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 strikingly different morphologies in response to environmental conditions encountered during development. Such polyphenisms have received particular attention from evolutionary biologists because the condition-dependent expression of alternative morphologies is believed to reflect the existence of discrete sets of adaptations to heterogeneous ecological or social conditions, which preclude the evolution of a single, optimal phenotype. Correct interpretation of the adaptive significance, if any, of facultative trait development requires a solid understanding of the determinative regime governing morph expression. Here I explore the environmental variables determining male morphology in the horn-dimorphic beetle Onthophagus taurus. I demonstrate that natural variation in both the quantity and quality of food that larvae receive from their parents determines body size in males and females, and, by means of a threshold response, the presence or absence of horns in males. In addition, results suggest that parent beetles adjust the amount of food they provision for their offspring according to diet quality, which may help to compensate for environmental variation induced by differential resource quality in the wild. I use these results to further characterize the selective regime responsible for the evolution of male polyphenism in onthophagine beetles and discuss its significance for understanding the origin and maintenance of morphological variation in the genus Onthophagus.