Socially mediated trade-offs between aggression and parental effort in competing color morphs

Individuals often face trade-offs between investment in parental care and alternative investments of time, energy, and re-sources into other life-history components, such as dominance, at-tractiveness, and health. Selection is thought to promote the optimal balance between the costs and the benefits of these conflicting ac-tivities by favoring individuals that adopt different tactics to maxi-mize their overall evolutionary fitness in different environments. To test this, we experimentally manipulate both aggression (i.e., com-petitive environment) and parental effort (i.e., brood size) in red and black morphs of the Gouldian finch {Erythrura gouldiae). Although aggressive red males provide parental effort comparable to that of black males in environments where competition is low, irrespective of their relative brood size, they severely reduce or abandon parental investment in highly competitive environments. In contrast, non-aggressive black males are largely unaffected by the competitive en-vironment and instead adaptively adjust their provisioning effort to the relative demands of their brood. Consequently, in highly com-petitive environments, although dominant red males defend higher-quality nest sites, they produce fewer and lower-quality offspring (in terms of mass and immunocompetence) than black males do. These opposing effects of frequency-dependent competitive environments on red and black males underlie their differential trade-offs between the costs and the benefits of aggression and parental effort.