Does Differential Susceptibility to Predation in Tadpoles Stabilize the Bombina Hybrid Zone?

Abstract

Despite substantial divergence, the European toads Bombina bombina and Bombina variegata (Anura: Discoglossidae) interbreed freely wherever their parapatric distributions adjoin. Natural selection that stabilizes the resulting hybrid zones should rest in part on the adaptation to different breeding habitat of the pure taxa. While B. bombina lays its eggs in semipermanent ponds, B. variegata is a typical puddle breeder. Here, we investigate whether selection for rapid larval development in B. variegata has resulted in the loss of effective antipredator defenses, thus excluding this species from predator-rich ponds. We collected adults from four populations in Romania (two for each taxon) and reared the offspring from four crosses per population in the laboratory either in the presence or in the absence of caged odonate predators (Aeshna cyanea). In predation trials, we found no taxon difference in mortality rate among tadpoles that had been reared with predators. The resilience of B. variegata tadpoles may have been due to their remarkable phenotypic plasticity. In both taxa, predator presence led to the development of a higher tail fin, which has been shown to reduce predation rates in other amphibians. This response was much stronger in B. variegata than in B. bombina. Moreover, differences between the two B. variegata populations in terms of laboratory predation rates and levels of plasticity correlated with predator abundance at the collection sites so as to suggest local adaptation in predator defenses. Finally, delayed metamorphosis in the predator-induced morphs of both taxa implies a cost to the defense. Given the heterogeneity of temporary habitat in terms of desiccation rate and predator occurrence, the greater amount of phenotypic plasticity in B. variegata fits predictions of life history theory. At the same time, our results leave the question unresolved as to why this species avoids ponds.