Habitat duration, predation risk and phenotypic plasticity in common frog (Rana temporaria) tadpoles

1. Common frogs (Rana temporaria) breed readily in small pools and thus expose their offspring to catastrophic mortality by desiccation, Amphibian larvae exhibit considerable phenotypic plasticity in metamorphic traits, and some species respond to environmental uncertainty by metamorphosing earlier and at smaller size. In a factorial laboratory experiment, we studied whether common frog tadpoles possess this ability. 2. We also studied the interaction between pool drying and predation risk, because in a previous study the presence of a predatory dragonfly larva delayed metamorphosis of the tadpoles. 3. We gradually removed water from half the experimental containers, while in the other half the water volume was kept constant. In the laboratory it was possible to remove the effect of increased water temperature in the decreasing volume treatment by using fluorescent lights. Tadpoles responded to decreasing volume by metamorphosing earlier and at smaller size. A greater proportion of the tadpoles metamorphosed in the decreasing volume treatment. 4. Tadpoles were less active at decreasing water level and there were significant positive correlations between activity late in the experiment and metamorphic size. This suggests that the metamorphic response to habitat drying is behaviourally mediated. 6. Early in the experiment, tadpoles developed slower in the presence of predators. At metamorphosis presence of a dragonfly larva had no effect in the whole data set, but when the constant volume treatment was analysed separately, larval period was longer in the presence of a predator. 7. Our results indicate that common frog larvae are able to respond to pond-drying adaptively by speeding up their development, and that temperature advantage is not needed to induce this adaptive plasticity. Furthermore, pond-drying seems to be a more important determinant of development rate than the presence of odonate predators.