The cost of conflict: agonistic encounters influence responses to chemical signals in the European shore crab

The determination of an individual's behaviour often relies upon complex signals that convey messages about the environment it inhabits. In aquatic environments such signals take varied and conflicting forms including chemical cues indicating food or potential mates being opposed by the presence of competitor or predator odour. Despite ever increasing knowledge of the chemical nature of signalling compounds, little is known of how the environment affects an animal's response towards such infochemicals. We examined how the European shore crab, Carcinus maenas, changes its behaviour in relation to recent social interactions. We hypothesized that agonistic encounters would affect males' behaviour towards two chemical signals: the female sex pheromone cue and a feeding odour (mussel-infused water). In a laboratory experiment, agonistic encounters significantly increased males' response times to food stimuli regardless of their dominance status. Additionally, after an aggressive interaction, 40% of subordinate males failed to respond, and both winners and losers showed a delayed response, to the female sex pheromone. Status-dependent responses by males are potential signs of social structures, enabling them to use social hierarchy-driven mating tactics. This could reduce the number and intensity of costly fights during the short reproductive window. This is a rare example of social interactions directly affecting an animal's response to chemical cues for feeding and reproduction, opening the field for further studies on the complexity of chemical communication.