Male-male competition and large size mating advantage in European earwigs,Forficula auricularia

European earwigs are sexually dimorphic in forceps shape and length. Male f orceps are thought to be weapons in male contests for access to females, bu t recent findings suggest that females choose males on the basis of their f orceps length. I investigated sexual selection on forceps length and body s ize and the occurrence of male-male competition. When I controlled for forc eps length experimentally and statistically, relatively heavy males had gre ater copulation success than relatively light males. When I controlled for body size, males with relatively longer forceps had no tendency for greater copulation success than males with shorter forceps. Relatively heavy males more often took over copulations from smaller males than vice versa. Male contests were important for the outcome of mate competition, as males commo nly interrupted and took over copulations. My results therefore suggest tha t intrasexual selection is significant in competition for copulations in ma le earwigs, and acts on body size. This contrasts with previous findings, w hich have shown intersexual selection on forceps length to be important. Ho wever, both modes of sexual selection may be acting through a two-stage pro cess, where male-male competition first determines which males have access to females, and then through female choice among available males. Morpholog ical measurements supported the conclusion that forceps length and body siz e are male secondary sexual characters, as these characters had large varia nce and skewed distributions in males, but were normally distributed in fem ales. (C) 2000 The Association for the Study of Animal Behaviour.