The colour 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, since fighting can involve vigorous activity, engaging in a fight can make the opponents more noticeable to predators. Thus, predation risk, alongside resource value, may interact with the effects of RHP difference. Hermit crab contests over the ownership of empty gastropod shells attackers perform vigorous shell rapping while 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 resource value their agonistic behaviour should vary with the colour of the defender’s shell and if they are sensitive to predation risk it should be influenced by the colour 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 resource value and potential predation risk. Moreover, this shows how even relatively subtle features of a contested resource, such as colour, and modify the effects of fighting ability during animal contests.