When and where to hatch? Red-eyed treefrog embryos use light cues in two contexts

Abstract

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, suggesting 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.

Heat-Induced Hatching of Red-Eyed Treefrog Embryos: Hydration and Clutch Structure Increase Behavioral Thermal Tolerance

“…The mechanism underlying this response is unknown but might involve increased osmolality of perivitelline and body fluids or the increased concentration of specific molecules, such as ammonia ( Méndez-Narvaez and Warkentin 2022 ). In addition, these embryos are known to combine information across cue properties and across sensory modalities for their hatching decisions ( Warkentin and Caldwell 2009 ; Güell and Warkentin 2018 ; Jung et al. 2020 ).…”

“…2005 ; Rogge and Warkentin 2008 ). This air-facing orientation also ensures that embryos will hatch into the air and can fall to the water rather than hatching into the jelly where they can be trapped between their siblings and the leaf ( Güell and Warkentin 2018 ). In addition to orienting in oxygen gradients ( Rogge and Warkentin 2008 ), A. callidryas embryos may, like turtle embryos ( Zhao et al.…”

Ontogeny of risk assessment and escape-hatching performance by red-eyed treefrog embryos in two threat contexts

“…They might also behaviorally facilitate rupture by pressing their head against the membrane to increase HGC-membrane contact. Flooded embryos lose the oxygen gradient that helps them orient-and hatch-toward the exposed side of their egg, and in glass cups they also lose directional light cues; this increases the frequency of hatching complications and need for a second rupture site (Güell and Warkentin, 2018;Salazar-Nicholls et al, 2017). Moreover, in both snake and wasp attacks embryos sometimes move-or are pushed-away from their initial rupture site (KMW, observations from video recordings).…”