Dominance hierarchies in group-living cleaning gobies: causes and foraging consequences

In species living in social groups, aggression among individuals to gain access to limiting resources can lead to the formation of stable social hierarchies. We tested whether dominance rank in social groups of sponge-dwelling cleaning gobies Elacatinus prochilos in Barbados was determined by physical attributes of individuals or by prior experience of dominance, and examined the foraging consequences of dominance rank. Intraspecific aggression within groups resulted in stable dominance hierarchies that were strongly correlated with fish length. Dominant individuals maintained exclusive territories while subordinate fish occupied broader home ranges. Larger, competitively dominant fish were able to monopolize areas inside the sponge lumen with the highest abundance of the polychaete Haplosyllis spp., a favoured prey item, and achieved the highest foraging rates. The removal of a territorial individual from large groups resulted in a domino-like effect in territory relocation of the remaining fish as individuals moved to the territory previously occupied by the individual just above them in the group hierarchy. Individuals added to existing groups generally failed to gain access to territories, despite being formerly dominant in their original groups. When given the opportunity to choose a location in the absence of larger competitors, gobies frequently preferred positions that were previously defended and that had abundant food. These results suggest that intraspecific competition for resources creates the observed dominance structures and provides support for the role of individual physical attributes in the formation and maintenance of dominance hierarchies.