Weapon strength and competitive success in the fights of shore crabs (Carcinus maenas)


*All correspondence to: L. U. Sneddon, Animal Welfare Research Group, Roslin Institute, Roslin, Midlothian EH 25 9PS, Scotland, U.K.


Weapon size has been shown to be a better predictor of competitive success than body size (carapace width) in fights between shore crabs, Carcinus maenas. However, when the weapon size disparity is small between two opponents, it is still difficult to predict the victor. The role of weapon strength in pairwise fights between male shore crabs was investigated, to determine if relative force influences contest content, duration and outcome. Weapon strength was ascertained using a force transducer on live crabs, then fights between crabs were staged between size matched males. Winning crabs had major (crusher) claws and minor (cutter) claws that exerted a significantly greater force than losing crabs even when claw length was the same. Winners and losers were matched for carapace width, claw length and dactyl length but not claw height or claw length minus the dactyl. Winners had greater claw height and claw length to the dactyl in the major claw giving them a higher mechanical advantage when closing the claw and thus exerting a greater force. The forces exerted by the major and minor claws were analysed for any relationship between force and morphological measurements. Winning crabs appear to be fitter in having a better claw structure which exerts a greater force and they are more successful in agonistic interactions.