Horn polyphenism in the beetle Onthophagus taurus: larval diet quality and plasticity in parental investment determine adult body size and male horn morphology

In a wide range of taxa, individuals are able to express strikinglydifferent morphologies in response to environmental conditionsencountered during development. Such polyphenisms have receivedparticular attention from evolutionary biologists because thecondition-dependent expression of alternative morphologies isbelieved to reflect the existence of discrete sets of adaptationsto heterogeneous ecological or social conditions, which precludethe evolution of a single, optimal phenotype. Correct interpretationof the adaptive significance, if any, of facultative trait developmentrequires a solid understanding of the determinative regime governingmorph expression. Here I explore the environmental variablesdetermining male morphology in the horn-dimorphic beetle Onthophagustaurus. I demonstrate that natural variation in both the quantityand quality of food that larvae receive from their parents determinesbody size in males and females, and, by means of a thresholdresponse, the presence or absence of horns in males. In addition,results suggest that parent beetles adjust the amount of foodthey provision for their offspring according to diet quality,which may help to compensate for environmental variation inducedby differential resource quality in the wild. I use these resultsto further characterize the selective regime responsible forthe evolution of male polyphenism in onthophagine beetles anddiscuss its significance for understanding the origin and maintenanceof morphological variation in the genus Onthophagus.