Experimentally-induced variation in neuroendocrine processes affects male reproductive behavior, sperm characteristics, and social interactions

While extensive research has focused on how social interactions evolve, the fitness consequences of the neuroendocrine mechanisms underlying these interactions have rarely been documented, especially in the wild. Here, we measure how the neuroendocrine mechanisms underlying male behavior affecting mating success and sperm competition in the ocellated wrasse (Symphodus ocellatus). In this species, males exhibit three alternative reproductive types. ‘Nesting males’ provide parental care, defend territories, and form cooperative associations with unrelated ‘satellites’, who cheat by sneaking fertilizations but help by reducing sperm competition from ‘sneakers’ who do not cooperate or provide care. To measure the fitness consequences of the mechanisms underlying these social interactions, we used “phenotypic engineering” that involved administering an androgen receptor antagonist (flutamide) to wild, free-living fish. Nesting males treated with flutamide shifted their aggression from sneake...