CALDWELL, M.S.*; MCDANIEL, J.G.; WARKENTIN, K.M.; Boston University: Do red-eyed treefrog embryos use frequency cues in distinguishing egg predators from benign disturbances?

Red-eyed treefrogs Agalychnis callidryas lay eggs on leaves overhanging seasonal ponds. If attacked by egg-eating snakes or wasps, the embryos hatch rapidly up to 30% prematurely, escaping to the water below. Other disturbances, such as rainstorms, do not induce early hatching. Embryos use vibrations transmitted through their gelatinous egg mass to detect predator attacks and discriminate between dangerous and benign disturbances. Here we investigate the role of vibration frequency in the early hatching decision. We used standard modal testing methods from mechanical engineering to assess the free vibration frequencies of A. callidryas egg clutches. We also analyzed vibration recordings from natural disturbances to determine their frequency content. We then used playbacks of synthetic vibrations to construct a frequency response curve for early hatching, across the range of frequencies in natural disturbances. Free vibration frequencies of A. callidryas egg clutches are low, with a dominant mode often near 20 Hz, and most energy in the 10-50 Hz range. Dangerous and benign natural disturbances both contain energy in this range, as well as at higher frequencies. Most energy in snake attacks is below 100 Hz, most energy in wasp attacks is below 200 Hz, and most energy in rainstorms is below 400 Hz. Disturbances caused by rain contain substantial energy across higher frequencies that are not strongly excited in predator attacks. This suggests that embryos could use frequency information to discriminate between dangerous and benign disturbances. Results from our playback experiments support this hypothesis. We found high levels of hatching in response to frequencies found in predator attacks and almost no hatching in response to the higher frequencies excited by rainstorms.