Temporal variation in predation risk: stage-dependency, graded responses and fitness costs in tadpole antipredator defences

Temporal variation in predation risk may be an important determinant of prey antipredator behaviours. According to the risk allocation hypothesis, the strongest antipredator behaviours are expected when periods of high risk are short and infrequent. We tested this prediction in a laboratory experiment where common frog Rana temporaria tadpoles were raised form early larval stages until metamorphosis. We manipulated the time a predatory Aeshna dragonfly larva was present and recorded behavioural responses (activity) of the tadpoles at three different time points during the tadpoles' development. We also investigated how tadpole shape, size and age at metamorphosis were affected by temporal variation in predation risk. We found that during the two first time points activity was always lowest in the constant high-risk situation. However, antipredator response in the two treatments with brief high-risk situation increased as tadpoles developed, and by the third time point, when the tadpoles were close to metamorphosis, activity was as low as in the constant high-risk situation. Exposure to chemical cues of a predation event tended to reduce activity during the first time period, but caused no response later on. Induced morphological changes (deeper tail and shorter relative body length) were graded the response being stronger as the time spent in the proximity of predator increased. Tadpoles in the brief risk and chemical cue treatments showed intermediate responses. Modification of life history was only found in the constant high-risk treatment in which tadpoles had longer larval period and larger metamorphic size. Our results indicate that both behavioural and morphological defences were sensitive to temporal variation in predation risk, but behaviour did not respond in the manner predicted by the risk allocation model. We discuss the roles of concentration of predator chemical cues and prey stage-dependency in determining these responses.