An Exploration of the Relationships Among Facial Dimensions, Age, Sex, Dominance Status, and Personality in Rhesus Macaques (Macaca mulatta)

Social Status as a Latent Variable in the Amygdala of Observers of Social Interactions

Successful integration into a hierarchical social group requires knowledge of the status of each individual and of the rules that govern social interactions within the group. In species that lack morphological indicators of status, social status can be inferred by observing the signals exchanged between individuals. We simulated social interactions between macaques by juxtaposing videos of aggressive and appeasing displays where two individuals appeared in each other's line of sight and their displays were timed to suggest the reciprocation of dominant and subordinate signals. Viewers of these videos successfully inferred the social status of the interacting characters. Dominant individuals attracted more social attention from viewers even when they were not engaged in social displays. Neurons in the viewers' amygdala signaled the status of both the attended (fixated) and the unattended individuals suggesting that in third party observers of social interactions, the amygdala signals jointly the status of interacting parties.

Highlights

Monkeys infer the social status of conspecifics from videos of simulated dyadic interactionsDuring fixations neural populations signal the social status of the attended individualsNeurons in the amygdala jointly encode the status of interacting individuals.

In brief

Third party-viewers of pairwise dominant-subordinate interactions infer social status from the observed behaviors. Neurons in the amygdala are tuned to the inferred dominant/subordinate status of both individuals.

Does the primate face cue personality?

When looking at others, primates primarily focus on the face - detecting the face first and looking at it longer than other parts of the body. This is because primate faces, even without expression, convey trait information crucial for navigating social relationships. Recent studies on primates, including humans, have linked facial features, specifically facial width-to-height ratio (fWHR), to rank and Dominance-related personality traits, suggesting these links' potential role in social decisions. However, studies on the association between dominance and fWHR report contradictory results in humans and variable patterns in nonhuman primates. It is also not clear whether and how nonhuman primates perceive different facial cues to personality traits and whether these may have evolved as social signals. This review summarises the variable facial-personality links, their underlying proximate and evolutionary mechanisms and their perception across primates. We emphasise the importance of employing comparative research, including various primate species and human populations, to disentangle phylogeny from socio-ecological drivers and to understand the selection pressures driving the facial-personality links in humans. Finally, we encourage researchers to move away from single facial measures and towards holistic measures and to complement perception studies using neuroscientific methods.

Oxytocin and testosterone administration amplify viewing preferences for sexual images in male rhesus macaques

Social stimuli, like faces, and sexual stimuli, like genitalia, spontaneously attract visual attention in both human and non-human primates. Social orienting behaviour is thought to be modulated by neuropeptides as well as sex hormones. Using a free viewing task in which paired images of monkey faces and anogenital regions were presented simultaneously, we found that male rhesus macaques overwhelmingly preferred to view images of anogenital regions over faces. They were more likely to make an initial gaze shift towards, and spent more time viewing, anogenital regions compared with faces, and this preference was accompanied by relatively constricted pupils. On face images, monkeys mostly fixated on the forehead and eyes. These viewing preferences were found for images of both males and females. Both oxytocin (OT), a neuropeptide linked to social bonding and affiliation, and testosterone (TE), a sex hormone implicated in mating and aggression, amplified the pre-existing orienting bias for female genitalia over female faces; neither treatment altered the viewing preference for male anogenital regions over male faces. Testosterone but not OT increased the probability of monkeys making the first gaze shift towards female anogenital rather than face pictures, with the strongest effects on anogenital images of young and unfamiliar females. Finally, both OT and TE promoted viewing of the forehead region of both female and male faces, which display sexual skins, but decreased the relative salience of the eyes of older males. Together, these results invite the hypothesis that both OT and TE regulate reproductive behaviours by acting as a gain control on the visual orienting network to increase attention to mating-relevant signals in the environment.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

Non-human contributions to personality neuroscience – from fish through primates. An introduction to the special issue

The most fundamental emotional systems that show trait control are evolutionarily old and extensively conserved. Psychology in general has benefited from non-human neuroscience and from the analytical simplicity of behaviour in those with simpler nervous systems. It has been argued that integration between personality, psychopathology, and neuroscience is particularly promising if we are to understand the neurobiology of human experience. Here, we provide some general arguments for a non-human approach being at least as productive in relation to personality, psychopathology, and their interface. Some early personality theories were directly linked to psychopathology (e.g., Eysenck, Panksepp, and Cloninger). They shared a common interest in brain systems that naturally led to the use of non-human data; behavioural, neural, and pharmacological. In Eysenck’s case, this also led to the selective breeding, at the Maudsley Institute, of emotionally reactive and non-reactive strains of rat as models of trait neuroticism or trait emotionality. Dimensional personality research and categorical approaches to clinical disorder then drifted apart from each other, from neuropsychology, and from non-human data. Recently, the conceptualizations of both healthy personality and psychopathology have moved towards a common hierarchical trait perspective. Indeed, the proposed two sets of trait dimensions appear similar and may even be eventually the same. We provide, here, an introduction to this special issue of Personality Neuroscience, where the authors provide overviews of detailed areas where non-human data inform human personality and its psychopathology or provide explicit models for translation to human neuroscience. Once all the papers in the issue have appeared, we will also provide a concluding summary of them.

Oral processing, sexual selection, and size variation in the circumorbital region of Colobus and Piliocolobus

OBJECTIVES

The function of the browridge in primates is a subject of enduring debate. Early studies argued for a role in resisting masticatory stresses, but recent studies have suggested sexual signaling as a biological role. We tested associations between circumorbital form, diet, oral processing, and social behavior in two species of colobus monkey-the king colobus (Colobus polykomos) and western red or bay colobus (Piliocolobus badius).

MATERIALS AND METHODS

We quantified circumorbital size and dimorphism in a sample of 98 crania. Controlling for age and facial size, we tested whether variation in circumorbital morphology can be explained by variation in diet, oral processing behavior, masticatory muscle size, and mating system. To contextualize our results, we included a broader sample of facial dimorphism for 67 anthropoid species.

RESULTS

Greater circumorbital thickness is unrelated to the stresses of food processing. King colobus engages in longer bouts of anterior tooth use, chews more per ingestive event, and processes a tougher diet, yet circumorbital thickness of C. polykomos is reduced compared to P. badius. Differences in circumorbital development do not vary with wear or facial size. Greater sexual dimorphism is present in P. badius; comparisons across anthropoids indicated patterns of circumorbital dimorphism were decoupled from overall size dimorphism.

CONCLUSIONS

The expanded circumorbits of male red colobus monkeys evolved in response to intense male-male competition. This hypothesis is consistent with the pattern across anthropoid primates and highlights the underappreciated role of sexual selection in shaping the primate face.

Was facial width-to-height ratio subject to sexual selection pressures? A life course approach

Sexual selection researchers have traditionally focused on adult sex differences; however, the schedule and pattern of sex-specific ontogeny can provide insights unobtainable from an exclusive focus on adults. Recently, it has been debated whether facial width-to-height ratio (fWHR; bi-zygomatic breadth divided by midface height) is a human secondary sexual characteristic (SSC). Here, we review current evidence, then address this debate using ontogenetic evidence, which has been under-explored in fWHR research. Facial measurements were collected from 3D surface images of males and females aged 3 to 40 (Study 1; US European-descent, n = 2449), and from 2D photographs of males and females aged 7 to 21 (Study 2; Bolivian Tsimane, n = 179), which were used to calculate three fWHR variants (which we call fWHRnasion, fWHRstomion, and fWHRbrow) and two other common facial masculinity ratios (facial width-to-lower-face-height ratio, fWHRlower, and cheekbone prominence). We test whether the observed pattern of facial development exhibits patterns indicative of SSCs, i.e., differential adolescent growth in either male or female facial morphology leading to an adult sex difference. Results showed that only fWHRlower exhibited both adult sex differences as well as the classic pattern of ontogeny for SSCs-greater lower-face growth in male adolescents relative to females. fWHRbrow was significantly wider among both pre- and post-pubertal males in the Bolivian Tsimane sample; post-hoc analyses revealed that the effect was driven by large sex differences in brow height, with females having higher placed brows than males across ages. In both samples, all fWHR measures were inversely associated with age; that is, human facial growth is characterized by greater relative elongation in the mid-face and lower face relative to facial width. This trend continues even into middle adulthood. BMI was also a positive predictor of most of the ratios across ages, with greater BMI associated with wider faces. Researchers collecting data on fWHR should target fWHRlower and fWHRbrow and should control for both age and BMI. Researchers should also compare ratio approaches with multivariate techniques, such as geometric morphometrics, to examine whether the latter have greater utility for understanding the evolution of facial sexual dimorphism.

Macaque Gaze Responses to the Primatar: A Virtual Macaque Head for Social Cognition Research

Following the expanding use and applications of virtual reality in everyday life, realistic virtual stimuli are of increasing interest in cognitive studies. They allow for control of features such as gaze, expression, appearance, and movement, which may help to overcome limitations of using photographs or video recordings to study social responses. In using virtual stimuli however, one must be careful to avoid the uncanny valley effect, where realistic stimuli can be perceived as eerie, and induce an aversion response. At the same time, it is important to establish whether responses to virtual stimuli mirror responses to depictions of a real conspecific. In the current study, we describe the development of a new virtual monkey head with realistic facial features for experiments with nonhuman primates, the "Primatar." As a first step toward validation, we assessed how monkeys respond to facial images of a prototype of this Primatar compared to images of real monkeys (RMs), and an unrealistic model. We also compared gaze responses between original images and scrambled as well as obfuscated versions of these images. We measured looking time to images in six freely moving long-tailed macaques (Macaca fascicularis) and gaze exploration behavior in three rhesus macaques (Macaca mulatta). Both groups showed more signs of overt attention to original images than scrambled or obfuscated images. In addition, we found no evidence for an uncanny valley effect; since for both groups, looking times did not differ between real, realistic, or unrealistic images. These results provide important data for further development of our Primatar for use in social cognition studies and more generally for cognitive research with virtual stimuli in nonhuman primates. Future research on the absence of an uncanny valley effect in macaques is needed, to elucidate the roots of this mechanism in humans.