Date Published: March 4, 2019
Publisher: Public Library of Science
Author(s): Jordan Raine, Katarzyna Pisanski, Rod Bond, Julia Simner, David Reby, Marcus Perlman.
Despite widespread evidence that nonverbal components of human speech (e.g., voice pitch) communicate information about physical attributes of vocalizers and that listeners can judge traits such as strength and body size from speech, few studies have examined the communicative functions of human nonverbal vocalizations (such as roars, screams, grunts and laughs). Critically, no previous study has yet to examine the acoustic correlates of strength in nonverbal vocalisations, including roars, nor identified reliable vocal cues to strength in human speech. In addition to being less acoustically constrained than articulated speech, agonistic nonverbal vocalizations function primarily to express motivation and emotion, such as threat, and may therefore communicate strength and body size more effectively than speech. Here, we investigated acoustic cues to strength and size in roars compared to screams and speech sentences produced in both aggressive and distress contexts. Using playback experiments, we then tested whether listeners can reliably infer a vocalizer’s actual strength and height from roars, screams, and valenced speech equivalents, and which acoustic features predicted listeners’ judgments. While there were no consistent acoustic cues to strength in any vocal stimuli, listeners accurately judged inter-individual differences in strength, and did so most effectively from aggressive voice stimuli (roars and aggressive speech). In addition, listeners more accurately judged strength from roars than from aggressive speech. In contrast, listeners’ judgments of height were most accurate for speech stimuli. These results support the prediction that vocalizers maximize impressions of physical strength in aggressive compared to distress contexts, and that inter-individual variation in strength may only be honestly communicated in vocalizations that function to communicate threat, particularly roars. Thus, in continuity with nonhuman mammals, the acoustic structure of human aggressive roars may have been selected to communicate, and to some extent exaggerate, functional cues to physical formidability.
In competitive contests, evolutionary selection processes favour vocal communication of resource holding potential to settle disputes without engaging in potentially costly combat . For example, many terrestrial mammalian species, including giant pandas , sea lions , fallow and red deer [4,5], and domestic dogs  use acoustic cues to body size or dominance rank in aggressive vocalizations to mediate agonistic interactions, particularly during male-male competition.
In Experiment 1, we acoustically analyzed aggressive roars, distress screams, aggressive speech, and distressed speech, testing whether the acoustic structure of these vocal stimuli follows Morton’s motivational-structure rules, and whether it reliably predicts a vocalizer’s strength and height.
Following acoustic analysis, we used playback experiments to assess the functional relevance of aggressive roars, aggressive speech, distress screams, and distressed speech in communicating strength and body size. Separate samples of listeners judged either the physical strength or height of the vocalizers whose voices we analyzed in Experiment 1.
We compared the acoustic structure of aggressive roars, distress screams, and their valenced speech equivalents (Experiment 1), and examined the effectiveness of these various speech stimuli in communicating physical strength (Experiment 2) and height (Experiment 3) to listeners. Our results provide strong evidence that the acoustic structure of human aggressive and distress vocal signals, particularly nonverbal vocalizations (roars and screams), varies according to Morton’s motivational-structural rules . Accordingly, aggressive stimuli exaggerated impressions of strength and body size relative to distress stimuli. Corroborating previous attempts [11,15,16], our acoustic analyses did not identify vocal features that reliably mediated the communication of strength, yet listeners could nevertheless accurately estimate strength from male and female aggressive (but not distress) vocal stimuli, and most reliably from aggressive roars. To a lesser degree, listeners could also estimate the height of vocalizers. Roars therefore conveyed honest inter-individual variation in strength more reliably than did any other type of vocal stimulus, and also exaggerated impressions of physical formidability most effectively.