Frogs have a hard time of it. They have a lot of predators, and creatures like snakes can come for them at any point in their lifecycle. It would seem safe to assume that embryonic tadpoles, still inside their eggs, would be especially vulnerable. But for red-eyed treefrogs, this isn’t the case: tadpoles can sense predators and hatch early in order to escape them.
Embryos from a number of species are constantly receiving information and nutrition as they grow, but some animals, like the red-eyed treefrog, show an ability to interact with the world around them. These embryos aren’t passive; they’re active, which creates a wonderful defense mechanism.
In the case of the red-eyed treefrog, which lives its adult life in trees, the eggs are laid on the undersides of leaves and branches close to water. Normally, the embryos would grow to full size in about a week and then drop into the water as tadpoles, where they go about the normal frog lifecycle. But eggs are vulnerable to snakes, wasps, and other predators, as well as environmental hazards. At least in the case of predators though, they have a defense.
The tadpoles inside the eggs use a special hatching enzyme that all frogs seem to have, but they can use that enzyme to break a hole in the egg before they would otherwise hatch, and then squirm out and drop into the water. The whole process takes on average 20 seconds in a laboratory environment with shaking motions to simulate approaching predators. In the wild though, the eggs can hatch even quicker when threatened by a real snake.
It’s not a foolproof plan, and when a snake or wasp attacks a bunch of eggs, some of them are likely going to be eaten before they have a chance to escape. But that’s why frogs lay big batches of eggs, around 40 or so in this case, so that at least some have a chance of surviving.