In many species, social interactions play a key role in deciding resource allocation. Aggression is one mechanism by which crayfish become dominant, which, in turn, enables them to acquire higher-quality resources. In crayfish, hierarchies and dominance structures are formed by dyadic interactions. In natural habitats, crayfish have agonistic interactions that often take place with other crayfish in the vicinity. There is a possibility for observers to gain information about potential future opponents. We were interested in examining the impact of observing agonistic interactions on the social behaviour of a bystander crayfish. Bystanders were visually exposed to one of four treatments followed by an agonistic interaction with a naive individual.

Treatments consisted of: (1) two size-matched crayfish fighting (fight treatment); (2) two size-matched crayfish not fighting (visual control); (3) an empty fight arena with a retractable wall (motion control); or (4) an empty tank with no retractable wall (handling control). The second interaction, where the bystander crayfish was fought against a naive size-matched individual, or tester crayfish, was analysed by a person, blind to treatment, for initiation, winner-loser and temporal dynamics of escalation. Our results indicate that bystander crayfish exposed to the fight treatment lost significantly more often to tester crayfish than bystander crayfish subjected to control treatments. In addition, there were changes in the fight dynamics for the bystander crayfish. These results show that observations of interactions by a third crayfish have implications for hierarchies and social behaviour in natural settings.