Consequences and Plasticity of the Specialized Predatory Behavior of Stream‐Dwelling Stonefly Larvae

The purpose of this study was to determine experimentally the potential mechanism(s) favoring specialized foraging behavior of stream—dwelling predatory stonefly larvae (Megarcys signata), and whether this specialized behavior was fixed or flexible. We measured stonefly growth rates after 10 d of conditioning on Baetis bicaudatus (Ephemeroptera) or one of two alternative mayfly prey species in replicated circular plexiglass flow—through chambers powered by water from a stream in western Colorado. We then compared attacks per encounter (as an index of predator choice) of conditioned stoneflies to those of unconditioned controls given equal densities of all three prey types. We conducted additional behavioral observations to determine whether predators differentially encountered the alternative prey types on natural substrates, and to determine whether predator—prey microhabitat overlap was mechanism explaining differential prey encounter rates among prey species. Megarcys exhibited a stereotyped attack bias toward swimming mayflies, such as Baetis, following the simple rule, "attack any prey that swim," which results in Megarcys' selection of prey types that swim in response to stonefly encounters. This behavior remained unmodified after extended experience with alternate but palatable nonswimming mayfly species. A potential fitness advantage (measured as relative growth rate) of this behavioral specialization occurred for female Megarcys, but not for males. Female Megarcys grew faster on Baetis diets because they ate more Baetis, not because Baetis was a more intrinsically profitable prey item (in terms of promoting predator growth). Further, high feeding rates on Baetis were a result of disproportionately high encounter rates and attacks per encounter with Baetis compared to the other mayfly species. Our data are consistent with the hypothesis that this predatory behavior arose and is maintained in female stoneflies due to a fitness advantage to individuals specializing on a relatively abundant, easily recognized prey resource. We speculate that this specialized behavior has not been lost in male stoneflies, for which we did not measure a fitness advantage, because there is no strong selection pressure against it. Effective pre—and—post—contact prey defenses may exert enough selection pressure to prevent stoneflies from evolving on alternative mayfly types.