Specific responses of sexual and hybridogenetic European waterfrog tadpoles to temperature

The European waterfrog, Rana esculenta, is a hemiclonal hybrid that must co exist with the parental species Rana lessonae in order to reproduce. It is not clear what allows the two morphologically, genetically, and ecologicall y similar forms to coexist, but differential success of the hybrid and its sexual host among environments suggests that these frogs may differ in thei r adaptive abilities, and that ecology plays an important role in determini ng the relative frequencies of the two related species. The objective of th is study was to identify factors that may promote coexistence. We investiga ted 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, wh ile 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 ou r 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 wit h increasing temperature. We use these results to evaluate the applicabilit y of the generalist and frozen niche variation models that had been propose d earlier as explanations for the coexistence of the sexual parental specie s, R. lessonae, and the hybridogen, R. esculenta.