Estimating the active space of male koala bellows: propagation of cues to size and identity in a Eucalyptus forest
Charlton, BD, Reby, D, Ellis, WAH, Brumm, J & Tecumseh Fitch, W 2012, PLoS ONE, vol. 7, no. 9, e45420.
The acoustic cues that most reliably distinguish the bellow of one male koala from another can be transmitted across a distance of up to 50 metres in a eucalypt forest. The acoustic features that indicate the size of a male, however, can be reliably communicated over a longer distance of up to 150 metres in the typical habitat of the koala.
As the distance increased between a sound signaller and receiver, the acoustic features of a koala bellow that remained the most stable were upper formant frequencies and formant frequency spacing. Koalas can use upper formant frequencies to distinguish between male callers, which each have a distinct vocal style. Formant frequency spacing provides cues to the body size of the male caller. The variation in this acoustic feature when transmitted over large distances was very low compared to variation between individuals, suggesting that body size can be reliably communicated over a larger distance than that at which identity can be communicated.
Information about a male caller’s identity and body size is beneficial for both male and female koalas, and the ability to communicate this information acoustically is important for a solitary and arboreal species such as the koala. Males, for instance, can use such information to establish or recognise dominance among other males, while females can use these cues to fulfil mating preferences. Given the importance of acoustic features in vocalisations for communicating this information between conspecifics, it is feasible that selection should favour those individuals with vocal features that publicise particular traits and resist degradation over large distances in that species’ typical habitat. Further, the acoustic advertisement of a male’s body size is potentially linked to the size of his home range during the breeding season. A circular territory of a typical size would have a radius of 145 metres; therefore, if the male is at the centre of his home range, he can effectively communicate his size across the extent of that range. This would allow conspecifics to decide whether or not to enter the territory based on the potential for rivalry or mating.
This initial evidence of the active space of koala vocalisations indicates that the communication of different types of information may also differ in importance over varying distances. Regarding koala conservation and management, the findings of this study not only suggest that bioacoustic monitoring techniques could be used for population monitoring but also inform the spacing and density at which recording devices should be deployed and the acoustic features that should be analysed to improve the accuracy of population surveys.
Summarised by Joanna Horsfall
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