Do developmental changes in fitness trade-offs predict how embryos use mechanosensory cues for escape-hatching decisions?

When defense is more costly, prey should differentiate more strongly between predator cues and benign stimuli and may therefore use more sources of information. Red-eyed treefrog (Agalychnis callidryas) embryos hatch prematurely to escape from egg-eating snakes and wasps, cued by physical disturbance in attacks. Missing predator cues is always costly, but false alarm costs decrease with development. We assessed developmental changes in how embryos use and combine information from two sensory modalities, using a playback system to present motion (shaking), tactile contact (rubbing), or both cue types to eggs in custom-made trays at two ages. Younger embryos showed a stronger hatching response to bimodal over unimodal cues. This synergistic effect disappeared in older embryos, which responded equally strongly to unimodal and bimodal cues and had a shorter latency to hatch. This indicates younger embryos – facing higher predation risk as tadpoles – use more information for their hatching decisions. We also investigated changes in response to tactile cues (simulated wasp attack) manually applied directly to embryos through the capsule (higher threat) or on the capsule away from embryo (lower threat). Younger embryos hatched faster in response to direct tactile contact than capsule-only contact, whereas older embryos responded equally to both. Both within and across sensory modalities, developmental changes in embryos’ cue use are consistent with ontogenetic adaptation, based on improved survival chances outside the egg. Embryo hatching timing can be crucial for survival, and the cognitive processes underlying their behavioral responses have likely been shaped by developmentally changing selection pressures.