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Hatching timing is under strong selection and environmentally cued in many species.Embryos use multiple sensory modalities to inform hatching timing and many have spontaneous hatching patterns adaptively synchronized to natural cycles. Embryos can also adaptively shift their hatching timing in response to environmental cues indicating immediate threats or opportunities. Such cued shifts in hatching are widespread among amphibians; however, we know little about what, if anything, regulates their spontaneous hatching. Moreover, in addition to selection on hatching timing, embryos may experience benefits or suffer costs due to the spatial orientation of hatching. Amphibian eggs generally lack internal constraints on hatching direction but embryos might, nonetheless, use external cues to inform hatching orientation. The terrestrial embryos of red-eyed treefrogs, Agalychnis callidryas, hatch rapidly and prematurely in response to vibrational cues in egg-predator attacks and hypoxia if flooded. Here we examined A. callidryas' use of light cues in hatching timing and orientation. To assess patterns of spontaneous hatching and the role of light cues in their diel timing, we recorded hatching times for siblings distributed across three light environments: continuous light, continuous dark, and a 12L:12D photoperiod. Under a natural photoperiod, embryos showed a clear diel pattern of synchronous hatching shortly after nightfall. Hatching was desynchronized in both continuous light and continuous darkness. It was also delayed by continuous light, but not accelerated by continuous dark, suggesting the onset of dark serves as a hatching cue. We examined hatching orientation and light as a potential directional cue for flooded embryos. Embryos flooded in their clutches almost always hatched toward open water, whereas individual eggs flooded in glass cups often failed to do so, suggesting the natural context provides a directional cue. To test if flooded embryos orient hatching toward light, we placed individual eggs in tubes with one end illuminated and the other dark, then flooded them and recorded hatching direction. Most embryos hatched toward the light, suggestingPeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27026v1 | CC BY 4.0 Open Access | recthey use light as a directional cue. Our results support that A. callidryas embryos use light cues to inform both when and where to hatch. Both the spatial orientation of hatching and the timing of spontaneous hatching may affect fitness and be informed by cues in a broader range of species than is currently appreciated. TitleWhen and where to hatch? Red-eyed treefrog embryos use light cues in two contexts Short titleRed-eyed treefrog embryos use light cues Abstract 2 3 Hatching timing is under strong selection and environmentally cued in many species. Embryos 4 use multiple sensory modalities to inform hatching timing and many have spontaneous hatching 5 patterns adaptively synchronized to natural cycles. Embryos can also adaptively shift their 6 hatching timing in resp...

Hatching timing is under strong selection and environmentally cued in many species.Embryos use multiple sensory modalities to inform hatching timing and many have spontaneous hatching patterns adaptively synchronized to natural cycles. Embryos can also adaptively shift their hatching timing in response to environmental cues indicating immediate threats or opportunities. Such cued shifts in hatching are widespread among amphibians; however, we know little about what, if anything, regulates their spontaneous hatching. Moreover, in addition to selection on hatching timing, embryos may experience benefits or suffer costs due to the spatial orientation of hatching. Amphibian eggs generally lack internal constraints on hatching direction but embryos might, nonetheless, use external cues to inform hatching orientation. The terrestrial embryos of red-eyed treefrogs, Agalychnis callidryas, hatch rapidly and prematurely in response to vibrational cues in egg-predator attacks and hypoxia if flooded. Here we examined A. callidryas' use of light cues in hatching timing and orientation. To assess patterns of spontaneous hatching and the role of light cues in their diel timing, we recorded hatching times for siblings distributed across three light environments: continuous light, continuous dark, and a 12L:12D photoperiod. Under a natural photoperiod, embryos showed a clear diel pattern of synchronous hatching shortly after nightfall. Hatching was desynchronized in both continuous light and continuous darkness. It was also delayed by continuous light, but not accelerated by continuous dark, suggesting the onset of dark serves as a hatching cue. We examined hatching orientation and light as a potential directional cue for flooded embryos. they use light as a directional cue. Our results support that A. callidryas embryos use light cues to inform both when and where to hatch. Both the spatial orientation of hatching and the timing of spontaneous hatching may affect fitness and be informed by cues in a broader range of species than is currently appreciated. TitleWhen and where to hatch? Red-eyed treefrog embryos use light cues in two contexts Short titleRed-eyed treefrog embryos use light cues We also used a test of equal proportions to test 283 for a difference in the incidence of hatching into the jelly or glass between whole-clutch and 284 individual egg flooding experiments, based on the null hypothesis that the proportion of hatching 285 into the glass in individual trials was equal to the observed average incidence of embryos 286 hatching into the jelly in whole-clutch flooding experiments (3.4%). We then tested for non-287 random hatching orientation in the presence of a light cue using a test of equal proportions, based 288 on the null hypothesis that the probability of hatching in each direction was 50%. We used a 289 GLMM to test whether the 'side of insertion into the tube' affected hatching direction, including 290 clutch as a random factor (N = 57 trials for which side of insertion was recorded). For all 291 statisti...