Mother-infant interactions were recorded for five dyads in a home environment. This report describes the vocalizations produced when the infants were 13 months old. Data are reported on syllable structure, phonetic properties of vowel-like and consonant-like segments, intonation contours and peak/ 0 values for individual syllables. In general, the acoustic-phonetic properties of the i3-month-olds' vocalizations were consistent with data reported in other studies for younger and older children. Hence, the results are seen as evidence for an overall continuity in early phonetic development.

The present study examined cardiac and behavioral reactions of infant chimpanzees to white noise and to conspecific screams and laughter. Chimpanzee screams evoked typical deceleratory cardiac orienting responses. Analysis of stimulus-evoked changes in respiratory sinus arrhythmia suggested that this cardiac deceleration arose from an increase in parasympathetic activity. In contrast, chimpanzee laughter evoked notable cardiac acceleration. Laughter also evoked vocalizations from the infant subjects, which were reminiscent of adult threat-barks. Analysis of respiratory sinus arrhythmia suggested that the cardioacceleratory response likely resulted from sympathetic activation, and was not associated with an inhibition of parasympathetic activity. The autonomic and vocal responses to laughter emerged early in development, were minimally dependent on social contact with adults, and declined in magnitude with increasing age. A consideration of the phylogeny of laughter raised the possibility that the functional reaction to this vocalization may be related to its origin in more primitive agonistic facio-vocal signals.

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In the human voice, lip retraction as in smiling can be associated with high fundamental (F₀) and formant frequencies. The aim of this study was to investigate under naturalistic conditions the cross-species generality of the frequency code hypothesis and related orofacial correlates in F₀. Digital spectral, spectrographic, and cinegraphic measurements were made of spontaneous chimpanzee vocalizations and orofacial movements produced and recorded in the field. A significant decline in F₀ was found in submissive scream-to-aggressive-waahbark transitions as predicted by the frequency code. Teeth and lip opening distances during sounds were positively correlated with their duration, F₀, frequency rise and tempo. These positive correlates and a trivariate regression between orofacial opening and F₀ give support to the coordination of these appeasing facial and vocal features proposed in the frequency code hypothesis.