Relative size and agonistic behaviour in the female velvet swimming crab, Necora puber (L.) (Brachyura, Portunidae).

Female velvet swimming crabs (Necora puber) fight readily in the laboratory, with interactions being initiated equally often by the larger and smaller of the two opponents, but with the larger usually being victorious. In 17 out of 65 interactions, however, a smaller crab won against a larger one, and possible reasons for this are discussed. In two respects our results are surprising in the context of insights gained from games theory: firstly, the fights do not show a gradual pattern of escalation through display to overt physical violence; and secondly, fights do not become more costly in terms of either potential for injury (intensity) or duration as the contestants became more evenly matched; indeed, as the contestants became more evenly matched, fight duration decreased.