Weak rappers rock more: hermit crabs assess their own agonistic behaviour.

Fighting animals use a variety of information sources to make strategic decisions. A neglected potential source of information is an individual's own performance during a fight. Surprisingly, this possibility has yet to be incorporated into the large body of theory concerning the evolution of aggressive behaviour. Here, by experimentally dampening the impact of their shell rapping behaviour, we test for the possibility that attacking hermit crabs monitor their own fight performance. Attackers with dampened raps did not show a reduction in the number of raps used. By contrast, they showed an increased frequency of a less intense agonistic behaviour, shell rocking. This change in behaviour, in attackers that are forced to rap weakly, indicates that they assess their own agonistic behaviour.