Morphological and life-history plastic responses to predators and competitors in two brown frogs, Rana dalmatina and R. Latastei.

Predation and competition are two major factors that drive natural selection. When they vary unpredictably, selection has promoted the evolution of plastic responses in behavioral, morphological and life-history traits. These responses are not independent of each other and often represent a trade-off between conflicting interests. We conducted a common-garden experiment to study the effects of predation and competition on the morphology and life history of R. dalmatina and R. latastei tadpoles. The experiment used a randomized-block design, where tadpoles were raised either with or without siblings and either with or without predators. Regardless of the treatments, R. dalmatina grew faster, developed proportionally larger tails, proportionally smaller bodies, and completed metamorphosis earlier than R. latastei. Both species developed relatively larger bodies with competitors, and relatively larger tails with predators. While the relative increase in body size with competitors was similar in the two species, the increase in tail size with predators was greater in R. dalmatina, suggesting that this species invested more in defense than R latastei. Competitors delayed metamorphosis in both species and in R. latastei they negatively affected froglet body size and shape. Conversely, predators delayed the metamorphosis only in R. dalmatina. This delay was the long-term cost paid by R. dalmatina for the short-term benefits of developing more effective behavioral and morphological defensive traits.