This variation in perceptual organization occurs because of differences in listeners' experiences with language.
The crucial point is that what has not been shown so far is that listeners perceive the variation in the sound quality of fricatives directly as a variation in aperiodic pitch.
If the listeners' approach to variation in aperiodic pitch was merely functionally oriented, then hypothesis 2 (a) and (b) would have not been confirmed and there would have been no significant F0 context effect at all.
Going beyond this basic relevance, evidence is provided here that listeners perceive the variation in the sound quality of fricatives indeed as a variation in aperiodic pitch, even if they were not explicitly asked to do so (i.e., unlike in the study of [27]).
When playback experiments systematically varied different features of the calls, listeners attended to variation in the two most prominent features that correlated with rank, namely the fundamental frequency and the length of the "hoo" syllable [Kitchen, D., Cheney, D.L., Engh, A., Fischer, J., Moscovice, L., Seyfarth, R.M., unpublished data].
The only evidence that listeners can perceive the variation in the sound quality of fricatives literally as "segmental intonation" comes from the study of Heeren [27].
Following a reviewer's request, we point out in this context that the assumption of a "subtraction mechanism" which underlies speech perception and allows listeners to compensate for variation in sound segments is still compatible with variation-driven sound change.
In summary, previous studies gathered converging cross-linguistic evidence that listeners can perceive the variation in the sound quality of fricatives that is caused by the F0 context and for which Niebuhr [25] introduced the term "segmentation intonation".
In Barbary macaques, such playback experiments have revealed that male listeners are sensitive to variation in relation to copulation success (Pfefferle et al. 2008a).
The results also show that listeners are sensitive to phonetic variation in Glasgow, and crucially, that they are easily able to categorise this variation according to associated conceptions of local social identities.
