Ecological constraints and benefits of philopatry promote group-living in a social but non-cooperatively breeding fish.

Why non-breeding subordinates of many animal societies tolerate group-living remains a pertinent question in evolutionary biology. The ecological constraints and benefits of philopatry hypotheses have the potential to explain the maintenance of group-living by specifying the ecological conditions favouring delayed dispersal over independent breeding by subordinates. In this study, I used field and laboratory experiments to investigate the role of ecological and social factors on the dispersal decisions of non-breeding subordinates in the coral-dwelling fish, Paragobiodon xanthosomus (Gobiidae). Subordinate dispersal was strongly influenced by ecological constraints (habitat saturation and risks of movement) and benefits of philopatry (relative coral size). Social factors, namely social rank and forcible eviction, did not affect the occurrence of subordinate dispersal. These results suggest that selection has favoured subordinate P. xanthosomus, which employ a mixed strategy-switching tactics in response to three ecological factors-despite having low mobility and extreme habitat-specific requirements. Furthermore, this study demonstrates the generality of the ecological constraints and benefits of philopatry hypotheses as explanations for group-living in species where subordinates are unrelated to breeders, provide no help and do not strictly delay dispersal.