In recent years there have been efforts to assess associations between personality, longevity and well-being in nonhuman primates. Currently, findings indicate that, as in humans, personality traits such as extraversion are associated with both higher well-being (in monkeys and apes) and longevity (in gorillas). Why certain traits seem to have a protective effect is not yet well understood. One hypothesis is that more extraverted individuals rely on the company of others to alleviate stress, and thus mediate physiological stressors, increasing potential life span. Individual differences in social behaviour are therefore an important consideration for increasing our understanding of the protective effects of personality traits. The role of personality in social interactions and well-being is the main focus of this thesis. In Chapter 2, I assess personality and well-being in two species of New World monkey - common (Saimiri sciureus) and Bolivian squirrel monkeys (Saimiri boliviensis) - for which these associations have not previously been studied. I assess differences in personality trait structure between the two species, and compare them to a close relative, Sapajus apella (brown capuchins). I also correlate personality with well-being. Results show that both squirrel monkey species are similar in personality structure, but Bolivian squirrel monkeys share more similar traits with brown capuchins than common squirrel monkeys do. Well-being is associated with low Neuroticism and high Openness. These comparisons inform our understanding of the phylogeny of personality traits, as well as the ancestral links between personality and well-being. In Chapter 3 I focus on examining individual differences in chimpanzee (Pan troglodytes) traits, through the use of previously collected personality data, which may reflect something akin to autism in nonhuman primates. In Chapter 4 I examine behavioural correlates of these traits, and assess personality in relation to behaviour. Results indicate that personality is a stronger predictor of individual differences in social behaviour than the scale measuring autism-like traits. Continuing with a focus in chimpanzees, in Chapter 5 I examine whether personality predicts how individual chimpanzees perceive emotion in conspecifics, using both behavioural and experimental data. Findings show that individuals differ in their attention and arousal in response to emotions in others, and that personality plays a role in these responses. I also report different ‘levels’ of response indicative of separate arousal and attention based processes. This is an understudied but important area of research that might help to elucidate differences in coping with stressful situations in a group environment. In Chapter 6 I shift from a focus of personality as a predictor of response, to potential signals of personality, by examining personality correlates of facial morphology in brown capuchins, Sapajus apella. Results indicate that face width is associated with higher Assertiveness, whilst lower face height is associated with higher Neuroticism/lower Attentiveness. To assess the theory that these associations may act as social cues, such as signals of status or mate quality, in Chapter 7 I assess whether capuchins perceive differences in face width of conspecifics by measuring response to facial images. Results suggest that capuchins do not differ between wide and narrow faces. I discuss the possibility that perception of these associations may be dependent on other variables such as age or knowledge of the signal receiver. Overall, this thesis takes a broad approach to understanding personality, by examining its role in social interactions, perception of others and well-being. These findings are discussed in light of both evolutionary theory and potential benefits to welfare.

For humans and non-human primates (NHPs) the face represents a particularly important source of social information providing a means of conspecific recognition and cues to personal details including sex, age, and emotional state. The human face may also be fundamental in the transmission to conspecifics of other forms of socially relevant information including the display of facial traits associated with sexual attraction and mate choice. A wealth of experimental literature indicates that humans display robust preferences for certain facial traits associated with facial attractiveness including preferences for bilateral facial symmetry, facial averageness and sexually dimorphic faces and facial features. It is thought that these preferences have evolved via sexual selection, and may be adaptive, due to the role that these specific facial features play in reliably signalling to others the possession of heritable genetic quality or ‘good genes’. Therefore, from an evolutionary perspective, it is possible that certain facial preferences may represent an evolutionary adaptation for the selection of potential mate quality. However, despite similarities between human and NHP face processing and recognition abilities, the shared evolutionary history and social importance of faces to primates in general, and the potential importance of these preferences in the mate choice decisions of NHPs, very little research has investigated the extent to which NHPs display comparable preferences to humans for these specific facial traits. Consequently, the aim of the following thesis was to comparatively assess the general and more specific preferences that humans and NHPs display for faces and for traits associated with facial attractiveness. Data was compiled from preference studies examining the visual preferences displayed by two species of NHP (brown capuchins (Cebus apella) and chimpanzees (Pan troglodytes)) for conspecific faces manipulated for those facial traits associated with attractiveness, and from a single study of brown capuchins examining their general visual preferences for various types of facial information. Comparative preference studies were also conducted upon human adults and infants examining the visual and declared preferences that they display for manipulations of facial attractiveness. Data showed that despite possessing general preferences for certain faces and facial information, generally NHPs displayed no significant preferences for those facial traits thought to influences judgements of attractiveness in humans. Possible reasons for this absence of preference for these particular facial traits and the evolutionary implications of these findings are discussed.