The role of neighbours in aggressive defence of territories in mixed-species breeding aggregations of cichlid fish

While territorial aggression is a prerequisite for reproduction in many animals, individuals need to balance its benefits against the costs. Territorial neighbours can influence these costs and benefits, yet their role is often poorly understood, especially in neighbourhoods with heterospecifics. To address this topic, we assessed both the overall (i.e. neighbourhood-level) and species-level aggression towards an intruder in naturally formed mixed-species neighbourhoods of Nicaraguan cichlid fish. We found that while the territories were tightly packed and territory holders were likely to benefit from neighbours that are aggressive towards intruders, the burden of intruder repellence fell mainly to the territory owners closest to the intruder. Moreover, the overall aggression did not markedly increase with the number of territories, further indicating that the investment by most territory holders decreased with territory density. While some species appeared better neighbours than others in terms of their contribution to the neighbourhood-level defensive aggression, the patterns of species co-occurrence did not reveal significant species pair-specific associations. Overall, these results are consistent with the selfish herd theory and suggest that territory defence against intruders can impact the composition of the local community.