Frog Embryos Escape From Snakes by Releasing Egg-Dissolving Enzymes From Their Faces

June 16, 2016 | Erica Tennenhouse

Treefrog eggs
Photo credit: Geoff Gallice/Wikipedia (CC BY 2.0)

Speedy hatching in the face of danger.

Frog embryos, still encased in their eggs, may seem defenseless against egg-eating snakes and wasps. But red-eyed treefrog babies have cooked up an effective method to escape these threats: they quickly hatch on demand.

"Most people think of embryos as fairly passive," said Boston University professor Karen Warkentin in a press release. "But evidence keeps accumulating that embryos of many species are actively engaged with their world, not only receiving information but also using it to do things that help them survive."

Normally, frog embryos hatch slowly, excising themselves from their protective coating over a period of hours. The discovery of their rapid emergence when under attack had scientists scratching their heads. How were the treefrog embryos orchestrating this premature hatching?

To get a better look at what the embryos were up to, Warkentin’s graduate student Kristina Cohen filmed them hatching in response to a simulated predator on high-speed video.

"They do a shaking behavior while releasing enzymes from glands concentrated on their snouts," Warkentin said. "That movement seems to push them snout-first against the hole the enzyme makes in the egg membrane. Then they muscle their way out by using big, S-shaped thrashing movements."

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The study, published in the Journal of Experimental Biology, reported that the premature tadpoles were able to hatch and drop from their perches in about 20 seconds when confronted by a threat.

"The process of getting out of the egg is the embryo's first, tiny, athletic event," Warkentin said.

It became apparent that the embryos were releasing an enzyme to break open their eggs when the researchers looked at the gland clustered in the embryos’ snouts under a powerful microscope. The glands of undisturbed embryos contained swollen sacs full of fluid, but immediately after hatching, the sacs appeared deflated.

The researchers concluded that the embryos use their snout glands to release a precise squirt of enzyme, burning a hole through the egg membrane, through which they burst to freedom.

Watch a video of the red-eyed treefrog embryos hatching here.

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