The role of vibration amplitude in the escape hatching response of red-eyed treefrog embryos

The function and adaptive significance of defensive behaviors depend on the contexts in which they naturally occur. Amplitude properties of predator cues are widely used by prey to assess predation risk, yet rarely studied in the context of the stimuli relevant to defensive decisions in nature. Red-eyed treefrog embryos, Agalychnis callidryas, hatch precociously in response to attacks on their arboreal egg clutches by snakes and wasps. They use vibrations excited during attacks to detect predators, but wind and rainstorms also excite intense vibrations. Past work has demonstrated that to avoid costly decision errors, A. callidryas non-redundantly combine information from the temporal and frequency properties of clutch vibrations. Here we demonstrate that embryos also use absolute amplitude and fine-scale amplitude modulation information to refine their hatching decision. We used vibration recordings to characterize the amplitude properties of the most common predator and benign-source disturbances to A. callidryas egg clutches in nature and tested whether embryos at three ages across the onset of mechanosensory-cued hatching (4-6 days) respond to amplitude variation during playback of synthetic vibrations to eggs. Older embryos responded to much lower-amplitude vibrations, reflecting a >88-fold decrease in response threshold from 4 to 5 days. To assess how embryos combine amplitude with other vibration properties, we played embryos recorded exemplars of snake attack and rain vibrations of varying amplitudes and patterns of amplitude modulation. The amplitude response curve was steeper for snake recordings than for rain. While amplitude information alone is insufficient to discriminate predator attack from benign-source vibrations, A. callidryas employ an impressively complex strategy combining absolute amplitude, amplitude modulation, temporal, and frequency information for their hatching decision.