Search

Aspects of personality in nonhuman primates have been linked to health, social relationships, and life history outcomes. In humans as well as nonhuman primates, facial morphology is associated with assertiveness, aggression, and measures of dominance status. In this study we aimed to examine the relationship among facial morphology, age, sex, dominance status, and ratings on the personality dimensions Confidence, Openness, Assertiveness, Friendliness, Activity, and Anxiety in rhesus macaques (Macaca mulatta). We measured facial width-to-height ratio (fWHR) and lower-height/full-height ratio (fLHFH) using photographs from 109 captive rhesus macaques, which observers also assessed for dominance status and personality, and explored the associations among facial morphology, age, sex, dominance status, and personality. fWHR and fLHFH personality associations depended on age category: Assertiveness was associated with higher fWHR and fLHFH, and Confidence was associated with lower fWHR and fLHFH, but all these associations were consistent only in individuals

Human language relies on a rich cognitive machinery, partially shared with other animals. One key mechanism, however, decomposing events into causally linked agent-patient roles, has remained elusive with no known animal equivalent. In humans, agent-patient relations in event cognition drive how languages are processed neurally and expressions structured syntactically. We compared visual event tracking between humans and great apes, using stimuli that would elicit causal processing in humans. After accounting for attention to background information, we found similar gaze patterns to agent-patient relations in all species, mostly alternating attention to agents and patients, presumably in order to learn the nature of the event, and occasionally privileging agents under specific conditions. Six-month-old infants, in contrast, did not follow agent-patient relations and attended mostly to background information. These findings raise the possibility that event role tracking, a cognitive foundation of syntax, has evolved long before language but requires time and experience to become ontogenetically available., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Wilson et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

When conversing, humans instantaneously predict meaning from fragmentary and ambiguous mspeech, long before utterance completion. They do this by integrating priors (initial assumptions about the world) with contextual evidence to rapidly decide on the most likely meaning. One powerful prior is attentional preference for agents, which biases sentence processing but universally so only if agents are animate. Here, we investigate the evolutionary origins of this preference, by allowing chimpanzees, gorillas, orangutans, human children, and adults to freely choose between agents and patients in still images, following video clips depicting their dyadic interaction. All participants preferred animate (and occasionally inanimate) agents, although the effect was attenuated if patients were also animate. The findings suggest that a preference for animate agents evolved before language and is not reducible to simple perceptual biases. To conclude, both humans and great apes prefer animate agents in decision tasks, echoing a universal prior in human language processing., Competing Interests: The authors declare no competing interests., (© 2024 The Author(s).)

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., Competing Interests: The authors have nothing to disclose., (© The Author(s) 2023.)

The study of gaze responses, typically using looking time paradigms, has become a popular approach to improving our understanding of cognitive processes in non-verbal individuals. Our interpretation of data derived from these paradigms, however, is constrained by how we conceptually and methodologically approach these problems. In this perspective paper, we outline the application of gaze studies in comparative cognitive and behavioral research and highlight current limitations in the interpretation of commonly used paradigms. Further, we propose potential solutions, including improvements to current experimental approaches, as well as broad-scale benefits of technology and collaboration. Finally, we outline the potential benefits of studying gaze responses from an animal welfare perspective. We advocate the implementation of these proposals across the field of animal behavior and cognition to aid experimental validity, and further advance our knowledge on a variety of cognitive processes and welfare outcomes., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Wilson, Bethell and Nawroth.)

Languages tend to encode events from the perspective of agents, placing them first and in simpler forms than patients. This agent bias is mirrored by cognition: Agents are more quickly recognized than patients and generally attract more attention. This leads to the hypothesis that key aspects of language structure are fundamentally rooted in a cognition that decomposes events into agents, actions, and patients, privileging agents. Although this type of event representation is almost certainly universal across languages, it remains unclear whether the underlying cognition is uniquely human or more widespread in animals. Here, we review a range of evidence from primates and other animals, which suggests that agent-based event decomposition is phylogenetically older than humans. We propose a research program to test this hypothesis in great apes and human infants, with the goal to resolve one of the major questions in the evolution of language, the origins of syntax.

Previous studies found associations between autism-related phenotypes and both rearing and V1A arginine vasopressin receptor (AVPR1A) genotypes. We tested whether these exposures as well as their interaction were associated with autism-related phenotypes in 121 laboratory-housed chimpanzees. We used expert-derived weights to obtain autism scores from ratings on the 43-item Chimpanzee Personality Questionnaire; higher scores indicated more autistic-like traits. The first model included fixed effects for sex, age, and rearing, and a random effect that addressed the relatedness of subjects. The second model was the same except that it also included the rearing × AVPR1A genotype interaction as a fixed effect. Both models indicated that the phenotype was moderately heritable and that chimpanzees reared by their mothers had lower scores on the scale. The effect of genotype in both models indicated that chimpanzees with an indel deletion had higher scores on the scale, although the credible interval included zero. Moreover, the rearing × genotype interaction in the second model indicated that chimpanzees who possessed the non-deletion genotype and who were reared by their mother were at even greater risk. The credible interval for this effect did not include zero, but fit statistics indicated that the model without the interaction was marginally better, and the interaction was in the opposite direction than we expected based on previous work. These findings highlight the importance of rearing effects in the typical social development of our closet-living nonhuman relative. LAY SUMMARY: We tested whether, in chimpanzees, scores on a scale comprising traits that resembled aspects of autism were related to a gene associated with autism in prior research and/or early rearing. Human-reared chimpanzees had higher scores (indicating more autistic-like traits). Chimpanzees that possessed the gene also had higher scores, but we could not exclude the possibility that there was no effect of genotype. These findings suggest that we can measure autism-like characteristics in chimpanzees, and so study it in this species., (© 2021 The Authors. Autism Research published by International Society for Autism Research and Wiley Periodicals LLC.)

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., (Copyright © 2020 Wilson, Kade, Moeller, Treue, Kagan and Fischer.)

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 ( Saimiri sciureus ) and 40 Bolivian squirrel monkeys ( Saimiri boliviensis ). 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, ((c) 2018 APA, all rights reserved).)