Members of a population often differ significantly in their parental investment. Such variation is generally believed to have important consequences for mating system evolution and has been suggested to play an important role in the evolution of some secondary sexual traits and displays. Recent studies suggest that individuals are able to adjust the intensity and kind of parental investment they provide according to the breeding conditions they encounter. As a consequence, between-individual variation in parental investment may depend more on external conditions than previously thought for these taxa. This may have important implications for current perspectives on the role of differential parental investment in the evolution and maintenance of certain mating systems and sexual selection regimes. Here I quantify patterns of variation in paternal investment as a function of social conditions in a species of beetle that is dimorphic for male horn morphology. I demonstrate that under certain conditions (namely, the absence of other males), paternal assistance covaries with male morphology, with horned males investing 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 other males, paternal assistance was negligible for both male morphs, who instead 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.
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