Parallel swims as a means of intra- and interspecific assessment in stream fishes

Synopsis Several North American minnow species are known to engage in the action pattern parallel swimming, an activity that allows males competing for nests or nesting sites an opportunity to assess each other. These agonistic displays have been hypothesized to occur only between males of similar size. We provide evidence to support this hypothesis for two minnow species and a sucker species. Competition is strong both intra- and interspecifically for nest sites and we document the occurrence of interspecific parallel swims between three species.