Simultaneous evaluation of pre- and post-metamorphic risk determines flexible timing of emergence and duration of metamorphosis in red-eyed treefrogs

Background/Question/Methods

Theory predicts that organisms with complex life cycles should minimize the ratio of growth rate (g) to risk of mortality (µ) when transitioning between life stages. However, the majority of studies have found no effect of predators on timing of or size at metamorphosis. Two things may contribute to this discrepancy. First, metamorphosis has traditionally been defined in relatively one-dimensional terms and secondly, both theory and empirical work have assumed that post-transition risk is unknown. Tadpoles of the red-eyed treefrog, Agalychnis callidryas, can detect predators of both larval and metamorph stages and have substantial plasticity in the timing of developmental and behavioral events within the process of metamorphosis itself. We reared A. callidryas tadpoles from hatching until metamorphosis in the presence or absence of free-roaming larval predators (giant waterbugs) and the presence or absence of free-roaming metamorph predators (fishing spiders) in order to test if plastic changes in metamorphosis itself could reconcile life-history optimization theory with empirical data. 

Results/Conclusions

Like many studies before, tadpoles did not alter the timing of the initiation of metamorphosis in response to larval or metamorph predators, despite a ~30% reduction in larval density due to predation. However, by examining aspects of metamorphosis itself, we found that tadpoles changed their timing of emergence from the water as well as the duration of metamorphosis, both in directions expected under the “minimize µ/g” framework. When organisms have information about post-metamorphic risk, they are able to better optimize of the timing of transition out of water. More importantly, our results demonstrate that adaptive changes in life-history transitions occur at timescales previously ignored. Such effects may be widespread, despite being currently underappreciated.