The color of combat: how subtleties of resource value alter the effects of fighting ability in hermit crab contests

During contests over resources, the intensity of fighting and the outcome is often influenced by an interaction between the difference between opponents in the fighting abilities (resource holding potential [RHP]) and the difference in how they subjectively value the resource. Additionally, because fighting can involve vigorous activity, engaging in a fight can make the opponents more noticeable to predators. Thus, predation risk, alongside resource value (RV), may interact with the effects of RHP difference. During hermit crab contests over the ownership of empty gastropod shells attackers perform vigorous shell rapping, whereas defenders adopt a protective posture inside their shells. This role asymmetry provides an opportunity to decompose the effects of motivation and predation risk on fighting as both will be influenced by the visual contrast of the shells against the substrate. If attackers are sensitive to RV, their agonistic behavior should vary with the color of the defender's shell and if they are sensitive to predation risk, it should be influenced by the color of their own shell, due to the vigorous way it moved during shell rapping. We found that the most vigorous fights, with the greatest chance of victory for attackers, occurred when both shells were of high contrast, indicating that RHP advantage is modulated by both RV and potential predation risk. Moreover, this shows how even relatively subtle features of a contested resource, such as color, and modify the effects of fighting ability during animal contests.