Group membership decisions of N. pulcher
reflect the capacity to discriminate and preferentially associate with familiar over unfamiliar conspecifics. Focal helpers preferred to associate with familiar individuals, a preference that may ameliorate aggression between group members, or facilitate advantageous reciprocal behaviours among familiar conspecifics (Croft et al. 2005
). However, focal helpers showed no preference for breeders or helpers from their original group, suggesting that either cues used to distinguish familiar individuals are shared by helpers and breeders, or helpers have no preference for associating with particular group members.
When choosing among unfamiliar groups, we expected individuals to base group-living decisions on their probability of attaining breeding status (Buston 2004
). However, N. pulcher
helpers did not preferentially join groups in which they would be larger than resident helpers and thereby increase their social position. Although hierarchies in some other social species function as strict queues (Buston 2004
), breeding vacancies are not always inherited by existing group members in N. pulcher
, occasionally being filled by conspecifics outside the group (Balshine-Earn et al. 1998
; Stiver et al. 2007
). Neolamprologus pulcher
subordinates may therefore be under weak selective pressure to join groups based on social rank relative to other social fishes, and instead base grouping decisions on other fitness currencies. Neolamprologus pulcher
subordinates showed a preference for joining groups containing larger, dominant helpers, despite initially increased aggressive interactions with these individuals. A preference for groups with larger helpers may indicate that factors besides the improvement of rank, such as relative predation risk in different groups (Heg et al. 2004
), may be of greater importance in group joining decisions in high predation environments such as Lake Tanganyika (Taborsky & Limberger 1981
). Although a preference for joining groups with large helpers resulted in focal helpers receiving more aggression within groups, the protection from predation offered by associating with larger group members (Heg et al. 2004
) may be sufficient to offset the increased aggression and lower social position within these groups. We did not detect a preference for groups of small or large helpers in the first 20 min after introduction, but observed a preference for groups with large helpers 3 h after introduction. This suggests that helpers may need more time to assess group characteristics and distinguish between demographically different groups (Doligez et al. 2002
), and is consistent with previous studies showing that helpers visit the same groups many times before finally joining them (Bergmüller et al. 2005
The pattern of aggression shown by existing group members provides insight into the internal mechanisms of group formation. Large helpers were significantly more aggressive to joining focal fish than were breeders or small helpers, suggesting that aggressive behaviours were not simply a territorial response shared equally among all group members. Rather, the observed pattern of aggression indicates that different group members have divergent interests concerning group augmentation and intra-group competition. For the breeders, acceptance of smaller subordinate helpers is likely to confer benefits associated with increased helping (Taborsky 1984
). For small helpers, acceptance of larger group members may increase survival prospects by providing better territory defence against predators (Heg et al. 2004
) or simply reflect an inability to prevent joining by larger conspecifics. In contrast, large resident helpers presumably will not immediately share the benefits of additional brood care provided by additional helpers, and may perceive medium-sized helpers only as competitors. These divergent reactions suggest that members of N. pulcher
groups in different social positions use contrasting currencies when evaluating membership options, and suggest that conflict between group members is an important factor shaping the structure and dynamics of animal groups.
We demonstrate that benefits of familiarity and association with large individuals influence group-living decisions in N. pulcher
. Because members of N. pulcher
groups are often related (Stiver et al. 2005
), familiarity and kinship may be interlinked, and recognition of familiar individuals may provide a mechanism for the evolution of kin-selected behaviours. Hence, the combination of kin- and familiarity-associated benefits may outweigh the benefits of grouping decisions based solely on the social rank. Further research into the interaction between direct and indirect benefits of grouping behaviour will clarify the relationships between familiarity, kinship and social evolution.