Human emotional expressions, such as laughter, are argued to have their origins in ancestral nonhuman primate displays. To test this hypothesis, the current work examined the acoustics of tickle-induced vocalizations from infant and juvenile orangutans, gorillas, chimpanzees, and bonobos, as well as tickle-induced laughter produced by human infants. Resulting acoustic data were then coded as character states and submitted to quantitative phylogenetic analysis. Acoustic outcomes revealed both important similarities and differences among the five species. Furthermore, phylogenetic trees reconstructed from the acoustic data matched the well-established trees based on comparative genetics. Taken together, the results provide strong evidence that tickling-induced laughter is homologous in great apes and humans and support the more general postulation of phylogenetic continuity from nonhuman displays to human emotional expressions. Findings also show that distinctively human laughter characteristics such as predominantly regular, stable voicing and consistently egressive airflow are nonetheless traceable to characteristics of shared ancestors with great apes.
The importance of individual identity and kinship has been demonstrated in the social behavior of many nonhuman primates, with some evidence suggesting that individually distinctive acoustic features are present in their vocalizations as well. In order to systematically test whether acoustic cues to identity are reliably present across the vocal repertoire of rhesus monkeys (Macaca mulatta), we examined coos, grunts, and noisy screams produced by adult females of two free-ranging groups. First, acoustic analyses were used to characterize spectral patterning, the fundamental frequency, and temporal characteristics of these three distinct call types. Vocalizations were then classified by caller identity, based on discriminant function analyses. Results showed that coos (rich, harmonically structured sounds) were markedly more distinctive by caller than were either grunts or noisy screams, and that spectral-patterning measures related to vocal tract filtering effects were the most reliable markers of individual identity. Grunts (pulsed, noisy calls) were classified at lower, but above-chance rates and spectral patterning cues were again critical in this sorting. Noisy screams (continuous, broadband noise bursts that could include a high-frequency, periodic component) could not be reliably sorted by caller. Playback experiments conducted with the screams showed no response differences when listening animals heard vocalizations produced by kin or nonkin individuals. This result was strikingly different from the corresponding outcome of a previous test with coo calls, but consistent with the acoustic analysis. Implications of these findings for vocal production mechanisms in nonhuman primates and previous studies of rhesus monkey vocalizations are discussed.
Remarkably little is known about the acoustic features of laughter. Here, acoustic outcomes are reported for 1024 naturally produced laugh bouts recorded from 97 young adults as they watched funny video clips. Analyses focused on temporal features, production modes, source- and filter-related effects, and indexical cues to laugher sex and individual identity. Although a number of researchers have previously emphasized stereotypy in laughter, its acoustics were found now to be variable and complex. Among the variety of findings reported, evident diversity in production modes, remarkable variability in fundamental frequency characteristics, and consistent lack of articulation effects in supralaryngeal filtering are of particular interest. In addition, formant-related filtering effects were found to be disproportionately important as acoustic correlates of laugher sex and individual identity. These outcomes are examined in light of existing data concerning laugh acoustics, as well as a number of hypotheses and conjectures previously advanced about this species-typical vocal signal.
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