Interference from a game theoretical perspective: shore crabs suffer most from equal competitors

In virtually all natural systems, interference competition among individuals is asymmetric. Here, we used game theoretical models on contest behavior to predict how time lost in agonistic interactions could affect strength of interference under asymmetric competition. We hypothesized that interference through time lost in agonistic interactions would result in a greater reduction in available foraging time and overall feeding rate under symmetric competition than under asymmetric competition. We tested this hypothesis for male shore crabs (Carcinus maenas) that foraged on mussels (Mytilus edulis) in an experiment where prey levels were kept constant. We varied absolute size of crabs (juveniles, small adults, large adults), mussel density (4, 16, 32 per 0.25 m(2)), and competitor size (smaller, equal, larger). Large adults spent more time in aggressive behaviors than juveniles or small adults, possibly because large adults were more persistent in interfering or because large adults were intrinsically more aggressive, as the experiment was conducted in the mating season. When handling prey, crabs mostly avoided competitors, but juveniles and small adults did so more than large adults. When searching for prey, crabs mostly displaced smaller competitors but threatened or avoided size-matched or larger competitors. By avoiding a competitor, the focal crab lost time but the competitor often did not, and this asymmetry in agonistic behavior is not yet incorporated in models on contest behavior. However, overall, negative effects of others were strongest with size-matched competitors, in line with our hypothesis.