Date Published: March 15, 2016
Publisher: Public Library of Science
Author(s): Linda Scheider, Bridget M. Waller, Leonardo Oña, Anne M. Burrows, Katja Liebal, Elke Zimmermann.
Non-human primates use various communicative means in interactions with others. While primate gestures are commonly considered to be intentionally and flexibly used signals, facial expressions are often referred to as inflexible, automatic expressions of affective internal states. To explore whether and how non-human primates use facial expressions in specific communicative interactions, we studied five species of small apes (gibbons) by employing a newly established Facial Action Coding System for hylobatid species (GibbonFACS). We found that, despite individuals often being in close proximity to each other, in social (as opposed to non-social contexts) the duration of facial expressions was significantly longer when gibbons were facing another individual compared to non-facing situations. Social contexts included grooming, agonistic interactions and play, whereas non-social contexts included resting and self-grooming. Additionally, gibbons used facial expressions while facing another individual more often in social contexts than non-social contexts where facial expressions were produced regardless of the attentional state of the partner. Also, facial expressions were more likely ‘responded to’ by the partner’s facial expressions when facing another individual than non-facing. Taken together, our results indicate that gibbons use their facial expressions differentially depending on the social context and are able to use them in a directed way in communicative interactions with other conspecifics.
In searching for the evolutionary roots of human communication, comparative researchers have dedicated much attention to the question whether communication of non-human primates is also characterized by voluntary and intentional use of different signal types, which is a key feature of human communication. In which case, non-human primates should use their signals purposefully, direct them to other group members and adjust them to the attentional state of the recipient. This would indicate that they have some voluntary control over the production of their signals. There are only a few studies investigating systematically whether and how non-human primates use facial expressions in social interactions. Waller et al.  found that orang-utans modify their facial expressions depending on the attentional state of the recipients. The authors offer a lower-level explanation for the differential use of facial expressions, since sensitivity to the attentional state of others could be the result of the salience of the face as a social stimulus. However, it is important to emphasize that currently there is only a small set of studies available that provide data on the use of facial expressions as a function of the recipient’s attentional state.
In this study, we investigated whether different gibbon species adjusted their usage of facial expressions in interactions with others to the recipients’ behaviour and thus use them differently when others are visually attending versus not attending. Gibbons directed at least some of their facial expressions to specific individuals when they were visually attending and facial expressions that were used when both individuals were facing each other lasted significantly longer than those not directed at others. A similar pattern has been reported for orangutans (Pongo pygmaeus) in the context of social play . Interestingly, our study shows that this pattern is present only in social but not in non-social contexts (social context included grooming, agonistic behaviour and play; non-social context included self-grooming and resting). It is important to note that in both social and non-social contexts, individuals were in close proximity to another, but only when in social contexts did facing another individual elicit longer durations of their facial expressions. The same pattern was not observed in non-social contexts. Therefore, we can exclude the possibility that a simple ‘looking at someone’s face’ causes longer durations of facial expressions.