Overwinter survival of Rana lessonae and its hemiclonal associate Rana esculenta

In central Europe, the hybridogenetic water frog Rana esculenta is a sexual parasite that only persists in the presence of the parental species, R. lessonae, with which it must mate in order to reproduce. R. esculenta is a superior larval competitor, and adult females are about three times more fecund than R. lessonae. This suggests that, in the absence of some balancing advantage to R. lessonae, R. esculenta should drive the parental species locally extinct, closely followed by itself. We measured annual survival rates over six years at two widely separated ponds using modern capture-mark-recapture methods to test whether differences in adult survival might contribute to the persistence of the water frog system. We marked 856 adult frogs and recovered 182 at least once. The data did not deviate significantly from the assumptions of the Cormack-Jolly-Seber model at either pond. There were no detectable differences in recapture probabilities between taxa, but females were, overall, less likely to be recaptured. Survival was higher for R. esculenta in all but one year. Therefore, some mechanism other than adult survival must be invoked to explain the persistence of this unusual breeding system. There was considerable year-to-year variation in survival, ranging from 6% over the winter of 1996-1997, to 98% over the winter of 1995-1996. Although the two ponds are separated by 35 km, pond identity did not contribute to the minimum models. A composite measure of winter severity constructed from a principal components analysis of weather data during the course of the study had a very close relationship with survival (R. lessonae, r = 0.905; R. esculenta, r = 0.889). Survival was lowest in winters with low minimum temperatures coupled with high and variable maximum temperatures.