Bat Echolocation Researc h - Bat Conservation International

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INTRODUCTION
Much of our knowledge about bat ecology, behavior,
and echolocation comes from studies conducted in the
field. Owing to continuous technological improvements
in equipment and an ever-increasing number of
researchers, bat field ecology is a growing and yet still
challenging area of research. In point, this symposium
provides an overview of the current state in field-based
bat echolocation studies. Another well of information
for bat behavioral biologists are psychophysical experiments
designed to investigate the performance and sensory
capacities of echolocation systems. Often, a 2-alternative
forced-choice paradigm is used in which a bat sits
on a Y-shaped platform and crawls either onto the left or
the right branch of the platform to indicate whether it
perceives an object left or right. Such objects might be
real targets like thin wires or virtual, so-called phantom
targets (i.e., echoes modified by a computer and played
back through a loudspeaker).
Whereas field studies allow us to learn about the natural
and complex behaviors of bats, they are limited by
the difficulty of creating standardized experimental conditions
that are necessary to answer many biological
questions. In psychophysical tests, the experimental conditions
can be controlled, and often reach a high level of
sophistication. However, experiments with sitting rather
than flying bats and highly specific, sometimes quite simplified
psychophysical tasks (for the sake of experimental
clarity) are different from a bat’s natural foraging situation,
where it flies in 3-dimensional space and has to cope
with complex echo scenes. In this paper, I discuss flightcage
studies as a method to bridge the gap between field
studies and psychophysical experiments. I outline the
advantages of working with bats in a flight cage and then
address more specifically the advantages and disadvantages
of recording echolocation calls in a flight cage.
Finally, I address the question of whether acoustic species
identifications can be based on flight-cage recordings as
a reference database.
WHY WORK WITH BATS IN A FLIGHT CAGE?
If one is interested in the study of hunting and associated
echolocation behavior in bats in the field, one has
to first know the natural foraging areas of the bats. This
is feasible for bats who prefer a specific and delimited
habitat type, such as the surface of calm water bodies.
Not surprisingly, several detailed field studies report the
prey-capture techniques and echolocation behavior of
trawling Myotis species; those that capture insects from
and at low heights above water surfaces (e.g., Britton et
al.1997; Jones and Rayner 1988, 1991; Kalko and
Schnitzler 1989). Likewise, bats that forage in open situations
using echolocation calls with high sound-pressure
level can, with considerable effort, be found,
observed, and recorded in the field (e.g., Jensen and
Miller 1999; Kalko 1995; Kalko and Schnitzler 1993).
However, the natural hunting behavior of bats that
habitually forage within or close to vegetation and who
often use low sound-pressure level echolocation signals
is difficult to see in the field. Even radio-tagged individuals
are not easy to be directly observed (e.g., Arlettaz
1999; Siemers et al. 1999). It is even more difficult to get
close enough to obtain high-quality call recordings
especially when the bats are actually capturing prey. For
these species, flight-cage studies may provide a suitable
approach to observe foraging behavior difficult to
observe in the wild (e.g., Trachops cirrhosus (Phylostomidae):
Barclay et al. 1981; Plecotus auritus (Vespertilionidae):
Anderson and Racey 1991; Myotis sp.: Arlettaz et
al. 2001; Faure and Barclay 1992, 1994; Siemers and
Schnitzler 2000; Swift and Racey 2002).
Most flight-cage studies focus on gleaning bats and
on those which hunt in or close to vegetation. First, only
anecdotal foraging observations are often obtainable for
these species. Second, flight-cage studies closely emulate
natural conditions for these kinds of bats. A flight cage is
a restricted and echo-cluttered “habitat” from a bat’s point
of view. To try and study prey capture behavior of open
space foragers in such a restricted environment will not
yield meaningful results. However, bats adapted to cluttered
habitats should cope well with a flight-cage environment.
They can be observed and experimentally tested
while performing natural behaviors in this semi-natural
setting. If necessary, flight cages can be equipped with
habitat elements such as leaf litter, grasses, branches,
small ponds, etc. (e.g., Arlettaz et al. 2001; Britton and
Jones 1999; Siemers and Schnitzler 2000; Siemers et al.
2001a, 2001b; Swift and Racey 2002).
In a flight cage, it is also feasible to study behavior
with a degree of detail difficult to achieve for animals in
the wild. This, for example, applies to the study of the
bats’ motor behavior during prey capture. Close-up photographic
or video documentation can be used, as prey
can be offered at a predefined location. Detailed behavioral
documentation with sufficient sample sizes are relatively
easy to obtain. For instance, the role of the tail
membrane and feet during prey retrieval from the water
surface by Myotis daubentonii was clarified in a flight cage
(Britton and Jones 1999; Siemers et al. 2001a) complementing
previous field observations (Jones and Rayner
1988; Kalko and Schnitzler 1989).
I referred to the difficulty of obtaining quantitative
experimental data in the field although there are a range
of excellent examples of experimental field studies
assessing the sensory basis of food detection with wild
bats (e.g., Barclay and Brigham 1994; Boonman et al.
1998; Fuzessery et al. 1993; von Helversen and von Helversen
1999). Two of these studies involved training wild
trawling bats to forage in a defined experimental area,
selecting and taking objects from the water surface. Von
Helversen and von Helversen (1999) worked with flower-visiting
glossophagine bats and experimentally
manipulated their natural food resource, the inflorescences
of the Neotropical liana, Mucuna holtonii. Where
Bat Echolocation Research: tools, techniques & analysis