Rhesus macaque personality, dominance, behavior, and health

Towards an Integrated Concept of Personality in Human and Nonhuman Animals

Every individual has an idiosyncratic way of feeling, thinking and behaving, which is relatively stable across time and situations. Usually known as Personality, today this phenomenon is recognized in many species, including arthropods, fish, avian or mammals. From an evolutionary perspective, research has shown that personality differences are manifest in distinctive forms of dealing with selective pressures, with consequences for fitness. Despite these facts, the study of personality in animals other than humans is relatively new. Only two decades ago, consistent behavioral individual differences were considered 'noise' around an optimal strategy for behavioral ecologists. Also, psychologists were not interested in animal personality as a consequence of the fear of anthropomorphization and the erroneous belief that humans are unique in nature. Fortunately, this misconception seems already overcome but there are still conceptual issues preventing a unified concept of personality. Throughout this review, we first explore the etymological origins of personality and other terminological issues. We further revise the historical course of the study of personality in humans and other animals, from the perspectives of Psychology and Behavioral Ecology, on the basis of the most used approach, the trait theory. We present the study of nonhuman primates as a paradigmatic example in between both frameworks. Finally, we discuss about the necessity of a unified science of personality.

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.

Measuring dominance certainty and assessing its impact on individual and societal health in a nonhuman primate model: a network approach

The notion of dominance is ubiquitous across the animal kingdom, wherein some species/groups such relationships are strictly hierarchical and others are not. Modern approaches for measuring dominance have emerged in recent years taking advantage of increased computational power. One such technique, named Percolation and Conductance (Perc), uses both direct and indirect information about the flow of dominance relationships to generate hierarchical rank order that makes no assumptions about the linearity of these relationships. It also provides a new metric, known as 'dominance certainty', which is a complimentary measure to dominance rank that assesses the degree of ambiguity of rank relationships at the individual, dyadic and group levels. In this focused review, we will (i) describe how Perc measures dominance rank while accounting for both nonlinear hierarchical structure as well as sparsity in data-here we also provide a metric of dominance certainty estimated by Perc, which can be used to compliment the information dominance rank supplies; (ii) summarize a series of studies by our research team reflecting the importance of 'dominance certainty' on individual and societal health in large captive rhesus macaque breeding groups; and (iii) provide some concluding remarks and suggestions for future directions for dominance hierarchy research. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Potential applications of personality assessments to the management of non-human primates: a review of 10 years of study

Studies of primate personality have become increasingly common over the past three decades. Recently, studies have begun to focus on the health, welfare and conservation implications of personality, and the potential applications of incorporating quantitative personality assessments into animal management programmes. However, this literature is dispersed across a multitude of settings and scientific disciplines. We conducted a review of nonhuman primate personality studies relevant to these issues published since 2010, following on from an earlier review. The databases ScienceDirect, PubMed and Web of Science were used to identify relevant articles. After eliminating irrelevant or duplicate papers, 69 studies were selected. Our review reveals that, while primate personality research is carried out on a range of species, there is strong taxonomic bias. While 28 species appeared within the reviewed literature, 52% of studies were carried out on just five species. Further, the most common research focus (43%) was validating new assessment methods or describing personality in different species, rather than exploring the links between personality and animal welfare using existing validated methods. However, among the remaining studies that did explore the role of animal personality in husbandry, health, and welfare, we identified progression towards integrating personality data into various aspects of animal management. Evidence suggests the assessment of personality may benefit social group management, enrichment practices, training protocols, health and welfare monitoring, and conservation planning for endangered species. We argue that further research which develops our understanding of primate personality and its influence in these areas will provide a valuable tool to inform animal management practices.

The relationship between personality, season, and wounding receipt in zoo-housed Japanese macaques (Macaca fuscata): A multi-institutional study

It is important to those managing Japanese macaques (Macaca fuscata) in captive settings to understand predictors of wounding. While studies have demonstrated that season (breeding or nonbreeding) and sex predict rates of wounding received by zoo-housed Japanese macaques, we investigated whether individual differences in personality ratings also might explain some of the observed interindividual variance in wounding. Such patterns were previously observed in rhesus macaques (M. mulatta), such that individuals rated higher on Anxiety and Confidence received greater wounding. Here, we collected wounding data over 24 months on 48 Japanese macaques from eight AZA-accredited zoos. Each macaque was also rated by keepers using a 26-item personality questionnaire. Principle components analysis of these ratings revealed four personality components: Openness, Friendliness, Dominance, and Anxiety/Reactivity. The model with the best fit revealed an interaction effect between season (breeding vs. nonbreeding) and the personality component Friendliness, such that individuals rated higher on Friendliness incurred fewer wounds in the nonbreeding season. The second-best model revealed both a main effect of the season as well as an interaction effect between season and Openness, such that macaques rated higher in Openness received more wounds in the nonbreeding season than those rated lower in Openness. Thus, as with rhesus macaques, personality mediated wounding receipt rate in Japanese macaques, although different personality components explained interindividual variance in wounding for these two species. These differences likely reflect species differences in behavior and personality structure, as well as the influence of differing management practices, highlighting the importance of species-specific approaches for captive primate care and welfare. This study provides further support for understanding primate personality to create individualized strategies for their care.

SARS-CoV-2 Infection Challenge Experiments in Nonhuman Primates: An Ethical Perspective

The COVID-19 pandemic has stimulated massive investment in biomedical research with the aims of understanding the disease and developing effective vaccine and therapeutic interventions. What role should animal research play in this scientific endeavor? Both the urgency to evaluate candidate interventions for human use and growing societal concern about ethical treatment of (nonhuman) animals put into question the justifiability of animal research as a precursor to clinical trials. Yet forgoing animal research in the rush to undertake human testing might expose human research participants to unacceptable risks. In this article, we apply a recently developed framework of principles for animal research ethics in exploring ethical questions raised by a SARS-CoV-2 infection challenge experiment involving rhesus macaques, which evaluated the protective efficacy of the mRNA-1273 vaccine that was recently approved for emergency use. Our aim is to illuminate the ethical issues while introducing, and illustrating the use of, the framework.

Linking behavioral thermoregulation, boldness, and individual state in male Carpetan rock lizards

Mechanisms affecting consistent interindividual behavioral variation (i.e., animal personality) are of wide scientific interest. In poikilotherms, ambient temperature is one of the most important environmental factors with a direct link to a variety of fitness-related traits. Recent empirical evidence suggests that individual differences in boldness are linked to behavioral thermoregulation strategy in heliothermic species, as individuals are regularly exposed to predators during basking. Here, we tested for links between behavioral thermoregulation strategy, boldness, and individual state in adult males of the high-mountain Carpetan rock lizard (Iberolacerta cyreni). Principal component analysis revealed the following latent links in our data: (i) a positive relationship of activity with relative limb length and color brightness (PC1, 23% variation explained), (ii) a negative relationship of thermoregulatory precision with parasite load and risk-taking (PC2, 20.98% variation explained), and (iii) a negative relationship between preferred body temperature and relative limb length (PC3, 19.23% variation explained). We conclude that differences in boldness and behavioral thermoregulatory strategy could be explained by both stable and labile state variables. The moderate link between behavioral thermoregulatory strategy and risk-taking personality in our system is plausibly the result of differences in reproductive state of individuals or variation in ecological conditions during the breeding season.

Personality, cognition and behavior in chimpanzees: a new approach based on Eysenck's model

Personality has been linked to individual variation in interest and performance in cognitive tasks. Nevertheless, this relationship is still poorly understood and has rarely been considered in animal cognition research. Here, we investigated the association between personality and interest, motivation and task performance in 13 sanctuary chimpanzees (Pan troglodytes) housed at Fundació Mona (Spain). Personality was assessed with a 12-item questionnaire based on Eysenck's Psychoticism-Extraversion-Neuroticism model completed by familiar keepers and researchers. Additionally, personality ratings were compared to behavioral observations conducted over an 11-year period. Experimental tasks consisted in several puzzle boxes that needed to be manipulated in order to obtain a food reward. Dependent variables included participation (as an indicator of interest), success and latency (as measures of performance), and losing contact with the task (as an indicator of motivation). As predicted, we obtained significant correlations between Eysenck's personality traits and observed behaviors, although some expected associations were absent. We then analyzed data using Generalized Linear Mixed Models, running a model for each dependent variable. In both sexes, lower Extraversion and lower Dominance were linked to a higher probability of success, but this effect was stronger in females. Furthermore, higher Neuropsychoticism predicted higher probability of success in females, but not in males. The probability of losing contact with the task was higher in young chimpanzees, and in those rated lower on Extraversion and higher on Dominance. Additionally, chimpanzees rated higher on Neuropsychoticism were also more likely to stop interacting with the task, but again this was more evident in females. Participation and latency were not linked to any personality trait. Our findings show that the PEN may be a good model to describe chimpanzee personality, and stress the importance of considering personality when interpreting the results of cognitive research in non-human primates.

Individual behavioral differences and health of golden-headed lion tamarins (Leontopithecus chrysomelas)

Individual behavioral differences may influence how animals cope with altered environments. Depending on their behavioral traits, individuals may thus vary in how their health is affected by environmental conditions. We investigated the relationship between individual behavior of free-living golden-headed lion tamarins (Leontopithecus chrysomelas) responding to a novel object (to assess exploration-avoidance), and their habitat use and health status (endoparasitism; clinical measures: biometric data, heart rate, respiratory frequency, and temperature; fecal glucocorticoid metabolites). As parasite transmission can be affected by individual variation in social contact and social grooming, we also evaluated whether more sociable individuals show higher endoparasite loads compared with less sociable animals. Four groups living in landscapes with different levels of human disturbance were investigated: two in degraded forest fragments in an agricultural matrix (DFAM-higher disturbance), and two in a cocoa agroforestry system (cabruca-lower disturbance) in the Atlantic forest of South Bahia, Brazil. Using a subjective ratings approach, highly correlated adjective descriptors were combined to produce z-score ratings of one derived variable ("confidence"), which was selected to characterize the tamarins' exploration/avoidance responses during a novel object test. The higher the confidence score, the longer female tamarins spent foraging for prey independent of landscape, and the greater their body mass independent of sex and landscape. Only DFAM individuals showed intestinal parasite infection. Endoparasite loads were positively correlated with the number of grooming partners, suggesting an association between social grooming and transmission (more groomers = more endoparasites). Individual behavior, including in a test situation, may thus have some predictive value for behavior in a free-living context, and for its health consequences.

Infant rhesus macaque (Macaca mulatta) personality and subjective well-being

Infant temperament is theorized to lay the foundation for adult personality; however, many questions remain regarding personality in infancy, including the number of dimensions, extent to which they are adult-like, and their relation to other outcomes, such as mental and physical health. Here we tested whether adult-like personality dimensions are already present in infancy in a nonhuman primate species. We measured personality and subjective well-being in 7-month-old rhesus macaques (N = 55) using the Hominoid Personality Questionnaire and Subjective Well-Being Questionnaire, both of which were developed for adult primates based on human measures. Multiple human raters, who provided infants with daily care since birth, independently rated each infant. We found high interrater reliability. Results from a parallel analysis and scree plot indicated a five component structure, which, using principal components analysis, we found to be comprised of dimensions relating to Openness (e.g., curiosity, inquisitive, playfulness), Assertiveness (e.g., dominance, bullying, aggressive), Anxiety (e.g., vigilance, fearful), Friendliness (e.g., sociable, affectionate, sympathetic), and Intellect (e.g., organized, not erratic). These components are largely analogous to those in adult macaques, suggesting remarkably stable structural personality components across the lifespan. Infant macaques' subjective well-being positively correlates with Openness and Assertiveness and negatively correlated with Anxiety, similar to findings in adult macaques and other primates. Together, these findings suggest that, in macaques, infant personality dimensions may be conceptually related to adult personality and challenge the view that infant temperament may be disorganized and not as meaningful as adult personality. Further research is necessary to explore the antecedents, predictive validity, and stability of these personality components across situations and with development.

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

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 <8 yr. of age. We found fWHR and fLHFH to not be consistently associated with sex or dominance status; compared to younger individuals, we found few associations with fWHR and fLHFH for individuals older than 8 yr., which may be due to limited sample size. Our results indicate that in macaques <8 yr. old, facial morphology is associated with the Assertiveness and Confidence personality dimensions, which is consistent with results suggesting a relationship between fWHR and trait aggression in humans and assertiveness in brown capuchins, all of which implies that fWHR might be a cue to assertive and aggressive traits.

Dolphins' Willingness to Participate (WtP) in Positive Reinforcement Training as a Potential Welfare Indicator, Where WtP Predicts Early Changes in Health Status

Welfare science has built its foundations on veterinary medicine and thus measures of health. Since bottlenose dolphins (Tursiops truncatus) tend to mask symptoms of poor health, management in captivity would benefit from advanced understanding on the links between health and behavioural parameters, and few studies exist on the topic. In this study, four representative behavioural and health measures were chosen: health status (as qualified by veterinarians), percentage of daily food eaten, occurrences of new rake marks (proxy measure of social activity), and willingness to participate (WtP) in Positive Reinforcement Training sessions as qualitatively measured by their caretakers. These data were collected multiple times a day, every day over the course of a year from a multi-facility, large sample size (n dolphins = 51), allowing powerful analyses of the relationships between measures. First, it was found that dolphins with a higher WtP score also had a significantly better health status, ate a higher percentage of their daily food, and a lower occurrence of new rake marks. In addition, the WtP score was significantly lower up to 3 days before the weekly veterinary diagnosis of a decrease in health state; the percentage of daily food eaten and new rake mark measures did not show any significant change before such a diagnosis. These results suggest that WtP in training sessions is a potential behavioural measure of dolphin welfare, and an indicator of early changes in the dolphins' health state. We therefore suggest measurement of WtP as a more practical and non-invasive tool to support veterinary care and general management. More work needs to be conducted to elucidate the influence of social behaviour on health, and to identify other potential welfare indicators, but this long-term study has shown that qualitative measures can be both practical and valid when assessing dolphin welfare.

Nonhuman primate welfare: Can there be a relationship between personality, lateralization and physiological indicators?

Measuring personality is being used to improved nonhuman primate welfare. To expand its use, it is important to identify traits that are shared between species and that measures are reliable, easy to use and less time consuming. Combining personality and other indicators strong validation of the results can be obtained. In the present study, we sought to determine if there is a link between physiological stress response (fecal cortisol metabolites), personality (ratings made by animal keepers and reaction to novel objects) and lateralization of the brain (hand preferences) on eight species of nonhuman primates: Callithrix jacchus, Callithrix geoffroyi, Cebuella Pygmaea, Saguinus imperator, Saguinus oedipus, Leontopithecus rosalia, Pithecia pithecia and Nycticebus pygmaeus. Personality assessments achieved good levels of interrater reliability and revealed three components of personality in our sample: fearfulness, activeness and aggressiveness. More exploratory individuals were more active, aggressive and showed higher cortisol metabolite levels. Right-handed subjects inspected novel objects sooner and the strength of the lateralization was linked with individual stress and the aggressiveness component. Our results highlight that there is a relation between personality, lateralization and physiological indicators in nonhuman primates, but although some aspects can be generalized across species and/or sexes others are species/sex dependent.

Future Directions for Personality Research: Contributing New Insights to the Understanding of Animal Behavior

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.

A multi-institutional assessment of a short-form personality questionnaire for use with macaques

There are two commonly-used methods for calculating primates' personality dimensions, behavioral assessments and surveys, which can be used separately or in conjunction. However, these methods have limitations. Behavioral assessments, such as the novel object test or human intruder test, often require subjects to be separated and demand highly-controlled conditions. This is likely not feasible in many zoological institutions. Furthermore, it may be difficult to replicate methods across institutions. While surveys are easier to implement, the most commonly used one - the Hominoid Personality Questionnaire (HPQ) - is long and can be time consuming to complete, especially if multiple individuals need assessing. Therefore, we developed a short-form personality questionnaire for use with macaques. We aimed to retain elements of published scales while creating a tool that would be less time consuming to complete. After creating a novel 26-item scale (28 fewer items than the HPQ), we gathered ratings for 63 Japanese macaques (Macaca fuscata) living in eight zoos accredited by the Association of Zoos and Aquariums. Using principal components analysis, we identified four components to describe the Japanese macaque personality: Openneness, Friendliness, Dominance, and Anxiety/Reactivity. While further testing is required to fully validate this scale, we tested for the convergent validity of our questionnaire by correlating the responses of a subset of the monkeys to a novel object and assessed the face validity of our scale in comparison to previously-published scales.