Assessing behavioural and morphological responses of frog tadpoles to temporal variability in predation risk

Finely tuned adjustment of an individual's phenotype can offer substantial fitness benefits when it is closely matched with environmental change. For instance, prey may be safeguarded against unnecessary costs to growth or development when their responses to temporally variable predation risk include plastic anti-predator defences. Yet, the correspondence between perceived predation risk and related responses should differ between behavioural and morphological phenotypes when risk fluctuates because behaviour can be modified quickly, whereas morphological phenotypes require time to build. Theoretical models predict intermediate expression when risk fluctuates rapidly relative to the time required to mount a response, whereas traits that can be modified relatively quickly should more closely track current conditions. Using a tadpole-dragonfly larva system, we sought to compare the expression of behavioural and morphological defences following exposure to constant versus variable predation risk. By varying the pattern and total duration of predator cue exposure, but not cue concentration, we quantified phenotypic plasticity and trait reversibility. Our results show that strong behavioural responses were limited to early ontogeny but closely matched current level of risk. The morphology of prey experiencing a weekly changing predator environment was intermediate to that of prey in the no-predator and constantly exposed treatments. Yet, prey exposed to a predator environment for the same total duration as the weekly changing environment, but in a different exposure pattern, was morphologically unresponsive to the onset of predation risk. Finally, unexposed tadpoles gained deeper tails and smaller relative body size in late development, coincident with limb bud development. Such changes are consistent with anti-predator response and represent either an innate response when prey are more vulnerable or shape optimization when faced with increased drag. We conclude that phenotypic expression depends critically on patterns of temporal variability in the environment, although the actual extent of expression depends on the specific trait in question.