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Behaviour & communication

Female koalas prefer bellows in which lower formants indicate larger males

Charlton, BD, Ellis, WAH, Brumm, J, Nilsson, K & Fitch, WT 2012, Animal Behaviour, vol. 84, no. 1, pp 1565-1571.

Formants, the frequencies produced by vibrating air in the vocal tract, are acoustic cues that female koalas can use to evaluate the size of a bellowing male. Oestrus female koalas at Lone Pine Koala Sanctuary looked significantly more towards and spent significantly more time in close proximity to speakers broadcasting bellows with lower formant values, indicating a large male caller, than to speakers broadcasting higher formant values produced by a smaller male.

  The functions of formants as a component of human speech are well understood, but only recently have we discovered that formant variation has a communicative function for nonhuman animals. Specifically, formants indicate the size of the vocalising animal because of the positive relationship between an animal’s vocal tract length and body size. Male koala bellowing is a sexual behaviour that assists in the attraction of a mate, and previous studies have demonstrated that formant pitches are reliable indicators of the size of the vocalising male. Oestrus female koalas are likely to use these acoustic cues to evaluate the body size of a potential mate.

  Female koalas are likely to be more attracted to the bellow of a larger- than a smaller-bodied male because of the benefits associated with producing larger and hence more competitive offspring. Because of this mating preference, large males may experience higher reproductive success compared to their smaller competitors, favouring the large body phenotype in males. This intersexual selection may partly explain the previously established correlation between body size and mating success in male koalas.

  This study may be the first to demonstrate that acoustic signals inform mate selection in marsupials and furthers our understanding of factors influencing the mating preferences and reproductive success of koalas.

 

Summarised by Joanna Horsfall

 

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