Abstract
Many animal species cooperate with conspecifics in various social contexts. While ultimate causes of cooperation are being studied extensively, its proximate causes, particularly endocrine mechanisms, have received comparatively little attention. Here, we present a study investigating the link between the hormone cortisol, cooperation and social bonds in long-tailed macaques (Macaca fascicularis). We tested 14 macaques in a dyadic cooperation task (loose-string paradigm), each with two partners of different social bond strength and measured their salivary cortisol before and after the task. We found no strong link between the macaques' cortisol level before the task and subsequent cooperative success. By contrast, we did find that the act of cooperating in itself led to a subsequent decrease in cortisol levels, but only when cooperating with closely bonded individuals. Two control conditions showed that this effect was not due to the mere presence of such an individual or the pulling task itself. Consequently, our study shows an intricate way in which the hypothalamic–pituitary–adrenal axis is involved in cooperation. Future studies should reveal whether and how our findings are driven by the anxiolytic effect of oxytocin, which has been associated with social bonding.
Highlights
Many animal species cooperate with conspecifics in various social contexts
Overall cooperation success rate ranged from 6 to 15 trials (40–100%) per session, with a median of 12 trials. When considering this cooperative success, the subset of the best models from all candidate models contained the null model, indicating that neither cortisol levels before the cooperation task, nor social bond or rank predict how successful individuals cooperated with each other (P1)
Whereas higher ranking individuals and individuals with a stronger social bond with their cooperation partner were more likely to experience a drop in cortisol, lower ranking individuals as well as weakly bonded partners were more likely to show stable cortisol levels or a slight increase
Summary
Many animal species cooperate with conspecifics in various social contexts. While ultimate causes of cooperation are being studied extensively, its proximate causes, endocrine mechanisms, have received comparatively little attention. Cooperating individuals may require a suite of cognitive traits, allowing them, for example, to recognize and assess their partners, to tolerate spatial proximity of others, to coordinate their actions with others to achieve a particular aim, and, at times, to accept lower immediate pay-offs [10] These cognitive abilities are tightly linked to physiological processes, including hormonal mechanisms modulating cooperative behaviour [10,11,12] that have, received considerably less attention. Close social bonds can buffer hypothalamic–pituitary– adrenal (HPA) axis activity, which means they can lower glucocorticoid levels [25] This has been found in several primate species, such as (female) yellow baboons, Papio cynocephalus [26,27]; Barbary macaques [28]; chimpanzees [29] and humans [30]. While many studies have shown that this applies to stressful situations in which responses to stressors are alleviated by interactions with and the mere presence of closely bonded individuals (social buffering hypothesis [25,28]), there is evidence that this is a general effect that applies to different contexts throughout daily life, independent of immediate stressors (main effects hypothesis [25,29])
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