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 life histories because it is difficult to manipulate most species during the terrestrial stage. I examined the terrestrial performance of Rana blairi, Rana sphenocephala, and four hybrid (two F1 and two advanced generation) genotypes in replicated experimental enclosures to test for differences in traits related to juvenile terrestrial fitness. I produced all genotypes by means of artificial fertilizations using frogs collected from natural populations in central Missouri, and juvenile frogs were obtained from larvae reared in experimental ponds. Following metamorphosis, froglets were raised in single-genotype groups in terrestrial enclosures through the first overwintering. The proportion surviving did not vary among genotypes, but the power to detect significant differences was low. F1 hybrid genotypes BS and SB demonstrated significantly higher growth rates than either parental species or advanced-generation hybrid genotypes. Observation of growth rates of advanced-generation hybrids equal to those of the parental species, and heterosis in F1 hybrids for growth rate, suggests that natural hybridization between R. blairi and R. sphenocephala can produce novel and relatively fit hybrid genotypes. Direct measurement of multiple fitness components for hybrid and parental genotypes is critical for assessing the evolutionary potential of natural hybridization in organisms with complex life cycles.

Jumping performance and short-term repeatability of newly metamorphosed hybrid and parental leopard frogs ( Rana sphenocephala and Rana blairi )

Differential fitness between hybrid and parental genotypes plays a critical role in explaining the maintenance of natural hybrid zones as well as the production of novel genetic variation that may lead to diversification. Because locomotor performance is a reliable and practical measure of potential fitness related to morphological variation, we tested for differences in jumping performance among parental and hybrid genotypes of newly metamorphosed leopard frogs (Rana sphenocephala and Rana blairi). Tadpoles of the parental species and primary and backcross hybrid tadpoles, generated from artificial crosses (a total of five genotypes), were reared at two initial larval densities. Locomotor performance of newly metamorphosed frogs, as measured by jumping ability, was tested three times over 6 days in the laboratory at 24-25°C. Maximum and average jump lengths were greater for metamorphs reared at low larval density than for those reared at high density. Regression analyses indicated that 70-79% of the variation in jump length was due to body mass. When reared at low density, metamorphs of two F1 backcross genotypes (HB and HS) and one primary hybrid genotype (SB) jumped shorter distances than either parental species. When reared at high density, hybrid performance was indistinguishable from that of the parentals, except for one backcross hybrid (HB). Moderately high short-term repeatabilities (0.47-0.66) of metamorphs reared at the high density indicate that measures of performance in newly metamorphosed frogs can be predictive. We suggest that, owing to poor jumping performance, some hybrid frogs would be at a selective disadvantage relative to their parental species in the terrestrial environment and thus would partially reinforce mechanisms of reproductive isolation in this leopard frog system. Yet equivalent performance of some hybrids relative to the parentals, at least when reared at low density, suggests that hybrid lineages also have the potential to evolve independently in some environments.

Terrestrial burrowing ecology of newly metamorphosed frogs (Rana pipiens complex)

I examined the burrowing performance of newly metamorphosed Rana areolata, Rana blairi, and Rana sphenocephala in laboratory aquaria. Three treatments were used to determine whether metamorphs, when deprived of a water source, actively dig their own burrows, and if they utilize natural irregularities in substrate profiles or preexisting burrows as shelter from evaporative desiccation. Analysis of covariance (initial body mass covariate) revealed that individuals of all three species actively burrowed and passively utilized preexisting experimental burrows, but active burrowing, which consisted of substrate excavation using only the hind limbs, took much longer. Metamorphs that used preexisting burrows conserved water more efficiently than those that actively dug their own burrows. Rana blairi experienced the highest percent water loss across all treatments and was the only species to suffer mortality during the experiment. These data suggest that burrowing, by minimizing evaporative water loss, may be a critical adaptive behavior for metamorphic frogs in semi-arid environments.