Determinants and co‐expression of anti‐predator responses in amphibian tadpoles: a meta‐analysis

A wide range of taxa respond to perceived predation risk (PPR) through inducible defenses, and many prey are capable of responding both behaviorally and morphologically to the same risk event. In cases where multiple defenses confer protection by independent means (i.e. they are mechanistically independent) responses will either be co‐expressed, or the expression of one defense will limit the capacity (or need) to respond along another axis. Our ability to generate a broad understanding of these patters has been limited, in part, by difficulties in comparing results across studies that employ distinct experimental protocols. Using the extensive literature on tadpole responses to PPR, we conducted a meta‐analysis to identify the ecological and experimental determinants of inducible defence expression. We then assessed whether the magnitude of response to PPR along behavioural versus morphological response axes was positively, or negatively, correlated. The most commonly quantified responses to perceived risk in tadpoles included reductions in movement and swimming behaviour, and altered tail morphology. Our analyses reveal that tadpole behavioural responses are strongly influenced by prey family, predator taxon, evolutionary history with the predator (native versus non‐native), amount of prey consumed by the predator, and how perceived risk was manipulated (e.g. presence versus absence of alarm cues). Tail morphology was similarly influenced by these factors, but also whether the target prey was palatable to predators. Thus, our results identify ecological and experimental features that critically influence the observed effect size in tadpole responses to PPR. A positive correlation between behavioural and morphological responses in studies where both were measured indicates that trait co‐specialization is the predominant pattern of defense deployment in larval amphibians. This positive relationship suggests that survival tends to be maximized in tadpoles through equivalent co‐activation of multiple independent axes of protection, opposed to maximal expression along any single axis. Synthesis Our understanding of plastic responses to perceived predation risk (PPR) has benefited substantially from the vast amount of experimental work examining inducible defences in anuran tadpoles. Indeed this research has illustrated the wide variety of ways that prey animals can respond to the same risk event. We conducted a metaanalysis to identify the key ecological and experimental determinants of inducible defence expression. We then show that, in most cases, behavioural and morphological responses to PPR tend to be co‐expressed suggesting that responding along one axis (moving behaviour) does not limit their ability to respond along another distinct axis (tail morphology).