Perception of size-related formant information in male koalas (Phascolarctos cinereus)

Advances in bioacoustics allow us to study the perceptual and functional relevance of individual acoustic parameters. Here, we use re-synthesised male koala bellows and a habituation–dishabituation paradigm to test the hypothesis that male koalas are sensitive to shifts in formant frequencies corresponding to the natural variation in body size between a large and small adult male. We found that males habituated to bellows, in which the formants had been shifted to simulate a large or small male displayed a significant increase in behavioural response (dishabituation) when they were presented with bellows simulating the alternate size variant. The rehabituation control, in which the behavioural response levels returned to that of the last playbacks of the habituation phase, indicates that this was not a chance increase in response levels. Our results provide clear evidence that male koalas perceive and attend to size-related formant information in their own species-specific vocalisations and suggest that formant perception is a widespread ability shared by marsupials and placental mammals, and perhaps by vertebrates more widely

Female koalas prefer bellows in which lower formants indicate larger males

Despite an extensive literature on the role of acoustic cues in mate choice little is known about the specific vocal traits that female mammals prefer. We used resynthesis techniques and playback experiments to examine the behavioural responses of oestrous female koalas, Phascolarctos cinereus, to male bellows in which a specific acoustic cue to body size, the formants, were modified to simulate callers of different body size. Oestrous females looked longer towards, and spent more time in close proximity to, loudspeakers broadcasting bellows simulating larger male koalas. These findings suggest that female koalas use formants (key components of human speech) to select larger males as mating partners, and represent the first evidence of a marsupial mating preference based on a vocal signal. More generally, these results indicate that intersexual selection pressures to lower formants and exaggerate size are present in a marsupial species, raising interesting questions about the evolutionary origins of formant perception

Estimating the active space of male koala bellows : propagation of cues to size and identity in a Eucalyptus forest

Examining how increasing distance affects the information content of vocal signals is fundamental for determining the active space of a given species’ vocal communication system. In the current study we played back male koala bellows in a Eucalyptus forest to determine the extent that individual classification of male koala bellows becomes less accurate over distance, and also to quantify how individually distinctive acoustic features of bellows and size-related information degrade over distance. Our results show that the formant frequencies of bellows derived from Linear Predictive Coding can be used to classify calls to male koalas over distances of 1–50 m. Further analysis revealed that the upper formant frequencies and formant frequency spacing were the most stable acoustic features of male bellows as they propagated through the Eucalyptus canopy. Taken together these findings suggest that koalas could recognise known individuals at distances of up to 50 m and indicate that they should attend to variation in the upper formant frequencies and formant frequency spacing when assessing the identity of callers. Furthermore, since the formant frequency spacing is also a cue to male body size in this species and its variation over distance remained very low compared to documented inter-individual variation, we suggest that male koalas would still be reliably classified as small, medium or large by receivers at distances of up to 150 m