Hybridization in leopard frogs (Rana pipiens complex): terrestrial performance of newly metamorphosed hybrid and parental genotypes in field enclosures

Terrestrial ecology has been largely neglected in the study of amphibian li fe histories because it is difficult to manipulate most species during the terrestrial stage. I examined the terrestrial performance of Rana blairi, R ana sphenocephala, and four hybrid (two F-1 and two advanced generation) ge notypes in replicated experimental enclosures to test for differences in tr aits related to juvenile terrestrial fitness. I produced all genotypes by m eans of artificial fertilizations using frogs collected from natural popula tions in central Missouri, and juvenile frogs were obtained from larvae rea red in experimental ponds. Following metamorphosis, froglets were raised in single-genotype groups in terrestrial enclosures through the first overwin tering. The proportion surviving did not vary among genotypes, but the powe r to detect significant differences was low. F-1 hybrid genotypes BS and SB demonstrated significantly higher growth rates than either parental specie s or advanced-generation hybrid genotypes. Observation of growth rates of a dvanced-generation hybrids equal to those of the parental species, and hete rosis in F-1 hybrids for growth rate, suggests that natural hybridization b etween R. blairi and R. sphenocephala can produce novel and relatively fit hybrid genotypes. Direct measurement of multiple fitness components for hyb rid and parental genotypes is critical for assessing the evolutionary poten tial of natural hybridization in organisms with complex life cycles.