Social dominance hierarchies play a pivotal role in shaping the behaviour of many species, and sex differences within these hierarchies often exist. To date, however, few physical markers of dominance have been identified. Such markers would be valuable in terms of understanding the etiology of dominant behaviour and changes in social hierarchies over time. Animals may also use such traits to evaluate the potential dominance of others relative to themselves (i.e. a physical “cue”). Facial width-to-height ratio (fWHR), for example, has been suggested as a cue to dominance in humans, with links to both dominant behaviour and the perception of dominance in other individuals. Whether this association is present in non-human animals is currently not known. Therefore, here we examine within-species links between fWHR and dominant behaviour in 64 brown capuchin monkeys (Sapajus spp.) aged between 2 and 40 years. fWHR was positively associated with alpha status and with a dimensional rating of assertive personality in both males and females. Moreover, fWHR showed significant sexual dimorphism in adults but not juveniles, suggesting a developmental change may occur during puberty. In a sub-sample, sex differences were mediated by weight, suggesting fWHR dimorphism does not exceed what would be expected by differences in body weight. This is the first report of an association between face shape and behaviour in a non-human species. Results are discussed in terms of the role that face-behaviour associations might play within capuchin societies, and the possible selective forces that might have led to the evolution of fWHR-dominance associations in humans.

Polymorphisms of the arginine vasopressin receptor 1a (AVPR1a) gene have been linked to various measures related to human social behavior, including sibling conflict and agreeableness. In chimpanzees, AVPR1a polymorphisms have been associated with traits important for social interactions, including sociability, joint attention, dominance, conscientiousness, and hierarchical personality dimensions named low alpha/stability, disinhibition, and negative emotionality/low dominance. We examined associations between AVPR1a and six personality domains and hierarchical personality dimensions in 129 chimpanzees (Pan troglodytes) living in Japan or in a sanctuary in Guinea. We fit three linear and three animal models. The first model included genotype, the second included sex and genotype, and the third included genotype, sex, and sex × genotype. All personality phenotypes were heritable. Chimpanzees possessing the long form of the allele were higher in conscientiousness, but only in models that did not include the other predictors; however, additional analyses suggested that this may have been a consequence of study design. In animal models that included sex and sex × genotype, chimpanzees homozygous for the short form of the allele were higher in extraversion. Taken with the findings of previous studies of chimpanzees and humans, the findings related to conscientiousness suggest that AVPR1a may be related to lower levels of impulsive aggression. The direction of the association between AVPR1a genotype and extraversion ran counter to what one would expect if AVPR1a was related to social behaviors. These results help us further understand the genetic basis of personality in chimpanzees.Electronic supplementary materialThe online version of this article (doi:10.1007/s10519-016-9822-2) contains supplementary material, which is available to authorized users.

Personality has been studied in all of the great apes, many Old World monkey species, but only a handful of New World monkey species. Because understanding the personalities of New World monkeys is crucial to understanding personality evolution in primates, we used the Hominoid Personality Questionnaire to assess personality in 55 common squirrel monkeys () and 40 Bolivian squirrel monkeys (). We found 4 personality components in each species, and labeled them Openness, Neuroticism, Assertiveness, and Agreeableness. We then, in a genus-level analysis, found 5 components, which we labeled Neuroticism, Openness, Assertiveness, Agreeableness, and Decisiveness. Comparisons of the genus- and species-level structures revealed that common squirrel monkeys had a personality structure that more closely resembled the genus-level structure than did Bolivian squirrel monkeys. We then compared the personality structures of common and Bolivian squirrel monkeys with that of brown capuchin monkeys, Sapajus apella. The personality structure of Bolivian squirrel monkeys more closely resembled that of brown capuchins. These findings suggest that the Bolivian squirrel monkey personality structure is ancestral and that Assertiveness and Openness are ancestral to both the Saimiri genus and brown capuchins; Agreeableness and Neuroticism seem to be derived in Saimiri. We discuss these findings in relation to differences in the social structures and ecologies of these species. (PsycINFO Database Record

As part of the European Conference on Behavioral Biology 2018, we organized a symposium entitled, “Animal personality: providing new insights into behavior?” The aims of this symposium were to address current research in the personality field, spanning both behavioral ecology and psychology, to highlight the future directions for this research, and to consider whether differential approaches to studying behavior contribute something new to the understanding of animal behavior. In this paper, we discuss the study of endocrinology and ontogeny in understanding how behavioral variation is generated and maintained, despite selection pressures assumed to reduce this variation. We consider the potential mechanisms that could link certain traits to fitness outcomes through longevity and cognition. We also address the role of individual differences in stress coping, mortality, and health risk, and how the study of these relationships could be applied to improve animal welfare. From the insights provided by these topics, we assert that studying individual differences through the lens of personality has provided new directions in behavioral research, and we encourage further research in these directions, across this interdisciplinary field.

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26Welfare questionnaires, which allow people who are familiar with individual animals 27 to rate the welfare of the animals, are an underutilised tool. We designed a 12-item welfare 28 questionnaire and tested its reliability and associations with subjective well-being (SWB), 29 locomotor stereotypy, and personality traits. The welfare questionnaire included questions 30 relating to physical health, stress and coping, satisfaction with social relationships, 31 psychological stimulation, and the display of positive and negative welfare indicators. We 32 collected ratings of 66 brown capuchins (Sapajus apella) living in three facilities. Each 33 capuchin was rated on the welfare questionnaire by an average of 2.8 raters. The interrater 34 reliability of the welfare questionnaire items ranged from ICC(3,k) 0.51 to 0.86. A principal 35 components analysis indicated that the 12 welfare items loaded onto one component. We 36 repeated this process with the welfare and subjective well-being items and found all the items 37 were defined by a single component (welfareSWB). We then conducted three sets of analyses, 38 one predicting the welfare component, one predicting the SWB component, and predicting the 39 welfareSWB component. The independent variables were frequency of locomotor stereotypy, 40 personality, age, and sex; facility was included as a random effect. In models including 41 stereotypy, age, and sex we found frequency of stereotypy to be significantly associated with 42 all three predicted components (p<0.01). After controlling for stereotypy (b=-0.25, p=0.17), 43 age (b=-0.54, p=0.01), and sex (b=-0.32, p=0.07), the personality traits of Sociability (b=1.02, 44 p<0.001), Assertiveness, (b=0.63, p<0.001), and Attentiveness (b=0.54, p=0.01) were 45 associated with higher scores on the joint welfareSWB component; Neuroticism was 46 negatively associated (b=-0.60, p=0.01). Our results suggest that welfare questionnaires may 47 be a useful, reliable, and valid tool for primate welfare assessment. 48 Keywords: Brown capuchin, personality, stereotypy, subjective well-being, welfare 49 50 131 experience working with animals. In addition, to prime raters for the welfare questions, this 132 section of the questionnaire asks which physical and behavioural indicators raters use to 157

Personality has important links to health, social status, and life history outcomes (e.g. longevity and reproductive success). Human facial morphology appears to signal aspects of one’s personality to others, raising questions about the evolutionary origins of such associations (e.g. signals of mate quality). Studies in non-human primates may help to achieve this goal: for instance, facial width-to-height ratio (fWHR) in the male face has been associated with dominance not only in humans but also in capuchin monkeys. Here we test the association of personality (assertiveness, openness, attentiveness, neuroticism, and sociability) with fWHR, face width/lower-face height, and lower face/face height ratio in 64 capuchins (Sapajus apella). In a structural model of personality and facial metrics, fWHR was associated with assertiveness, while lower face/face height ratio was associated with neuroticism (erratic vs. stable behaviour) and attentiveness (helpfulness vs. distractibility). Facial morphology thus appears to associate with three personality domains, which may act as a signal of status in capuchins.

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