Hormonal control of male horn length dimorphism in the dung beetle Onthophagus taurus (Coleoptera : Scarabaeidae)

Male dung beetles (Onthaphagus taurus) facultatively produce a pair of horn s that extend from the base of the head: males growing larger than a thresh old body size develop long horns, whereas males that do not achieve this si ze grow only rudimentary hems or no hems at all. Here we characterize the p ostembryonic development of these beetles, and begin to explore the hormona l regulation of horn growth. Using radioimmune assays to compare the ecdyst eroid titers of horned males, hornless males, and females, we identify a sm all pulse of ecdysteroid which is present in both hornless males and female s, but not in horned males. In addition, we identify a brief period near th e end of the final (third) larval instar when topical applications of the j uvenile hormone analog methoprene can switch the morphology of developing m ales. Small, normally hornless, males receiving methoprene during this sens itive period were induced to produce horns in 80% of the cases. We summariz e this information in two models for the hormonal control of male dimorphis m in horn length. (C) 1998 Elsevier Science Ltd. All rights reserved.