Mysterious Mystacina: how the New Zealand short-tailed bat (Mystacina tuberculata) locates insect prey
1 School of Biological Sciences, University of Bristol, Woodland Road,
Bristol BS8 1UG, UK
2 Department of Zoology, University of Otago, PO Box 56, Dunedin, New
Zealand
3 Science & Research Unit, Department of Conservation, Private Bag,
Christchurch, New Zealand
* Author for correspondence (e-mail: gareth.jones{at}bris.ac.uk)
Accepted 19 August 2003
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Summary |
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Key words: echolocation, predation, clutter, sensory ecology, bat, Mystacina tuberculata
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Introduction |
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The varied diet and terrestrial adaptations of M. tuberculata make
it interesting from a sensory ecology perspective. M. tuberculata has
relatively large ears (O'Donnell et al.,
1999) and prominent nostrils
(Daniel, 1979
), implying that
it may listen for prey-generated sounds and may use olfaction in the detection
of food. Because the bats eat both volant and nonvolant arthropod prey
(Arkins et al., 1999
), we
expect that they will face different sensory challenges for the detection and
localization of prey in cluttered (clutter echoes are echoes other than those
from the target of interest) and non-cluttered space
(Faure and Barclay, 1994
). In
highly cluttered space, background echoes overlap with prey echoes, and
masking of prey echoes by clutter echoes makes detection of prey problematic.
Bats that emit frequency-modulated (FM) calls mainly use prey-generated
acoustic cues for the detection and localization of prey in clutter
(Schnitzler and Kalko, 1998
).
The Indian false vampire bat Megaderma lyra may use echolocation to
detect prey in limited clutter (Schmidt et
al., 2000
), but one feeding situation presented here (prey buried
under leaf litter) will preclude the use of echolocation for detection of
prey. Recent work on mouse-eared bats Myotis myotis and Myotis
blythii showed experimentally how bats used echolocation to detect aerial
prey but required prey movement to rapidly detect prey in leaf litter. The
bats also greatly reduced the intensity of echolocation calls emitted
immediately prior to prey capture in leaf litter
(Arlettaz et al., 2001
).
We therefore predict that the diversity of sensory challenges that M.
tuberculata faces when foraging will result in it adopting a range of
mechanisms for the detection of arthropod prey. We first describe the
echolocation calls of M. tuberculata from field recordings, and we
show how this species has a wing shape suitable for foraging in habitats that
contain considerable physical clutter, such as forest interiors. We then
determine how M. tuberculata detects and locates arthropod prey in
uncluttered space (by aerial hawking) and in leaf litter, where echolocation
calls will not be able to reach prey buried in clutter. Specifically, we
predict that, like another FM bat (Myotis evotis;
Faure and Barclay, 1994),
M. tuberculata will use echolocation to detect and localize aerial
prey and will switch off echolocation when hunting for prey in clutter. Some
bat species detect prey in clutter by listening for prey-generated sounds
(Plecotus auritus: Anderson and
Racey, 1991
; Antrozous pallidus:
Fiedler, 1979
;
Bell, 1982
;
Fuzessery et al., 1993
), while
another species (Macrotus californicus) locates terrestrial prey by
vision (Bell, 1985
). We
therefore isolated cues available to the bats to determine how they found prey
hidden in leaf litter. We investigated whether M. tuberculata
detected buried prey by listening for sounds generated by prey movement. We
also investigated whether olfaction plays a role in prey detection, given that
the bats may locate nectar by this means.
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Materials and methods |
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Wing shape analysis
We captured bats emerging from roosts by harp traps and by mist netting
along flight paths. We traced wing outlines onto paper, digitized the tracings
with a Summasketch III bitpad (Summagraphics, Seymour, CT, USA) connected to a
PC. We measured the wing shape parameters described by Norberg and Rayner
(1987). We measured forearm
length to the nearest 0.1 mm with dial calipers, and body mass to the nearest
0.1 g with a spring balance.
Recording and analysis of echolocation calls
We used a time-expansion (10x) bat detector (D-980; Pettersson
Elektronik AB, Uppsala, Sweden; frequency response ±3 dB, 20120
kHz; Waters 1995) linked to a
Sony WM-D6C Professional Walkman cassette recorder to record echolocation
calls. We analysed sounds using BatSound (Pettersson Elektronik AB). Temporal
characters were measured from waveforms, frequency parameters from
spectrograms, with the exception of frequency of most energy, which was
measured from power spectra [fast Fourier transform (FFT) size 512, Hanning
window for all spectral analyses]. Maximal and minimal frequencies were
measured at 30 dB below the power at the frequency of most energy.
Laboratory observations of foraging behaviour
Laboratory observations were made in a room 2.9 mx3.2 mx2.4 m.
Bats were released into the room during their natural foraging hours and were
observed and recorded from a covered recess on one side of the room. The room
was dark, and bats were recorded under infrared illumination using a Sony
TRV9E digital video camera in `nightshot' mode. Bat echolocation calls were
recorded using the equipment described in `Recording and analysis of
echolocation calls' above, with a 5 m cable leading from the bat detector to
the microphone and 12 s of recording time. The microphone of the video camera
recorded the frequency-divided output of the bat detector, and this allowed us
to synchronise the bats' behaviour with the time-expanded recordings of
echolocation calls. Synchronisation was facilitated by recording single
flashes from a flashgun (Nikon SB12 Speedlight) on both audio (ultrasound from
flash firing) and video tapes (light output recorded). To record bats
capturing aerial prey, insects [moths (Lepidoptera), dobsonflies (Megaloptera:
Corydalidae) and stoneflies (Plecoptera)] were suspended from fishing line
(<1 mm diameter), and the microphone placed about 5 cmbehind the prey item.
To encourage foraging on the ground, a tray 120 cmx70 cm, with 7 cm-high
walls, was filled with natural Nothofagus leaf litter. Bats learnt to
land in the tray rapidly and began searching for food there without
training.
Isolating cues used for the detection of prey in leaf litter
We predicted that bats might detect prey by echolocation, by vision, by
listening for prey-generated sounds or by olfaction. For the detection of prey
in leaf litter, we aimed to isolate cues as much as possible to determine
which sensory mechanisms the bats used to detect prey hidden under leaf
litter. We removed the possibility of the bats using echolocation by hiding
prey under 34 cm of leaf litter. We argue that vision is also of no use
in these situations, as the prey are concealed. Our experiments were conducted
in complete darkness, under infra-red illumination. We therefore tried to
isolate cues available from prey-generated sounds and smell experimentally. To
isolate prey-generated sounds, we placed 25 mealworms in plastic dishes
(diameter 15 cm)among leaf litter. In one dish, the mealworms were dead
(killed by placing in boiling water), and hence generated no sound, in the
other they were alive. The dish was covered with foil, and 25 dead mealworms
were placed on the foil of both dishes as a reward for the bats finding the
dish. Thus, both dishes were identical except that one contained live
mealworms under the foil while the other contained dead mealworms. It is
likely that the only difference in cues available to the bats under these
treatments was the presence or absence of prey-generated sounds. Of course, in
nature, prey movements might cause leaf litter to move, and such movement
might be detectable by echolocation or by vision. The purpose of the
experiment described here was to determine whether bats could locate prey by
acoustic cues alone. The dishes were placed randomly in the tray of leaf
litter, and we recorded which dish the released bat found first. To isolate
olfactory cues, we tethered 10 dead mealworms (no prey-generated sounds) under
the leaf litter (no echolocation cues) and recorded whether the bats found
these in the dark (no visual cues) under infra-red lighting. The mealworms
were placed at random positions within the tray. Tethering mealworms beneath
the leaf litter allowed prey to be relocated easily by the experimenter and
precluded the use of vision or echolocation by the bats for finding the
tether. We used nine bats (adults or fully grown juveniles: seven males, two
post-lactating females) and released bats at the site of capture within 10
days. All experimental sessions were performed with single bats, although
groups of bats were initially allowed to familiarize themselves with the
feeding arenas.
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Results |
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Field recordings of echolocation
We recorded 31 calls with good signal:noise ratio from nine bats flying in
the forest. In addition, we analysed 11 calls from three bats recorded
circling above us after release in open space away from trees
(Table 2). Pulses and pulse
intervals were substantially longer from bats released in the open, but
frequency characteristics were similar in open and forest environments. In
both situations, the calls were relatively brief and multiharmonic
(Fig. 1). The frequency of most
energy in the call was usually in the fundamental harmonic (64% of cases) but
was often in the second harmonic (remaining cases). Pulse repetition rate was
12.413.8 Hz. The predicted wingbeat frequency for a bat with the body
mass of M. tuberculata is 10 Hz
(Jones, 1994), so it appears
that M. tuberculata usually emits one pulse, sometimes two pulses,
per wingbeat during search phase.
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Detection of prey in uncluttered space
We sometimes heard terminal buzzes emitted by bats flying around roost
sites, suggesting that M. tuberculata hunts for aerial prey using
echolocation. These buzzes were of relatively long duration and were clearly
associated with foraging rather than obstacle negotiation. In the laboratory,
M. tuberculata always emitted terminal buzzes when attacking prey
suspended by fishing wire, showing that bats (50 captures from five
individuals) detect prey in uncluttered prey by echolocation
(Fig. 2). As in the field,
search-phase calls were multiharmonic. Distinct approach and terminal phases
were detectable during aerial feeding sequences, with calling terminated about
100 ms before striking the prey (Fig.
2). Typical of feeding sequences during aerial captures by bats,
pulse duration and pulse interval decreased in the time leading up to prey
capture (Fig. 2). Energy also
became more concentrated in the fundamental harmonic relative to higher
harmonics during the terminal phase of the buzz
(Fig. 2C,D).
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Detection of prey in leaf litter
Bats rapidly learnt to land on the litter tray and to search for food
there. They never appeared to detect prey from the air and only started
searching after landing on the ground. On the ground, bats moved rapidly and
adeptly. They would frequently dig into the leaf litter with their forelimbs
to find prey, sometimes disappearing completely under the litter. A typical
echolocation call sequence from a bat finding prey with the presence of
prey-generated sounds in a dish is shown in
Fig. 3. The bat emitted
search-phase calls during flight and increased call repetition rate prior to
landing. When on the ground, call repetition rate was low (typically <5
Hz), showing that prey are unlikely to be located by echolocation. After the
bat found the prey, it took off with the prey in its mouth (sometimes prey
were eaten on the ground), and call repetition rate increased again
(Fig. 3).
|
So, as predicted, M. tuberculata detected prey in leaf litter by
methods other than echolocation. In our experiment where rustling prey were in
one dish, dead prey in the other, eight of nine bats first found the dead
`reward' mealworms above the dish containing live mealworms (2
with Yates' correction = 4.0, P<0.05), suggesting that they were
attracted to the dish by the sounds made by mealworms moving in leaf litter.
Over time, however, bats often found prey in the other dish containing dead
mealworms, suggesting that they did so by means other than listening. In
total, five of eight bats found at least half of the buried dead mealworms
overnight. One bat found six of the 10 dead buried mealworms within 10 min.
These results suggest that M. tuberculata found dead mealworms by
using olfaction.
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Discussion |
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The frequency and time parameters reported here for search-phase
echolocation calls resemble those given by Parsons
(2001) for short-tailed bats
of a different subspecies (M. tuberculata aupourica) recorded on
Little Barrier Island off North Island, New Zealand. Here, we provide
additional information on pulse repetition rates and show that calls and pulse
intervals are longer from bats flying in open areas compared with forests.
Longer pulses can be produced in open habitats because echoes return later,
and avoidance of pulse-echo overlap imposes fewer constraints on pulse
duration (Kalko and Schnitzler,
1993
). Extending pulse interval in the open may increase the time
window for processing echoes from more distant targets
(Fenton et al., 1998
). The
brief, multiharmonic calls of Mystacina resemble the echolocation
calls of many phyllostomid bats (e.g.
Belwood, 1988
;
Thies et al., 1998
). Such
similarities may reflect the close phylogenetic affinities between bats in the
families Mystacinidae and Phyllostomidae
(Pierson et al., 1986
;
Kirsch et al., 1998
;
Kennedy et al., 1999
;
Van Den Bussche and Hoofer,
2000
). If bats in these families shared a common ancestor,
descendant taxa may have inherited similar constraints that shaped the
evolution of their echolocation calls. Alternatively, similarities may arise
through convergent evolution, although this seems unlikely given that there
are no terrestrial phyllostomids, with the exception of the vampire bats.
We showed that M. tuberculata uses echolocation to detect and
localize aerial prey. Feeding buzzes are similar to those described for aerial
feeding bats (e.g. Kalko,
1995; Surlykke et al.,
1993
; Britton et al.,
1997
) in that pulse duration and pulse interval decrease as the
prey is approached. Harmonics remain prominent during the approach and
terminal phases, maintaining a broad bandwidth for the calls. A switch from
echolocation for the detection of aerial prey to using other cues to detect
prey in clutter has been shown in other bats. We know of no other bat [with
the possible exception of vampire bats (Desmodus rotundus);
Altenbach, 1979
] that shows
terrestrial locomotion that is as agile as that of Mystacina,
however.
Species that glean prey from surfaces often listen for prey-generated
sounds to detect and localize prey
(Fiedler, 1979;
Anderson and Racey, 1991
;
Faure and Barclay, 1992
;
Faure et al., 1993
;
Marimuthu, 1997
). M.
tuberculata is unable to locate prey in leaf litter by echolocation
because echoes from leaves will mask prey echoes. Instead, the bats locate
mealworms beneath leaf litter by listening for prey-generated sounds and by
olfaction. Although gleaning bats such as M. myotis and M.
blythii seem to locate prey in leaf litter by listening for prey sounds
while flying (Arlettaz et al.,
2001
), M. tuberculata seemed unable to do this. Instead,
the bats seemed to locate prey while on the ground. Whether this terrestrial
location of prey is because of an inability of M. tuberculata to fly
very slowly or hover or whether it is caused by sensory constraints remains
unclear. In nature, passive cues generated by moving prey may be more
conspicuous than those generated by the mealworms in our experiments.
Once on the ground, the bats often dug deep into the litter to find the
mealworms. Although many frugivorous and nectarivorous bats find food by
olfaction (review in Bloss,
1999), some insectivorous species can locate covered prey on the
ground by olfaction (Kolb,
1961
,
1973
), and mealworm odours
appear to stimulate prey-searching behaviour at close range in M.
emarginatus and P. auritus (Dijkgraaf,
1946
,
1957
). Because M.
tuberculata eats nectar as well as arthropods
(Arkins et al., 1999
), it is
probably highly adapted for the detection of plant odours and may also use
these adaptations for the detection of buried prey. We were surprised with the
ease by which M. tuberculata found buried prey in the laboratory.
This, coupled with the observation that M. tuberculata digs for wild
beetle larvae in captivity (J. McCartney, unpublished), suggests that M.
tuberculata may use olfaction for prey detection under natural
conditions.
M. tuberculata eats a wide range of foods
(Arkins et al., 1999) and is
endowed with a range of sensory adaptations that allow the bats to exploit
this diversity. The species is therefore a useful model to investigate how
prey detection depends on ecological situation in echolocating bats. We have
shown that echolocation, listening for prey-generated sounds and olfaction can
all be used in the detection and localization of arthropod prey. The use of
echolocation is dependent on clutter echoes not masking echoes from the prey
item. We expect that olfaction is the most important sense used for the
detection of nectar-producing plants and fruit. Cues used by M.
tuberculata for the detection of plant foods, and field observations on
foraging tactics, are important challenges for future researchers.
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Acknowledgments |
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