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Abstract Group members do not always act cohesively when facing extra-group rivals. When benefits such as group-defence are not monopolizable, it poses an economics problem: who should contribute to public goods and who should freeload? A collective action framework compliments existing theoretical explanations for cooperation, and provides testable hypotheses about group-level behaviour based on individual costs and benefits. Using this approach, we review research on intergroup encounters in non-human primates published over the last 20 years, focusing on participation by different classes of individuals. While food- and mate-defence explain much between-sex variation in participation, rank and reproductive access frequently explain within-sex variation. In some species, individuals may use intergroup interactions to survey potential transfer locations and mating options, which might coincidently intimidate rivals. Experimental evidence suggests that when intergroup dominance is based on relative number of fighters, individual participation still varies with sex, rank, companion behaviour and dependent offspring presence. Relatively few studies have examined how factors such as relationships within and between groups or individual temperament mediate aggression. Long-term studies of multiple habituated groups and methodological advances (e.g., playback experiments) will continue to improve our understanding of how complex group-level patterns are predictable when viewed from an individual perspective.

When assessing a group's overall fighting ability, functional group size (i. e. the number of cooperative members willing to confront opponents) may be more important than actual group size. Despite obvious benefits, group members do not always act in a collective manner. For example, participation by subordinate male black howler monkeys (Alouatta pigra) in natural group-level contests is highly variable. Using playback experiments, we examined whether alpha-subordinate relationships, number of intruders, or age affected subordinate contest participation. Long-term census data were used to estimate the nature and duration of intra-group male associations. Some subordinate males had long-term relationships with the alpha that existed prior to residence in the current group or they were the alpha male's grown sons. Other subordinate males were in more recent associations with the alpha that formed under seemingly antagonistic circumstances (e. g. the deposed alpha or his grown sons). We found that subordinate males in long-term alpha-subordinate associations had stronger howling and approach responses than males in short-term associations. Younger long-term associates had the strongest reactions to simulated intruders, similarly aged shortterm associates rarely responded, and older males in both association categories responded at intermediate levels. We discuss the variable strategies of subordinate male black howlers and suggest that males who were more likely to participate in group-defence might be gaining direct or indirect fitness benefits by group living.

Inter-group encounters among baboons range from peaceful to aggressive. During 23 months we observed 110 inter-group interactions involving four groups of chacma baboons in the Okavango Delta, Botswana. Results supported the hypothesis that male behavior functions to prevent extra-group males from gaining access to sexually receptive females. Males were more likely to chase females in their own group when estrous females were present, and their chases targeted estrous females more often than expected. Males also chased members of the opposing group more when estrous females were present. When estrous females were absent, male displays were shorter in duration, involved fewer participants, were less intense, and were more likely to result in peaceful mingling between groups. The alpha male was the individual most actively involved in inter-group chases and displays, but males of all ranks participated, especially when they were in consort with a female. However, males did not cooperate in group defense. While behavior during encounters was affected by the presence of estrous females, the outcome of encounters was affected by location and rival group identity. Groups were more likely to approach and displace opponents in the core of their range and more likely to retreat in the periphery. Correcting for location, we also found some evidence for an inter-group dominance hierarchy based on the relative number of males.