Relative fitness of parental and hybrid Leopard Frogs was examined by measuring larval performance in artificial ponds and laboratory containers. Effect of a variable hydroperiod on growth, development, and survival was used to assess larval responses to a desiccating aquatic environment. Larvae for the artificial pond experiment were obtained from field-collected egg masses of Rana blairi and R. sphenocephala. Artificial crosses with adult animals produced larvae of one parental and three hybrid genotypes for laboratory performance assays. Experimental pond drying significantly reduced survival for both parental species. Rana blairi had higher survival than R. sphenocephala across all treatments. Both species reduced larval period lengths when exposed to a drying environment relative to constant water levels. The laboratory experiment revealed Backcross2 hybrid larvae exhibited longer larval period lengths than parental R. sphenocephala and certain F1 hybrid genotypes in the drying environment. Proportion of survivors metamorphosing was greatest for the F1 hybrid genotype SB and lowest for the Backcross2 hybrid genotype. Consequently, advanced-generation hybrid genotypes (i.e., Backcross2) did not perform well in desiccating environments. Direct measurements of important fitness components of parental and hybrid genotypes are critical for determining the evolutionary potential of natural hybridization.
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Vol. 2000 • No. 1