Who’s what? Prompt recognition of social status in crayfish.

Crayfish are excellent model organisms to study the proximate mechanisms underlying the maintenance of dominance hierarchies in invertebrates. Our aim here was to investigate whether Procambarus clarkii males use social eavesdropping to discriminate dominant from subordinate crayfish. To this end we conducted an experiment composed of a ‘passive’ and an ‘active’ phase. In the passive phase, ‘focal’ individuals were allowed (Treatment 1) or not (Treatment 2) to see and smell two size-matched crayfish fighting, while in the subsequent active phase they were allowed to freely interact with the fighting dyad. All the recorded variables did not show any significant difference between the two treatments but invariably focal individuals were able to promptly discriminate dominant from subordinate crayfish. This study provides evidence that male crayfish, differently from female of the same species in the context of mate choice, recognise the social status of a conspecific without the need of direct or indirect experience with it and avoid dominants --and thus dangerous opponents-- by the means of a badge of status. A form of ‘winner and loser effects’ seems also to contribute to the structuring of dominance/subordinate relationships. The implication of these results in understanding the maintenance of dominance hierarchies in invertebrates are finally discussed and compared with findings previously achieved in the context of mate choice by P. clarkii females.