Echolocation call intensity in the aerial hawking bat Eptesicus bottae (Vespertilionidae) studied using stereo videogrammetry
1 School of Biological Sciences, University of Bristol, Bristol,
UK
2 Mitrani Department of Desert Ecology, Blaustein Institute for Desert
Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion,
Israel
3 Department of Biology, York University, Canada
4 School of Biological Sciences, University of Auckland, Auckland, New
Zealand
5 Department of Geomatic Engineering, University College London,
UK
* Author for correspondence (e-mail: mholderi{at}biologie.uni-erlangen.de)
Accepted 31 January 2005
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: bat echolocation, source level, flight paths, videogrammetry, flight speed
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
There have been few measurements of call intensities from bats flying in
open spaces, due to the technical difficulties of implementing such methods in
the field (Holderied and von Helversen,
2003; Surlykke et al.,
1993
, Jensen and Miller,
1999
). Recent measurements made with microphone arrays have shown
that even small aerial feeding species, such as Pipistrellus
pipistrellus, are calling with intensities as high as 128 dB
peak-equivalent sound pressure level (peSPL) at 10 cm
(Holderied and von Helversen,
2003
). These estimates are considerably higher than previously
published estimates of call intensity in aerial feeding bats, and are among
the most intense airborne animal vocalisations recorded in nature.
The knowledge of the source level of an echolocation call, allows
estimating the echolocation range i.e. the distance range of targets (e.g.
flying insect prey) accessible with this call. Yet, the echolocation range is
not an exclusively spatial aspect of an echolocation system but also affects
temporal aspects: due to the constant speed of sound in air, echolocation
range corresponds to a time window of possible echo delays. For low duty-cycle
bats (such as E. bottae), which remain silent while listening for
returning echoes, this affects the decision when to produce the next call.
Choosing a pause between two calls longer than the maximum possible echo delay
means waiting for further echoes in vain and wasting time better invested to
update echolocation information. Calling more frequently and thus before the
last echo from the previous call would have arrived, brings callecho
assignation problems arising from late echoes arriving after the next call.
Indeed, Holderied and von Helversen
(2003) found that the window of
possible echo delays for flying insect prey matches the preferred time
interval between calls in eleven European bat species. By this match these
bats maximise calling rate, and consequently temporal information flow while
searching for flying insect prey, without risking callecho assignation
problems. Surprisingly, this window of possible echo delays also matches the
average duration of the wing beat in these species the so-called `wing
beat window' (Holderied and von Helversen,
2003
). This second match helps bats to reduce the costs for the
production of their intense echolocation signals: by coupling call emission to
their wing beat, they can utilize the increased lung pressure generated by the
work of the flight muscles for their vocalizations
(Heblich, 1986
;
Speakman and Racey, 1991
;
Wong and Waters, 2001
).
In this study we describe the echolocation and search flight behaviour of
the medium-sized (89 g) bat species Eptesicus bottae
(Vespertilionidae). Specifically, we test the predictions: (1) that the bats
will call at intensities within the range of aerial feeding, open space
foraging species of similar sizes studied in Europe, i.e. between
120133 dB peSPL at 10 cm (Holderied
and von Helversen, 2003
); and (2) that the effective range of
echolocation calls of E. bottae matches its pulse interval and
`wing-beat window'. A major aim of this study is to introduce videogrammetry
as a valid method for flight path tracking of bats in the field, and to
determine whether our estimates of call intensity correspond with measurements
from other species studied with well-established acoustic tracking
methods.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
To record bat calls we used a Brüel & Kjær 2670 preamplifier (Nærum, Denmark), 5935L power supply and a National Instruments (Austin, TX, USA) analog to digital converter card (DAQCard 6062E) in combination with a Toshiba SP6100 Laptop running the Avisoft Triggering Harddisk Recorder (AVISOFT, Berlin, Germany). Calls were sampled with 12-bit precision at 29,4117 Hz. For synchronisation purposes a 31 Hz rectangular sound pulse of 460 ms duration was produced by a custom-built pulse generator, and played after each bat pass simultaneously to the ultrasound input of the DAQCard 6062E and sound channels of both cameras.
Cameras were supported on separate tripods to confer flexibility in their
on-site orientation (positioning and pointing direction) to optimise the
measurement accuracy and volumetric coverage of the system to the specific
recording situation. This flexibility was achieved using multiple images of a
3D calibration frame in combination with photogrammetric bundle adjustment
techniques (Granshaw, 1980).
These enabled the simultaneous mathematical determination of the relative
orientation and internal geometric imaging properties of each camera to be
determined (calibration). For the purposes of this work, the calibration frame
consisted of a cross with 2 m diameter carrying 48 reflecting point targets at
pre-determined locations. To achieve accurate measurements over the complete
volume of interest, the target array was imaged in many different places in
the field of view of both cameras. Coverage of the whole volume where bats are
expected could be achieved with the exception of the limit set by the edge of
the gorge. The relative orientation of both cameras were then derived from
each pair of corresponding images from the two cameras by evaluating the
position of all visible targets in both images. Combining information from a
large number of image pairs not only increases the accuracy of the relative
orientation of the two cameras, but also ensures that the geometric
characteristics (particularly lens distortion) of the video cameras is known
at the time that the images are taken. These accurate camera orientations can
then be used for triangulating the position of flying bats, or any other
objects of interest, visible in both cameras' field of view by the method of
intersection (Shortis et al.,
2000
). All photogrammetric computations were carried out within
Vision Measurement System VMS software
(www.geomsoft.com).
The two video recordings of each bat pass were synchronised to the frame level (25 Hz) and grabbed using the software EditDV 2.01 (Digital Origin). To increase contrast between bat and background the pixelwise difference between consecutive frames was calculated. Then the two fields of each frame were separated (deinterlaced) and the missing lines interpolated resulting in an actual frame rate of 50 Hz. Finally the head of the bat was indicated manually in each frame in both videos and the xy-image coordinates stored. The remaining phase shift between the two videos, which could amount up to 1/25 s, i.e. 40 ms, was assessed from the synchronisation pulse on the audio tracks with an accuracy of 1 ms. The actual xy-image coordinates of the bat in the second video at the exact times the corresponding frames in the first video were taken was then interpolated accordingly. These steps were all performed with a custom-written program created using the freeware software packages VirtualDub v1.4.10 (by Avery Lee; www.virtualdub.org) and Avisynth v0.3 (by Ben Rudiak-Gould; www.avisynth.org).
Triangulation, based on the list of xy-image coordinates from both cameras, was performed with VMS software resulting in a series of consecutive 3D localisations for the period of time a bat was visible to both video cameras. The calculated bat localisation precision is of the order of a few mm. A spline was fitted to localisations using MATLAB (The MathWorks Inc., Natick, MA, USA) functions thus creating an interpolated flight path. x-, y- and z-coordinates over time were interpolated separately. Instantaneous flight speed as well as the relative position and flight speed of the bat towards the microphone could then be calculated. In the synchronised sound recordings all bat calls were selected manually with their respective recording time. The actual time and position the bat had produced each individual call was then determined taking the flight speed of the bat and the speed of sound in air into account.
Call source levels were determined as described by Holderied and von
Helversen (2003). The
microphone was amplitude calibrated at the beginning and the end of each
recording session with an acoustical calibrator (D-1411E; Dawe Instruments,
England). Geometric and atmospheric attenuation of the sound on its way from
the bat to the microphone were calculated for the peak frequency of the call
(Bazley, 1976
) using
temperature and relative humidity as measured at the study site at the time of
the recording. The directionality of the recording microphone at the relevant
frequencies (
30 kHz) is very broad (e.g.
Pye, 1993
) and thus results in
a maximum potential underestimation of 2 dB in the measured source levels.
Even though we selected for bats approaching the microphone (11 out of 17
flight paths), it remains uncertain whether they also directed the acoustic
axis of their sonar beam towards the microphone. Due to this effect, some of
the measured call source levels will be slight underestimates. Unless we know
the exact shape and orientation of the sonar beam for every single call it is
not possible to control for this error. Yet, the maximum source level will, if
at all, only be moderately affected: first because call beam patterns in this
genus are rather broad (Ghose and Moss
2003
) and second because it is very likely that bats approaching
the microphone occasionally have pointed their acoustic beam axis in this
direction by chance. All source levels are given in dB peSPL (peak equivalent
sound pressure level) re 20 µPa, i.e. the sound pressure level of a
continuous pure tone of the same amplitude, and were related to a reference
distance of 10 cm in front of the sound source.
We calculated detection distances for two types of targets that differ in
their echo spreading: point sources, which generate a spherically spreading
echo (e.g. flying insects or conspecifics) and large plane background objects,
which mirror the sound back to the source (e.g. a water surface from above or
a wall from the front). In both cases, we assumed a target strength of 0 dB in
10 cm, i.e. the target sends all the impinging acoustic energy back into the
direction of sound incidence, which serves as an approximation to the largest
possible targets of each type (e.g. conspecifics or flying predators). Maximum
detection distances for calls were calculated using the sonar equation
(Møhl, 1988) for the
loudest call encountered in each species as described in Holderied and von
Helversen (2003
). Spreading
losses of call and echo were determined according to Bazley
(1976
). The detection threshold
of the bats was assumed to be 0 dB SPL
(Coles et al., 1989
;
Kick, 1982
;
Neuweiler et al., 1984
).
Call duration was determined from the spectrogram using Avisoft SASLab Pro
v4.2 (window length 512 Hanning; 100% frame; 93.75% overlap). Pulse interval
was measured from the start of one pulse to the start of the next. Peak
frequency and bandwidth 15 dB below the peak frequency were measured from the
mean spectrum of the entire call (window length 512, Hanning). Aerial hawking
bats frequently couple call emission to their wing beat to reduce the
energetic costs for echolocation (Heblich,
1986; Speakman and Racey,
1991
; Wong and Waters,
2001
). As they also skip calls, the distribution of pulse interval
has several peaks reflecting integer multiples of the wing beat period
(Holderied and von Helversen,
2003
). The lowest peak was taken as a measure of the mean wing
beat period. Approach phase signals were distinguished from search phase ones
qualitatively from a prolonged increase in pulse repetition rate (usually
leading to a `terminal buzz'), an associated decrease in call duration, and a
change in call shape to more broadband signals.
Wing morphology was measured for E. bottae from bats captured in
the study area (data from Korine and
Pinshow, 2004). Wing shape parameters follow definitions by
Norberg and Rayner (1987
).
Theoretical optimum flight speeds (minimum power speed and maximum range
speed) were calculated based on these measurements using the software Flight
for windows v1.12 (Pennycuick,
1975
; Pennycuick,
1989
). This program does not aim to calculate the full airflow
around the wing or to model the aerodynamic forces directly but to provide a
simplified model based on flight mechanics `to represent those features and
processes that mostly determine the work and power.' One advantage of this
model is that most of the necessary morphological parameters are easily
obtained from live animals (i.e. mass, wing span and wing area).
Eptesicus bottae (Peters 1869) is a small/medium-sized
vespertilionid bat (body mass 79 g, average forearm length 41.4 mm,
N=21, Korine and Pinshow,
2004) found throughout Egypt, Middle East, Iraq, Turkestan and
Afghanistan (Qumsiyeh, 1985
).
Recently it has been recorded in Rhodes, Greece
(von Helversen, 1998
) and
Turkey (Spitzenberger, 1994
).
In the study area, E. bottae typically foraged within 25 m of
the gorge edge, and also around streetlights and above water. It is found in
the study area all year round (Korine and
Pinshow, 2004
).
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Echolocation behaviour
Representative calls in different situations (search phase, approach phase
and feeding buzz) are illustrated in Fig.
2, with summary data from search phase calls in
Table 1. In search flight
E. bottae emitted calls of 7 ms duration and a peak frequency of
32.5 kHz, a bandwidth of 8.7 kHz, and a mean pulse interval of 155.6 ms,
giving an average call repetition rate of 8.42 Hz and a duty cycle of 7.6%.
The distribution of pulse intervals was multimodal
(Fig. 3): 67% of calls were
followed with a pulse interval at the lowest peak; the remainder of calls had
double (29%) or sometimes triple (4%) this interval showing that the bats
skipped emitting calls relatively frequently. We did not observe multiple
calls per wing beat during search phase. Feeding buzzes were emitted during
pursuit of aerial prey (Fig.
4). Calls decreased in peak frequency towards the end of the buzz,
though there was no evidence in any of five buzzes with high signal:noise
ratios of a clearly defined `buzz II'
(Kalko and Schnitzler, 1993
).
Calls became shorter and more broadband during the approach and terminal
phases, with shortest calls recorded at the end of terminal buzzes
(Fig. 2).
|
|
|
|
Flight morphology and behaviour
Eptesicus bottae had a mean mass of 8.1±1.2 g, wingspan of
28.16±0.7 cm and wing area of 112.6±6.7 cm2
(N=6). This species flew at an average speed of 5.70 m
s1 (17 flight paths)
(Fig. 5). This is close to the
minimum power speed (5.3 m s1), and well below the maximum
range speed (mechanical power: 8.7 m s1) predicted according
to Pennycuick (1975,
1989
).
|
Call intensities
All calls from 11 individual flight paths recorded on several days were
analysed. During the recordings always more than one bat was present at the
recording site. Thus, we are confident that source levels originate from
several individuals. For E. bottae, calls recorded close to the
microphone showed relatively low source levels (typically 105115 dB
peSPL in 10 cm with the bat 23 m from the microphone;
Fig. 6). These lower
intensities were recorded from approach and early terminal phase calls as the
bat unsuccessfully pursued an insect flying near the microphone. Beyond 3 m
from the microphone, source levels were independent of the distance between
bat and microphone (E. bottae: distance to microphone 3.18.7
m;regression coefficient 0.198 dB m1;
R2=0.002; F1,44=0.0889;
P=0.77), giving confidence that there were no distance-related errors
in our intensity estimates. Source levels during search phase frequently
exceeded 120 dB, with maximum levels of 133 dB peSPL (121±7.8 dB mean
± S.D.). Intensity showed a significant relationship with
call duration: approach phase calls of short duration, emitted close to
targets, had lowest intensities (Fig.
6; R2=0.50; F1,64=64,
P<0.0001).
|
Maximum detection distances and echo delays
The calculated maximum detection distance of E. bottae for very
large flying targets (0 dB target strength) is 20.8 m, which corresponds to a
maximum echo delay of 120 ms (temperature and relative humidity at the time of
recording at the recording microphone: 25.2°C, 54%). This is in almost
perfect agreement with the lowest peak in the frequency distribution of pulse
intervals (119 ms, Table 1),
which in aerial hawking bats corresponds to the mean wing beat period
(Jones, 1994). The maximum
detection distance for background targets is 38.8 m (maximum echo delay = 224
ms).
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Eptesicus bottae shows echolocation behaviour similar to that of
other aerial-feeding vespertilionid bats. In open space, calls are relatively
narrowband and adapted for detection
(Parsons et al., 1997;
Schnitzler and Kalko, 2001
).
The call design resembles that of its larger (
23 g) congener E.
serotinus (e.g. Jensen and Miller,
1999
), except call frequencies are higher, and pulse duration and
intervals are shorter; trends expected for a bat of smaller size
(Jones, 1999
). The mean call
peak frequency of 32.5 kHz is considerably higher than in the individuals from
Rhodes (28 kHz; von Helversen,
1998
) perhaps because in Israel this species is smaller than in
Turkey (Spitzenberger, 1994
).
As insect prey is approached during foraging, call repetition rate increases
and calls become shorter (so pulseecho overlap is avoided), as well as
more broadband, favouring localisation rather than detection
(Parsons et al., 1997
;
Schnitzler and Kalko,
2001
).
Our main aim was to determine call intensities for bats foraging in open
spaces. The values measured for E. bottae (up to 133 dB;
121±7.8 dB mean ± S.D.) are similar to those reported
by Holderied and von Helversen
(2003) for other aerial hawking
species: these values are among the highest source levels recorded for
vocalising animals. Source levels are similar to and sometimes higher than
those reported by Jensen and Miller
(1999
) for E.
serotinus.
The interval between consecutive echolocation pulses gives insight into
understanding echolocation ranges, as it makes no sense for a bat to delay the
production of a call if the echo from the outer limit of its echolocation
range has already arrived. In E. bottae, we found a very good
agreement between the calculated maximum detection distance for flying targets
and the mean wing beat period (assuming the bat produces one call per wing
beat in search phase). This corroborates the finding of Holderied and von
Helversen (2003; their fig. 3c)
that aerial-hawking vespertilionids generally match wing beat period and
detection range for flying targets. The tendency of E. bottae to skip
one call in 29% of cases, can be interpreted as an adjustment to more distant
planar background targets: twice the wing beat period is 238 ms, which
corresponds well to the maximum echo delay for background targets of 224 ms.
This species cannot expect echoes to arrive later than that, therefore it
makes sense that it very rarely skips more than one call (4% of calls,
N=5). E. bottae fits well with other vespertilionids studied
in this respect (Holderied and von
Helversen, 2003
). The full corroboration of findings from other
species obtained with different tracking methods gives further evidence that
stereo videogrammetry is a functional method for the study of free-ranging
bats in field.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bazley, E. N. (1976). Sound absorption in air at frequencies up to 100 kHz. In National Physics Laboratory Acoustics Report. No. Ac 74. National Physics Laboratory, Teddington, UK.
Britton, A. R. C. and Jones, G. (1999).
Echolocation behaviour and prey-capture success in foraging bats: laboratory
and field experiments on Myotis daubentonii. J. Exp.
Biol. 202,1793
-1801.
Britton, A. R. C., Jones, G., Rayner, J. M. V., Boonman, A. M. and Verboom, B. (1997). Flight performance, echolocation and foraging behaviour in pond bats, Myotis dasycneme (Chiroptera: Vespertilionidae). J. Zool. Lond. 241,503 -522.
Coles, R. B., Guppy, A., Anderson, M. E. and Schlegel, P. (1989). Frequency sensitivity and directional hearing in the gleaning bat, Plecotus auritus. J. Comp. Physiol. A 165,269 -280.[Medline]
Ghose, K. and Moss, C. F. (2003). The sonar beam pattern of a flying bat as it tracks tethered insects. J. Acoust. Soc. Am. 114,1120 -1131.[CrossRef][Medline]
Granshaw, S. I. (1980). Bundle adjustment methods in engineering photogrammetry. Photogramm. Rec. 10,181 -207.
Heblich, K. (1986). Flügelschlag und Lautaussendung bei fliegenden und landenden Fledermäusen. In Bat Flight Fledermausflug, vol.5 (ed. W. Nachtigall), pp.139 -156. Stuttgart, New York: Fischer.
Holderied, M. W. (2001). Akustische Flugbahnverfolgung von Fledermäusen: Artvergleich des Verhaltens beim Suchflug und Richtcharakteristik der Schallabstrahlung. PhD thesis, Friedrich-Alexander-Universität Erlangen-Nürnberg.
Holderied, M. W. and von Helversen, O. (2003). Echolocation range and wingbeat period match in aerial-hawking bats. Proc. R. Soc. Lond. 270,2293 -2300.[CrossRef][Medline]
Jensen, M. E. and Miller, L. A. (1999). Echolocation signals of the bat Eptesicus serotinus recorded using a vertical microphone array: effect of flight altitude on searching signals. Behav. Ecol. Sociobiol. 47, 60-69.[CrossRef]
Jones, G. (1994). Scaling of wingbeat and echolocation pulse emission rates in bats: why are aerial insectivorous bats so small? Func. Ecol. 8,450 -457.
Jones, G. (1999). Scaling of echolocation call
parameters in bats. J. Exp. Biol.
202,3359
-3367.
Kalko, E. K. V. (1995). Echolocation signal design, foraging habitats and guild structure in six Neotropical sheath-tailed bats (Emballonuridae). Symp. Zool. Soc. Lond. 67,259 -273.
Kalko, E. K. V. and Schnitzler, H.-U. (1993). Plasticity in echolocation signals of European pipistrelle bats in search flight: implications for habitat use and prey detection. Behav. Ecol. Sociobiol. 33,415 -428.
Kalko, E. K. V., Schnitzler, H.-U., Kaipf, I. and Grinnell, A. D. (1998). Echolocation and foraging behaviour of the lesser bulldog bat, Noctilio albiventris: preadaptations for piscivory? Behav. Ecol. Sociobiol. 42,305 -319.[CrossRef]
Kick, S. A. (1982). Target-detection by the echolocating bat, Eptesicus fuscus. J. Comp. Physiol. A 145,431 -435.
Korine, C. and Pinshow, B. (2004). Community of insectivorous bats in the Negev Desert: guild structure, foraging space use, and distribution. J. Zool. Lond. 262,187 -196.
Møhl, B. (1988). Target detection by echolocating bats. In Animal Sonar: Processes and Performance, vol. 156 (ed. P. E. Nachtigall and P. W. B. Moore), pp 435-450. New York: Plenum Press.
Neuweiler, G., Singh, S. and Sripathi, K. (1984). Audiograms of a south indian bat community. J. Comp. Physiol. A 145,133 -142.
Norberg, U. M. and Rayner, J. M. V. (1987). Ecological morphology and flight in bats (Mammalia; Chiroptera): wing adaptations, flight performance, foraging strategy and echolocation. Phil. Trans. R. Soc. Lond. B 316,337 -419.
Parsons, S., Thorpe, C. W. and Dawson, S. M. (1997). Echolocation calls of the long-tailed bat A quantitative analysis of types of call. J. Mammal. 78,964 -976.
Pennycuick, C. J. (1975). Mechanics of flight. In Avian Biology, vol. 5 (ed. D. S. Farner and J. R. King), pp. 1-75. New York: Academic Press.
Pennycuick, C. J. (1989). Bat flight performance: a practical calculation manual. Oxford, New York: Oxford University Press.
Pye, J. D. (1993). Is fidelity futile? The `true' signal is illusory, especially with ultrasound. Bioacoustics 4,271 -286.
Qumsiyeh, M. B. (1985). The bats of Egypt. Special Publications the Museum Texas Tech University No. 23. Lubbock, Texas: Texas Tech Press.
Rydell, J., Miller, L. A. and Jensen, M. E. (1999). Echolocation constraints of Daubenton's bats foraging over water. Func. Ecol. 13,247 -255.[CrossRef]
Schnitzler, H.-U. and Kalko, E. K. V. (2001). Echolocation by insect-eating bats. Bioscience 51,557 -569.
Schnitzler, H.-U., Kalko, E. K. V., Kaipf, I. and Grinnell, A. D. (1994). Fishing and echolocation behaviour of the greater bulldog bat, Noctilio leporinus, in the field. Behav. Ecol. Sociobiol. 35,327 -345.[CrossRef]
Schnitzler, H.-U., Moss, C. F. and Denzinger, A. (2003). From spatial orientation to food acquisition in echolocating bats. Trends Ecol. Evol. 18,386 -394.[CrossRef]
Shortis, M. R., Robson, S. and Harvey, E. S. (2000). The Design, Calibration and Stability of an Underwater Stereo-video System. In Proceedings of Direct Sensing of the Size Frequency and Abundance of Target and Non-Target Fauna in Australian Fisheries A National Workshop (ed. E. S. Harvey and M. Cappo). Fisheries Research Development Corporation, Rottnest Island, Western Australia. 47 September, CD-ROM.
Siemers, B. M. and Schnitzler, H.-U. (2000). Natterer's bat (Myotis nattereri Kuhl, 1818) hawks for prey close to vegetation using echolocation signals of very broad bandwidth. Behav. Ecol. Sociobiol. 47,400 -412.[CrossRef]
Speakman, J. R. and Racey, P. A. (1991). No cost of echolocation for bats in flight. Nature 350,421 -423.[CrossRef][Medline]
Spitzenberger, F. (1994). The genus Eptesicus (Mammalia, Chiroptera) in Southern Anatolia. Folia Zool. 43,437 -454.
Surlykke, A., Miller, L. A., Møhl, B., Andersen, B. B., Christensen-Dalsgaard, J. and Jørgensen, M. B. (1993). Echolocation in two very small bats from Thailand Craseonycteris thonglongyai and Myotis siligorensis. Behav. Ecol. Sociobiol. 33,1 -12.
von Helversen, O. (1998): Eptesicus bottae (Mammalia, Chiroptera) auf der Insel Rhodos. Bonner Zool. Beitr. 48,1 -6.
Wong, J. and Waters, D. (2001). The
synchronisation of signal emission with wingbeat during the approach phase in
soprano pipistrelles (Pipistrellus pygmaeus). J. Exp.
Biol. 204,575
-583.