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. Significance statement Tadpoles develop plastic morphological and life-history traits in responses to predators and competitors. This plasticity is costly in terms of both performance and function. Our study shows that two closely related brown frogs have evolved different trade-offs in their plastic responses to predators and competitors. Rana dalmatina , which grew and developed faster than R. latastei , was more sensitive to predators and developed more effective morphological defenses. Both species delayed metamorphosis at high density, but R. dalmatina did it more than R. latastei . Predators did not affect the duration of larval development in R. latastei , whereas in R. dalmatina , they further delayed metamorphosis. This delay was the long-term cost of the R. dalmatina ’s larger investment in short-term defensive traits.