Chimpanzees (Pan troglodytes) Produce the Same Types of ‘Laugh Faces’ when They Emit Laughter and when They Are Silent | PLOS One
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Abstract<br>The ability to flexibly produce facial expressions and vocalizations has a strong impact on the way humans communicate, as it promotes more explicit and versatile forms of communication. Whereas facial expressions and vocalizations are unarguably closely linked in primates, the extent to which these expressions can be produced independently in nonhuman primates is unknown. The present work, thus, examined if chimpanzees produce the same types of facial expressions with and without accompanying vocalizations, as do humans. Forty-six chimpanzees (Pan troglodytes) were video-recorded during spontaneous play with conspecifics at the Chimfunshi Wildlife Orphanage. ChimpFACS was applied, a standardized coding system to measure chimpanzee facial movements, based on FACS developed for humans. Data showed that the chimpanzees produced the same 14 configurations of open-mouth faces when laugh sounds were present and when they were absent. Chimpanzees, thus, produce these facial expressions flexibly without being morphologically constrained by the accompanying vocalizations. Furthermore, the data indicated that the facial expression plus vocalization and the facial expression alone were used differently in social play, i.e., when in physical contact with the playmates and when matching the playmates’ open-mouth faces. These findings provide empirical evidence that chimpanzees produce distinctive facial expressions independently from a vocalization, and that their multimodal use affects communicative meaning, important traits for a more explicit and versatile way of communication. As it is still uncertain how human laugh faces evolved, the ChimpFACS data were also used to empirically examine the evolutionary relation between open-mouth faces with laugh sounds of chimpanzees and laugh faces of humans. The ChimpFACS results revealed that laugh faces of humans must have gradually emerged from laughing open-mouth faces of ancestral apes. This work examines the main evolutionary changes of laugh faces since the last common ancestor of chimpanzees and humans.
Citation: Davila-Ross M, Jesus G, Osborne J, Bard KA (2015) Chimpanzees (Pan troglodytes) Produce the Same Types of ‘Laugh Faces’ when They Emit Laughter and when They Are Silent. PLoS ONE 10(6):<br>e0127337.
https://doi.org/10.1371/journal.pone.0127337<br>Academic Editor: Roscoe Stanyon, University of Florence, ITALY<br>Received: June 3, 2014; Accepted: April 14, 2015; Published: June 10, 2015<br>Copyright: © 2015 Davila-Ross et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited<br>Data Availability: All relevant data are within the paper and its Supporting Information files.<br>Funding: The work was funded in part by European Commission-Framework Programme 6, Information Society Technologies grant #045169, http://ec.europa.eu/research/fp6/index_en.cfm (to KAB), and in part by the Research Committee, Department of Psychology, University of Portsmouth (to MDR). The funders had no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript.<br>Competing interests: Kim Bard is a PLOS ONE Editorial Board member, but this does not alter the authors' adherence to PLOS ONE Editorial policies and criteria.
Introduction<br>The ability to use facial expressions and vocalizations flexibly has a significant impact on how humans communicate and it must have presented a foundation for a more comprehensive communication system throughout evolution [1–3]. This ability to flexibly combine facial and vocal expressions leads to a more explicit form of communication via versatile means [4] and contributes to the communicative tool kit of humans. For instance, people may show laugh faces while speaking or laughing, and they may also produce them silently, corresponding to a facial expression that may provide the individual with advantages, e.g., in emotional intelligence (see [5]).
However, little is known about the extent to which nonhuman primate facial expressions, which are also often closely linked to vocalizations [6–8], can be produced independently from vocalizations, despite the growing research interest in nonhuman primate multimodal communication (see [9–11]). Therefore, the present work tested whether nonhuman primates are able to produce their facial expressions in such a flexible way, by assessing whether their facial expressions are morphologically constrained by accompanying vocalizations....