Perception of size-related formant information in male koalas (Phascolarctos cinereus)
Charlton, BD, Ellis, WAH, Larkin, R & Tecumseh Fitch, W 2012, Animal Cognition, vol. 15, no. 1, pp. 999-1006.
This report is the first to document evidence of male koalas being able to perceive variations in formant spacing in the bellows of other males that convey the caller’s body size.
Formants are the resonance frequencies produced by vibrating air in the vocal tract. In mammals, formants contribute to phonemic variation and, in this regard, are thought to have functional relevance in vocal communication between conspecifics. It has recently been demonstrated that formants in koala bellows vary in frequency spacing with body size and that receivers can interpret these acoustic cues. In this study, recordings of male koala bellows were digitally manipulated by shifting the call’s formant spacing to simulate a variation in body size. All other acoustic parameters, such as pitch and duration, were unaltered. Playback experiments were conducted in which male koalas heard the repeated calls of a single male five times (habituation stimulus) before hearing the same male’s digitally altered call (dishabituation stimulus) and finally the unaltered call once again (rehabituation stimulus). The koalas demonstrated a significant renewal of response when presented with the dishabituation stimulus, indicating that male koalas perceive and attend to changes in formant frequencies in bellows. The responses of the koalas diminished when exposed to the rehabituation playback. Despite perceiving variations in body size, the particular formant spacing of the digitally altered stimulus had no noticeable effect on dishabituation response patterns.
As male koalas bellow most frequently during breeding season, these unique vocalisations are likely associated with reproductive behaviours. Certainly, it would be beneficial for male koalas to communicate their size to attract potential mates or deter rivals. Likewise, the ability to interpret acoustic cues in koala bellows can assist females in mate selection and males in sizing up their competition.
The findings of this and related studies suggest that formants play a key functional role in the acoustic communication of phenotypic characteristics between conspecifics, not only in marsupials and mammals but perhaps in many vertebrate species. To further this inquiry, the authors suggest that similar playback experiments are conducted to explore whether male koalas use acoustic cues in bellows to make judgements about engaging in or avoiding male-male competition.
Summarised by Joanna Horsfall
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