Paternal aggression towards a brood predator during parental care in wild smallmouth bass is not correlated with circulating testosterone and cortisol concentrations

Male smallmouth bass (Micropterus dolomieu) provide sole parental care including frequent aggressive actions towards conspecifics and potential brood predators. Failure to defend the brood through continual vigilance results in predation reducing the number of offspring and promoting abandonment by the nesting male. However, little is known about how biochemical and endocrine factors and brood size collectively influence paternal aggression. Behavioral assays were conducted during the egg stage of offspring development by placing a brood predator in a jar on the nest to quantify aggression (number of attacks on the potential brood predator in a minute). To determine the correlates of parental aggression, we temporarily removed fish from their nests and measured circulating levels of testosterone and indicators of the primary (plasma cortisol) and secondary stress response (plasma glucose, Cl⁻, Na⁺, K⁺) from non-lethal blood samples. While the male was removed from the nest, a snorkeler quantified the size of the brood. Brood size was positively correlated with male aggression. The only biochemical correlate of parental aggression was plasma glucose, which also had a positive relationship with brood size. When the effect of brood size was removed, no biochemical or endocrine factors were predictive of male aggression. Hence, brood value appeared to influence parental aggression independent of biochemical or endocrine status. While several-fold individual differences in aggression towards brood predators were noted, the role of androgens and glucocorticoids in mediating these behaviors is currently not well understood.