Induction, acclimation, and behavioral phenotypes: Predator cues change flight initiation distance in hatchling red-eyed treefrogs

Background/Question/Methods Many amphibians respond plastically to predator cues, changing behavior, morphology, and hatching or metamorphic timing. These responses are often predator-specific or modulated by factors indicative of the level and nature of risk. The terrestrial embryos of red-eyed treefrogs, Agalychis callidryas, hatch prematurely to escape from egg predators. Premature hatchlings enter the water smaller and less developed than full-term hatchlings, and are more vulnerable to several aquatic predators, in part because they are less behaviorally responsive. Nonetheless, development through the plastic hatching period is faster in the water than in the egg. This might reduce the mortality cost of hatching early, particularly if larvae developing in the water can use cues from aquatic predators to improve their defenses. In this study we measured flight initiation distance (FID) of A. callidryas larvae in response to an approaching model predator to assay antipredator behavior. We assessed effects of hatching age (4 or 6 days), 30 min exposure to predator cues (acclimation) and, for early hatchlings, 48 h exposure to cues (induction treatment) on FID. All tadpoles were tested at age six days, when spontaneous hatching peaks in our study population, we compared responses to cues from three predators: an odonate, belostomatid, and poeciliid.
Results/Conclusions After 2 days in the water, belostomatid- and odonate-induced tadpoles showed increased FID, whereas fish-induced tadpoles showed decreased FID, compared with tadpoles that developed in predator-free water. Animals newly hatched at 6 d showed similar FID to the predator-naïve tadpoles that hatched 2 d earlier. After 2 days in the water, a 30 min acclimation period with odonate cues was sufficient to significantly increase FID; however equal-aged, newly hatched tadpoles showed no such change in FID with acclimation. Prematurely hatched A. callidryas face an initial high risk period in the water that later hatching tadpoles avoid. This study suggests, however, that by the time all eggs have hatched the surviving early-hatched animals may have an advantage over the less behaviorally responsive late-hatching members of their age cohort. More rapid development in the water may facilitate enhanced sensitivity to predator cues. Moreover, the opposite changes of tadpole FID induced by fish and insect cues are consistent with differences in the predator's foraging behavior and suggest that A. callidryas tadpoles have predator-specific responses.