Probing aggressive motivation during territorial contests in a group-living cichlid fish.

Many animals fight to win resources, repel competitors or establish dominance in a social group. Mutual-assessment of fighting ability, where competitors gather and compare information about their opponent's as well as their own fighting ability has been the dominant theoretical framework for understanding decision-making during fights. However, self-assessment, where each individual has a cost threshold and fights up until that point, may be more common than previously appreciated. In this study, we attempted to discriminate between these two potential assessment mechanisms in a group-living cichlid fish, Neolamprologus pulcher by probing aggressive motivation during a territorial contest. We measured aggressive motivation, and used this metric to investigate assessment rules during an ongoing contest. We predicted that if these social fish use self-assessment, we would observe a positive correlation between the fighting ability of the probed animal and its aggressive motivation. Alternatively, if mutual-assessment is used then we predicted we would find a negative effect of the opponent's fighting ability on the aggressive motivation of the probed fish because fish should be less motivated to fight against formidable opponents. Our results did not support either of these predictions. In contrast we found that small individuals were more aggressively motivated regardless of their opponent's size. We discuss this result in the context of theoretical models of aggression in individuals of small body size.