Compensatory larval responses shift trade‐offs associated with predator‐induced hatching plasticity

Many species with complex life histories can respond to risk by adaptively altering the timing of key life history switch points, including hatching. It is generally thought that such hatching plasticity involves a trade‐off between embryonic and hatchling predation risk, e.g., hatching early to escape egg predation comes at the cost of increased vulnerability to hatchling predators. However, most empirical work has focused on simply detecting predator‐induced hatching responses or on the short‐term consequences of hatching plasticity. Short‐term studies may not allow sufficient time for hatchlings to exhibit compensatory responses, which may extend to subsequent life stages and could alter the nature of the trade‐offs associated with hatching plasticity. In this study, we examine the consequences of predator‐induced hatching plasticity through the larval stage to metamorphosis in the East African reed frog, Hyperolius spinigularis. To do this we conducted an experiment in which we manipulated initial larval size and density (mimicking the effects of egg predators) and the presence of aquatic predators. We expected that predator‐induced hatchlings (because they are less developed and smaller) would experience higher per capita predation rates and a longer larval period and thus would exhibit lower survival to metamorphosis in the presence of aquatic predators than larger, more developed, later hatched larvae. Surprisingly, we found that predator‐induced hatchlings survived better, not worse, than hatchlings from undisturbed clutches. These results motivated us to develop a model parameterized from additional experiments to explore whether a combination of mechanisms, compensatory growth, and density‐ and size‐specific predation, could give rise to this pattern. Predicted survival probabilities from the model with compensatory growth were consistent with those from the field experiment: early hatched larvae grew more rapidly through vulnerable size classes than later hatched larvae, resulting in higher survival at metamorphosis. Thus, in this system, there does not appear to be a trade‐off in vulnerability between egg and larval predators. Instead, our results suggest that the cost that balances the survival benefit of hatching early to evade egg predators arises later in the life history, as a result of smaller size at metamorphosis.