Sexual dimorphism, fighting success and mating tactics of male Onymacris plana Peringuey (Coleoptera : Tenebrionidae) in the Namib desert

Sexually dimorphic characters of Onymacris plana, a dune-living, solitary t enebrionid beetle of the Namib Desert, were tested for their roles in male- male fighting over females. Males were smaller than females but had extraor dinarily wide elytra, with great variance in this characteristic. In males, but not in females, elytra width increased with body length at an allometr ic scale. Male beetles were often aggressive towards each other, especially when mating or guarding females after mating or waiting for females at sha dy spots. Interactions were less intense when contesting over females on th e open surface, where these fast-running beetles often overran each other i n their attempts to retain their positions behind females until the females retreated into the sand, where mating took place. Winners of intrasexual f ights and the successful mates of females tended to have longer bodies and wider elytra than the losers. Sexual selection appears to be the best expla nation of the allometric scaling of the lateral extensions of male elytra. Sexual selection may furthermore contribute to other characteristics, such as large body length and long legs, that have ecological and ecophysiologic al significance.