Do patterns of phenotypic variation in nature meet predictions from experiments? A test with wood frog (Rana sylvatica) larvae

Background/Question/Methods

Phenotypic plasticity plays a pivotal role in aiding organisms to survive in their environment. Amphibians have been a model organism for studies of phenotypic plasticity, and exhibit adaptive responses to many environmental conditions. However, while phenotypic plasticity of amphibian larvae has been studied intensively in laboratory and mesocosm settings, only a few studies have tested whether correlations between environmental variables and tadpole morphology in natural ponds match expectations from experiments. We conducted a field survey to test for relationships between environmental variables and tadpole morphology. We sampled wetlands in Northeast Ohio to collect wood frog tadpoles, and measure predation risk, tadpole density, and forest canopy cover over each pond. We quantified size and shape of tadpoles using geometric morphometrics. We tested three hypotheses based on previous experimental work on amphibian phenotypic plasticity. First, tadpoles inhabiting ponds with higher predator risk are predicted to have short bodies and deep tail fins. Second, tadpoles inhabiting ponds with abundant competitors are predicted to have relatively smaller tails and relatively larger bodies. Third, tadpoles in ponds with increased canopy cover are predicted to be smaller in size but to have relatively large bodies.

Results/Conclusions

Phenotypic variation in wood frog tadpoles was associated with canopy cover and predation risk, which met our expectations from previous experiments. Tadpoles from ponds with high canopy cover had large overall size and relatively larger bodies and smaller tails. Tadpoles from ponds with higher predation risk had relatively deeper tails. Surprisingly, there was no correlation between tadpole density and tadpole shape, which is different from previous experimental results. Previous experiments have shown that rearing tadpoles at high densities causes tadpoles to develop relatively small tails and large bodies.  Competitor-induced changes in shape appear to be an adaptive response which improves growth when food is limited. Our failure to find a relationship between density and morphology suggests that density is not a good indicator of per-capita food availability across wetlands. Instead, individual ponds may vary greatly in available resources. It has been argued that the artificial conditions of laboratory and mesocosm experiments lead to conclusions that do not correspond to natural patterns.  Our finding that two of the three environmental variables had consistent effects in our survey of natural ponds as of experiments suggests that experimental venues do provide a good representation of natural conditions for at least some variables.