EMLEN, Douglas/J; University of Montana: Endocrine regulation of beetle horn expression: insights for horn evolution

Males of many beetle species develop elaborate weapons called horns. In dung beetles (Onthophagus; Coleoptera: Scarabaeidae), expression of horns is regulated by a threshold: males growing larger than a critical body size produce a pair of full-length horns, and males not attaining this critical size do not (they produce rudimentary horns or no horns at all). Artificial selection experiments and comparative studies both suggest that this developmental threshold can evolve rapidly. Yet, appreciating HOW such genetic changes are brought about requires knowledge of the physiological processes underlying polyphenic expression of horns. Here I summarize our understanding of the endocrine regulation of beetle horn expression, and use this information to identify likely avenues for horn evolution.