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Behaviour & communication

The acoustic structure and information content of female koala vocal signals

Charlton, BD 2015, PLoS ONE, vol. 10, no. 10, e0138670.

Female koalas produce distinct categories of vocalisations that differ in acoustic structure. Some of these vocalisations contain acoustic cues to the caller’s identity, age and sex, which is likely of great adaptive significance to male koalas.

  Female koalas produce a number of unique vocalisations during the breeding season and especially during oestrus. In this study, female koala vocalisations were recorded and grouped into call types based on similarities in acoustic structure. Subsequently, the calls were analysed to identify acoustic features that may provide cues to the caller’s identity, age or sex. One temporal feature, twelve source- and filter-related acoustic features, and three types of nonlinear phenomena in the calls were examined, making this study on the elements of female koala vocalisations the most comprehensive to date. Vocalisations were categorised into three groups: bellows, snarls, and tonal rejection calls. Bellows were the longest in duration of all call types and were the most individually distinctive. Despite being structurally similar to male koala bellows, they were shorter in duration and had higher fundamental frequencies, formant frequencies, and formant frequency spacing. Snarls were the second longest in duration of the call types and were not individually distinctive, giving no reliable cues to the caller’s identity or age. Tonal rejection calls comprise the vocalisations that were previously termed squeaks, squawks, wails and screams. These rejection calls were individually distinctive and also varied according to the age of the caller, with older females producing calls with lower mean, minimum and maximum fundamental frequencies than their younger counterparts. Although female koalas are known to produce barks or grunts associated with oestrus, such vocalisations were not captured in this study. The author describes the anatomical basis of the production of each call type in great detail.

  The distinctiveness of male versus female koala bellows is advantageous for male koalas to discriminate between potential mates and potential rivals. The ability to discriminate between individual females based on their bellows and tonal rejection calls may be beneficial for males seeking a particular female with which they are familiar, as prior familiarity between koalas has been demonstrated to increase the success of copulation. Males may also enhance their reproductive success by discriminating between older and younger female koalas based on their tonal rejection calls as female fecundity is reduced with age.

  As most previous analyses of koala vocalisations focused on those of males, this study provides valuable new information about female vocalisations and their potential functions. As a result of the findings presented here, the author recommends future studies to evaluate how koalas in the wild perceive and respond to cues to a female caller’s identity, age and sex.

 

Summarised by Joanna Horsfall

 

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