Asymmetry and fighting performance in the shore crab Carcinus maenas

Fluctuating asymmetries (left-right differences in symmetric traits) can be negatively related to fitness parameters in a number of biological systems . Hence, it has been suggested that symmetric individuals should outcompete asymmetric individuals during intraspecific agonistic encounters. However, there is a lack of experimental evidence for such a relationship. We inves tigated the relationship between trait asymmetry (both directional and fluc tuating asymmetry) and the outcome of agonistic encounters among size-match ed male shore crabs. Our findings indicate that cheliped ('weapon claw') di rectional asymmetry is not related to the outcome of fights, whereas fluctu ating asymmetry in the fifth pereiopod, but not the second pereiopod, is ne gatively related to the likelihood of winning conspecific aggressive encoun ters. This relationship is most readily explained by a biomechanical advant age in symmetric individuals, as the fifth pereiopod is likely to be mechan ically important in maintaining stability and balance during fighting. Ther e is no evidence that asymmetry (in traits that display fluctuating asymmet ry) is related to an intrinsic individual quality factor. None the less, th e relative mechanical advantage of low asymmetry may give rise to fitness b enefits in symmetric crabs that may have evolutionary consequences. (C) 199 9 The Association for the Study of Animal Behaviour.