ENVIRONMENTAL-CONTROL OF HORN LENGTH DIMORPHISM IN THE BEETLE ONTHOPHAGUS-ACUMINATUS (COLEOPTERA, SCARABAEIDAE)

Impressive examples of male ornamentation occur in horned beetles. Many beetle species are characterized by substantial amounts of phenotypic variation in horn length, and in some species this variation is biomodally distributed so that males may be separated into two groups on the basis of horn length. Two discrete male morphs are present in natural populations of Onthophagus acuminatus (Coleoptera: Scarabaeidae), a dung beetle common to lowland tropical forests of Panama. Large males possess a pair of frontal horns, which in small males are greatly reduced in length or are lacking. This paper presents results from experiments designed to assess the relative importance of genetic and environmental factors as determinants of male horn morphology. Experimental manipulation of food quantity, a factor known to influence body size, showed that male horn length variation in O. acuminalus was influenced primarily by environmental factors. Horn lengths of male progeny were a function of individual differences in body size (the manipulated variable) and not of the horn lengths of their fathers, in both experimental and control populations. These results support recent theories on sexual selection which predict that male ornaments will evolve to be reliable indicators of male quality. The utility of incorporating studies of developmental mechanism into analyses of morphological evolution is discussed.