Repeatable glucocorticoid expression is associated with behavioural syndromes in males but not females in a wild primate.

Behavioural syndromes are a well-established phenomenon in human and non-human animal behavioural ecology. However, the mechanisms that lead to correlations among behaviours and individual consistency in their expression at the apparent expense of behavioural plasticity remain unclear. The 'state-dependent' hypothesis posits that inter-individual variation in behaviour arises from inter-individual variation in state and that the relative stability of these states within an individual leads to consistency of behaviour. The endocrine stress response, in part mediated by glucocorticoids (GCs), is a proposed behavioural syndrome-associated state as GC levels are linked to an individual's behavioural responses to stressors. In this study, in wild Barbary macaques (Macaca sylvanus), consistent inter-individual differences were observed in both sexes for GC activity (faecal glucocorticoid, fGC concentrations), but not GC variation (coefficient of variation in fGC concentrations). The expression of the behavioural syndrome 'Excitability' (characterized by the frequencies of brief affiliation or aggressive interactions) was related to GC activity in males but not in females; more 'excitable' males had lower GC activity. There was no relationship in females between any of the behavioural syndromes and GC activity, nor in either sex with GC variation. The negative relationship between GC activity and Excitability in males provides some support for GC expression as a behavioural syndrome-generating state under the state-dependent framework. The absence of this relationship in females highlights that state-behavioural syndrome associations may not be generalizable within a species and that broader sex differences in state need to be considered for understanding the emergence and maintenance of behavioural syndromes.

Consistency of dominance rank order: a comparison of David's Scores with I&SI and Bayesian methods in macaques.

In nonhuman primate social groups, dominance ranks are usually assigned to individuals based on outcomes of dyadic agonistic encounters. Multiple approaches have been used, but currently there is no consensus. One approach, David's Scores (DS), offers dual advantages of yielding cardinal scores that may in turn be used to compute hierarchical steepness. Here we correlate rank orders yielded by DS with those yielded by both the traditionally used I&SI approach and the recently proposed parametric Bayesian approach. We use six datasets for female macaques (three despotic and three tolerant groups), and 90 artificially generated datasets modeling macaque groups. We also use the artificial datasets to determine the impact of three characteristics (group size, interaction frequency, and directional asymmetry of aggression) on the magnitude of correlation coefficients, and assess the relative utility of two indices used to compute DS: Dij versus Pij. DS-based rank orders were strongly positively correlated with those yielded by the other two approaches for five out of the six macaque datasets, and for the majority of artificial datasets. Magnitudes of correlation coefficients were unrelated to group size or interaction frequency, but increased with directional asymmetry, suggesting methodological inconsistencies were more likely when dyads had more frequent reversals in directions of aggression. Finally, rank orders calculated using the Dij and Pij indices were similarly consistent with orders from other methods. We conclude that DS offers consistent estimates of rank orders, except perhaps in groups with very low levels of aggression asymmetry. In such "tolerant" groups, we suggest that the relatively greater methodological variability in rank orders may reflect behavioral characteristics of tolerant groups rather than computational inconsistencies between methods. We hypothesize that this quality may be quantified using posterior probability scores of Bayesian rank orders and may also index macaque social styles.

Fitness-related benefits of dominance in primates.

Dominance hierarchies are thought to provide various fitness-related benefits to dominant individuals (e.g., preferential access to food or mating partners). Remarkably, however, different studies on this topic have produced contradictory results, with some showing strong positive association between rank and fitness (i.e., dominants gain benefits over subordinates), others weak associations, and some others even revealing negative associations. Here, we investigate dominance-related benefits across primate species while controlling for phylogenetic effects. We extracted data from 94 published studies, representing 25 primate species (2 lemur species, 4 New World monkeys, 16 Old World monkeys, and 3 apes), to assess how dominance affects life-history and behavior. We used standard and phylogenetic meta-analyses to analyze the benefits of dominance in primates. Dominant females had higher infant survival to first year, although we found no significant effect of dominance on female feeding success. Results for female fecundity differed between the two meta-analytical approaches, with no effect of dominance on female fecundity after controlling for phylogeny. Dominant males had a higher fecundity and mating success than subordinate males. Finally, the benefits of dominance for female fecundity were stronger in species with a longer lifespan. Our study supports the view that dominance hierarchies are a key aspect of primate societies as they indeed provide a number of fitness-related benefits to individuals.