Subharmonics increase the auditory impact of female koala rejection calls
Charlton, BD, Watchorn, DJ & Whisson, DA 2017, Ethology, vol. 123, no. 1, pp. 571-579.
Subharmonics in female koala vocalisations made when rejecting a mating attempt create an unpredictable acoustic pattern that may serve to attract a higher-quality mate.
Mammalian vocal signals often contain ‘non-linear phenomena’, which are the sounds produced by irregular or chaotic vocal fold vibration patterns. The functions of these potential acoustic cues are not fully understood. Although the vocalisations of her male counterpart are more frequently examined, the female koala also vocalises during the breeding season and especially during oestrus. A particularly notable female vocalisation occurs when rejecting a mating attempt from a male. These ‘rejection calls’ are characterised by screams, wails and snarls. The purpose of this experiment was to test the hypothesis that the acoustic elements of female rejection calls are functional for attracting male attention using playback experiments. Male koalas displayed a greater interest in the sound stimulus of a sequence of male bellows followed by female rejection calls, imitating a typical call sequence heard during the breeding season, than the stimulus of male bellows only. Males demonstrated a greater interest in female rejection calls that contained more subharmonics (vibrations detected at frequencies lower than the fundamental frequency of the vocalisation), more periodicity (the regularity of successive vibrations), and less biphonation (rapid jumps in call frequency). It was predicted that males would show greater interest in harsh and chaotic vocalisations, which was true for the interest demonstrated towards vocalisations with more subharmonics. Greater periodicity and less biphonation, however, are both associated with less abrasive vocalisations, and therefore the males’ interest in these acoustic characteristics was unexpected. Subharmonics are therefore thought to be the major acoustic element contributing to the high auditory impact of rejection calls.
Several mammalian vocalisations convey information about the caller via acoustic cues, and analysis of these cues and responses to them can reveal information about the selection pressures that result in the evolution of distinct vocal signals. In particular, non-linear vocal phenomena produce harsh and unpredictable calls that are thought to reflect the aroused state of the caller. The unpredictability of the call may serve to attract attention and prevent habituation to the stimulus. In koalas, subharmonics in female rejection calls may attract nearby males other than the rejected mate in order to provoke male-male competition. The adaptive significance of this behaviour for the female may be to increase her chances of attracting a high-quality mate.
Of all mammalian groups, marsupials typically produce particularly harsh vocalisations. Further studies of non-linear phenomena in the vocalisations of other marsupial species may reveal similar vocal patterns to those observed in the koala, and such studies can provide insights into the repertoire of vocalisations and associated selective pressures that have shaped the evolution of the species.
Summarised by Joanna Horsfall
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