Specific responses of sexual and hybridogenetic European waterfrog tadpoles to temperature

The European waterfrog, Rana esculenta, is a hemiclonal hybrid that must coexist with the parental species Rana lessonae in order to reproduce. It is not clear what allows the two morphologically, genetically, and ecologically similar forms to coexist, but differential success of the hybrid and its sexual host among environments suggests that these frogs may differ in their adaptive abilities, and that ecology plays an important role in determining the relative frequencies of the two related species. The objective of this study was to identify factors that may promote coexistence. We investigated the effect of temperature, food level, and food quality on a variety of life history traits in a laboratory experiment. Our results indicated that tadpoles of the two forms respond differently to temperature. Probability of metamorphosis and survival of R. lessonae were higher at 24 degreesC, while the hybrid, R. esculenta, had a better survival rate and a much larger body mass at metamorphosis at 18 degreesC. We then tested the results of our laboratory experiment by assessing the distribution of the hybridogen and the parental species in natural populations as a function of temperature, and found that the relative frequency of R. esculenta tadpoles declined with increasing temperature. We use these results to evaluate the applicability of the generalist and frozen niche variation models that had been proposed earlier as explanations for the coexistence of the sexual parental species, R. lessonae, and the hybridogen, R. esculenta.