Resource availability and predation risk influence contest behavior and dominance hierarchies in crayfish

Theory-based predictions on asymmetries in animal contests have seldom been tested in situations where payoff asymmetries exist among contestants otherwise clearly matched for physical fighting capacity (i.e., resource holding potential, RHP). We examined the effects of food availability and predation risk on contest outcomes and the maintenance of dominance hierarchies in size-matched male noble crayfish pairs (Astacus astacus). Following theoretical predictions, dominant status established via contests staged in a non-resource context successfully predicted contest outcome (resource use) once resources were made available. Still, resource availability significantly increased the aggressive motivation of subordinates and sometimes resulted in hierarchy reversals. In subsequent resource-based contests, subordinates increased their fighting intensity, whereas dominants reduced their fighting motivation, potentially reflecting hunger-driven payoff asymmetries. Moreover, the data support the "asset-protection" principle by showing for the first time that predation risk lowered the aggressive motivation of dominants. At the same time, the percentage of fighting bouts won by subordinates increased significantly, leading to reversal of prior dominance relationships in 30 % of pairs. Compared to pairs where the hierarchy was retained through the different contests, pairs where dominance was reversed were characterized by smaller RHP asymmetries, which is again consistent with theory. These results show how animal contest behavior can be sensitive to the complex interactive effects of prior experience, resource value and RHP. Furthermore, in systems where repeated contests lead to dominance hierarchies, reversals within the hierarchy may occur more often when the RHP asymmetry between contestants is small. Contest outcome and resulting dominance status of contestants may be influenced by relative resource value and perceived costs involved in animal contests, especially when individuals are similar in major correlates of fighting ability, i.e., often body and weapon size. However, empirical evidence is scant. Using size-matched male crayfish pairs, we investigated the effects of food availability and predation threat on fighting behavior on individual and pair level and on hierarchy maintenance in repeated contests. Our study reveals novel evidence, indicating that crayfish are capable of changing their decisions during contests in response to food availability and predation threat, which can alter the hierarchy. For example, dominants reduced their aggressiveness in the presence of predation threat. Overall, hierarchy reversals were more likely in pairs where the difference in fighting ability between contestants was smaller.