Sound Absorption in Air at Frequencies up to 100 kHz. NPL Acoustics report Ac74. Teddington, UK: National Physics Laboratory.
Fay, R. R. (1994). Comparative auditory research. In Comparative Hearing: Mammals (ed. R. R. Fay and A. N. Popper), pp. 1-17. New York: Springer.
Fenton, M. B. (1995). Natural history and biosonar signals. In Hearing by Bats (ed. A. N. Popper and R. R. Fay), pp. 37-86. New York: Springer.
Griffin, D. R. (1958). Listening in the Dark. New Haven: Yale University Press.
Hartley, D. J. (1992a). Stabilization of perceived echo amplitudes in echolocating bats. I. Echo detection and automatic gain control in the big brown bat, Eptesicus fuscus, and the fishing bat, Noctilio leporinus. J. Acoust. Soc. Am. 91,1120 -1132.[Medline]
Hartley, D. J. (1992b). Stabilization of perceived echo amplitudes in echolocating bats. II. The acoustic behavior of the big brown bat, Eptesicus fuscus, when tracking moving prey. J. Acoust. Soc. Am. 91,1133 -1149.[Medline]
Hartley, D. J., Campbell, K. A. and Suthers, R. A. (1989). The acoustic behavior of the fish-catching bat, Noctilio leporinus, during prey capture. J. Acoust. Soc. Am. 86,8 -27.
Heisenberg, M. and Wolf, R. (1988). Reafferent control of optomotor yaw torque in Drosophila melanogaster. J. Comp. Physiol. 163,373 -388.
Henson, O. W. (1965). The activity and function of the middle-ear muscles in echo-locating bats. J. Physiol. 180,871 -887.[Medline]
Holderied, M. (2001). Akustische Flugbahnverfolgung von Fledermäusen: Artvergleich des Verhaltens beim Suchflug und Richtkarakteristik der Schallabstrahlung. PhD thesis, University of Erlangen-Nürnberg, Germany.
Hope, G. M. and Bhatnagar, K. P. (1979). Electrical response of bat retina to spectral stimulation: Comparison of four microchiropteran species. Experientia 35,1189 -1191.[Medline]
Jones, G. and Rayner, J. M. V. (1988). Flight performance, foraging tactics and echolocation in free-living Daubenton's bat, Myotis daubentoni (Chiroptera: Vespertilionidae). J. Zool. 215,113 -132.
Kalko, E. K. V. and Schnitzler, H. U. (1989). The echolocation and hunting behavior of Daubenton's bat, Myotis daubentoni. Behav. Ecol. Sociobiol. 24,225 -238.
Kick, S. A. and Simmons, J. A. (1984). Automatic gain control in the bat's sonar receiver and the neuroethology of echolocation. J. Neurosci. 4,2725 -2737.[Abstract]
Kobler, J. B., Wilson, B. S., Henson, O. W. and Bishop, A. L. (1985). Echo intensity compensation by echolocating bats. Hear. Res. 20,99 -108.[Medline]
Lee, D. N., van der Weel, F. R., Hitchcock, T., Matejowski, E. and Pettigrew, J. D. (1992). Common principle of guidance by echolocation and vision. J. Comp. Physiol. A 171,563 -571.[Medline]
Moss, C. F. and Schnitzler, H. U. (1995). Behavioral studies of auditory information processing. In Hearing by Bats (ed. A. N. Popper and R. R. Fay), pp.87 -145. New York: Springer.
Moss, C. F. and Surlykke, A. (2001). Auditory scene analysis by echolocation in bats. J. Acoust. Soc. Am. 110,2207 -2226.[Medline]
Müller, R. and Schnitzler, H. U. (1999). Acoustic flow perception in cf-bats: properties of the available cues. J. Acoust. Soc. Am. 105,2958 -2966.[Medline]
Neuweiler, G. (1990). Auditory adaptations for
prey capture in echolocating bats. Physiol. Rev.
70,615
-641.
Norman, A. P., Teagle, L. and Jones, G. (1998). A method for the synchronisation and control of ultrasound recording and stereophotogrammetry for the reconstruction of animal flight. Bioacoustics 9,207 -212.
Patheiger, S. (1998). Die Detektionsschwelle von Eptesicus fuscus bei verschiedenen Zielentfernungen. `Diplomarbeit' (Masters thesis) University of Tübingen, Germany.
Rayner, J. M. V. and Aldridge, H. D. J. N. (1985). Three-dimensional reconstruction of animal flight paths and the turning flight of microchiropteran bats. J. Exp. Biol. 118,247 -265.
Rydell, J., Miller, L. A. and Jensen, M. E. (1999). Echolocation constraints of Daubenton's bat foraging over water. Funct. Ecol. 13,247 -255.
Schnitzler, H. U. and Kalko, E. K. V. (2001). Echolocation by insect-eating bats. Bioscience 51, 557-569
Schuller, G. (1977). Echo delay and overlap with emitted orientation sounds and Doppler-shift compensation in the bat, Rhinolophus ferrumequinum. J. Comp. Physiol. 114,103 -114.
Siemers, B. M., Stilz, P. and Schnitzler, H. U.
(2001). The acoustic advantage of hunting at low heights above
water: behavioural experiments on the European `trawling' bats Myotis
capaccinii, M. dasycneme and M. daubentonii. J. Exp.
Biol. 204,3843
-3854.
Simmons, J. A., Moffat, A. J. M. and Masters, W. M. (1992). Sonar gain control and echo detection thresholds in the echolocating bat, Eptesicus fuscus. J. Acoust. Soc. Am. 91,1150 -1163.[Medline]
Spedding, G. R., Rayner, J. M. V. and Pennycuick, C. J. (1984). Momentum and energy in the wake of a pigeon (Columba livia) in slow flight. J. Exp. Biol. 111,81 -102.
Stanton, T. K. (1990). Sound scattering by spherical and elongated shelled bodies. J. Acoust. Soc. Am. 88,1619 -1633.
Suga, N. and Jen, P. (1975). Peripheral control of acoustic signals in the auditory system of echolocating bats. J. Exp. Biol. 62,277 -311.[Abstract]
Urick, R. J. (1983). Principles of Underwater Sound. New York: McGraw-Hill.
Vaughan, N. (1997). The diets of British bats (Chiroptera). Mammal. Rev. 27, 77-94.
Waters, D. A. and Jones, G. (1995).
Echolocation call structure and intensity in five species of insectivorous
bats. J. Exp. Biol. 198,475
-489.
Wong, J. G. and Waters, D. A. (2001). The
synchronisation of signal emission with wingbeat during the approach phase in
soprano pipistrelles (Pipistrellus pygmaeus). J. Exp.
Biol. 204,575
-583.