Every red-eyed tree frog must confront this dilemma. The frogs lay their eggs on plants that lie over ponds. After a week, the tadpoles hatch and drop into the water—at least in theory. In practice, many of the defenceless, immobile, exposed, yummy eggs are devoured by snakes or wasps. Fortunately, they have a solution: super-fast hatching.
For most frogs, hatching is a slow process. The tadpoles release enzymes that break down the jelly coatings of their eggs over the course of several hours. But red-eyed tree frog eggs can hatch in seconds, if the need arises. Karen Warkentin discovered this ability in 1995, and she has spent the last 20 years exploring it. She has shown that the frogs can hatch early to escape snakes, wasps, flooding, drought, and infectious fungi.
The trick comes at a cost: the premature hatchlings are smaller and more vulnerable to threats in the water, but at least they survive aerial dangers some 80 percent of the time.
That explains why the frogs have evolved their rapid-hatching trick, but not how the trick works. Throughout her work, Warkentin didn’t know. She assumed that since the process was so fast, the embryos couldn’t be releasing enzymes as other frogs do. Instead, by thrashing about inside the egg, they were probably strong-arming their way out.
Credit: Karen M. Warkentin
To see exactly what they do, graduate student Kristina Cohen filmed the youngsters using a high-speed video camera. On the slowed footage, she noticed that an embryo can create a hole in its egg without touching anything. It starts shaking, while opening and closing its mouths. Soon, fluid starts to leak from the part of the egg directly in front of the embryo’s snout. It then lodges its snout against the point of rupture, and expands it by wriggling, eventually propelling itself through.
To confirm that the whole sequence begins without contact between the tadpole and its egg wall, Cohen waited until the embryos started to shake, and then turned them around inside their eggs by nudging them with a blunt rod. Even though they had moved, the rupture would form at the place where their snouts used to be.
The frogs, it turned out, were using enzymes after all. But rather than releasing those enzymes gradually, they stockpile their supply. By studying the embryos under a powerful microscope, Cohen found that they have a dense cluster of glands in their snout. Each is full of small packets that contain egg-dissolving enzymes. When danger threatens, the embryos can release these all at once, pressing fast-forward on the hatching process.
“They could do it in six seconds,” says Cohen—and in other experiments, “we’ve recorded them getting out in less than that.”
This is one of several studies showing that embryos are not just passively awaiting their emergence into the world. They are already part of it.
Before they hatch, for example, cuttlefish embryos can learn about potential prey, shark embryos can use their electric senses to avoid danger by freezing, and turtle embryos can shimmy to the warmest sides of their eggs. “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,” says Warkentin.