Meeting Abstract
Eggs face many biotic and abiotic threats, but embryos can escape some dangers by hatching early. Arboreal embryos of red-eyed treefrogs hatch early to escape from acute threats, such as egg-predators and flooding. They can also respond to slow-acting threats such as dehydration, in which the timing of impending mortality is less clear. We compared the timing and process of hatching from dry and well-hydrated eggs. To hydrate clutches, we sprayed them hourly with an automatic mister. We dried clutches at 5–10% below ambient humidity, with infrequent manual spraying. All eggs initially absorbed water from their jelly and swelled, but by age 3 days dry eggs were shrinking. We monitored hatching hourly (age 4.4–6.9 days) and measured the first hatchling from each clutch from photos. We induced other hydrated controls to hatch when dry eggs did, for an age-matched comparison. All clutches hatched gradually, with dry clutches hatching about a day earlier than wet ones. Hatchlings from dry eggs were smaller than age-matched wet controls, and much smaller than later-hatched wet ones. We video-recorded hatching from dry and wet clutches during their peak hatching times. In wet eggs, embryos moved easily in their capsule, showed little clear pre-hatching behavior, and simply ruptured the capsule then rapidly slid out. In dry eggs, most embryos became constrained in one of two positions, and position strongly affected ability to move and hatching success. Many struggled repeatedly in their shrunken capsules before breaking free, but exhibited no body compression during exit, suggesting they made large exit holes. Hydration-dependent differences in the physical structure of egg capsules combine with ontogenetic changes in embryo abilities to generate variation in the hatching process.
