Chemical communication is a widespread phenomenon in aquatic animals but is difficult to investigate because the signals are not visible. Here, we present the results of a study into chemical communication in blindfolded fighting crayfish (Astacus leptodactylus) in which we employed a new method: visualisation of urine using the dye Fluorescein. The probability of urine release is greater during fights than during non-social activities or inactivity. The eventual winners are more likely to

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... se urine during fights than the eventual losers. In both winners and losers, urine release is coupled to offensive behaviours, and the probability of urine release increases with increasing levels of aggression. In A. leptodactylus, urine is carried to the opponent by the forward-projecting gill currents. During spontaneous release, urine is fanned laterally with the aid of the exopodites of the maxillipeds. Aggressive behaviour is effective in intimidating blindfolded opponents only in conjunction with urine release: receivers decrease offensive behaviour and increase defensive behaviour. Aggressive behaviour alone does not intimidate opponents. The loser of a recent fight is deterred equally well by a familiar and an unfamiliar opponent. Hence, in crayfish, individual recognition of the urine scent of a dominant individual does not appear to be significant for the maintenance of dominance hierarchies. Our results suggest that urine contains information about the fighting ability and/or aggressiveness of the signaller. The chemical signals thus far unidentified appear to be important in determining the outcome of a fight.