Evolution of fighting behavior under asymmetric competition: an experimental test with juvenile salmonids

Large-dominant and small-subordinate species engaging in asymmetric interference competition may optimize behavior under different trade-offs between the chance of winning and the cost of fighting. If fighting behavior is heritable and under selection, theory suggests that large-dominant and small-subordinate species should evolve aggressive and passive fighting behaviors, respectively. To test this prediction, I manipulated the size and competitive asymmetry of juveniles from sympatric populations of large-dominant coho salmon (Oncorhynchus kisutch) and small-subordinate steelhead trout (O. mykiss) and asked whether differences in fighting behavior persisted independently of competitive ability I observed fighting behavior during dyadic contests in two habitats, mutually preferred pools and energetically demanding riffles, under each of three size treatments: natural size asymmetry, asymmetry removed, and reversed size asymmetry. The results supported the prediction. Competitive ability depended primarily on size; large individuals of both species dominated smaller heterospecifics, and neither species dominated when size matched. Fighting behavior depended primarily on species identity; coho salmon used a higher proportion of aggressive chases, whereas steelhead trout used a higher proportion of passive displays. Large individuals were more likely to chase, and small individuals were more likely to display. As evidence that asymmetric competition is associated with behavioral divergence, these results complement previous work on morphological divergence under asymmetric competition and provide a richer context for other features of the coho-steelhead system.