Meeting Abstract
Vibrations cue red-eyed treefrog (Agalychnis callidryas) embryos to hatch and escape in attacks by egg-eating snakes. We hypothesize that inner ear mechanoreceptors serve a key sensory role in this process. As a first step to testing this, we used the roll-induced gravitational vestibulo-ocular reflex (VOR) to assess the ontogeny of ear function. VOR generates eye movements that compensate for head movements to stabilize the visual field, and depends critically on vestibular function. To measure VOR, we placed newly hatched A. callidryas in a horizontal water-filled tube and rolled them around their body axis for 180° in both directions, in 15° increments. We photographed their faces at each angle, used ImageJ to measure eye and body angles, and fit sine curves to calculate the peak-to-peak amplitude of eye motion. Embryos begin escaping from predators early in Gosner stage 23 (usually age 4.3 d) with an initially weak response, which strengthens and becomes more consistent over the next day. We found a similar, temporally matched ontogenetic pattern of VOR. Next, we quantified hatching responses in vibration playbacks to clutches across the period of VOR onset and increase, and tested the VOR of 3–6 individuals per clutch immediately following playback. Our vibrational stimulus was 0–60 Hz noise, resembling snake vibrations, played in an intermittent temporal pattern that elicits nearly 100% hatching of older embryos. All embryos that hatched in playback showed VOR, consistent with a critical role for otic mechanoreceptors in vibration-cued hatching. However, across clutches showing VOR, the hatching response ranged from 0–88% and was not correlated with VOR magnitude, suggesting that additional factors contribute to the variation in embryo response.
