Frog embryos use multiple levels of temporal pattern in risk assessment for vibration-cued escape hatching

The Role of Vibration Amplitude in the Escape-Hatching Response of Red-Eyed Treefrog Embryos

The function and adaptive significance of defensive behaviors depend on the contexts in which they naturally occur. Amplitude properties of predator cues are widely used by prey to assess predation risk, yet rarely studied in the context of the stimuli relevant to defensive decisions in nature. Red-eyed treefrog embryos, Agalychnis callidryas, hatch precociously in response to attacks on their arboreal egg clutches by snakes and wasps. They use vibrations excited during attacks to detect predators, but wind and rainstorms also excite intense vibrations. Past work has demonstrated that to avoid costly decision errors, A. callidryas nonredundantly combine information from the temporal and frequency properties of clutch vibrations. Here, we demonstrate that embryos also use absolute amplitude and fine-scale amplitude modulation information to refine their hatching decision. We used vibration recordings to characterize the amplitude properties of the most common predator and benign-source disturbances to A. callidryas egg clutches in nature and tested whether embryos at 3 ages across the onset of mechanosensory-cued hatching (4-6 days) respond to amplitude variation during playback of synthetic vibrations to eggs. Older embryos responded to much lower-amplitude vibrations, reflecting a >88-fold decrease in response threshold from 4 to 5 days. To assess how embryos combine amplitude with other vibration properties, we played embryos recorded exemplars of snake attack and rain vibrations of varying amplitudes and patterns of amplitude modulation. The amplitude response curve was steeper for snake recordings than for rain. While amplitude information alone is insufficient to discriminate predator attack from benign-source vibrations, A. callidryas employ an impressively complex strategy combining absolute amplitude, amplitude modulation, temporal, and frequency information for their hatching decision.

Egg-Clutch Biomechanics Affect Escape-Hatching Behavior and Performance

Arboreal embryos of phyllomedusine treefrogs hatch prematurely to escape snake predation, cued by vibrations in their egg clutches during attacks. However, escape success varies between species, from ∼77% in Agalychnis callidryas to just ∼9% in A. spurrelli at 1 day premature. Both species begin responding to snake attacks at similar developmental stages, when vestibular mechanosensory function begins, suggesting that sensory ability does not limit the hatching response in A. spurrelli. Agalychnis callidryas clutches are thick and gelatinous, while A. spurrelli clutches are thinner and stiffer. We hypothesized that this structural difference alters the egg motion excited by attacks. Since vibrations excited by snakes must propagate through clutches to reach embryos, we hypothesized that the species difference in attack-induced hatching may reflect effects of clutch biomechanics on the cues available to embryos. Mechanics predicts that thinner, stiffer structures have higher free vibration frequencies, greater spatial attenuation, and faster vibration damping than thicker, more flexible structures. We assessed clutch biomechanics by embedding small accelerometers in clutches of both species and recording vibrations during standardized excitation tests at two distances from the accelerometer. Analyses of recorded vibrations showed that A. spurrelli clutches have higher free vibration frequencies and greater vibration damping than A. callidryas clutches. Higher frequencies elicit less hatching in A. callidryas, and greater damping could reduce the amount of vibration embryos can perceive. To directly test if clutch structure affects escape success in snake attacks, we transplanted A. spurrelli eggs into A. callidryas clutches and compared their escape rates with untransplanted, age-matched conspecific controls. We also performed reciprocal transplantation of eggs between pairs of A. callidryas clutches as a method control. Transplanting A. spurrelli embryos into A. callidryas clutches nearly tripled their escape success (44%) compared to conspecific controls (15%), whereas transplanting A. callidryas embryos into different A. callidryas clutches only increased escape success by 10%. At hatching competence, A. callidryas eggs are no longer jelly-encapsulated, while A. spurrelli eggs retain their jelly coat. Therefore, we compared the hatching response and latency of A. spurrelli in de-jellied eggs and their control, jelly-encapsulated siblings using manual egg-jiggling to simulate predation cues. Embryos in de-jellied eggs were more likely to hatch and hatched faster than control siblings. Together, our results suggest that the properties of parentally produced egg-clutch structures, including their vibration biomechanics, constrain the information available to A. spurrelli embryos and contribute to interspecific differences in hatching responses to predator attacks.

Early onset of urea synthesis and ammonia detoxification pathways in three terrestrially developing frogs

Frogs evolved terrestrial development multiple times, necessitating mechanisms to avoid ammonia toxicity at early stages. Urea synthesis from ammonia is a key adaptation that reduces water dependence after metamorphosis. We tested for early expression and plasticity of enzymatic mechanisms of ammonia detoxification in three terrestrial-breeding frogs: foam-nest-dwelling larvae of Leptodactylus fragilis (Lf) and arboreal embryos of Hyalinobatrachium fleischmanni (Hf) and Agalychnis callidryas (Ac). Activity of two ornithine-urea cycle (OUC) enzymes, arginase and CPSase, and levels of their products urea and CP in tissues were high in Lf regardless of nest hydration, but reduced in experimental low- vs. high-ammonia environments. High OUC activity in wet and dry nests, comparable to that under experimental high ammonia, suggests terrestrial Lf larvae maintain high capacity for urea excretion regardless of their immediate risk of ammonia toxicity. This may aid survival through unpredictably long waiting periods before rain enables their transition to water. Moderate levels of urea and CP were present in Hf and Ac tissues and enzymatic activities were lower than in Lf. In both species, embryos in drying clutches can hatch and enter the water early, behaviorally avoiding ammonia toxicity. Moreover, glutamine synthetase was active in early stages of all three species, condensing ammonia and glutamate to glutamine as another mechanism of detoxification. Enzyme activity appeared highest in Lf, although substrate and product levels were higher in Ac and Lf. Our results reveal that multiple biochemical mechanisms of ammonia detoxification occur in early life stages of anuran lineages that evolved terrestrial development.