How do embryos assess risk? Vibrational cues in predator-induced hatching of red-eyed treefrogs

I examined the role of vibrations in predator-induced early hatching of red-eyed treefrogs, Agalychnis callidryas. The arboreal eggs of A. callidryas hatch up to 30% early if attacked by egg-eating snakes. This induced hatching is a behavioural response that occurs once snakes begin physically disturbing the clutch, and is sufficiently rapid to allow most embryos to escape. Other intense but benign disturbances, such as tropical storms, do not induce such hatching. I used a miniature accelerometer to record vibrations in egg clutches during snake attacks and rainstorms, and analysed the recordings to identify parameters that distinguished disturbance types. Snake-induced vibrations were on average longer, more widely spaced, and of lower frequency than rain-induced vibrations. I performed three sets of vibration playbacks to examine the hatching response of embryos to different vibration patterns. (1) Playbacks of recorded snake attacks elicited more hatching than did rain recordings. (2) I edited snake and rain recordings by moving periods of stillness to clump together rain vibrations and extend intervals, and divide snake vibrations into shorter, more tightly spaced bits. In playbacks, clumped rain elicited more, and divided snake vibrations less hatching than did the original recordings. (3) Bursts of white noise with a constant vibration-to-interval ratio but different cycle durations elicited different levels of hatching. Vibrations alone were sufficient to induce early hatching, without chemical or visual cues from predators. Embryos also clearly distinguished among different vibration patterns and used cues in the gross temporal pattern in making their hatching decision.