Hatching out of an egg is normally a long, laborious process. Most frog eggs take at least a few hours to hatch as the frog embryos release enzymes that slowly dissolve the egg membrane. They can then push and wiggle their way out. Recently, scientists have discovered that the red-eyed treefrog is an exception. Red-eyed treefrog eggs can hatch in seconds if there’s danger, such as a nearby treesnake.
When a red-eyed treefrog egg is shaken or otherwise moved around, the embryo can hatch out in less than 10 seconds. The newly hatched embryo can then drop down into water, escaping the predator. Previously, it was assumed that they were forcing their way out of the egg somehow but no one had studied the exact mechanism.
Researchers from Boston University and the Smithsonian Tropical Research Institute collaborated to better understand this process. They collected red-eyed treefrog eggs and allowed them to mature for five days. Mimicking the presence of a predator, the scientists pushed the eggs around to start the hatching process. Oddly, the embryos began opening and closing their mouths. A hole appeared before they even touched the membrane, discounting the theory that they were breaking out by force.
Using a high speed camera, the researchers were able to solve the mystery. The frog embryos were releasing a special enzyme from their snouts. This enzyme rapidly eats away at the egg membrane, allowing the tadpoles to easily push their way out. Further analysis using electron microscopy confirmed that the frogs had glands in their snouts that release the enzyme just before hatching.
This is the first recorded case of an amphibian hatching early in order to escape predation and other dangers. The authors note that other amphibians may have similar mechanisms. Amphibian eggs normally hatch after receiving environmental cues (for example, weather and temperature); the red-eyed treefrog escape hatching is simply an adaptation of this process.
Cohen, K. L. et al. How embryos escape from danger: the mechanism of rapid, plastic hatching in red-eyed treefrogs. Journal of Experimental Biology (2016).