Chimpanzees (Pan troglodytes) Produce the Same Types of 'Laugh Faces' when They Emit Laughter and when They Are Silent - PubMed (original) (raw)

Chimpanzees (Pan troglodytes) Produce the Same Types of 'Laugh Faces' when They Emit Laughter and when They Are Silent

Marina Davila-Ross et al. PLoS One. 2015.

Abstract

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.

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Conflict of interest statement

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.

Figures

Fig 1

Fig 1. Action units of open-mouth faces.

Total number of subjects producing the action units of open-mouth faces (with laughter and silently). AU10: Pulls the upper lip toward the nose, exposing the upper teeth; based on action of levator labii superioris; AU12: Pulls the lip corners upwards and backwards; based on action of zygomatic major; AU16: Depresses the lower lip, exposing the lower teeth; based on action of depressor labii inferioris; AU 25: Opens the lips; may involve action of different muscles (e.g., depressor labii inferioris, levator labii superioris, orbicularis oris); AU26 and AU27 are mutually exclusive. Prototypical open-mouth faces of chimpanzees consist of AU25 as well as AU26 and they may also include different combinations of AU10, AU12, and AU16 (for the action unit configurations, see Table 1). The individual in the picture has given informed consent.

Fig 2

Fig 2. Open-mouth faces across play contexts.

Number of open-mouth faces (with laughter and without laughter) produced by the subjects during rough and gentle play, with and without physical contact. The total number of subjects is shown in parentheses. C = play with physical contact; N = play with no physical contact. The box plots depict medians, upper and lower quartiles, and maximum and minimum range values.

Fig 3

Fig 3. Matching of open-mouth faces.

Number of matching open-mouth faces (with laughter and without laughter) of the subjects. The total number of subjects is shown in parentheses. The box plots depict medians, upper and lower quartiles, and maximum and minimum range values.

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Grants and 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.

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