Bat Echolocation Researc h - Bat Conservation International

Figure 3: Spectrogram representation of Myotis nigricans (Panama) search calls: Individual M. nigricans change from narrow band to broadband and
back to narrow-band signals again, as the individual flies from open space to a forest edge and back into the open again. Note that the pulse intervals
have been cut out. Figure adapted from Siemers et al. 2001c.
Bats of the Old World families Rhinolophidae and
Hipposideridae and the Neotropical mormoopid Pteronotus
parnellii produce long (> 20 ms; hipposiderids > 10
ms) constant frequency (CF) calls with high duty cycle
(Fig. 2). They are characterized by anatomical and physiological
specializations of the hearing system that
allows a fine frequency resolution around their CF frequency
– the so-called “acoustic fovea” (for review see
Moss and Schnitzler 1995). These specializations are
used to detect fluttering prey by evaluating the “acoustic
glints” (rhythmical amplitude and frequency shifts) modulated
onto the echoes by insect wing beats (Schnitzler
1987). Therefore, these bats always center the returning
echoes in the high-resolution frequency range of the
acoustic fovea. When hanging from a perch, they emit
their CF call component at their “resting frequency”.
When flying, they compensate for frequency shifts
caused by Doppler shift due to their own flight speed by
lowering the emission frequency by up to 3 kHz,
depending on flight speed and absolute value of resting
frequency (reviewed in Moss and Schnitzler 1995). In
essence, there is variability in CF frequency within individuals
caused by Doppler compensation, and there is
individual variation within a population (e.g., Jones et al.
1992; Russo et al. 2001). However, this variability is not
situation-specific. Call duration varies with the echolocation
task faced, but emitted CF frequency is the same
at different distances to clutter (rhinolophids and hipposiderids
usually do not fly in open space), in order to
match the returning echoes with the acoustic fovea. This
means that CF frequency parameters measured in the
flight tent can be used for field identification in rhinolophids
and hipposiderids. Depending on the distribution
of CF frequencies among rhinolophid and hipposiderid
species in the local community, this might or
might not allow unequivocal identification of wild bats
from their CF frequency (e.g., Heller and von Helversen
1989, Kingston et al. 2000; Russo et al. 2001; Siemers,
Dietz, Schunger and Ivanova, unpublished data on Bulgarian
Rhinolophus community). In the case of rhinolophids
and hipposiderids, field biologists benefit
Section 3: Ultrasound Species Identification
from the biological specialization of the acoustic fovea
and associated CF constancy, such that flight-tent data,
can be used for field identification.
Most bats lack the anatomical and physiological specialization
of an acoustic fovea and broadcast signals
with situation-specific frequency content (and duration).
Likewise, these differences have an adaptive value and
serve to solve different orientation tasks (Fig. 3; for
review see Fenton 1990; Neuweiler 1989, 1990;
Schnitzler and Kalko 2001; Schnitzler et al., 2003). For
these species, it is impossible to record the entire species
repertoire of echolocation signals in a flight tent. In a
tent, they typically produce short broadband signals (<
4 ms, > 40 kHz frequency range), whereas in open space
they might well broadcast long and narrowband signals
(> 10 ms, < 10 kHz frequency range). The natural transition
in calls by bats moving from clutter to open space
is usually characterized by a gradual change in call
parameters. While (in downward FM calls) duration
gradually increases and starting frequency gradually
decreases, terminal frequency remains relatively constant
for many species. In some species terminal frequency
decreases slightly with increasing call duration;
generally by < 5 kHz across the search-call repertoire.
Additionally, terminal frequency is less affected by
atmospheric attenuation than the higher starting frequency.
Consequently, field recordings give more reliable
measurements of terminal frequency, as they are less
dependent on the distance and the angle of the bat to
the microphone than starting frequency.
For both, biological and technical reasons, terminal
frequency recorded in a flight cage is the most suitable
parameter to use to predict natural signal structure in
open space. This is of no use for species that introduce
new call types when flying in the open (e.g., Barbastella
barbastellus: Denzinger et al. 2001; Parsons and Jones
2000). But even such a prediction based on terminal frequency
does not provide a sound basis for field identification,
and I reiterate that, for many species, field discrimination
depends on a comprehensive reference
database that is not collected solely in a flight cage.
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