Intranasal Oxytocin Failed to Affect Chimpanzee (Pan troglodytes) Social Behavior

Conserved and differing functions of the endocrine system across different social systems - oxytocin as a case study

A key goal of the field of endocrinology has been to understand the hormonal mechanisms that drive social behavior and influence reactions to others, such as oxytocin. However, it has sometimes been challenging to understand which aspects and influences of hormonal action are conserved and common among mammalian species, and which effects differ based on features of these species, such as social system. This challenge has been exacerbated by a focus on a relatively small number of traditional model species. In this review, we first demonstrate the benefits of using non-traditional models for the study of hormones, with a focus on oxytocin as a case study in adding species with diverse social systems. We then expand our discussion to explore differing effects of oxytocin (and its response to behavior) within a species, with a particular focus on relationship context and social environment among primate species. Finally, we suggest key areas for future exploration of oxytocin's action centrally and peripherally, and how non-traditional models can be an important resource for understanding the breadth of oxytocin's potential effects.

Characterizing the personality and gray matter volume of chimpanzees that exhibit autism-related socio-communicative phenotypes

Autism spectrum disorder (ASD) is a developmental disorder characterized by stereotypies or repetitive behaviors and impairments in social behavior and socio-communicative skills. One hallmark phenotype of ASD is poor joint attention skills compared to neurotypical controls. In addition, individuals with ASD have lower scores on several of the Big 5 personality dimensions, including Extraversion. Here, we examine these traits in a nonhuman primate model (chimpanzees; Pan troglodytes) to further understand the relationship between personality and joint attention skills, as well as the genetic and neural systems that contribute to these phenotypes. We used archival data including receptive joint attention (RJA) performance, personality based on caretaker ratings, and magnetic resonance images from 189 chimpanzees. We found that, like humans, chimpanzees who performed worse on the RJA task had lower Extraversion scores. We also found that joint attention skills and several personality dimensions, including Extraversion, were significantly heritable. There was also a borderline significant genetic correlation between RJA and Extraversion. A conjunction analysis examining gray matter volume showed that there were five main brain regions associated with both higher levels of Extraversion and social cognition. These regions included the right posterior middle and superior temporal gyrus, bilateral inferior frontal gyrus, left inferior frontal sulcus, and left superior frontal sulcus, all regions within the social brain network. Altogether, these findings provide further evidence that chimpanzees serve as an excellent model for understanding the mechanisms underlying social impairment related to ASD. Future research should further examine the relationship between social cognition, personality, genetics, and neuroanatomy and function in nonhuman primate models.

Comparative economics: how studying other primates helps us better understand the evolution of our own economic decision making

The origins of evolutionary games are rooted in both economics and animal behaviour, but economics has, until recently, focused primarily on humans. Although historically, specific games were used in targeted circumstances with non-human species (i.e. the Prisoner's Dilemma), experimental economics has been increasingly recognized as a valuable method for directly comparing both the outcomes of economic decisions and their underlying mechanisms across species, particularly in comparison with humans, thanks to the structured procedures that allow for them to be instantiated across a variety of animals. So far, results in non-human primates suggest that even when outcomes are shared, underlying proximate mechanisms can vary substantially. Intriguingly, in some contexts non-human primates more easily find a Nash equilibrium than do humans, possibly owing to their greater willingness to explore the parameter space, but humans excel at more complex outcomes, such as alternating between two Nash equilibria, even when deprived of language or instruction, suggesting potential mechanisms that humans have evolved to allow us to solve complex social problems. We consider what these results suggest about the evolution of economic decision-making and suggest future directions, in particular the need to expand taxonomic diversity, to expand this promising approach. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Testing the effect of oxytocin on social grooming in bonobos

Oxytocin has attracted research attention due to its role in promoting social bonding. In bonobos and chimpanzees, the two species most closely related to humans, urinary oxytocin is known to correlate with key behaviours related to social bonding such as social grooming in chimpanzees and female-female sexual behaviour in bonobos. However, no study has demonstrated that the administration of oxytocin promotes real-life social interactions in Pan, leaving it unclear whether oxytocin is merely correlated with social behaviors or does affect them in these species. To test this, we administered nebulized oxytocin or saline placebo to a group of female bonobos and subsequently observed changes in their gross behavior during free interaction. We found an overall effect of more frequent grooming in the oxytocin condition. However, on the individual level this effect remained significant for only one participant in our follow-up models, suggesting future work should explore interindividual variation. Our results provide some experimental support for the biobehavioural feedback loop hypothesis, which posits that some functions of the oxytocin system support the formation and maintenance of social bonds through a positive feedback loop; however, further tests with a larger number of individuals are required. Our results, at a minimum, demonstrated that oxytocin affects spontaneous, naturalistic social interactions of at least some female bonobos, adding to accumulating evidence that oxytocin modulates complex social behaviors of Pan.

Oxytocin promotes species-relevant outgroup attention in bonobos and chimpanzees

Previous research has found that oxytocin (OT) is associated with intergroup behaviour in humans as well as wild chimpanzees, and that exogenous OT affects Pan social attention. The two Pan species, bonobos and chimpanzees, differ drastically from one another in their intensity of intergroup competition, with lethal intergroup aggression often led by males in chimpanzees and more tolerant associations often centered around females in bonobos. However, it remains unclear how exogenous OT changes the two species' responses to ingroup and outgroup individuals. In this study, after intranasal administration of nebulized OT or placebo control, chimpanzees and bonobos viewed image pairs of ingroup and outgroup conspecifics while their eye movements were tracked with an eye-tracker. Although the overall effect of OT was small, we found that OT shifted bonobos' and chimpanzees' attention to outgroup images of the sex primarily involved in intergroup encounters in each species. Specifically, OT selectively shifted attention towards outgroup photos of female conspecifics in bonobos, and those of outgroup male conspecifics in chimpanzees. This suggests that OT generally promotes outgroup attention in both bonobos and chimpanzees but this effect is restricted to the sex most relevant in intergroup relations. These results suggest that, although OT may have a generally conserved role in hominid intergroup behaviour, it may act in species-relevant ways under the influence of their socio-ecological backgrounds.

Divergent effects of oxytocin on eye contact in bonobos and chimpanzees

Oxytocin has drawn significant research attention for its role in modulating mammalian social behavior. Despite generally conserved roles, oxytocin can function differently even in closely related species. Previous studies have shown that bonobos and chimpanzees, humans' two closest relatives, demonstrate considerable behavioral differences, including that bonobos look more at others' eyes than chimpanzees. Oxytocin is known to increase attention to another's eyes in many mammalian species (e.g. dogs, monkeys, and humans), yet this effect has not been tested in any nonhuman great ape species. This study examined how intranasally-administered oxytocin affects eye contact in bonobos and chimpanzees using eye tracking. Following administration of either oxytocin or saline control with a nebulizer, chimpanzees (n = 6) and bonobos (n = 5) were shown images of conspecific faces while their eye movement was recorded. Oxytocin changed the eye-looking behavior of bonobos and chimpanzees differently. We found that oxytocin increased eye contact in bonobos but not chimpanzees; while one chimpanzee showed an increase, interestingly, 5 out of 6 chimpanzees showed decreased looking to the eyes compared to the mouth, suggesting moderate eye avoidance. Given the importance of eye contact in their social interactions, our results suggest that oxytocin may play modulatory roles in bonobos' and chimpanzees' species-specific social behavior and underscore the importance of oxytocin in hominid social evolution.

Chimpanzees Rarely Settle on Consistent Patterns of Play in the Hawk Dove, Assurance, and Prisoner's Dilemma Games, in a Token Exchange Task

Games derived from experimental economics can be used to directly compare decision-making behavior across primate species, including humans. For example, the use of coordination games, such as the Assurance game, has shown that a variety of primate species can coordinate; however, the mechanism by which they do so appears to differ across species. Recently, these games have been extended to explore anti-coordination and cooperation in monkeys, with evidence that they play the Nash equilibria in sequential games in these other contexts. In the current paper, we use the same methods to explore chimpanzees' behavior in the Assurance Game; an anti-coordination game, the Hawk Dove game; and a cooperation game with a temptation to defect, the Prisoner's Dilemma game. We predicted that they would consistently play the Nash equilibria, as do the monkeys, and that, as in previous work, the subjects' level of experience with cognitive experiments would impact performance. Surprisingly, few of our pairs consistently played the same outcome (i.e., no statistically significant preferences), although those who did showed evidence consistent with Nash equilibria play, the same pattern seen more consistently in the monkeys. We consider reasons for their inconsistent performance; for instance, perhaps it was due to lack of interest in a task that rewarded them almost every trial no matter what option they chose, although this does not explain why they were inconsistent when the monkeys were not. A second goal of our study was to ascertain the effects of exogenous oxytocin in their decision making in one population. In line with recent work showing complex effects of oxytocin on social behavior, we found no effect on subjects' outcomes. We consider possible explanations for this as well.