Group structure in a restricted entry system is mediated by both resident and joiner preferences
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The benefits of grouping behaviour may not be equally distributed across all individuals within a group, leading to conflict over group membership among established group members, and between residents and outsiders attempting to join a group. Although the interaction between the preferences of joining individuals and existing group members may exert considerable pressure on group structure, empirical work on group living to date has focussed on free entry groups, in which all individuals are permitted entry. Using the humbug damselfish, Dascyllus aruanus, we examined a restricted entry grouping system, in which group residents control membership by aggressively rejecting potential new members. We found that the preferences shown by joining members were not always aligned with strategies that incurred the least harm from resident group members, suggesting a conflict between the preferences of residents and preferences of group joiners. Solitary fish preferred to join familiar groups and groups of size-matched residents. Residents were less aggressive towards familiar group joiners. However, resident aggression towards unfamiliar individuals depended on the size of the joining individual, the size of the resident and the composition of the group. These results demonstrate that animal group structure is mediated by both the preferences of joining individuals and the preferences of residents.
KeywordsGroup living Social organisation Dascyllus aruanus Membership preferences
We wish to thank Prof. David Booth for advice on the biology and behaviour of Dascyllus spp., and Kylie, Dave, Russ and Jen at One Tree Island Research Station for their assistance in the field. We also thank two anonymous reviewers for comments that greatly improved the manuscript. LAJ, JEHR, CA, DIR and AJWW were supported by funds supplied by University of Sydney. DIR was also supported by the Class of 1877 Research Fund. Australian ethics approval for this study was granted by the University of Sydney’s Animal Ethics Committee (L04/9-2008/1/4877). After experiments were completed, fishes were returned to where they were caught. Fishes were kept in captivity for a maximum of 4 days.
Conflict of interest
The authors declare that they have no conflict of interest.
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