Facultative paternal investment in the polyphenic beetle Onthophagus taurus: the role of male morphology and social context

Members of a population often differ significantly in their parental invest ment. Such variation is generally believed to have important consequences f or mating system evolution and has been suggested to play an important role in the evolution of some secondary sexual traits and displays. Recent stud ies suggest that individuals are able to adjust the intensity and kind of p arental investment they provide according to the breeding conditions they e ncounter. As a consequence, between-individual variation in parental invest ment may depend more on external conditions than previously thought for the se taxa. This may have important implications for current perspectives on t he role of differential parental investment in the evolution and maintenanc e of certain mating systems and sexual selection regimes. Here I quantify p atterns of variation in paternal investment as a function of social conditi ons in a species of beetle that is dimorphic for male horn morphology. I de monstrate that under certain conditions (namely, the absence of other males ), paternal assistance covaries with male morphology, with horned males inv esting substantially more time in assisting females than hornless males. I also show that the magnitude of differences in paternal investment between male morphs varies in response to external conditions. In the presence of o ther males, paternal assistance was negligible for both male morphs, who in stead invested substantially and equally in mate-securing behaviors. I use my findings to discuss the significance of variation in paternal assistance for onthophagine mating systems and evaluate ideas proposed to explain the evolution of alternative morphologies in the genus Onthophagus.