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Darwin observed that form, and in his view, meaning, of facial behaviour (observable changes in the appearance of the face, often termed facial ‘expression’) is similar between a wide range of species and concluded that this must be due to a shared ancestral origin. Yet, as with all social behaviours, exactly how to define similarity and determine homology is debated. Facial behaviour is linked to specific facial muscle movements, so one important factor in determining homology is the anatomical basis of facial behaviours that appear similar in both appearance and social function. The Facial Action Coding System (FACS) was developed for the scientific measurement of human facial behaviour and is based on individual facial muscle movements (Ekman and Friesen, 1978). FACS has since been modified for use with various non-human primate species (chimpanzees, macaques, hylobatids, orangutans) and domestic species (dogs, cats, horses). These FACS can be used to trace continuity of form in facial behaviour across species and build a better understanding of the evolution of facial communication in mammals.

Objectives - While it has been demonstrated that even subtle variation in human facial expressions can lead to significant changes in the meaning and function of expressions, relatively few studies have examined primate facial expressions using similarly objective and rigorous analysis. Construction of primate facial expression repertoires may, therefore, be oversimplified, with expressions often arbitrarily pooled and/or split into subjective pigeonholes. Our objective is to assess whether subtle variation in primate facial expressions is linked to variation in function, and hence to inform future attempts to quantify complexity of facial communication.Materials and Methods - We used Macaque Facial Action Coding System, an anatomically based and hence more objective tool, to quantify “silent bared‐teeth” (SBT) expressions produced by wild crested macaques engaging in spontaneous behavior, and utilized discriminant analysis and bootstrapping analysis to look for morphological differences between SBT produced in four different contexts, defined by the outcome of interactions: Affiliation, Copulation, Play, and Submission.Results - We found that SBT produced in these contexts could be distinguished at significantly above‐chance rates, indicating that the expressions produced in these four contexts differ morphologically. We identified the specific facial movements that were typically used in each context, and found that the variability and intensity of facial movements also varied between contexts.Discussion - These results indicate that nonhuman primate facial expressions share the human characteristic of exhibiting meaningful subtle differences. Complexity of facial communication may not be accurately represented simply by building repertoires of distinct expressions, so further work should attempt to take this subtle variability into account.

The studies were funded by a Leverhulme Trust Research Project Grant Cultural variation in the social function and expression of guilt awarded to the seventh and fourth authors (RPG-2016-206) and the Leverhulme Trust Research Project Grant Rethinking complexity in facial communication systems awarded to the sixth author (RPG-2018-334). Guilt is a complex emotion with a potentially important social function of stimulating cooperative behaviours towards and from others, but whether the feeling of guilt is associated with a recognisable pattern of nonverbal behaviour is unknown. We examined the production and perception of guilt in two different studies, with a total of 238 participants with various places of origin. Guilt was induced experimentally, eliciting patterns of movement that were associated with both the participants’ self-reported feelings of guilt and judges’ impressions of their guilt. Guilt was most closely associated with frowning and neck touching. While there were differences between self-reported guilt and perception of guilt the findings suggest that there are consistent patterns that could be considered a non-verbal signal of guilt in humans. Publisher PDF

Research has shown that individuals can recognise personality traits from photographs of others’ faces. It is suggested that this is possible as faces contain a biometric ‘kernel of truth’ for personality traits. If biometric facial features facilitate person judgments, then those adept at face memory (super-recognisers) could show heightened ability at recognising personality traits. This study evaluates the links between face memory accuracy and trait judgment accuracy. We investigated the relationship between participants’ face memory, Big Five personality traits and their accuracy in judging the Big Five personality traits (from 50 self-selected photographs of unknown individuals) in a sample of 792 individuals. We replicated previous findings that participant extraversion relates to face memory ability. We find no relationship between other personality traits, personality judgment accuracy and face memory. Individual differences in detecting personality traits do not relate to their face memory abilities. We do replicate the relationship between extraversion and face memory ability in the largest sample to date. This suggests that super-recognising faces and traits are domain-specific abilities. If surveillance operators are expected to detect impending incidents, then our findings would suggest that this is something that super-recognisers would need training in.

Ants are known to use the terrestrial visual panorama in navigation. Recent evidence has accumulated for the use of the currently perceived visual panorama to determine a direction to head in. The pattern of the height of the terrestrial surround, the skyline, is one key cue for the Central Australian red honey ant Melophorus bagoti in determining a direction of travel. But ants might also possess some mechanism to match the skyline heights encountered during training, which functions to steer away from regions whose skyline is too high and towards regions whose skyline is too low. We made an initial test of this hypothesis by training ants to visit a feeder centred between two experimentally constructed walls of black cloth. Trained ants were then tested for their initial homing direction with the walls retaining their heights as encountered in training (controls), with one of the walls lowered or raised in height, or with one wall lowered and the opposite wall raised. Wall-height manipulations deflected the initial headings of ants towards the lower wall, with combined wall lowering and wall raising changing the initial headings by ~30° when compared with controls. The results suggest that the ants combined the dictates of the panorama in determining the best direction of travel (a heading towards the nest) with some attractor mechanism that functions to establish the skyline heights of training conditions (a heading towards the lower wall).

Human and non-human primates exhibit facial movements or displays to communicate with one another. The evolution of form and function of those displays could be better understood through multispecies comparisons. Anatomically based coding systems (Facial Action Coding Systems: FACS) are developed to enable such comparisons because they are standardized and systematic and aid identification of homologous expressions underpinned by similar muscle contractions. To date, FACS has been developed for humans, and subsequently modified for chimpanzees, rhesus macaques, orangutans, hylobatids, dogs, and cats. Here, we wanted to test whether the MaqFACS system developed in rhesus macaques (Macaca mulatta) could be used to code facial movements in Barbary macaques (M. sylvanus), a species phylogenetically close to the rhesus macaques. The findings show that the facial movement capacity of Barbary macaques can be reliably coded using the MaqFACS. We found differences in use and form of some movements, most likely due to specializations in the communicative repertoire of each species, rather than morphological differences.

Humans are uniquely cooperative and form crucial short- and long-term social bonds between individuals that ultimately shape human societies. The need for such intense cooperation may have provided a particularly powerful selection pressure on the emotional and communicative behaviours regulating cooperative processes, such as guilt. Guilt is a social, other-oriented moral emotion that promotes relationship repair and pro-sociality. For example, people can be more lenient towards wrongdoers who display guilt than towards those who do not. Here, we examined the social consequences of guilt in a novel experimental setting with pairs of friends differing in relationship quality. Pairs of participants took part in a cooperative game with a mutual goal. We then induced guilt in one of the participants and informed the other participant of their partner's wrongdoing. We examined the outcome using a dictator game to see how they split a joint reward. We found that guilty people were motivated to repair wrongdoing regardless of friendship. Observing guilt in others led to a punishment effect and a victim of wrongdoing punished close friends who appeared guilty more so than acquaintances. We suggest, therefore, that guilt has a stronger function between close friends as the costs of relationship breakdown are greater. Relationship context, therefore, is crucial to the functional relevance of moral emotions.