Julia Ostner and Oliver Schülke

Social bonds, here defined as strong, equitable and enduring social relationships, increase fitness in both male and female primates irrespective of their dispersal regime. Despite the benefits they carry for some, social bonds evolved more often among female than among male primates which is thought to be caused by the unsharable nature of males’ limiting resource, fertilizations. Here we present a structured review of variation in primate male social relationships, mating systems, and social organization. In addition to classical socio-ecological reasoning and recent models on the evolution of male coalitions, we consider the phylogenetic history of species living in multi-male groups and alternative evolutionary routes to male co-residency, which may constrain the evolution of male social bonds in some cases. We summarize our results in a conceptual framework that represents the effects of male contest competition within and between groups on male social organization, affiliation and cooperation. We conclude that male social bonds evolved as long-term alliances that gain their adaptive function in within group contests and, thus, that the evolution of male social bonds is driven by variation in within group contest competition. Between group contest competition may select for large male group size but in the end it is the narrow window of medium to low within group contest competition that promotes the evolution of political coalitions and thus is responsible for the rarity of social bonds among primate males.

Julia Ostner, Andreas Berghänel and Oliver Schülke

Abstract

Dyadic agonistic dominance relationships are thought to result from asymmetries in both intrinsic and extrinsic power. One form of extrinsic power is the ability to solicit agonistic support from other individuals. In extreme cases extrinsic power differences may override intrinsic power differences so that physically inferior individuals attain rank positions above stronger competitors. In other cases superior extrinsic power in physically inferior individuals may destabilize the otherwise clear dominance relationships. We tested this prediction with observational data on adult males in one of three free-ranging groups of Barbary macaques at Affenberg Salem, Germany. All prime males that were subjects of this study were at least 5–8 years (average 10 years) younger than the old post-prime males that were all subordinate to them. Assuming large age differences to reflect large intrinsic/physical power differences, interactions between these prime and the old males allowed investigation of the separate effects of intrinsic and extrinsic power asymmetries on dominance relationships. We estimated relationship instability using four different estimates (counter aggression, conflicts initiated by aggression from the subordinate, spontaneous submissions by dominants, and decided conflicts won by the subordinate). Relationship instability did not decrease with increasing asymmetry in intrinsic power. Instead, all four measures of relationship instability were positively related to the number of times the dominant in a dyad was target of destabilizing coalitions. Destabilizing coalitions targeted dominant males in old male–prime male dyads more often than in old male–old male dyads. Consequently, old males had less stable relationships with the much stronger prime males than with each other. We conclude that extrinsic power asymmetries predicted the instability of dyadic dominance relationships and may systematically override intrinsic power asymmetries.

Christopher Young, Oliver Schülke and Julia Ostner

In group living animals, there is pronounced variation in the formation and function of cooperation between males via coalitionary aggression. Pandit, van Schaik and colleagues developed a mathematical model to predict the evolution of different coalition types in group-living male primates, the PvS model. Coalitions are classified into five types dependent on the ranks of the participants and the function of the aggression. The main factor determining the coalition types expected to evolve is contest potential, an estimate of female monopolisability by individual males. We examined the model using groups of Assamese (Macaca assamensis) and Barbary macaques (Macaca sylvanus) to gain a full range of contest potentials. We observed, across groups, 393 coalitions during 3645 h of data collection. We measured contest potential on a species-specific basis dependent on the information males can infer about female reproductive state. By examining coalition formation in different populations and species, but also in the same groups over time, we showed the strengths and weaknesses of the PvS model. We discuss why our results do not fully fit the model’s predictions, including differing costs/benefits of coalition formation, such as delayed benefits via increased status, making rank-changing coalitions viable at mid–low contest potential. Alternative factors not considered by the model include the effect of male social bonds on coalition partner choice and the effect of female mate-choice on coalition target selection. Finally, we suggest possible improvements to the model and provide information on how best to test the current predictions of the PvS model.