Bat Echolocation Researc h - Bat Conservation International

and van Parijs 1993).
Early studies using bat detectors indicated that different
species of bats use different echolocation calls,
thus suggesting that free-flying individuals could be
identified on the basis of their calls (Ahlén 1981; Fenton
and Bell 1981). However, one problem in using echolocation
calls to identify species of bats is that the characteristics
of the calls of each species (e.g., duration, bandwidth,
frequency, harmonic structure) vary. To address
questions requiring species identification, researchers
rely on the fact that variation in call structure is generally
greater among species than within species. Within
many bat communities, the calls of some species are distinctive
(Fig. 1A), and even though there is variation
within each species, calls can be assigned unambiguously
to particular species. In most cases, however, the call
characteristics of various species overlap because on
average the calls of different species are very similar to
one another, or because there is greater variation in the
calls of each species (Fig. 1B). Assigning calls to a particular
species is less certain under these circumstances.
Calls that fall within the zone of overlap (Fig. 1B) may
be assigned to a species with a probability less than one,
with the probability increasing the closer the combination
of traits is to the mean value for that species. By definition,
there is error involved in making such classifications.
Often, multivariate statistical techniques such as
discriminant function analysis (DFA) or neural networks
are used to determine the probability of assignment
(e.g., Jones et al. 2000; Parsons and Jones 2000; Vaughan
et al. 1997).
Variation in the echolocation-call characteristics of a
particular species of bat clearly increases the uncertainty
of identifying calls of unknown, free-flying individuals
to that species. Such variation occurs for a number of
reasons, including variation within a population related
to characteristics of the individual, such as age, gender,
and body mass (Betts 1998; Brigham et al. 1989; Jones et
al. 1992; Jones and Ransome 1993; Obrist 1995). Some
Section 4: Resources, Research and Study
Figure 1: Variation in two
characteristics of echolocation
calls within and between
two species of bats. A: Variation
within each species,
depicted as the shaded circles,
is relatively small and
calls can be unambiguously
assigned to one species or
the other. B: The mean values
for the two call variables
are the same as in A, for each
species, but there is greater
within-species variation. This
means that there is overlap
among the calls of the two
species. Calls with characteristics
in the zone of overlap
cannot be assigned to a particular
species with 100%
certainty.
of these factors may cause variation in calls from different
locations (e.g., body mass), while others will not
(e.g., age and gender), unless population structure varies
(e.g., Barclay 1991). The purpose of this review is to
consider geographic variation, especially among populations
within a species, and to make recommendations as
to how to account for it when conducting studies that
depend on identifying bats by their echolocation calls.
By geographic variation we mean variation in the
structure of echolocation calls of individuals of the same
species recorded in different locations. It is a potential
problem if call characteristics from one population are
used to identify the calls of unknown individuals from
another population (Law et al. 2002). Most studies rely
on a set of calls from known individuals (a reference
library) to establish the characteristics used to identify
unknown bats recorded by the monitoring system.
Suppose there are two species in our study area and
we base species identification on the call characteristics
determined from another location (Fig. 2). Suppose further
that the characteristics of the calls of the two
species are distinct, but call characteristics from individuals
in our study populations differ from those in the reference
library. Such geographic variation could result in
the inability to identify some calls and misidentification
of others, depending on the amount of geographic variation
(Fig. 2).
While geographic variation is most commonly
thought of on a large scale, an analogy being the dialects
of humans and birds, there is relatively little information
regarding this for the echolocation calls of bats. A large
part of the problem is a lack of consistency in recording
techniques and equipment among studies. There are,
however, considerable data regarding smaller-scale geographic
variation and variation caused by other factors
that may result in large-scale geographic differences. We
thus examine these sorts of variation, make predictions
Figure 2: Potential problems associated with geographic variation in the
characteristics of the echolocation calls of two species of bats. Characteristics
from the reference library for each species (shaded circles) are
distinctive, but differ from the characteristics of the calls from the study
populations (open circles). Because of this, some calls will be unidentifiable
(they do not fall within either reference set), and others will be
misidentified (they fall within the reference set of the other species).
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