Free-ranging male koalas use size-related variation in formant frequencies to assess rival males
Charlton, BD, Whisson, DA & Reby, D 2013, PLoS ONE, vol. 8, no. 7, e70279.
Male koalas detect acoustic cues in the bellows of their competitors to the caller’s body size and change their own vocal response accordingly. When exposed to a bellow from a large, as opposed to a small, caller during the breeding season, male koalas produce a delayed bellow of greater duration.
Formants are the resonance frequencies produced by vibrating air in the vocal tract. It was recently recognised that the formant frequency spacing of koala bellows is indicative of the male caller’s body size, and that captive males can perceive and attend to these acoustic cues to the size of their conspecifics. The purpose of this study was to evaluate the occurrence of the same phenomenon in free-ranging males, as well as to examine receivers’ behavioural responses to bellows from males of different sizes. A group of male koalas in the wild were presented with recordings of bellows of unfamiliar males during the breeding season. The recordings were digitally manipulated by shifting the formant spacing of the call to simulate a variation in body size. All other acoustic parameters were unaltered. Each koala was exposed to two stimuli from the same caller: one bellow simulating a smaller male, and another simulating a larger male. Although looking responses towards the stimuli did not differ between calls simulating males of different sizes, the koalas produced longer bellows and spent a greater amount of time bellowing overall in response to a stimulus simulating a large caller than that of a small caller. Males were also slower to respond vocally to the bellows of larger males than to those of smaller males.
In many mammals, including koalas, body size is positively correlated with competitive fitness. The observations made in this study suggest that male koalas can use acoustic cues conveyed by formants in the bellows of conspecifics to evaluate the threat they may pose. Additionally, male koalas were observed to invest greater effort in their vocal responses to larger, rather than smaller, potential competitors. A possible reason for the greater duration of bellows produced in response to larger males is to signal competitive characteristics such as high motivation or testosterone levels. The delayed vocal response of male koalas to their larger competitors may imply that males are more hesitant to engage in agonistic behaviour with these potential rivals than with smaller males.
The finding that male koalas use formants in bellows as cues to the size and hence the threat of their potential competitors in the breeding season sheds light on the role of intra-sexual competition as an evolutionary pressure that shapes male koala vocalisations. The authors suggest that future studies explore the extent to which the duration of male koala bellows indicates the caller’s motivational state and testosterone levels, as suggested here.
Summarised by Joanna Horsfall
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