Cues to body size in the formant spacing of male koala (Phascolarctos cinereus) bellows: honesty in an exaggerated trait
Benjamin D. Charlton1,*, William A. H. Ellis2, Allan J. McKinnon3, Gary J. Cowin4, Jacqui Brumm5, Karen Nilsson5 and W. Tecumseh Fitch1
1Department of Cognitive Biology, University of Vienna, A-1090 Vienna, Austria,
2Koala Ecology Group, School of Biological Sciences, University of Queensland, Brisbane, Queensland 4072, Australia,
3Moggill Koala Hospital, Department of Environment and Resource Management, Bellbowrie, Queensland 4070, Australia,
4Centre for Advanced Imaging, University of Queensland, Brisbane, Queensland 4072, Australia and 5Lone Pine Koala Sanctuary, Brisbane, Queensland 4069, Australia
*Author for correspondence ()
Determining the information content of vocal signals and understanding morphological modifications of vocal anatomy are key steps towards revealing the selection pressures acting on a given species’ vocal communication system. Here, we used a combination of acoustic and anatomical data to investigate whether male koala bellows provide reliable information on the caller’s body size, and to confirm whether male koalas have a permanently descended larynx. Our results indicate that the spectral prominences of male koala bellows are formants (vocal tract resonances), and show that larger males have lower formant spacing. In contrast, no relationship between body size and the fundamental frequency was found. Anatomical investigations revealed that male koalas have a permanently descended larynx: the first example of this in a marsupial. Furthermore, we found a deeply anchored sternothyroid muscle that could allow male koalas to retract their larynx into the thorax. While this would explain the low formant spacing of the exhalation and initial inhalation phases of male bellows, further research will be required to reveal the anatomical basis for the formant spacing of the later inhalation phases, which is predictive of vocal tract lengths of around 50cm (nearly the length of an adult koala’s body). Taken together, these findings show that the formant spacing of male koala bellows has the potential to provide receivers with reliable information on the caller’s body size, and reveal that vocal adaptations allowing callers to exaggerate (or maximise) the acoustic impression of their size have evolved independently in marsupials and placental mammals.