Experimental size- and sex-specific predation on dung flies by amphibian and arthropod predators

Because predator-prey interactions in nature are multifarious, linking phenomenological predation rates to underlying behavioral or ecological mechanisms is challenging. Size- and sex-specific predation has been implicated as a major selective force keeping animals small, directing the evolution of body size and sexual size dimorphism. We experimentally assessed predation by 4 amphibian and 3 arthropod species (bug, fly, spider) on 3 species of dung flies with similar ecology but contrasting body sizes, sexual size dimorphism, and coloration. Predators were offered a size range of male and/or female individuals of one prey species. As expected based on optimal foraging theory, some anurans (Bufo bufo) preferred larger prey individuals, others (Bombina variagata, Rana esculenta) showed no preference. Small juvenile Rana temporaria metamorphs preferred small prey, as did all arthropod predators, a pattern that may be explained by gape limitation or larger prey escaping better. Presumably more mobile males were not preyed upon more frequently or faster than cryptic females, even when conspicuously colored. Contrary to expectation, predation rates on flies in mixed groups facilitating mating activity were not higher, nor was predation generally sex-specific, hence contributing little to sexual dimorphism. We conclude that the size-selectivity of predators, and hence the viability selection pattern exerted on their prey, depends foremost on the relative body sizes of the two in a continuous fashion, in addition to any specific prey defense mechanisms. Therefore, the mechanistic study of predation requires integration of both the predator and prey perspectives, and phenomenological field studies of predation remain indispensable.