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Video reveals how iconic frog’s embryos escape death

Red-eyed tree frog embryos are famous for their Houdini-like behavior. The unhatched young of this neon green Central American rainforest amphibian (Agalychnis callidryas)—well known for its big, bulging eyes and fat, orange toes—can break out of their eggs at a moment’s notice if attacked by a snake, even before they are fully developed. Now, biologists have learned the secret of this rapid egress (see video). Researchers in Panama collected branches with newly deposited egg masses attached to the leaves, and used high-speed video in the lab to watch the escape in slow motion. When they jiggled the egg mass as an attacking snake would, the eggs started to shake. Inside each egg, the embryo thrashed around as if using force to break out. But the researchers also noticed the embryos were repeatedly opening and closing their mouths and that fluid began to leak out just in front of the embryo’s snout, even though the snout hadn’t made contact with the membrane until after the leaking started. Then the embryo stuck its snout through and squeezed out. Suspecting enzymes were creating this escape hatch (when most frog embryos are ready to hatch, they secrete enzymes from their heads that slowly degrade the egg membrane), the team scoured red-eyed tree frog embryo heads using electron microscopy and found clusters of glands on the snouts that were packed prior to hatching but were empty in newly hatched tadpoles. Thus, the embryos use both physical force and chemistry to foil the snake, the scientists report today in the Journal of Experimental Biology. Up to 80% can escape unharmed. By studying how the embryos coordinated both the secretion of the enzyme and the body movements, the researchers hope to better understand how the frogs—and other animals—are able to respond so quickly to threats.