Conspecific alarm cues affect interspecific aggression in cichlid fishes

Individuals have to respond simultaneously to different environmental factors often making trade-offs between conflicting demands necessary. Many freshwater ecosystems are resource-limited and both intra- and interspecific competitiveness is a common requirement to gain and defend resources necessary for reproduction. Although predation risk is an important selective force affecting behavioral decisions, little is known about the impact of predation risk on interspecific competition. Here, we investigate whether chemically mediated predation risk affects interspecific territorial aggression by the freshwater cichlid Pelvicachromis taeniatus. In our experiments, territorial P. taeniatus males were visually confronted with a territorial intruder: a heterospecific, sympatric cichlid (Benitochromis nigrodorsalis) which generally induced aggression in P. taeniatus. Predation risk for P. taeniatus was simulated by a concurrent release of conspecific chemical alarm cues. In control treatments, no chemical cues, dissolved heterospecific alarm cues, or aliquots of distilled water were provided during these aggressive encounters. The results show that interspecific aggression of territorial male P. taeniatus is significantly decreased under predation risk compared to the control treatments. This suggests that interspecific competition becomes less intense under concurrent predation risk. As this process could hinder competitive exclusion, predation risk may indirectly promote and stabilize biodiversity in natural ecosystems.