Chelicerae as male grasping organs in scorpions: sexual dimorphism and associated behaviour

Specialised structures that enable males to grasp females during sexual interactions are highly susceptible to selection and thus diverge relatively rapidly over evolutionary time. These structures are often used to test hypotheses regarding sexual selection such as sexually antagonistic co-evolution and sexual selection by female choice. In the present study, we determine whether there is a relationship between a novel record of scorpion sexual dimorphism, the sexual dimorphism of chelicerae (CSD), and the presence of the mating behaviour termed cheliceral grip (CG). The presence of both traits in the order Scorpiones is also reviewed from a phylogenetic perspective. The results confirm a strong relationship between CSD and the presence of CG. The morphological and behavioural patterns associated with CSD-CG are opposed to the predictions postulated by the hypothesis of sexually antagonistic co-evolution. However, if the female shows resistance after the deposition of the spermatophore, the possibility that the male exerts pressure as a cryptic form of coercion to prevent the interruption of mating cannot be ruled out completely. Female choice by mechanical fit could be another explanation for some aspects of the CG's contact zone. The possibility that the CG-CSD complex has evolved under natural selection in order to ensure sperm transfer is also considered. (C) 2009 Elsevier GmbH. All rights reserved.