Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera : Scarabaeidae)

The existence of discrete phenotypic variation within one sex poses interes ting questions regarding how such intrasexual polymorphisms are produced an d modified during the course of evolution. Approaching these kinds of quest ions requires insights into the genetic architecture underlying a polymorph ism and an understanding of the proximate mechanisms determining phenotype expression. Here we explore the genetic underpinnings and proximate factors influencing the expression of beetle horns - a dramatic sexually selected trait exhibiting intramale dimorphism in many species. Two relatively discr ete male morphs are present in natural populations of the dung beetle Ontho phagus taurus (Scarabaeidae, Onthophagini). Males exceeding a critical body size develop a pair of long, curved horns on their heads, while those smal ler than this critical body size remain essentially hornless. We present re sults from laboratory breeding experiments designed to assess the relative importance of inherited and environmental factors as determinants oi male m orphology. Using father-son regressions, our findings demonstrate that horn length and body size of male progeny are not predicted from paternal morph ology. Instead, natural variation in an environmental factor, the amount of food available to larvae, determined both the body sizes exhibited by male s as adults and the presence or absence of horns. The nonlinear scaling rel ationship between the body size and horn length of males bred in the labora tory did not differ from the pattern of variation present in natural popula tions, suggesting that nutritional conditions account for variation in male morphology in natural populations as well. We discuss our results by exten ding ideas proposed to explain the evolution of conditional expression of a lternative phenotypes in physically heterogeneous environments toward incor porating facultative expression of secondary sexual traits. We use this syn thesis to begin characterizing the potential origin and subsequent evolutio n of facultative horn expression in onthophagine beetles.