Territorial aggression in male bower-building cichlids Nyassachromis cf. microcephalus is disrupted by exposure to environmental concentrations of fluoxetine

Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed drugs for treating human depression, but the role of serotonin—often called the happy chemical—is complex, associated with the regulation of appetite, anxiety, aggression, and more. With such psychoactive pharmaceuticals being increasingly detected in aquatic environments and their effects on non-target species an uncertainty, this study seeks to investigate how inhibiting the serotonin pathway with the SSRI fluoxetine affects territorial aggression, using the cichlid fish Nyassachromis cf. microcephalus as a model. Males of this sand-dwelling Lake Malawi species build bowers to attract females within a lekking system, where male-male competition is intense. Being aggressive in defending against intruders would serve to maximise mating opportunities and reproductive success for the territory holding male. However, after a one-week exposure to fluoxetine at environmentally relevant concentrations, a decline in aggression was observed in the male cichlids. This implies the serotonergic system plays an important role in modulating aggression and therefore sexual selection in cichlid fishes, and that environmental pollution has the potential to disrupt these behaviours. ### Competing Interest Statement The authors have declared no competing interest.