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Eavesdropping by bats on the feeding buzzes of conspecifics

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Echolocation calls of most bats are emitted at high intensities and subject to eavesdropping by nearby conspecifics. Bats may be especially attentive to "feeding buzz" calls, which are emitted immediately before attack oil airborne insects and indicate the potential presence of prey in the nearby area. Although previous work has shown that some species are attracted to feeding buzzes, these studies did not provide a well-controlled test of eavesdropping, as comparisons were made between responses to natural and altered signals (e.g., forward versus backward broadcasts of calls). In this study, I assessed the importance of feeding buzzes by conducting playbacks of controlled echolocation stimuli. I presented free-flying Brazilian free-tailed bats, Tadarida brasiliensis (I. Geoffroy, 1824), with echolocation call sequences in which feeding buzz calls were either present or absent, as well as a silence control. I determined level, of bat activity by Counting the number of echolocation calls and bat passes recorded in the presence of each stimulus, and found significantly greater bat activity in response to broadcasts that contained feeding buzzes than to broadcasts without feeding buzzes or to the silence control. These results indicate that bats are especially attentive to conspecific feeding buzz calls and that eavesdropping may allow a bat to more readily locate rich patches of insect prey.
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Eavesdropping by bats on the feeding buzzes of
conspecifics
E.H. Gillam
Abstract: Echolocation calls of most bats are emitted at high intensities and subject to eavesdropping by nearby conspe-
cifics. Bats may be especially attentive to ‘‘feeding buzz’’ calls, which are emitted immediately before attack on airborne
insects and indicate the potential presence of prey in the nearby area. Although previous work has shown that some spe-
cies are attracted to feeding buzzes, these studies did not provide a well-controlled test of eavesdropping, as comparisons
were made between responses to natural and altered signals (e.g., forward versus backward broadcasts of calls). In this
study, I assessed the importance of feeding buzzes by conducting playbacks of controlled echolocation stimuli. I presented
free-flying Brazilian free-tailed bats, Tadarida brasiliensis (I. Geoffroy, 1824), with echolocation call sequences in which
feeding buzz calls were either present or absent, as well as a silence control. I determined levels of bat activity by count-
ing the number of echolocation calls and bat passes recorded in the presence of each stimulus, and found significantly
greater bat activity in response to broadcasts that contained feeding buzzes than to broadcasts without feeding buzzes or to
the silence control. These results indicate that bats are especially attentive to conspecific feeding buzz calls and that eaves-
dropping may allow a bat to more readily locate rich patches of insect prey.
Re
´sume
´:Les appels d’e
´cholocation de la majorite
´des chauves-souris sont e
´mis a
`de hautes intensite
´s et sont sujets a
`
l’e
´coute des autres chauves-souris de me
ˆme espe
`ce pre
´sentes a
`proximite
´. Les chauves-souris sont sans doute particulie
`re-
ment attentives aux « bourdonnements alimentaires » qui sont e
´mis imme
´diatement avant l’attaque d’un insecte au vol et
qui indiquent la pre
´sence de proies dans le milieu environnant. Alors que des e
´tudes ante
´rieures ont montre
´que certaines
espe
`ces sont attire
´es par les bourdonnements alimentaires, ces travaux ne fournissent pas de test bien contro
ˆle
´de l’e
´coute,
puisque les comparaisons ont e
´te
´faites entre des re
´actions a
`des signaux naturels et alte
´re
´s (par ex., la rediffusion vers
l’avant et l’arrie
`re d’enregistrements d’appels). La pre
´sente e
´tude e
´value l’importance des bourdonnements alimentaires a
`
l’aide de rediffusions de stimulus contro
ˆle
´s d’e
´cholocation. Des tadarides du Bre
´sil, Tadarida brasiliensis (I. Geoffroy,
1824), ont e
´te
´confronte
´es a
`des se
´quences d’appels d’e
´cholocation contenant ou non des bourdonnements alimentaires,
ainsi qu’a
`un silence comme te
´moin. Les niveaux d’activite
´des chauves-souris ont e
´te
´mesure
´s par le de
´nombrement des
appels d’e
´cholocation et des passages en vol en re
´action a
`chacun des stimulus; l’activite
´des chauves-souris est significa-
tivement plus importante en re
´ponse a
`des rediffusions qui contiennent des bourdonnements alimentaires qu’a
`des rediffu-
sions sans ces bourdonnements alimentaires ou qu’au silence qui sert de te
´moin. Ces re
´sultats indiquent que les chauves-
souris sont particulie
`rement attentives aux bourdonnements alimentaires des chauves-souris de leur espe
`ce et que l’e
´coute
peut leur permettre de de
´couvrir plus facilement de riches taches d’insectes proies.
[Traduit par la Re
´daction]
Introduction
Food resources often occur in small, ephemeral patches
that are separated by larger areas of poor quality. Although
such an uneven distribution may increase the time an animal
must dedicate to foraging (Stephens and Krebs 1986), efforts
to locate areas of high resource density may be enhanced by
monitoring conspecific cues associated with feeding activity
(McGregor 2005). For example, the conspicuous sounds of
an agouti, Dasyprocta punctata Gray, 1842, chewing a nut
attract other agoutis to a feeding site (Smythe 1970). Such
eavesdropping on conspecific foraging cues can occur at the
feeding site (McQuoid and Galef 1992, 1993; Nieh et al.
2004), or at a colony or roost, which may serve as an ‘‘in-
formation center’’ (Wright et al. 2003; Chauvin and Thierry
2005; Ratcliffe and ter Hofstede 2005).
Several species of bats gain knowledge of food resources
by attending to the cues of conspecifics. Seba’s short-tailed
bats, Carollia perspicillata (L., 1758), alter their food pref-
erences based on olfactory cues obtained in the roost from
conspecifics that have recently fed (Ratcliffe and ter Hof-
stede 2005). Evening bats, Nycticeius humeralis (Rafin-
esque, 1818), follow successful foragers from roosts to rich
feeding areas, with bats alternating the roles of leader and
follower on subsequent trips (Wilkinson 1992). Group de-
partures from a roost have been observed in several other
species (Racey and Swift 1985; Fenton et al. 2004),
although it is often unclear if such behavior is due to passive
information transfer (Wilkinson 1992), active recruitment of
roostmates via communication calls (Wilkinson and Bough-
man 1998), or bottlenecks at the roost exit (Speakman et al.
1992).
For insectivorous bats, echolocation is a critical sensory
process for gathering detailed information about airborne
prey. A typical echolocation sequence contains three call
Received 25 January 2007. Accepted 4 June 2007. Published on
the NRC Research Press Web site at cjz.nrc.ca on 8 August
2007.
E.H. Gillam. Department of Ecology and Evolutionary Biology,
University of Tennessee, 569 Dabney Hall, Knoxville,
TN 37996-1610, USA (e-mail: egillam@utk.edu).
795
Can. J. Zool. 85: 795–801 (2007) doi:10.1139/Z07-060 #2007 NRC Canada
phases, with each phase shift characterized by a decrease in
call duration and an increase in repetition rate (Schnitzler
and Kalko 2001). Search-phase calls are emitted before
prey detection, followed by approach-phase calls that are
produced after detection and as a bat moves toward a prey
target. The final terminal-phase, or ‘‘feeding buzz’’, calls
are emitted in rapid succession immediately before insect at-
tack (Griffin 1958; Simmons et al. 1979; Moss and Surlykke
2001). Since changes in echolocation call structure reveal
information about the foraging activity of an individual, at-
tention to the signals of conspecifics may allow bats to more
readily locate rich patches of insect prey. Furthermore, the
echolocation calls of aerial-hawking bats are very intense
compared with other natural sounds (Waters and Jones
1995), particularly those above 15 kHz, and these signals
may be especially susceptible to eavesdropping. As a result,
monitoring of calls produced by nearby conspecifics poten-
tially can lead to opportunistic aggregations of bats at insect-
rich locations (Fenton et al. 1976; Bell 1980; Vaughan 1980).
Bats may be especially attracted to feeding buzzes, as
these call sequences are typically only produced during the
final moments of attack on airborne insects (Surlykke et al.
2003). Feeding buzzes are not a direct indicator of foraging
success, as bats often exhibit low rates of successful capture
(e.g., 31% in Lasiurus borealis (Mu
¨ller, 1776), Reddy and
Fenton 2003; 35% in Eptesicus nilssonii (Keyserling and
Blasius, 1839), Rydell 1998). Alternatively, these distinctive
call sequences indicate the presence of potential prey in an
area, which may be important information for bats foraging
in the near vicinity. Three previous studies have investigated
the response of bats to echolocation call sequences contain-
ing feeding buzzes. Barclay (1982) broadcast signals of
feeding Myotis lucifugus (LeConte, 1831) (which contained
search, approach, and feeding buzz calls) to free-flying con-
specifics and he found that bat activity was significantly
higher when the foraging signal was played forwards com-
pared with trials in which the signal was played backwards.
Leonard and Fenton (1984) found similar results when per-
forming a forward–backward playback experiment with the
foraging calls of Euderma maculatum (J.A. Allen, 1891)
and suggested that this species may use echolocation to reg-
ulate individual spacing within a feeding area. Balcombe
and Fenton (1988) performed the most direct test of eaves-
dropping on feeding buzz calls, showing that foraging activ-
ity of L. borealis greatly increased in the presence of
repeated conspecific feeding buzzes compared with presen-
tations of an unedited foraging sequence that contained all
three phases of bat echolocation.
The objective of this study was to further test the hypoth-
esis that bats eavesdrop on the echolocation calls of nearby
conspecifics and are especially attracted to feeding buzzes.
Although this question has been investigated in the past,
previous studies have not examined responses to controlled
and realistic playback signals in which terminal-phase calls
were either present or absent. The backward broadcasts con-
ducted by Barclay (1982) and Leonard and Fenton (1984)
altered important information about the echolocation signal,
including the direction of a call’s frequency sweep and the
temporal pattern of the call sequence (Barclay 1982). It is
possible that the altered signal was not recognized by bats
as a sequence of foraging calls, resulting in lower responses
compared with the forward broadcast, independent of the
presence or absence of feeding buzzes. Balcombe and Fen-
ton (1988) used a ‘‘super stimulus’’ of 51 repeated feeding
buzzes that did not contain the search- and approach-phase
signals which almost always occur between consecutive
buzzes (Schnitzler and Kalko 2001). In this study, I per-
formed a controlled experiment in which I compared bat ac-
tivity in response to two echolocation playback stimuli: (1) a
call sequence that contained only search-phase calls and
(2) the same search-phase sequence with a typical series of
approach-phase and feeding buzz calls added at regular in-
tervals. This design also allowed me to control for other
stimulus characteristics such as call frequency and duration.
I chose to investigate the effects of conspecific feeding
buzzes with Brazilian free-tailed bats, Tadarida brasiliensis
(I. Geoffroy, 1824). This species is highly gregarious, form-
ing colonies that reach into the millions in south-central
Texas (Davis et al. 1962). Despite their ability to disperse
25 km or more from the roost (Davis et al. 1962; Williams
et al. 1973), these bats experience a high interaction rate
with conspecifics while foraging (Ratcliffe et al. 2004). The
echolocation calls of T. brasiliensis are generally narrow-
band and relatively low in frequency (mean minimum fre-
quency of 22.3 kHz; Gillam and McCracken 2007); as a
result, calls will propagate substantial distances in the envi-
ronment and be audible to nearby bats. This foraging behav-
ior makes T. brasiliensis an optimal species for studying the
response of bats to the feeding buzzes of conspecifics.
Materials and methods
Field experiments with echolocation playbacks
I performed playback experiments between 2120 and
0020 on eight nights from 3 June to 12 June 2006. All ex-
periments were performed on a cotton farm in the vicinity
of Uvalde, Texas, which is close to several large Brazilian
free-tailed bat colonies, and bats were often observed forag-
ing on insects found in high densities over the crop fields
where I conducted the study. Bat activity during this time
period was relatively steady, generally with one to two bats
foraging in the area for a short period of time (~1 min), sep-
arated by varying periods of no activity (E.H. Gillam, per-
sonal observation).
Playback stimuli were constructed from previously ob-
tained recordings of bats foraging at the study site. The first
playback signal, or ‘‘feeding buzzes present’’, contained
calls from the search, approach, and terminal phases of bat
echolocation (Fig. 1a). I assembled this signal by repeating
one typical search-phase call at 200 ms intervals for 8.8 s
and appending a 1.45 s sequence of approach-phase and
feeding buzz calls. This 10.25 s composite sequence was re-
peated to create a 10 min playback. The second playback
signal, or ‘‘feeding buzzes absent’’, contained only search-
phase calls (Fig. 1b) and was constructed by repeating the
same search-phase call from the first playback at 200 ms in-
tervals to create a 10 min signal. I also used a 10 min con-
trol broadcast containing no sound, referred to as ‘‘silence’’.
Each night, I broadcast six replicates of each stimulus in a
mixed order and changed the playback order on successive
nights to control for temporal effects. This design ensured
an even distribution of the stimulus presentations through-
796 Can. J. Zool. Vol. 85, 2007
#2007 NRC Canada
out the evenings of the study period. I broadcast stimuli
through an omnidirectional ultrasonic speaker (Avisoft
60401, Avisoft Bioacoustics, Berlin, Germany; flat fre-
quency response ± 5 dB between 15 and 43 kHz) mounted
on a tripod 3 m from the ground. Broadcast amplitude was
assessed by converting the intensity of an equal-amplitude
continual tone to the peak equivalent sound pressure level
(peSPL; Stapells et al. 1982) with a B&K ¼ condensor micro-
phone No. 4939 and a B&K measuring amplifier No. 2606
(Bru
¨el and Kjær, Nærum, Denmark). Broadcast amplitude
was determined to be 88 dB peSPL, which is lower than
the typical call amplitude of many insectivorous bats
(>100 dB; Lawrence and Simmons 1982; Waters and Jones
1995), but was the highest amplitude possible without
overloading the speaker. A solid dielectric microphone
(Avisoft CM16; flat frequency response ±3 dB between 10
and 100 kHz) was positioned 2 m to the left of the speaker
at a height of 3 m and oriented directly upward. Stimuli
were generated from a Dell Inspiron Laptop through a
high-speed sound card (DAQCard-6062E; National Instru-
ments, Austin, Texas) and an amplifier (Avisoft 70101)
powered by three 12 V, 7.2 A gel cell batteries. High-
speed data acquisition was accomplished with Avisoft’s
Ultrasound Gate 416 through the same laptop that was
used for broadcasts. Both playback and recording were
conducted with Avisoft RECORDER. Recordings were
5 min long, but sampling was continuous because there
was no time gap between consecutive recording files. Re-
cordings were made with 16-bit resolution and a 166 kHz
sampling rate, and included both the playback signal and
the calls of free-flying bats in the area.
Measurements of bat activity
All acoustic measurements and analyses were conducted
with Avisoft-SasLab Pro. I digitally high-pass-filtered all re-
cordings to remove background noise, using a finite impulse
response filter with a 5 kHz cutoff. I excluded from analysis
files that contained high levels of wind noise. I assessed lev-
els of bat activity in the presence of the three playback sig-
nals by counting the following: (i) number of bat calls and
(ii) number of bat passes. Both of these measurements pro-
vided information about relative bat activity in the presence
of each playback stimulus, but they could not be used to es-
timate the absolute number of bats in the recording area.
A pulse-train analysis was used to automatically detect
and count echolocation calls. This analysis used a hystere-
sis searching method to detect calls, which involved count-
ing an amplitude peak only if it exceeded the pre-peak
amplitude by a pre-defined threshold (Specht 2004). The
value of this hysteresis threshold influences the pulse
counts produced by the program. To ensure that I chose an
appropriate value, I counted several recordings by hand and
compared my counts with those produced by the pulse-train
analysis at different hysteresis settings. A 20 dB hysteresis
threshold yielded the most accurate pulse counts, and thus
was used for all analyses.
The pulse-count analysis also was influenced by the am-
plitude threshold setting, with lower thresholds resulting in
increased detection of weak signals and a higher final pulse
count. I counted the calls in each recording file using three
amplitude thresholds: 100, 300, and 500 mV (Fig. 2). This
allowed me to assess the relative amplitude of detected calls
and to gain insight into how close bats were flying to the
Fig. 1. Spectrogram of the final 2 s portion of the playback stimulus sequences. (a) ‘‘Feeding buzzes present’’ stimulus containing search-,
approach-, and terminal-phase calls of Tadarida brasiliensis. The preceding 8.25 s of the call sequence that is not shown is composed of
search-phase calls that are identical to the first two signals in this shortened sequence. (b) ‘‘Feeding buzzes absent’’ stimulus containing
search-phase calls only. The preceding 8.25 s of the call sequence that is not shown is identical to the depicted calls.
Gillam 797
#2007 NRC Canada
recording system. Whereas the 100 mV analysis detected the
greatest number of calls (including weak calls from more
distant bats), the 300 and 500 mV analyses counted a de-
creasing number of pulses, only detecting higher amplitude
signals (Fig. 2).
Playback calls were present in recordings but were only
detectable by the 100 mV analysis. To obtain a pulse count
that excluded the playback calls, I broadcast each stimulus
before bats arrived at the study site and counted the number
of detected pulses in these ‘‘bat-free’’ recordings. I then sub-
tracted the appropriate playback pulse count (feeding buzzes
present or feeding buzzes absent) from the counts produced
by the 100 mV analyses. Another issue was that some bat
calls overlapped with playback calls and were not counted.
Although this led to lower pulse counts, I chose not to in-
clude a correction for this overlap error in the final analysis,
as corrections resulted in only small changes to the final
counts and the unadjusted value was more conservative.
Assessing bat activity by counting individual calls can
lead to issues of pseudoreplication, as calls emitted in a se-
quence by the same bat are not independent of each other
(Hurlbert 1984). To address this issue, I conducted a manual
count of bat passes in addition to the automated pulse-count
analyses. Since a bat pass consists of a series of adjacent
calls that are likely produced by one bat, issues of depend-
ence are substantially reduced. A bat pass was defined as a
sequence of 2 or more echolocation calls (Furlonger et al.
1987) exceeding an amplitude of 300 mV, with adjacent
passes separated by at least 1 s of silence (Seidman and Za-
bel 2001). A 300 mV amplitude threshold allowed me to ex-
clude weak calls of distant bats and to only focus on the calls
of animals in the immediate vicinity of the experimental
setup.
Statistical analyses
I tested if the number of detected bat passes and bat calls
differed between the three playback stimuli (feeding buzzes
present, feeding buzzes absent, silence) by conducting GLM
ANCOVAs and post hoc Tukey–Kramer multiple compari-
son tests. Because of the potential influence of differences
in weather conditions between recording nights, date was in-
cluded as a covariate in all analyses. A significance level of
0.05 was used for all tests.
Results
I analyzed thirty-eight 10 min recordings for each of the
three playback signals (n= 114 total). I excluded thirty
10 min recordings in which spurious noise caused by strong
winds was present and obscured the signals of calling bats.
The number of recorded bat calls was significantly different
between stimuli for detection thresholds of 100 mV
(F[2,114] = 3.21, P= 0.044), 300 mV (F[2,114] = 13.63, P<
0.0001), and 500 mV (F[2,114] = 14.94, P< 0.0001). Date
was not a significant covariate for any count analysis. For
the 300 and 500 mV analyses, the Tukey–Kramer tests re-
vealed that bat activity was significantly greater in response
to the feeding buzzes present broadcast compared with the
feeding buzzes absent or silence stimulus (Figs. 3B, 3C).
Despite a significant Pvalue for the ANCOVA test, no dif-
ferential response between broadcast stimuli was observed
for the 100 mV analysis, although there is an obvious trend
for the same pattern of increased activity in response to the
feeding buzzes present stimulus (Fig. 3A). The number of
recorded bat passes was also significantly different between
the three playback stimuli (F[2,114] = 4.83, P= 0.009), with
date as a significant covariate (P= 0.018). Mean number of
bat passes in a 10 min recording period was greatest for the
Fig. 2. Amplitude envelope depicting the detection of five calls of T. brasiliensis with inter-call intervals of approximately 210 ms. In this
example, the 100 mV amplitude threshold detects all five calls, the 300 mV detects three calls, and the 500 mV threshold only detects the
loudest call.
798 Can. J. Zool. Vol. 85, 2007
#2007 NRC Canada
feeding buzzes present stimulus and lowest for the silence
control, with an intermediate value for the feeding buzzes
absent signal (Fig. 3D).
Discussion
The results of this study support the hypothesis that bats
eavesdrop on the echolocation calls of conspecifics and are
attracted to terminal-phase feeding buzzes which indicate
the potential presence of insect prey. Although significant
differences existed between stimuli for all three pulse-count
analyses, it is interesting that the largest differences were
observed in the 300 and 500 mV analyses (Figs. 3B and
3C, respectively). This suggests that the playback stimulus
containing feeding buzzes not only attracted more bats, but
that these bats more closely approached our speaker system,
as revealed by the high amplitude of the detected echoloca-
tion calls. Furthermore, the playback amplitudes used here
were lower than the natural call amplitudes of many insec-
tivorous species (88 vs. >100 dB; Lawrence and Simmons
1982; Waters and Jones 1995), suggesting that results from
this study are likely conservative compared with eavesdrop-
ping among multiple free-flying bats. Overall, these results
provide evidence that bats approach conspecifics emitting
terminal-phase calls, likely in an attempt to enhance feeding
success or to gain more detailed information about the sig-
naling animal and its foraging area.
Our findings agree with those of previous studies demon-
strating that bats are attracted to terminal-phase feeding
buzzes (Barclay 1982; Leonard and Fenton 1984; Balcombe
and Fenton 1988). Although Barclay (1982) reported that
more bats responded to foraging calls played forward com-
pared with calls played backward, he also indicated feeding
buzzes may not be critical to eavesdropping, as there were
no differences in the response of M. lucifugus to conspecific
‘foraging’’ and ‘‘non-foraging’’ sequences. However, this
non-foraging playback was recorded during swarming, when
bats aggregate for mating (McCracken and Wilkinson 2000),
and could possibly have contained other signals (e.g., social
communication calls) that attracted bats to the playback de-
spite the absence of feeding buzzes. Here I demonstrate that
when exposed to two realistic and otherwise identical echo-
location stimuli, the signal containing approach-phase and
feeding buzz calls was more attractive, suggesting that bats
pay particular attention to the portion of a call sequence
which is associated with insect attack.
Fig. 3. Mean activity of T. brasiliensis during a 10 min broadcast of each playback stimulus. All ANCOVA tests were significant at the 0.05
level. Letters indicate results of the Tukey–Kramer multiple comparison tests; counts from stimuli labeled aare not significantly different
from each other, but are significantly different from stimuli labeled b. (A–C) Mean (±SE) number of bat calls as detected by the automated
pulse-count analyses using amplitude thresholds of 100 mV (A), 300 mV (B), and 500 mV (C). (D) Mean (±SE) number of bat passes
detected by manual counts of call sequences containing two or more echolocation pulses that exceeded an amplitude threshold of 300 mV.
Gillam 799
#2007 NRC Canada
The question remains as to whether eavesdropping on
conspecific feeding buzzes represents information parasitism
or information transfer. Information parasitism occurs if for-
aging success decreases when conspecifics are attracted to a
bat’s foraging area, whereas information transfer occurs if
sharing knowledge of foraging spots either does not affect
or increases an individual’s foraging success (Wilkinson
1992, 1995). Since eavesdropping is a passive process by
which individuals attend and respond to the foraging cues
of conspecifics, either information parasitism or information
transfer could be occurring.
The incidence of aggressive territorial behaviors among
conspecifics may indicate if the aggregation of bats at a
feeding sight owing to eavesdropping affects individual for-
aging success. Reddy and Fenton (2003) reported that
L. borealis exhibit aggressive, intraspecific chases at high
bat densities, although this behavior may not be associated
with lower prey densities (Hickey and Fenton 1990). Racey
and Swift (1985) found that aggressive interactions between
foraging Pipistrellus pipistrellus (Schreber, 1774) were high-
est when insect density at the feeding site was low. Alterna-
tively, at very high insect densities, >30 bats foraged in a
small space and showed no aggressive behavior, suggesting
that in P. pipistrellus aggression is primarily influenced by
insect density instead of bat density. Brazilian free-tailed
bats aggregate in large numbers and ultimately encounter
many conspecifics while foraging (Ratcliffe et al. 2004). De-
spite this high encounter rate, agonistic interactions are
rarely observed (G.F. McCracken, personal communication)
and large numbers of bats are commonly seen foraging over
crop fields in close proximity to one another. This lack of
aggression between foraging T. brasiliensis suggests that
eavesdropping on the echolocation calls of conspecifics
does not lead to reduced individual foraging success and is
best described as passive information transfer.
The rich food sources exploited by Brazilian free-tailed
bats also compliment the hypothesis of passive information
transfer. Moths are a major food source for T. brasiliensis,
sometimes comprising over 80% of their diet (Whitaker et
al. 1996; Lee and McCracken 2002). Noctuid moths, such
as Helicoverpa zea (Boddie, 1850), are very abundant in
south-central Texas and their distribution is highly variable
in space and time (Fitt 1989). Mass emergences of billions
of moths occur asynchronously over crop fields within brief
time windows (Raulston et al. 1990), resulting in strong spa-
tial and temporal heterogeneities in resource availability
(J.K. Westbrook and E.H. Gillam, unpublished data (2004)).
Such rich, ephemeral patches of insects have previously
been suggested to be ideal conditions for information trans-
fer, as patches contain sufficient prey to support successful
foraging by multiple bats but do not persist long enough to
warrant territoriality and defense (Wilkinson 1992). Although
high bat densities may lead to increased acoustic interfer-
ence from the calls of nearby bats, some species appear to
forage effectively in the presence of many conspecifics
(Racey and Swift 1985). Furthermore, T. brasiliensis exhibit
jamming avoidance responses (Gillam et al. 2007), which
should reduce the amount of interference caused by the calls
of other bats. Overall, eavesdropping by Brazilian free-tailed
bats should allow individuals to enhance foraging success by
decreasing the amount of time spent in a poor area and gain-
ing information about the presence of new, ephemeral patches
that are rich in insect prey (Galef and Giraldeau 2001).
Although echolocation signals in bats are primarily used
for orientation and prey detection, it has been suggested
that echolocation evolved from social communication calls
(Fenton 1984). Most bats emit a wide range of social calls
that are associated with several behaviors, including mating
(Bradbury 1977), mother–young interactions (Balcombe and
McCracken 1992), and alarm signaling (Russ et al. 1998).
The results of this study enhance the link between echoloca-
tion and social calls by further demonstrating that echoloca-
tion can have a communicative function.
Acknowledgements
I thank Gary McCracken for help with the experimental
design and critical reading of earlier versions of the manu-
script, as well as Jim Hall for assistance with measurements
of signal intensity. This research was supported by an Envi-
ronmental Protection Agency Science to Achieve Results
Graduate Research Fellowship, a University of Tennessee
SARIF grant, National Science Foundation – Experimental
and Integrative Activities award (NSF–EIA) 0326483 (T.H.
Kunz, principal investigator; M. Betke, G.F. McCracken,
P. Morton, and J.K. Westbrook, co-principal investigators),
and a research award from the Department of Ecology and
Evolutionary Biology at the University of Tennessee.
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... Insectivorous bats increase their call repetition rate to produce a feeding buzz in the final moments before attacking insect prey. Other bats eavesdrop on these feeding buzzes to find ephemeral food patches, thus setting the stage for potential interference competition between bats hunting in the same food patch (Gillam, 2007;Dechmann et al., 2009). ...
... Bats of this species disperse from colonies across the landscape to search for ephemeral patches of insects while traveling distances of over 100 km (Best and Geluso, 2003). Mexican free-tailed bats eavesdrop on feeding buzzes of conspecifics to find food (Gillam, 2007). They also produce sinusoidal Frequency Modulated (sinFM) signals to jam the echolocation of conspecifics (Corcoran and Conner, 2014). ...
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Organisms compete for food in many ways, but it is often difficult to know why they use certain competition strategies over others. Bats compete for food either through aggression coupled with food-claiming signals or by actively interfering with a competitor’s sensory processing during prey pursuit (i.e., jamming). It is not known why these different behaviors are exhibited. I studied food competition between Mexican free-tailed bats ( Tadarida brasiliensis ) at foraging sites in Arizona and New Mexico using passive acoustic recording, insect sampling and 3-D infrared videography with or without supplemental lighting that concentrated prey. Bat activity was quantified by the number of recorded echolocation calls, while feeding behavior was indicated by feeding buzzes. Two competitive behaviors were observed—song, which was produced by bats chasing conspecifics, and sinFM calls, which jam echolocation of competitors pursuing prey. Song production was most common when few bats were present and feeding at low rates. In contrast, jamming signals were most common with many bats present and feeding at high rates. Supplemental lighting increased the numbers of bats, feeding buzzes and sinFM calls, but not song. These results indicate that bats employ different strategies—singing and chasing competitors at low bat densities but jamming competitors at high bat densities. Food claiming signals (song) may only be effective with few competitors present, whereas jamming can be effective with many bats at a foraging site. Multiple competition strategies appear to have evolved in bats that are used under different densities of competitors.
... However, prey abundances and distributions change over time and space, creating a complex, rapidly changing environment (Pyke et al. 1977). Efficient foraging decisions can be based on previous experience and individual search behaviour (Krebs 1973), but there is also extensive literature showing that animals use social information to locate high-quality food patches (Elgar 1986, Templeton and Giraldeau 1995, Laland and Williams 1997, Coolen et al. 2003, Clark 2007, Gillam 2007, Farine et al. 2015. This includes inadvertent information, such as visual or chemical cues, or echolocation calls to locate prey patches (Clark 2007, Gillam 2007, and signals that have evolved to recruit others to food sites (e.g. ...
... Efficient foraging decisions can be based on previous experience and individual search behaviour (Krebs 1973), but there is also extensive literature showing that animals use social information to locate high-quality food patches (Elgar 1986, Templeton and Giraldeau 1995, Laland and Williams 1997, Coolen et al. 2003, Clark 2007, Gillam 2007, Farine et al. 2015. This includes inadvertent information, such as visual or chemical cues, or echolocation calls to locate prey patches (Clark 2007, Gillam 2007, and signals that have evolved to recruit others to food sites (e.g. food calls and odour trails; Elgar 1986, Judd and Sherman 1996, Hillemann et al. 2019). ...
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Social information use is well documented across the animal kingdom, but how it influences ecological and evolutionary processes is only just beginning to be investigated. Here we evaluate how social transmission may influence species interactions and potentially change or create novel selection pressures by focusing on predator–prey interactions, one of the best studied examples of species coevolution. There is extensive research into how prey can use social information to avoid predators, but little synthesis of how social transmission among predators can influence the outcome of different stages of predation. Here we review evidence that predators use social information during 1) encounter, 2) detection, 3) identification, 4) approach, 5) subjugation and 6) consumption. We use this predation sequence framework to evaluate the implications of social information use on current theoretical predictions about predator–prey dynamics, and find that social transmission has the potential to alter selection pressures for prey defences at each predation stage. This suggests that considering social interactions can help answer open questions about species coevolution, and also predict how populations and communities respond to rapid human-induced changes in the environment.
... Although they are not cooperative foragers, insectivorous bats still have a propensity to aggregate, likely attracted by the echolocation sounds associated with feeding and increased detection distance of insect swarms (Boonman et al., 2019;Cvikel et al., 2015a,b;Dechmann et al., 2009;Gillam, 2007). Despite rapid attenuation in air, high-frequency echolocation calls can be detectable by bats at distances of many meters. ...
... Recent advances in technology such as largescale, long-term recording arrays and computational methods for tracking individuals (Henninger et al., 2020) are just now allowing researchers to glimpse the nuances of communication in the wild. Likewise, studies of bats and odontocetes suggest that these animals have the potential to glean information from the echolocation signals of others, but it is unclear how this information is encoded, how reliable the encoded information is and whether bats actually use this information to operate in the natural environment (see Balcombe and Fenton;Barclay, 1982;Gillam, 2007). ...
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Animals that rely on electrolocation and echolocation for navigation and prey detection benefit from sensory systems that can operate in the dark, allowing them to exploit sensory niches with few competitors. Active sensing has been characterized as a highly specialized form of communication, whereby an echolocating or electrolocating animal serves as both the sender and receiver of sensory information. This characterization inspires a framework to explore the functions of sensory channels that communicate information with the self and with others. Overlapping communication functions create challenges for signal privacy and fidelity by leaving active-sensing animals vulnerable to eavesdropping, jamming and masking. Here, we present an overview of active-sensing systems used by weakly electric fish, bats and odontocetes, and consider their susceptibility to heterospecific and conspecific jamming signals and eavesdropping. Susceptibility to interference from signals produced by both conspecifics and prey animals reduces the fidelity of electrolocation and echolocation for prey capture and foraging. Likewise, active-sensing signals may be eavesdropped, increasing the risk of alerting prey to the threat of predation or the risk of predation to the sender, or drawing competition to productive foraging sites. The evolutionary success of electrolocating and echolocating animals suggests that they effectively counter the costs of active sensing through rich and diverse adaptive behaviors that allow them to mitigate the effects of competition for signal space and the exploitation of their signals.
... Large populations of prey are spatio-temporally structured, meaning that successful predators should be able to reach bountiful feeding patches. Bats can forage over many kilometres in a single night (Müller et al., 2012), but they also eavesdrop on successful foragers, resulting in an increasing number of bats that feed rapidly on newly available resources (Gillam, 2007;Cvikel et al., 2015). However, because flying is energetically expensive, some bats may have limited commuting range, which can limit their choice of foraging areas. ...
... serotinus. We also counted the "feeding buzzes" (calls emitted by bats before tackling the prey) as a measure of bat foraging activity (Gillam, 2007). ...
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There is growing evidence about the role of insectivorous bats against agricultural pests in various crops. Nevertheless, little research addressed the aggregational and functional responses of bat assemblages to changes in pest availability across a spatio-temporal scale. Therefore, we examined the activity and diet habits of different bat species using DNA metabarcoding by simultaneously monitoring the relative abundance of two major pests (the European grapevine moth, Lobesia botrana, and the leaf rolling tortrix, Sparganothis pilleriana) through the grape growing season, in a vineyard region of the Iberian Peninsula. During pest major irruptions, we found the highest bat activity levels and frequencies of grape pests in the diet of bats, although not all bat species contributed equally to pest suppression. Bats of different foraging guilds positively responded to pest abundances , indicating distinct bat species may synergistically play a role at suppressing agricultural pests at broad scales of the aerospace. For instance, narrow space foragers exploiting major irruptions in grape interior, edge space foragers hampering pest dispersion at local scale, and open space foragers preventing infestations of new grapevine patches at broader scales. Yet, our study exposed the current methodological constraints regarding pest dispersion dynamics, acoustic monitoring of bats' foraging activity or the unfeasibility of metabarcoding to reliably quantify prey abundance in bats diet, and thus further improvement in these issues is required in order to gain insight on the agroecological interactions between bats and pests.
... As active sensors, bats' normal use of echolocation can provide conspecifics with ample information about their whereabouts, condition, and the nature of the behavior they are engaged in Barclay (1982, Gillam 2007, Cvikel et al. 2015, and Lewanzik et al. 2019. The characteristic approach and buzz echolocation sequences are a tell-tale sign that an individual bat has located prey and may alert conspecifics who would then try to utilize the same food patch, thus directly competing with it for resources (Gillam 2007;Corcoran and Conner 2014;Lewanzik et al. 2019). ...
... As active sensors, bats' normal use of echolocation can provide conspecifics with ample information about their whereabouts, condition, and the nature of the behavior they are engaged in Barclay (1982, Gillam 2007, Cvikel et al. 2015, and Lewanzik et al. 2019. The characteristic approach and buzz echolocation sequences are a tell-tale sign that an individual bat has located prey and may alert conspecifics who would then try to utilize the same food patch, thus directly competing with it for resources (Gillam 2007;Corcoran and Conner 2014;Lewanzik et al. 2019). Similar to its congeneric common pipistrelle, Kuhl's pipistrelle often hunts around streetlights, and individuals defend their foraging grounds using chase flights and deterrence calls (Russo and Jones 1998). ...
Chapter
This comprehensive species-specific chapter covers all aspects of the mammalian biology, including paleontology, physiology, genetics, reproduction and development, ecology, habitat, diet, mortality, and behavior. The economic significance and management of mammals and future challenges for research and conservation are addressed as well. The chapter includes a distribution map, a photograph of the animal, and a list of key literature.
... Kerivoula are "narrow space passive gleaning foragers" (Denzinger & Schnitzler, 2013) with particularly high-frequency calls compared to other species in the bat assemblage, it may have been that the acoustic stimulus indicated to these species, in contrast to the horseshoe bats, a profitable foraging site or similar source of interest. Gaining such information through eavesdropping is a known behavior in other bats (Fenton, 2003;Gillam, 2007;Ubernickel et al., 2013). ...
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Bats are the most diverse mammalian order second to rodents, with 1400+ species globally. In the tropics, it is possible to find more than 60 bat species at a single site. However, monitoring bats is challenging due to their small size, ability to fly, cryptic nature, and nocturnal activity. Recently, bioacoustic techniques have been incorporated into survey methods, either through passive acoustic monitoring or acoustic bat lures. Lures have been developed on the premise that broadcasting acoustic stimuli increases the number of captures in harp traps or mist nets. However, this is a relatively new, niche method. This study tested the efficacy of two commonly used acoustic bat lure devices, broadcasting two different acoustic stimuli, to increase forest understory bat captures in the tropics. This is the first time an acoustic bat lure has been systematically tested in a tropical rainforest, and the first study to compare two lure devices (Sussex AutoBat and Apodemus BatLure). Using a paired experimental design, two synthesized acoustic stimuli were broadcasted, a feeding call and a social call, to understand the importance of the call type used on capture rates and genus-specific responses. Using an acoustic lure significantly increased capture rates, while the type of device did not impact capture rates. The two acoustic stimuli had an almost even distribution of captures, suggesting that the type of call may be less important than previously thought. Results indicate a possible deterrent effect on Rhinolophous sp., while being particularly effective for attracting bats in the genera Murina and Kerivoula. This study highlights the effectiveness of lures, however, also indicates that lure effects can vary across genera. Therefore, lures may bias survey results by altering the species composition of bats caught. Future research should focus on a single species or genus, using synthesized calls of conspecifics, to fully understand the effect of lures.
... Locating and identifying these areas rapidly through the feeding buzzes of other individuals, of the same or another species, can be very advantageous. Bats' ability to eavesdrop the feeding buzzes of conspecifics is already recognised [72,73]. The higher similarity of the two species' vocalisations compared with their ancestors [14] may have contributed to a higher overlap in the prey spectrum and the recognition of successful feeding activity between species. ...
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One of the fundamental interests in ecology is understanding which factors drive species’ distribution. We aimed to understand the drivers of bat distribution and co-occurrence patterns in a remote, insular system. The two bat species known to occur in the Azores archipelago were used as a model. Echolocation calls were recorded at 414 point-locations haphazardly distributed across the archipelago. Calls were analysed and assigned to each species. Binominal generalised linear models were adjusted using different descriptors at two scales: archipelago and island. The presence of the co-occurring species was included at both scales. The results show that island isolation, habitat and climate play an essential role on the archipelago and island scales, respectively. However, the positive interaction between bat species was the most critical driver of species’ distribution at the island scale. This high co-occurrence pattern at the island scale may result from both species’ maximising foraging profit in a region where prey abundance may be highly variable. However, further research is necessary to clarify the mechanisms behind this positive interaction. Both species are threatened and lack specific management and protection measures. Maintaining this positive interaction between the two species may prove to be fundamental for their conservation.
... Nonetheless, echolocation calls can possess group, age, sex and individual signatures, which may facilitate individual recognition and interactions among group members and may be used as cues when seeking resources (review in Fenton 2003;Gillam and Fenton 2016). Some bats eavesdrop on echolocation calls of conspecifics to locate patchily distributed food or to coordinate social foraging (Barclay 1982;Balcombe and Fenton 1988;Gillam 2007;Dechmann et al. 2009;Cvikel et al. 2015). Eavesdropping on both echolocation and social calls of conspecifics may also reduce the costs of roost location, because it decreases search time (Ruczyński et al. 2007(Ruczyński et al. , 2009Schöner et al. 2010;Furmankiewicz et al. 2011). ...
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Communication between group members is mediated by a diverse range of signals. Contact calls are produced by many species of birds and mammals to maintain group cohesion and associations among individuals. Contact calls in bats are typically relatively low-frequency social calls, produced only for communication. However, echolocation calls (higher in frequency and used primarily for orientation and prey detection) can also facilitate interaction among individuals and location of conspecifics in the roost. We studied calling behaviour of brown long-eared bats ( Plecotus auritus ) during return to maternity roosts in response to playbacks of social and echolocation calls. We hypothesised that calling by conspecifics would elicit responses in colony members. Bat responses (inspection flights and social calls production) were significantly highest during social call and echolocation call playbacks than during noise (control) playbacks. We suggest that social calling in maternity roosts of brown long-eared bat evolved to maintain associations among roostmates, rather than to find roosts or roostmates, because this species is strongly faithful to roosts and the social groups and roosts are stable over time and space. Living in a stable social group requires recognition of group members and affiliation of social bonds with group members, features that may be mediated by vocal signals.
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As a migratory, cryptic, foliage-roosting bat with a mostly solitary roosting behavior we have an incomplete understanding of the social behavior of the hoary bat, Lasiurus cinereus. In this species most social interactions between conspecifics are thought to involve mating behavior or territorial disputes. Developing a more complete understanding of the social behavior of this species would provide critical insight to address conservation challenges including high fatality rates from wind turbines during the period of fall migration. We tested the response of hoary bats to conspecific social call playback during the spring and fall migration to: (1) test whether conspecific social call broadcasting attracts or repels individual bats; (2) examine whether there are seasonal differences in these responses; (3) describe the structure and variation of recorded social calls; and (4) test whether conspecific social call playback can increase capture success. Hoary bats were attracted to social call broadcasting during both the spring and fall migration. Hoary bats produced social calls during the spring and fall migration, and when only males were present, suggesting a social function not associated with mating. While calls were variable in frequency and length, social calls tended to be a consistent upsloping shape. Attraction to social calls suggests social interactions not associated with mating behavior in hoary bats, and this technique proved successful as an acoustic lure to aid in capture and study of this elusive species.
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The extent to which organic farming can support biodiversity has been extensively studied. However, most of the research has been conducted on organic farms in temperate regions, with the focus mainly being on birds, insects, and plants and rarely on insectivorous bats, especially in Southeast Asia. We studied pairs of matched organic and conventional rice fields along a gradient of landscape complexity in the Songkhla Lake Basin and conducted acoustic surveys using bat detectors to analyze the influence of farming system and landscape characteristics on bat activity and prey availability. We also tested the “intermediate landscape complexity” hypothesis, which states that local conservation efforts are most effective in landscapes of intermediate complexity compared to extremely simple or extremely complex landscapes. We detected no difference in bat species richness, total bat activity, feeding activity, and insect prey abundance between organic fields and conventional fields. Even though organic farming did not increase bat activity on its own, it was most beneficial to bat activity in landscapes of intermediate complexity. Our findings suggest that landscape traits contribute more to bat activity than farm management and that insectivorous bats have species- and guild-specific responses to various landscape contexts. We also found that disturbance caused by tropical storms negatively impacts the activity of insectivorous bat.
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We studied foraging activity and food resource use of Brazilian free-tailed bats. Tadarida brasiliensis, from a large maternity colony near Uvalde, Texas. We recorded echolocation calls of bats at replicate sites in towns, cropland, and ranches and quantified the proportions of signal-receiving time, the numbers of feeding buzzes, and the mean attack attempts per unit of activity time to evaluate habitat use by bats. Food habits of the bats were determined from fecal samples, and the composition and relative abundance of various insect orders in the three habitats were assessed using light traps. Towns with mercury-vapor type street lamps appeared to be important feeding areas for the bats, where they exhibited higher foraging activity and attack rates than over cropland or ranches. The common use of street lamps over towns, however, could have affected the efficiency of our traps in insect assessment; towns exhibited lower relative insect abundance than the other two habitats. Insect abundance did not differ between ranches and cropland, but a decline in relative insect abundance occurred in pre-dawn periods over cropland, which correlated to a higher bat activity per unit time over ranches than on cropland. The bats also displayed higher attack rates, and had a broader diet, in evening than in pre-dawn periods. The diet of bats contained 12 orders of insects, which accounted for 80% of insect orders captured in traps. At the ordinal level, the relative importance of these prey in the bats' diet strongly correlated to those sampled by the traps. The differences in relative abundance of major insect orders between midnight and pre-dawn sampling also correlated to those in percent volumes of respective insect orders in the bats' diet. These results agree with our prediction and suggest an opportunistic foraging mode for this species.
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Naive, adolescent Burmese red jungle fowl (Gallus gallus spadiceus) observed trained conspecifics feeding in a large enclosure. When tested 48 hr later, observers exhibited significantly enhanced preferences both for the type of foraging site and for the area in the enclosure where they had observed conspecifics foraging successfully. Such delayed influences of observation of foraging success on the orientation of feeding by an observer can be explained as an instance of stimulus enhancement (Spence, 1937) but not as an example of local enhancement (Thorpe, 1963).
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Full text download available at: http://www.jstor.org/stable/1942378 For Further Information: http://www.esajournals.org/doi/abs/10.2307/1942378
Article
Bats are known to use areas above perennial streams and rivers for foraging and traveling; however, little is known about bat use of smaller streams that flow intermittently. We compared bat activity among 3 size classes of streams and upland sites in a northwestern California watershed during summers 1996 and 1997. Stream size was classified based on channel width. Ultrasonic Anabat II(R) bat detectors were placed in stream channels and at upland sites, and bat activity was recorded remotely at night. Analysis of bat detector data revealed a significant difference in activity among the 4 habitat types in both years. In 1996, bat activity was greatest along medium and large intermittent streams, was intermediate at small intermittent streams. and was least at upland sites. In 1997, a similar pattern was found, but no significant difference was found in bat activity between small stream and upland sites. To determine species presence, bats were captured in mist nets at stream sites with the highest bat activity. Results are presented indicating differences in number of captures by species between medium and large streams.
Article
In a lowland agricultural area where the habitat suitable for foraging was extensive, mean distance between the roost and bats during pregnancy was 1.8 km and maximum recorded distance was 5.1 km. These distances were reduced to 1.3 km and 3.7 km respectively during lactation. In an upland area, bats foraged in all habitats where insects were abundant both before and after parturition. Average distance between the roost and foraging sites was 1.0 km and maximum distance 2.5 km during both pregnancy and lactation. Pipistrelles moved between foraging sites on a regular route, and were sighted at the same time and place on successive nights. Pipistrelles left the roost and travelled between foraging sites in groups of 2-6 individuals: 'following' behaviour involving 2 bats was frequently observed. Bats foraged on beats which were seldom defended. Intra-specific aggression was evident only at low insect densities: at high insect densities large numbers of bats foraged in small areas without showing any overt aggression. The rate at which P. pipistrellus attacked insects was proportional to insect density until a maximum rate of 10 'feeding buzzes' per minute was reached. Bats did not remain in an area or attempt to forage if the insect density was <300-1000-3 m. Juvenile P. pipistrellus moved progressively further from the roost over a 3 wk period after they first began to fly. The recorded rate of attempted feeding also increased progressively during this period.-from Authors
Article
Both search and height-finding radars were used to observe the airborne behavior of free-tailed bats, Tadarida brasiliensis mexicana, near several caves in the southwestern United States. Radar echoes from dense groups of bats covered areas as large as 400 square kilometers and rose to altitudes of more than 3000 meters. The presence of large numbers of bats within these areas was confirmed by visual observation from a helicopter. Bat flights appeared on radar at dusk and at dawn as a slowly expanding or contracting target, usually located near a known roost. The direction in which the echo expanded most rapidly was not due to drift of the bats by winds. This leading edge often moved at more than 40 kilometers per hour, indicating the capacity for rapid, well-directed, high altitude flight in these animals. Bats flying at such high altitudes must employ sensory systems other than echolocation for orientation and navigation.
Article
In a field experiment designed to evaluate dietary variation in Mexican free-tailed bats (Tadarida brasiliensis mexicana) we found that lactating females fed largely on coleopterans and lygaeid bugs during evening feeding bouts and mostly on moths during morning feeding bouts. These results suggest that interpretations of food habits in this and other species may be biased unless samples from both nightly feeding bouts are included in the analyses. Diets of different individuals during the same feeding bout were strikingly similar, suggesting that lactating females either fed in the same general habitats or that they encountered and preferentially fed on similar prey items among those available. Bats captured upon return from evening feeding bouts produced significantly more fecal pellets than those captured following second feeding bouts. This difference suggests that either more food is eaten in the first feeding bout or, alternatively, highly chitinous insects such as coleopterans and lygaeids contribute more to fecal matter than relatively soft-bodied moths. We found no significant relationship between hardness of prey and number of pellets produced. Individual bats produced an average of 2-3.6 insects/pellet, but no consistent relationship was found between the number of insects eaten and the number of fecal pellets produced. Our analysis indicates that at least five pellets are needed to establish the number of insect taxa (families) consumed by a bat. Results from this study suggests that future research on food habits of insectivorous bats should examine fecal pellets or stomach contents from evening and morning feeding bouts to fully characterize the diet of a given species.