In highly social species, like primates, oxytocin plays an important role in cooperation, and in the formation and maintenance of social relationships. Despite recent interest in the relationship between oxytocin and social behavior in nonhuman primates, relatively little is known about endogenous oxytocin in social New World Monkeys. In this paper, we investigate the relationship between oxytocin and affiliative behaviors in socially-housed captive capuchin monkeys (Sapajus [Cebus] apella) by first validating methods of analysis of urinary oxytocin in this species and, second, examining the effects of grooming and fur-rubbing behavior on oxytocin concentrations and further affiliative behavior. In the validation, we found that intranasal exogenous oxytocin significantly increased urinary oxytocin 15-60 min post-administration. Oxytocin was also implicated in both grooming and fur-rubbing behaviors. We found that oxytocin concentrations increased after subjects engaged in grooming or fur-rubbing. In addition, we found that fur-rubbing influenced affiliative behaviors, both during and after a social fur-rubbing bout. While individuals spent more time in contact and proximity while fur-rubbing, immediately following the fur-rubbing event (15-30 min afterwards) all affiliative behaviors decreased. This supports previous research that oxytocin may, in fact, initially be related to increased social distance in this species. Yet, an increase in all affiliative behaviors 30-45 min after the onset of fur-rubbing suggests that fur-rubbing, like grooming, may ultimately function to strengthen social relationships. Overall, these results support a critical role for oxytocin in affiliative behaviors that maintain and strengthen social relationships in capuchin monkeys, and highlight the complexity of the interactions among oxytocin, affiliative behaviors, and social bonding.

Although individuals in some species refuse foods they normally accept if their partner receives a more preferred one, this is not true across all species. The cooperation hypothesis proposes that this species‐level variability evolved because inequity aversion is a mechanism to identify situations in which cooperation is not paying off, and that species regularly observed cooperating should be more likely to be averse to inequity. To rule out other potential explanations of inequity aversion, we need to test the converse as well: species rarely observed cooperating, especially those phylogenetically close to more cooperative species, should be less likely to be inequity averse. To this end, we tested eight zoo‐housed Western lowland gorillas (Gorilla gorilla gorilla) on a token exchange task in which subjects received either the same food reward or a less‐preferred reward for the same or more effort than their partner, recording both refusals to participate in the exchange and refusals to accept the reward. Supporting the cooperation hypothesis, even with procedural differences across sessions, gorillas were significantly more likely to refuse in all conditions in which they received a low‐value food reward after completing an exchange, regardless of what their partner received, suggesting that gorillas were not inequity averse, but instead would not work for a low‐value reward. Additionally, gorillas were more likely to refuse later in the session; while the pattern of refusals remained unchanged after accounting for this, this suggests that species should be tested on as many trials as is practical.

Humans often experience striking performance deficits when their outcomes are determined by their own performance, colloquially referred to as “choking under pressure.” Physiological stress responses that have been linked to both choking and thriving are well-conserved in primates, but it is unknown whether other primates experience similar effects of pressure. Understanding whether this occurs and, if so, its physiological correlates, will help clarify the evolution and proximate causes of choking in humans. To address this, we trained capuchin monkeys on a computer game that had clearly denoted high- and low-pressure trials, then tested them on trials with the same signals of high pressure, but no difference in task difficulty. Monkeys significantly varied in whether they performed worse or better on high-pressure testing trials and performance improved as monkeys gained experience with performing under pressure. Baseline levels of cortisol were significantly negatively related to performance on high-pressure trials as compared to low-pressure trials. Taken together, this indicates that less experience with pressure may interact with long-term stress to produce choking behavior in early sessions of a task. Our results suggest that performance deficits (or improvements) under pressure are not solely due to human specific factors but are rooted in evolutionarily conserved biological factors.

Visual attention to facial features is an important way that group-living primate species gain knowledge about others. However, where this attention is focused on the face is influenced by contextual and social features, and emerging evidence in Pan species suggests that oxytocin, a hormone involved in forming and maintaining affiliative bonds among members of the same group, influences social attention as measured by eye gaze. Specifically, bonobos tend to focus on conspecifics’ eyes when viewing two-dimensional images, whereas chimpanzees focus more on the edges of the face. Moreover, exogenous oxytocin, which was hypothesized to increase eye contact in both species, instead enhanced this existing difference. We follow up on this to (1) determine the degree to which this Pan pattern generalizes across highly social, cooperative non-ape primates and (2) explore the impact of exogenously administered vs. endogenously released oxytocin in impacting this behavior. To do so, we tracked gaze direction on a computerized social categorization task using conspecific faces in tufted capuchin monkeys (Sapajus [Cebus] apella) after (1) exogenously administering intranasal oxytocin using a nebulizer or (2) inducing an endogenous increase in oxytocin using fur-rubbing, previously validated to increase oxytocin in capuchins. Overall, we did not find a general tendency in the capuchins to look toward the eyes or mouth, but we found that oxytocin was related to looking behavior toward these regions, albeit not in a straightforward way. Considering frequency of looking per trial, monkeys were more likely to look at the eye region in the fur-rubbing condition as compared to either the saline or exogenous oxytocin conditions. However, in terms of duration of looking during trials in which they did look at the eye region, monkeys spent significantly less time looking at the eyes in both oxytocin conditions as compared to the saline condition. These results suggest that oxytocin did not necessarily enhance eye looking in capuchins, which is consistent with the results from Pan species, and that endogenous and exogenous oxytocin may behave differently in their effect on how social attention is allocated.

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