Out-Group Threat Promotes Within-Group Affiliation in a Cooperative Fish

Abstract

In social species, conflict with outsiders is predicted to affect within-group interactions and thus influence group dynamics and the evolution and maintenance of sociality. Although empirical evidence exists for a relationship between out-group conflict and intragroup behavior in humans, experimental tests in other animals are rare. In a model fish system, we show that simulated out-group intrusions cause postconflict increases in intragroup affiliation but no changes in postconflict intragroup aggression. Postconflict affiliation was greater following intrusions by neighboring compared with nonneighboring individuals; neighbors represent greater threats to the dominance rank and breeding success of residents, and they are visible in the aftermath of the intrusion. By providing strong evidence of a link between out-group conflict and postconflict intragroup behavior and demonstrating that intragroup affiliation is affected by the nature of the out-group intrusion, our study shows the importance of considering postconflict behavior for our understanding of cooperation and social structure.

Within-group behavioural consequences of between-group conflict: a prospective review

“…Female bonnet macaques groomed and mated with males that had participated more in recent between-group conflict [56], which suggests that they might have been rewarding combatants. Experimental manipulations inducing aggressive interactions between focal groups and single out-group individuals led to post-conflict increases in within-group affiliation in cooperatively breeding cichlid fish ( Neolamprologus pulcher [25]) and Wied's black tufted-ear marmosets ( Callithrix kuhli [74]); in the latter study, there was a greater increase in larger compared with smaller groups. Post-conflict within-group allo-preening increased in the green woodhoopoe ( Phoeniculus purpureus ), a cooperatively breeding bird in which all group members participate in between-group conflicts and are thus combatants [23,37].…”

“…A playback experiment on green woodhoopoes demonstrated a greater increase in within-group allo-preening following simulated territorial intrusions by unfamiliar groups compared to neighbours [37]; while neighbours probably only intrude temporarily, unfamiliar groups may usurp residents permanently. By contrast, there was a greater increase in post-conflict affiliation by N. pulcher (cichlid fish) group members following simulated intrusions by neighbours than strangers [25]; in this species, neighbouring individuals are more likely than unfamiliar individuals to take over breeding or dominance positions.…”

Expanding the Link between Out-Group Threats and In-Group Behavior: (A Reply to Kavaliers and Choleris)

“…Individuals or same-sex coalitions may challenge the position or breeding success of particular group members (Mares et al 2011;Bruintjes et al 2016), while groups may attempt to acquire certain resources or the whole territory of rivals (Radford 2003;Wilson and Wrangham 2003;Kitchen and Beehner 2007). As Kavaliers and Choleris (In press) point out, pathogen exposure and the risk of infection may represent an additional threat posed by outsiders.…”

“…As Kavaliers and Choleris (In press) point out, pathogen exposure and the risk of infection may represent an additional threat posed by outsiders. Stronger immediate responses might be expected to infected as opposed to healthy outsiders, in the same way that there are stronger responses to individuals or groups who represent a greater threat in other contexts -for example, differences in the relative threat posed by groups of different size (Radford and du Plessis 2004) and by neighbours and strangers (Radford 2005;Müller and Manser 2007) have been shown to be important -with knock-on consequences for in-group behaviour (Radford 2008b;Bruintjes et al 2016;Christensen et al 2016). Moreover, exposure to pathogens can influence the response to subsequent out-group threats; pathogen 'priming' can lead to more negative reactions towards outsiders (Navarette and Fessler 2006;Fincher et al 2011).…”

“…The starting premise for our work on Neolamprologus pulcher (Bruintjes et al 2016) was that outgroup threats should influence in-group behavioural interactions. However, various feedback loops are likely to be involved.…”