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Methods to discriminate echolocation calls between male and female big brown bats (Eptesicus fuscus).

Article Excerpt
ABSTRACT

Methods to discriminate echolocation calls of male and female big brown bats (Eptesicus fuscus) during the non-mating season were investigated. A total of 4,018 calls from 23 bats (12 males and 11 females) were analyzed. The bat calls were recorded in natural settings in Georgia (13 bats) and Ohio (10 bats). Both hand-held and flying calls were analyzed. Calls were further divided into multiple classes based on duration. A discriminant function analysis (DFA) detected sexual differences between the calls in some situations. In particular, when calls of similar durations were compared, the results indicated that short calls may be especially useful in differentiating the sexes.

Keywords: big brown bats, echolocation, Eptesicus fuscus, sex differences

INTRODUCTION

Bats use echolocation for many purposes, such as targeting and capturing prey, communicating between individual bats, and navigating through cluttered environments (1). There are many ways to characterize the variation in bat echolocation calls as calls can differ in their duration, their harmonic structure, their frequency pattern, and their amplitude (1). An echolocation call is classified as a constant-frequency (CF) call when the entire call is made at the same frequency (1). Conversely, an echolocation call classified as a frequency-modulated (FM) call sweeps downward through a range of frequencies (1). This variation in echolocation calls is associated with differences in hunting behavior as well as correlating with the species of bat (2). For example, bats living in cluttered environments tend to use more FM calls, as these types of calls can provide better multidimensional acoustic images (1). There can also be variation in a bat's repertoire as calls change in response to environmental and social factors (3). There is also debate on the importance of geographic variation, where calls of a given species change throughout its geographic range, because such variation may complicate attempts to classify calls (4-7).

Acoustical differences between males and females have been shown in other mammalian species (8). Because bats fly at night when visual acuity is impaired, and male and female bats are generally monomorphic in their physical appearance, they have limited ability to tell the sexes apart by sight (9). Bats are also not likely to use olfactory signals alone, as chemicals have a slow speed of transmission through the air, and would not be very accurate for locating a bat in flight (9). Differences between the echolocation calls of male and female bats have been identified in some species (10-13), but these have typically been species that use CF calls (14). Big brown bats use FM calls, and there is evidence that females of the species are able to discriminate between male and female bat sonar calls (9). Kazial and Masters (2004) performed playback experiments in which a female E. fuscus was presented with playback of echolocation calls from unfamiliar conspecifics. Their results showed that the subject animals responded differently to calls of the two sexes, which further reinforces the hypothesis that bats can determine sex difference through echolocation calls alone (9). Although this behavioral study has shown that female big brown bats can indeed recognize members of the opposite sex, researchers thus far have been unable to identify reliable differences in the echolocation calls of males versus females (14,15). Current research has identified some variables that distinguish male and female E. fuscus calls, such as duration and frequency-related variables, but they have proven useful only during the mating season (16).

The purpose of our project was to statistically examine the differences between echolocation calls of male and female big brown bats, extending the results of previous work. The study done by Masters et al. (1995) looked only at the differences between juvenile males and females of E. fuscus born in captivity. Kazial et al. (2001) looked at the differences between adult male and female E. fuscus, all from Ohio. Research at Auburn University by M. Grilliot (16) has looked at bats from Georgia and Alabama in a captive colony during mating season. All of these studies used calls recorded from bats in enclosed spaces, which may result in calls that differ from those produced by a bat in open areas (17). Additionally, the calls were used from bats that came from a fairly localized region. Our study examined bat echolocation calls from a wider geographical range using calls recorded in the open, rather than using recordings from enclosed spaces. Analysis of these calls may allow more reliable differentiation of male and female echolocation calls.

MATERIALS AND METHODS

Capture of the bats. -- We captured bats in Ohio and Georgia. The ten Ohio bats were captured in separate locations on the campus of the Ohio...

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