Breeding habitat partitioning in the Rana esculenta complex: The intermediate niche hypothesis supported

The hypothesis of hybrids occupying intermediate niche was tested in the wa ter frog hybridogenetic complex by investigating the variation of assemblag e compositions in different habitats along a gradient of river influence in a large floodplain (upper Rhone, France). The parental species strongly di ffered in their habitat use. Whereas Rana ridibunda Pallas occupied dead ar ms close to the active channels, Rana lessonae Camerano occupied marsh pond s. The hybrid Rana kl. esculenta Linnaeus was found in the same ponds as Ra na lessonae, as expected in L-E systems where Rana kl. esculenta females ac t as sexual parasites of Rana lessonae males. However, the proportions of e ach of these two taxa in mixed assemblages varied according to the degree o f river influence, with Rana kl. esculenta predominating in ponds experienc ing an intermediate level of flooding (alluvial marsh) and Rana lessonae pr edominating in less frequently flooded ponds (peat marsh). These results co nverge with other studies in demonstrating that the success of hybrid frogs depends on niche partitioning along an ecological gradient, which is proba bly a relevant templet for water frog evolution. In this evolutionary conte xt, hybrid frogs have to deal with a trade-off between habitat selection an d mate choice.