Fundamental frequency declination is not unique to human speech: evidence from nonhuman primates.

In human speech, declination of the fundamental frequency (F0) of the voice spans coherent units of an utterance and, therefore, signals where units begin and end. A rapid final fall at the end of an utterance provides a further indication of an utterance's ending. The occurrence of declination is sufficiently widespread across languages that several investigators have suggested it as a language universal. Language universals may be universal because they are part of a species-specific specialization for language or, alternatively, they may constitute conventionalizations of natural dispositions of the vocal tract that may serve a communicative function. Evidence is offered favoring the latter account for declination and the final fall by showing that vocal productions of vervet monkeys (Cercopithecus aethiops) and rhesus macaques (Macaca mulatta) show declination, and vervets show clear evidence of a final fall. Interestingly, the fall in F0 may serve some communicative role in the vocal exchanges of vervets and rhesus, analogous to its signalling function in human language.