The evolution of sexually dimorphic earwig forceps: social interactions among adults of the toothed earwig, Vostox apicedentatus

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Abstract
Earwigs (Insecta, Dermaptera) are characterized by uniquely elaborated cerci, commonly called forceps, the function of which remains unclear. We studied intrasexual and intersexual interactions in the laboratory to examine the context and pattern of forceps use in the toothed earwig. Vostox apictdenlatus (Caudell). Interactions between pairs of earwigs were recorded in four social situations: (1) two males, (2) two males plus a virgin female, (3) two females, and (4) one male and one female. Forceps were used as both weapons and display structures by males and females in all of these social contexts. During pairwise male-male interactions, one male clearly dominated the other male. Dominant males were more active and more likely to use their forceps in intrasexual interactions than were subordinate males. In interactions where there were two males and one female present, the male that dominated male-male interactions was able to maintain exclusive access to the female. There was no indication of active female choice during or after courtship. During intersexual interactions, only males used their forceps during courtship. The behavioral repertoire involving forceps was greater for males than for females, especially in intrasexual contests. There was no clear outcome of intrasexual interactions among females. These results suggest that forceps function mainly as weapons in male-male interactions and may have evolved, at least in part, as a result of sexual selection. Further research is required to test for female mate choice and to separate the various mechanisms of sexual selection if mate choice exists. Comparative studies are needed to determine if sexual selection was the original evolutionary mechanism leading to the development of these unusual structures or if sexual selection is relegated to a secondary effect, leading to the elaboration and sexualdimorphism of these structures in selected groups of earwigs.

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