Hierarchical competitive ability and phenotypic investments in prey: inferior competitors compete and defend

It is commonly assumed that prey must make growth-defense trade-offs when in the presence of predators under resource-limited conditions. Thus, it is predicted that prey will allocate limited resources either to competitive phenotypes to grow or to defensive phenotypes to defend, but not both. Evidence from plants, however, has suggested that under strong selection prey might evolve phenotypes that allow them to simultaneously compete and defend. In a controlled laboratory experiment, we investigated phenotypic investment in asymmetrically competing tadpoles of Rhacophorus schlegelii (RS) and Pelophylax nigromaculatus (PN) in the presence and absence of predatory dragonfly larvae. We predicted that, because of differences in resource acquisition abilities, tadpoles would invest in competitive and defensive phenotypes differently, depending on their relative competitive ability in the presence of predators. RS was the superior competitor and depressed the growth of PN whether predators were present or absent. As expected, the inferior competitor (PN) responded to competition by elongating its gut in predator-free environments. In contrast, the superior competitor (RS) did not respond to competition. In the presence of predators, both tadpoles invested in larger tails, a common defensive trait, while the presence of competitors did not influence tail size. When reared alone in the presence of predators, PN grew faster and tended to have a longer gut. In contrast to theoretical predictions, however, in the presence of both predators and competitors the inferior competitor (PN) still maintained the competitive (long gut) and defensive (large tail) phenotypes despite being depressed in growth. Optimal digestion theory suggests that gut elongation should enhance assimilation efficiency and energy gain. In this view, when a competitive phenotype also serves to benefit prey in predator environments, prey may be able to both compete and defend.