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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 investigated the relationship between trait asymmetry (both directional and fluctuating asymmetry) and the outcome of agonistic encounters among size-matched male shore crabs. Our findings indicate that cheliped (‘weapon claw’) directional asymmetry is not related to the outcome of fights, whereas fluctuating asymmetry in the fifth pereiopod, but not the second pereiopod, is negatively related to the likelihood of winning conspecific aggressive encounters. This relationship is most readily explained by a biomechanical advantage in symmetric individuals, as the fifth pereiopod is likely to be mechanically important in maintaining stability and balance during fighting. There is no evidence that asymmetry (in traits that display fluctuating asymmetry) is related to an intrinsic individual quality factor. None the less, the relative mechanical advantage of low asymmetry may give rise to fitness benefits in symmetric crabs that may have evolutionary consequences.


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