Agonistic support towards victims buffers aggression in geladas (Theropithecus gelada)

Agonistic support occurs when a bystander intervenes in an ongoing conflict. The consequences of agonistic support may differ when provided to victims or aggressors. Supporting victims may not only protect them, but also limit the escalation of aggression among group members. Our results onTheropithecus geladashowed that support was preferentially directed towards victims and high-ranking individuals provided the highest levels of support. Whereas the support towards the aggressor had no effect in reducing its renewed aggression, it increased the frequency of subsequent conflicts among fellows. The support towards victims significantly reduced subsequent aggression both towards the victim and among other group members. The support was biased towards victims who were unrelated and shared weak bonds with the aggressors. In conclusion, victim support may be a social tool, which intervenes when other mechanisms are less likely to occur such as the case when the opponents are not kin or friends.

The roles of food quality and sex in chimpanzee sharing behavior (Pan troglodytes)

Both wild and captive chimpanzees (Pan troglodytes) share food with non-relatives. Researchers have proposed several hypotheses to explain this behavior, including ‘food for sex’, ‘food for grooming or agonistic support’, and ‘sharing under pressure’. We examined food sharing in two captive groups of socially-housed chimpanzees. In contrast to previous captive studies, which only examined transfers of low-quality foods, we conducted seven trials with high-quality food and seven with low-quality food for each group to directly compare transfers of different food qualities. We recorded how male chimpanzees shared food, including active transfers, passive transfers, and co-feeding. We also noted all instances of copulations, female estrous states, benign attempts to access food (termed ‘perseverance’), and aggressive attempts (termed ‘harassment’) to examine whether any of these factors influenced food sharing. Male food possessors shared at the same rate in both food quality conditions, but seemingly for different reasons, indicating that food quality may affect the exchange of social benefits in chimpanzees. In the low-quality condition, there was an interaction with rank and perseverance: while low- and middle-ranking females received more food the more they persevered, high-ranking females received more food without perseverance and gained relatively little benefit from persevering. In the high-quality condition, there was an interaction between copulations and perseverance: females who copulated with the male food possessor received more food during that trial with less perseverance. Non-copulating females received more transfers the more they persevered. This result was only observed in the short-term — copulations over the previous year were not correlated with food transfers. Further, the copulations observed here were unusual for these chimpanzees in that they were not confined to peak fertility, suggesting a non-conceptive function for copulations in chimpanzees. Copulations in this study may have functioned to reduce tension and increase short-term tolerance, allowing females better access to food.

Coalitions destabilize dyadic dominance relationships in male Barbary macaques (Macaca sylvanus)

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.

Preparing for battle? Potential intergroup conflict promotes current intragroup affiliation

Groups of human soldiers increase their affiliative behaviour when moving into combat zones. Despite numerous other species also competing as groups, little is known about how potential intergroup conflict might influence current intragroup affiliative behaviour in non-human animals. Here, I show that allopreening (when one individual preens another) increases in groups of cooperatively breeding green woodhoopoes ( Phoeniculus purpureus ) when they enter areas where conflicts with neighbours are more likely. Self-preening, which is an indicator of stress in other species, did not increase in conflict areas, suggesting that the change in affiliative behaviour is not the simple consequence of greater stress. Instead, because it is the dominant breeding pair that increase their preening of subordinate helpers, it is possible that current affiliative behaviour is being exchanged for agonistic support in any intergroup conflicts that might ensue. These results are important for our understanding of group dynamics, cooperation and the evolution of sociality, but also bring to mind the intriguing possibilities of social contracts and future planning in birds.

Grooming reciprocation among female primates: a meta-analysis

The theory of reciprocal altruism offers an explanation for the evolution of altruistic behaviours among unrelated animals. Among primates, grooming is one of the most common altruistic behaviours. Primates have been suggested to exchange grooming both for itself and for rank-related benefits. While previous meta-analyses have shown that they direct their grooming up the hierarchy and exchange it for agonistic support, no comprehensive evaluation of grooming reciprocation has been made. Here we report on a meta-analysis of grooming reciprocation among female primates based on 48 social groups belonging to 22 different species and 12 genera. The results of this meta-analysis showed that female primates groom preferentially those group mates that groom them most. To the extent allowed by the availability of kinship data, this result holds true when controlling for maternal kinship. These results, together with previous findings, suggest that primates are indeed able to exchange grooming both for itself and for different rank-related benefits.