How do red-eyed treefrog embryos sense motion in predator attacks? Assessing the role of vestibular mechanoreception

Abstract

Red-eyed treefrogs' hatching responses to predator attacks, vibration playbacks, and egg-jiggling 1 appear when vestibular function develops. Ear development may be a key limiting factor in the 2 onset of mechanosensory-cued hatching. 3 ABSTRACT 4 5The widespread ability to alter hatching timing in response to environmental cues can 6 serve as a defense against threats to eggs. Arboreal embryos of red-eyed treefrogs, Agalychnis 7 callidryas, hatch up to 30% prematurely to escape predation. This escape-hatching response is 8 cued by physical disturbance of eggs during attacks, including vibrations or motion, and thus 9 depends critically on mechanosensory ability. Predator-induced hatching appears later in 10 development than flooding-induced, hypoxia-cued hatching; thus, its onset is not constrained by 11 the development of hatching ability. It may, instead, reflect the development of mechanosensor 12 function. We hypothesize that vestibular mechanoreception mediates escape-hatching in snake 13 attacks, and that the developmental period when hatching-competent embryos fail to flee from 14 snakes reflects a sensory constraint. We assessed the ontogenetic congruence of escape-hatching 15 responses and an indicator of vestibular function, the vestibulo-ocular reflex (VOR), in three 16ways. First, we measured VOR in two developmental series of embryos 3-7 days old to compare 17 with the published ontogeny of escape success in attacks. Second, during the period of greatest 18 variation in VOR and escape success, we compared hatching responses and VOR across 19 sibships. Finally, in developmental series, we compared the response of individual embryos to a 20 simulated attack cue with their VOR. The onset of VOR and hatching responses were largely 21 concurrent at all three scales. Moreover, latency to hatch in simulated attacks decreased with 22 increasing VOR. These results are consistent with a key role of the vestibular system in the 23 escape-hatching response of A. callidryas embryos to attacks. 24 25