doi: 10.1098/rspb.2009.0473
 published online 6 May 2009Proc. R. Soc. B
?
Wikelski
Dina K.N. Dechmann, Silke L. Heucke, Luca Giuggioli, Kamran Safi, Christian C. Voigt and Martin
?
echolocating bats
Experimental evidence for group hunting via eavesdropping in
?
References
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 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from 
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insectivorous bat Noctilio albiventris. Females of this species live in small groups, forage over water bodies
1. INTR
Althou
groups
2002),
socialit
cues,
uckley
ing in
s was
l cues
Findley 1980; Gonc?alves et al. 2007), but little information
beyond the level of order or about prey ecology is available.
Proc. R. Soc. B
Biology, University of Konstanz, 78457 Konstanz, Germany
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from (ddechmann@orn.mpg.de).living in groups is access to socially acquirable infor-
mation, such as about the best mating partners, and
information on the presence or absence of predators
and food (Danchin et al. 2004; Dall 2005). Socially
acquired information can be gained either in the form of
evolved signals where individuals ?deliberately? provide
personal information to others or as cues, which are
inadvertent by-products of individual activities, e.g.
feeding behaviour can convey information about the
presence or quality of food (Dall 2005). In fact, sharing
information about food is thought to represent a major
driving factor for the evolution of sociality, especially in
species exploiting resources with a patchy and ephemeral
distribution (Barta 1992; Beauchamp 1997; Buckley
1997b; Safi & Kerth 2007).
in information transfer (Beauchamp (2007), but see
Wilkinson (1992)). Contrary to most other nocturnal
animals, echolocating bats produce auditory cues while
foraging, which contain information that could potentially
be used by other foraging bats: ?search-phase? echoloca-
tion calls that are emitted while a bat is searching for prey,
which change to ?feeding buzzes? (FBs) when prey is
encountered. Indeed, bats are attracted to the FBs of
conspecifics (Barclay 1982; Balcombe 1988; Fenton
2003; Gillam 2007), but, thus far, it remained unclear
whether eavesdropping affects group foraging behaviour
and hence sociality of bats.
We investigated social foraging in the lesser bulldog bat,
Noctilio albiventris. The species is fairly common, lives in
social groups year-round and is often seen flying in small
groups over open bodies of water. The diet of N. albiventris
consists mainly of insects (Tamsitt & Valdivieso 1963;
Hooper & Brown 1968; Howell & Burch 1973; Whitaker &
* Author and address for correspondence: Department ofReceived
Accepted(i) free-ranging bats are attracted by playbacks of echolocation calls produced during prey capture, and
that (ii) bats of the same social unit forage together to benefit from passive information transfer via the
change in group members? echolocation calls upon finding prey. Network analysis of high-resolution
automated radio telemetry confirmed that group members flew within the predicted maximum hearing
distance 94G6 per cent of the time. Thus, echolocation calls also serve as intraspecific communication
cues. Sociality appears to allow for more effective group foraging strategies via eavesdropping on acoustical
cues of group members in nocturnal mammals.
Keywords: information transfer; network; Noctilio albiventris; sociality
ODUCTION
gh many animals from a multitude of taxa live in
, at least during part of their lives (Krause & Ruxton
the ultimate and proximate factors underlying
y often remain unresolved. One main advantage of
Several taxa of social diurnal birds commonly use
usually visual cues, during foraging (Brown 1986; B
1997a,b; Beauchamp 2001). However, social forag
strictly nocturnal animals such as owls and bat
thought to be limited due to the importance of visuafor swarming insects and have an extremely short daily activity period. We predicted and confirmed thatExperimental eviden
via eavesdropping i
Dina K. N. Dechmann1,2,3,*, Silk
Kamran Saf i7, Christian C. V
1Leibniz Institute for Zoo and Wildlife Research,
2Smithsonian Tropical Research Institute, Apartad
3Department of Biology, University o
4University of Vienna, Althan
5Bristol Centre for Complexity Sciences, Department of E
University of Bristol,
6Department of Ecology and Evolutionary Biolog
7Zoological Society of London, Institute of Z
8Department of Migration and Immuno-Ecology, Max P
Group foraging has been suggested as an importan
cues are predominantly used to gain information
foraging animals such as birds, where group fora
nocturnal animals, such as bats, would have to rel
investigated the role of echolocation calls as ina20 March 2009
17 April 2009 1e for group hunting
echolocating bats
L. Heucke1,4, Luca Giuggioli5,6,
igt1 and Martin Wikelski2,3,8
ed-Kowalke-Strasse 17, 10315 Berlin, Germany
843-03092, Balboa, Ancon, Republic of Panama
nstanz, 78457 Konstanz, Germany
asse 14, 1090 Wien, Austria
neering Mathematics and School of Biological Sciences,
istol BS8 1TR, UK
rinceton University, Princeton, NJ 08544, USA
gy, Regent?s Park, London NW1 4RY, UK
ck Institute for Ornithology, 78315 Radolfzell, Germany
ctor for the evolution of sociality. However, visual
ut group members? foraging success in diurnally
has been studied most intensively. By contrast,
n other cues or signals to coordinate foraging. We
rtently produced cues for social foraging in the
doi:10.1098/rspb.2009.0473
Published onlineThis journal is q 2009 The Royal Society
efficiently, enabling them to catch enough of their small
2 D. K. N. Dechmann et al. Experimental evidence for group hunting
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from and ephemeral prey during the short foraging period. To
test our hypotheses, we performed playback experiments
and expected N. albiventris to be (i) attracted to playbacks
of conspecific FBs. In addition, we monitored bats with an
automated telemetry system expecting that (ii) individuals
spend a high proportion of time foraging within hearing
distance of group members.
2. MATERIAL AND METHODS
(a) Study site
Our main study site was the village Gamboa (09.078 N;
079.418 W) in Panama, and surrounding water bodies, the
Panama Canal and the Chagres River. An additional study
site for recording and playbacks was Barro Colorado Island
(BCI, 09.108 N; 079.518 W). The entire study area is covered
by semi-deciduous tropical lowland rainforest with a distinct
wet and dry season (Windsor 1990; Leigh 1999).
(b) Playback experiments
We conducted playback experiments from 6 October to 30
November 2007 and 4?26 June 2008. We recorded foraging
N. albiventris, whose calls were discernible from those of other
bat species flying over water by their distinctive patterns and
frequencies, from the shores of the Chagres River (four sites)
and BCI (two sites) with a condenser ultrasound microphone
(CM16/CMPA) and the software RECORDER USGH v. 3.4
(Avisoft, Berlin, Germany) onto the hard disk of a laptop
computer. All recordings were made with a sampling rate of
250 kHz and a bit rate of 16. In the software SASLAB PRO v.
4.40, we then identified six sequences of 1.5 s duration of FBs
and search calls (SCs) of single individuals from each site,
which were free of heterospecific calls. So each 1.5 s element
contained varying numbers of calls as they naturally occurred.
We filtered out background noise outside of the species-
specific frequency range (bandpass filter: 35?75 kHz) fromHowever, due to the fact thatN. albiventris forages over open
water, the diet was expected to consist of ephemeral but
locally abundant swarming insects (Jones & Rydell 2003).
Finding ephemeral prey, such as insect swarms, is difficult as
their occurrence is unpredictable in space and time. At the
same time, insect swarms are a rich and nonmonopolizable
resource, reducing effects of intraspecific competition.
Ephemeral and nonmonopolizable diets have in fact been
linked with social foraging and group living in theoretical
and comparative studies of birds and male bats from the
temperate zones (Beauchamp 1997; Buckley 1997b;
Beauchamp 2002; Safi & Kerth 2007). This makes
N. albiventris an ideal candidate for the studies of social
foraging in echolocating bats. Noctilio albiventris has an
extremely short-activity period just after sunset (Hooper &
Brown 1968), which implies that foraging must be very
efficient in spite of the fact that unlike many other insect
species, those swarming over water often do not have ears
or behavioural strategies against bat attacks (Jones &
Rydell 2003).
We hypothesized that N. albiventris uses the acoustical
cues that group members inadvertently emit during prey
detection and capture, presumably to increase foraging
efficiency. Furthermore, we predicted that individuals,
who emerge from their roosts together, maintain social
cohesion by flying within hearing distance of each other.
This would allow them to find food sources moreProc. R. Soc. Bthese sequences to ensure that the animals were really reacting
to the presented stimulus. For each of the six recording sites, we
composed one SC file and one FB file of 3 min duration each,
composed of the repeated six fragments in random order. As
controls, we created a file consisting of white noise (WN)
covering the call frequencies of the bats (32?75 kHz) and a
silence file, both also lasting 3 min.
For the playback trials, we selected five sites, along the
shores of the Chagres River (four sites) and on BCI (one site)
where foraging N. albiventris had been observed previously.
Each site had direct access to the water that was clear of
floating vegetation. We limited playback trials to nights
without wind or rain. During experiments, we placed an
Ultrasonic Dynamic Speaker (ScanSpeak) as close to the
water as possible and connected to the laptop computer via an
Ultrasound Gate Player 116. Playbacks were started as soon
as the first Noctilio sp. were seen over the water or 5 min after
they had first been seen on the previous playback night.
During each playback session, a selection of the 3 min files
was then played back in the following order: silence?
stimulus 1?silence?stimulus 2?silence?stimulus 3. The
order of the stimuli, i.e. FB, SC and WN, was random. We
played back at the same site only after a minimum of 5 and a
maximum of 108 days, and a maximum of six times, spread
over a period of eight months to avoid habituation of the bats.
We played back the files from all recording sites, but one from
a different recording site each time at every playback site to
avoid the fact that the bats were reacting to playback of calls of
familiar individuals. We calibrated the loudspeaker prior to
each experiment to ensure that files were played back at
approximately the same sound pressure level (SPL). For this
purpose, we played the SC file of the respective session at a
distance of 5 m from the microphone and simultaneously
recorded it. We then adjusted the volume of the loudspeaker
until the intensity (derived from the spectrogram) of the
playback matched that of the original file in sound intensity.
For detailed information on the frequency response of
microphone and loudspeaker, see product descriptions on
www.avisoft.com.
Simultaneously with the playback experiments, we visually
recorded the behaviour of the bats according to the following
categories: (i) far pass, i.e. aNoctilio sp. passing in a direct line in
front of the loudspeaker at a distance of 2?10 m. This was used
as a means to assess general background activityof bats; (ii) near
pass, i.e. a Noctilio sp. passing the loudspeaker at a distance of
2 m or less; and (iii) approach, i.e. a Noctilio sp. flying directly
towards the loudspeaker to a distance of 1 m or less. In addition
to the visual observations, we also recorded echolocation calls
during each entire playback session. The same microphone as
had been used for recording the playback files was placed at a
short distance behind the speaker. Playbacks lasted 21 min.
Longer playback sessions proved fruitless, as bats were only
present over the water for a short-time period.
For analysis of the visual observations, we selected only
those playbacks of stimuli where bats had been present and
thus within the predicted maximum hearing distance during
the immediate pre-playback silence. We compared the
number of far passes, near passes and approaches during
the pre-playback silence with those during the stimulus in a
paired-sample sign test. In addition, we visually scanned the
spectrograms of all recorded audio files in SASLAB PRO and
noted for every 2 s window whether ?N. albiventris (NA)? had
been recorded or not. Recordings of very closely approaching
bats are ?overloaded?, which is visible from clipped waveforms
minute. We used the maximum-likelihood estimator by Lenth
(1981) as implemented in LOAS (Ecological Software
Solutions, Inc.) to tri-, quadr- or quintangulate individual
locations. We tested the location accuracy by using precisely
known locations of bats (i.e. roosts) and by placing radio
transmitters at five terrestrial locations within the study site.
We used the triangulation method with one mobile observer
and the two ARU?s to estimate the locations of these
transmitters from 300 to 800 m distance to each receiver.
We determined five locations during each trial and averaged
their error in metres. The location accuracy was determined
as the average location error of the five trials and amounted to
30G3.7 m (distanceGs.d.).
Only the recorded positions of foraging bats observed over
intervals of at least 5 min were compared to avoid pseudo-
replication (Otis 1999). Based on the calculated maximum
hearing distance, bats were counted as having foraged in a
group, when they had been localized within 40 m or less. We
determined the total number of co-locations (C) between
individual female bats as well as the total number of
Experimental evidence for group hunting D. K. N. Dechmann et al. 3
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from albiventris by adding K50 dB target strength to the model
(Waters et al. 1995). Target strength represents the loss of
sound energy as a consequence of the absorption and
reflective properties of small insects in the field. In our
estimate, we assumed that the SPL of FBs is the same as that
of SCs, as published SPLs for the latter are not available (but
see Boonman & Jones 2002). Thus, our distance estimates are
maximum values, especially also considering the high
directionality of echolocation calls.
We calculated a maximum hearing distance of 35?40 m
between echolocating bats, depending on the assumed
hearing threshold (0 or 20 dB SPL). By contrast, calculated
maximum detection distance for a small prey item such as an
insect was approximately 4?6 m.
(d) Capture and radio-tracking
We caught groups of bats with a handmade harp trap in the
wet season of 2007 during evening emergence from twohand, less precise, as all bats in the range of the microphone
(which was close to but not identical with the loudspeaker)
were recorded regardless of whether they were approaching or
passing. On the other hand, we were able to distinguish the
calls of N. albiventris and Noctilio leporinus, which was not
always possible with certainty during the visual observations.
Finally, we were able to exclude the possibility that additional
bat species were attracted by the playbacks. We used non-
parametric (Wilcoxon matched-pairs signed-ranks test) tests
to analyse the results. We analysed visual observations from
14 playbacks of FBs, 10 of SCs and 8 of WN where Noctilio
sp. had been visually observed during the pre-playback
silence. Calculations were made in INSTAT GRAPHPAD v. 3.0
for Macintosh.
(c) Calculation of hearing distance
We calculated maximum hearing distance based on the
attenuation of the sound pressure of echolocation signals
using a simplified version of the formula by Stilz (2004). Both
geometric and atmospheric attenuations reduce the amount
of energy contained in a call as a function of call frequency
and distance between sender and receiver. Therefore, the
hearing distance is the distance after which the initial sound
pressure of an emitted call falls below the hearing threshold of
a potential receiver, which is determined by the auditory
abilities of the bats and background noise levels. This
threshold ranges between 0 and 20 dB SPL in bats (Kick 1982).
We determined the maximum distance of hearing by using
the following equation:
sound pressure levelZ initial sound pressure
Cgeometric attenuationCatmospheric attenuation:
We obtained the environmental values needed to calculate
atmospheric attenuation in the model for temperature
(tZ27.1258C), barometric pressure ( pZ109 125 pa) and
relative air humidity (RH rZ93.625%) from average weather
data during the nights we radio-tracked bats (http://www.
wunderground.com). Main echolocation frequency was
fZ70 kHz (own data and Surlykke & Kalko 2008), and
SPL of SCs at a distance of 10 cm from the bat was SPLZ
134 dB (Surlykke & Kalko 2008). We calculated detection
distance for a small insect, such as the typical prey of N.in the spectrogram and this was noted as a second category.
Finally, we also included ?other bat species? as a third
category. This way of analysing the data was on the oneProc. R. Soc. Bknown roosts in buildings in Gamboa. We distinguished
social groups by the fact that they emerged simultaneously.
Group A (two individuals), group B (three individuals) and
group C (three individuals) were all captured on 9 June at
roost 1. Group D (five individuals) was captured at roost 2 on
14 June. We glued a 0.6 g transmitter to the dorsal fur of
fully grown, non-pregnant females and released them at the
capture site during the same night. Data collection started
the following evening and always lasted from emergence just
after sunset until all bats had returned to the roost
approximately 1 hour later. Radio-tracking continued until
22 June. Using an automated telemetry system (see below),
we confirmed that no bat emerged from the roost again later
during the night.
After emerging in the evening, bats foraged over the
nearby Panama Canal and Chagres River. Two automated
recording units (ARU?s) were placed at an elevated point and
near the water to determine bearings towards bat radio
signals (figure 1; Crofoot 2008). In addition, two to three
persons functioned as mobile-tracking units, locating the bats
continuously by scanning through the radio-transmitter
frequencies, taking compass bearings (G58) for each
individual. Whenever the signal of a bat was located, we
scanned through all the other frequencies within half a
far pass
0
1
2
3
4
5
6
7
av
er
ag
e 
nu
m
be
r o
f c
ou
nt
s
near pass approach
**
n.s.
n.s.
Figure 1. Mean number of visual observations (Gs.d.) of far
passes, near passes and approaches contrasting between
pre-playback silence (left hand values of each category) and
playback of FBs (right hand value). Only the difference
between approaches during pre-playback silence and FBs was
significant ( asterisks, p!0.007; nZ14).
represented the difference between the number of edges
3. RESULTS
The network analysis of radio-tracked individuals
suggested a structure identical to that observed at capture,
with a modularity QZ0.48, well above the value of 0.3
considered tobea gooddiscriminatorbetween real (QO0.3)
and random subdivision (Q!0.3, figure 4a). The same was
true for the visualization method with Kruskal?Wallis? non-
metric scaling (figure 4b). The four groups that clustered
together in both visualization methods were identical with
N
N
Gamboa
Panama
Canal
500 m
river
(b)
(c)
Figure 2. Bat foraging movements during one representative
evening (15 June 2007); (a) group A (two bats), (b) group B
(three bats) and (c) group C (three bats). For explanation of
symbols, see (a) (crosses, automated tracking site; circles,
hand-tracking site; squares, main bat roost). Points in the
figure represent fixes where all the animals? signal was heard
from the same direction within a maximum of 30 s.
4 D. K. N. Dechmann et al. Experimental evidence for group hunting
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from (a) Playback experiments
We never visually observed a response to playback of WN,
and only non-significant responses to playback of SCs. By
contrast, playback of FBs caused a significant increase in
approaches by Noctilio sp. ( p!0.007; Wilcoxon matched-
pairs signed-ranks test; figure 1).
The analysis of the simultaneously recorded audio files
confirmed this finding. Only the category ?overloaded?, i.e.
bats in the immediate vicinity of the loudspeaker, revealed
a significant increase in the number of bats approaching
the loudspeaker between pre-playback silence
(nZ20; meanZ0.364G0.79) and playback of FBs
(meanZ2.409G4.687, pZ0.0156; Wilcoxon matched-
pairs signed-ranks test). The only other comparison that
came close to significance was between pre-SC silence
?NA? (nZ19; meanZ6.64G11.03) and SC playback
(meanZ11.06G19.10; pZ0.08), all other comparisons
yielded a pZ0.27 or greater.
(b) Radio-tracking
Several individuals?including all five members of group
D?were always triangulated within maximum hearing
distance (!40 m) of at least one other bat (for a
representative example of a radio-tracking night, see
figure 2); the bats foraged socially 94G6 per cent of
the time (figure 3). Some of the groups switched
roosts after capture, but continued to forage together.
Flight trajectories changed from night to night, thus we
can be sure that the bats were not simply following
established routes.within a ?community? (in our case a social group) and the
expected number of edges from a random network, but with
the same degree (number of edges from a vertex) distribution
of the original network (Newman 2004). The highest Q-value
(0!Q!1) indicates the most probable structure.
In addition, we visualized group foraging, with Kruskal?s
non-metric multidimensional scaling using the R statistical
package v. 2.7.1 (R Development Core Team 2008). This
method estimates the orthogonal coordinates of a set of
objects from data measuring distances between them
(Venables & Ripley 2002). We used the inverse of the ranked
pairwise number of co-locations resulting in a matrix where
individuals with few co-locations received a large distance
and individuals with many co-locations a short distance.
In our example, bats that had foraged together had low
pairwise distances, i.e. low dissimilarity. The result was a two-
dimensional representation of thepairwise matrix of co-locations
of the radio-tracked individuals.observations (P ) for each bat. We then calculated the
percentage of observations when each bat was foraging with
at least one other bat (C/(PCC )).
From the observed co-locations, we created an undirected
weighted network representative of the social ties between
individuals. Vertices represented each bat, while the presence
of an edge between two vertices meant that the two bats were
observed foraging in a group. The weight of each edge was
given by the value C. We constructed a 13!13 adjacency
matrix for all bats and calculated the modularity Q for various
possible subdivisions of the network. The modularityProc. R. Soc. BN
Chagres
(a)
members that have found an insect swarm, indirectly
Experimental evidence for group hunting D. K. N. Dechmann et al. 5
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from increasing potential detection distance of prey. Even if our
10-fold estimate of increase in detection distance is toothe groups that had emerged simultaneously from the
same roost, showing that roost members foraged together.
4. DISCUSSION
Social foraging has been postulated as a major reason for
the evolution of sociality in animals, yet most studies are
either of a theoretical nature (Barta 1992; Buckley 1997b;
Sernland 2003; Jackson et al. 2008) or observational
(e.g. Wilkinson 1992;Buckley1996;Wilkinson &Boughman
1998; but see Buckley 1997a). Our data strongly suggest
that passive information transfer between group members via
acoustical rather than visual cues may coordinate foraging of
nocturnal flying animals.
Our playback experiments confirm our first prediction
that FBs, produced when prey has been detected and is
being attacked, attract conspecific bats. This implies that
foraging within hearing distance enables N. albiventris to
profit from inadvertent or passive information transmitted
through the change in echolocation call structure of group
1
gr
ou
p 
fe
ed
in
g 
tim
e 
fra
ct
io
n 
(%
)
80
85
90
95
100
2 3 4 5 6 7
individual bats
8 9 10 11 12 13
Figure 3. Bar plot of the fraction of time (no. of times a bat
was co-located/total no. of observations) bats from roost 1
(groups A?C, bats 1?8) and roost 2 (group D, bats 9?13)
foraged with at least one other bat.high because FBs are not as loud as SCs, the fact that
indirect detection distance can be increased into every
direction where there is another bat flying in close
proximity probably increases the probability of finding a
food resource.
Confirming our second hypothesis as well, we showed
that female N. albiventris, which emerged from their roost
as a group, also foraged together. A similar behaviour had
been inferred by monitoring emergence from and return
to the roost in female evening bats (Wilkinson 1992).
Social foraging was also suspected to be a reason for the
formation of stable female groups in N. leporinus (Brooke
1997). However, due to methodological limitations, it was
not possible to confirm this hypothesis. Our use of an
automated telemetry system allowed the simultaneous
radio-tracking of groups of bats. Group members were
found to be flying within 34.7 m or less of each other
between 85 per cent and 100 per cent of the time.
Members of groups A?C were occasionally observed
leaving their group and foraging on their own. However,
usually, they joined the remainder of the group again later
Proc. R. Soc. Brather than completing the foraging period and returning
to the roost on their own. This suggests that although it is
important to emerge together from the roost as a group to
facilitate social foraging, bats are also able to distinguish
their roost mates from other N. albiventris foraging in the
same area through some unknown cue, probably by their
call structure (Masters 1995; S. L. Heucke 2008,
unpublished results) and/or olfactory cues (Bouchard
2001). The non-significant trend towards increased bat
activity during playbacks of SCs indicates that maybe the
louder SCs are used for group cohesion and possibly also
finding the right group again.
Group foraging could only be beneficial if the increased
foraging efficiency outweighs the potential costs of more
rapid exploitation of insect swarms (Beauchamp 2005).
The latter may not be relevant for our study species, since
insect swarms over the water occur mainly during a very
narrow time window just around sunset, and because
swarms quickly start to scatter as an avoidance mechanism
once one or several bats start exploiting them (Jones &
Rydell 2003). Quite possibly as a response to this short
peak in prey availability, N. albiventris spends only
approximately 1 hour outside the roost every day,
including time needed to commute to and from the
foraging areas. Faecal analysis shows that the insects
consumed by N. albiventris are very small (2?3 mm, D. K.
N. Dechmann 2008, personal observation). In addition,
the weight difference between emerging and returning bats
is 6 g or more (nZ5, D. K. N. Dechmann 2008, personal
observation), indicating that a large number of insects are
being caught during this short foraging period, further
corroborating the need for efficient foraging.
Competition for insect swarms with other species might
potentially be more important than accelerated patch
exploitation. However, there were very few and often no
other bat species foraging directly over the water surface
while N. albiventris was present, as became evident from
the audio recordings. The larger and ecologically similar
N. leporinus forages in the same areas, but appeared later
and all other bats we recorded on our audio files foraged
higher up in the air, while N. albiventris was always seen
within approximately 1 m of the water during our
experiments. Similarly, on our recordings, N. albiventris?
main frequency range showed little overlap with calls of
other simultaneously foraging bat species, which could
mask and interfere with their own calls, making both active
and passive foraging more difficult.
Coordination of foraging with group members via
screech calls, specially produced for this purpose, and thus
active information transfer, has been described in another
bat, Phyllostomus hastatus (Wilkinson & Boughman 1998).
However, in Ph. hastatus, roosts function as information
centres from which successful foragers can be followed to
fruiting trees during the next foraging bout of the same
night. Noctilio albiventris feeds on a short-lived food source
that constantly changes in time and space; insect
swarms that cannot be exploited repeatedly. In addition,
there is usually only one foraging bout per night in
N. albiventris. Thus, roost members have to stay together
and simultaneously exploit food sources. Any bat, which
encounters an insect swarm, starts catching prey and thus
producing FBs. The information content of these FBs is
an unavoidable by-product and useful for other group
members (and other N. albiventris that happen to be within
6 D. K. N. Dechmann et al. Experimental evidence for group hunting
 on 7 May 2009rspb.royalsocietypublishing.orgDownloaded from group A
group B
4
18
311
5
5
259
2
54
1
(a) (i)earshot). Additional active communication via social calls
does not seem to be necessary, but cannot be excluded
based on our data, even though we never recorded social
calls during our experiments.
It had been previously shown that FBs attract
conspecific and heterospecific bats (Fenton 2003; Gillam
2007), but it remained unclear whether this was an
opportunistic behaviour or the main foraging strategy,
and thus potentially linked to the evolution of sociality.
In fact, some species may use eavesdropping opportunis-
tically or seasonally, depending on the occurrence of
14
6
group C
7
15195
8
7
2
2
15
21
16
5
?0.02
?0.02
?0.01
0
0.01
0.02
?0.01
first pri
se
co
n
d 
pr
in
ci
pa
l c
oo
rd
in
at
e
(b)
Figure 4. (a) Graphical representation of the structure emerging
Thick-dashed rectangles cluster animals from the same roost. Thi
shown subdivision is obtained as the maximum value of the netw
structure inferred at capture. The bats are represented by a nu
connected with a thin line. The number of co-locations between tw
line connecting them. Co-located bats from the same group are co
dotted line ((i) roost 1 and (ii) roost 2). (b) Two-dimensional rep
a non-metric multidimensional scaling analysis. The distance be
ranked pairwise number of co-locations. Large distances there
distances symbolize a large number of co-locations (red circle
black circles, group D).
Proc. R. Soc. Bgroup D
6
16
9
10
11
3
20
4
9
(ii)insect swarms. However, our data from the wet season
in Panama make a clear link between emerging to forage
as a group and attraction by conspecific echolocation
calls. Relatedness between members of socially foraging
groups remains unknown, as does the mechanism through
which this behaviour is learned and both warrant
further investigation.
In summary, we show that social foraging in
N. albiventris is non-opportunistic and allows a manifold
increase in the detection distance of insects and thus
potentially more efficient exploitation of patchily
5 6
12
8
13
2
6
0
ncipal coordinate
0.01 0.02
from the network of co-locations of foraging individual bats.
n-dashed rectangles cluster animals from the same group. The
ork modularity (QZ0.48), and it corresponds to the social
mber in a thick oval shape. Bats that foraged together are
o bats is depicted by the number in a thin ellipse breaking the
nnected by a solid line, bats from different social groups by a
resentation co-locations of bats during radio tracking, using
tween individual points is proportional to the inverse of the
fore represent a low number of co-locations, while small
s, group A; blue circles, group B; yellow circles, group C;
foraging: what happens to group size, vigilance, spacing,
aggression and habitat use in birds and mammals that
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Experimental evidence for group hunting D. K. N. Dechmann et al. 7
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appraoch in echolocating bats; stereotypical sensori-motor
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Biol. 205, 2865?2874.
Bouchard, S. 2001 Sex discrimination and roostmate
recognition by olfactory cues in the African bats, Mops
condylurus andChaerephon pumilus (Chiroptera: Molossidae).
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Brown, C. R. 1986 Cliff swallow colonies as information-
centers. Science 234, 83?85. (doi:10.1126/science.234.
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Buckley, N. J. 1996 Food finding and the influence of
information, local enhancement, and communal roosting
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Buckley, N. J. 1997a Experimental tests of the information-
center hypothesis with black vultures (Coragyps atratus)distributed, but rich food sources, which are available only
during a narrow time window. Similar to the findings of
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results confirm the important role of passive information
transfer via auditory cues for sociality in bats and
potentially many other animals.
All capture and handling of animals were done in con-
cordance with Panamanian laws and under permits from
Panamanian authorities.
For help with fieldwork, we want to thank Nicolas Ory,
Daniel Obando (and ART-S) as well as Edgar Perez, general
manager of the Gamboa Rainforest Resort. Thanks to Bjo?rn
Siemers for letting us to use his playback equipment. Patrick
Kelley, Corey Tarwater and two referees provided valuable
comments to the manuscript. Funding was received from:
Max Planck Institute for Ornithology, Radolfzell; German
Science Foundation, grant VO 890/11-1 to C.C.V. and
D.K.N.D.; NSF grant DEB-0083566 and DARPA grant
HR0011-05-1-0057 funded L.G.
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