Sensory biases underlie sex differences in tamarin long call structure

Within the vocal repertoires of many species, both males and females produce acoustic variants of the same call type. Despite the relative salience of the acoustic features that correlate with the caller's sex, little is known about the functional significance of this acoustic cue. Specifically, do conspecifics use this information when making behavioural decisions, implying that it is meaningful to signal receivers? And, if so, what selective pressures underlie the salience of the sex differences in call structure? Here we present a series of phonotaxis experiments designed to address these issues in the cottontop tamarin, Saguinus oedipus. We focus on the tamarin combination long call (CLC), a species-specific vocalization consisting of one to two chirps followed by three to four whistles, that functions in the context of social isolation and mate attraction. CLCs produced by males had significantly shorter whistles than those produced by females, although there was some overlap in the sex-specific ranges. Sex differences in the acoustic morphology of CLCs elicited consistent selective phonotaxis, suggesting that this acoustic cue is meaningful to signal receivers and that whistle duration is the primary acoustic feature used to determine the sex of the caller. Males preferred female CLCs consisting of the longest whistles, whereas females preferred male CLCs with the shortest whistles, suggesting that sensory biases in tamarins underlie sex differences in CLCs. These preferences persisted even when stimulus calls contained whistle durations that fell outside the natural range.