Developmental onset of escape-hatching responses in red-eyed treefrogs depends on cue type

Hatching is an essential and often behaviourally mediated process. Many animals can hatch at different developmental stages, and embryos time hatching based on cues indicating threats to eggs or opportunities outside them. However, specific mechanisms enabling such responses, and how their ontogenies combine to determine when environmentally cued hatching is possible, are largely unknown. Many embryos use hatching cues in multiple sensory modalities. Thus, comparing response onset across cue types can distinguish shared ontogenetic constraints, such as hatching ability, from modality-specific constraints, such as sensor development. The arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch rapidly in response to physical disturbance in predator attacks and hypoxia if flooded. Prior research documented both responses beginning at age 4 days. Because embryos orient in oxygen gradients long before this, we hypothesized the onset of hypoxia-cued hatching is limited by development of hatching ability. The onset of mechanosensory-cued hatching might share this constraint or be limited by a later-developing sensory mechanism. We tested developmental series for hypoxia-cued hatching, by submerging eggs in degassed water to impose strong hypoxia, and for mechanosensory-cued hatching, by manually jiggling eggs as a simulated attack. We identified morphological markers to distinguish developmental stages across the onset of hatching. Hatching competence begins substantially earlier than previously reported. Across sibships, hypoxia-cued hatching began at a smaller size and less developed stage, and on average 8 h earlier than mechanosensory-cued hatching. Both responses increased from 0% to 100% over just a few hours. Latency to hatch after stimulation was longer in hypoxia-cued hatching and uncorrelated with stage, whereas latency in mechanosensory-cued hatching decreased with development. Hypoxia-cued hatching appears constrained by the development of hatching ability, while mechanosensory-cued hatching appears constrained by mechanosensor development. Hatching ability is not the sole constraint on the onset of escape-hatching responses to attacks.