Scientists Speak 24.1
Red-eyed treefrogs, they’re like the poster frog of the rain forest. They’re very beautiful, they’re much photographed. Bright green, red eyes, orange feet.
Where we work with them is at the ponds where they come to breed.
The pairs of frogs lay their eggs in gelatinous egg masses, stuck on leaves, overhanging the water. They don’t have parents looking after them. The parents just, like, lay them and go away, so the eggs are really kind of on their own in the world, with all sorts of threats and enemies. They can’t really fight back or defend themselves. They can’t do anything bad to a predator. But if they’re about to be eaten by a snake or succumb to a fungus infection, or suffocate because their egg clutch has fallen into the pond and they’re no longer getting air, they can hatch early and escape from all of those threats.
I’ve been studying hatching in these frogs since 1991. We used to think that hatching was basically a developmental event that just kind of occurred just like gastrulation or the onset of a heartbeat. We now know that the embryos are extremely responsive to their environment, and they can hatch prematurely basically in response to anything that would kill them as an embryo in the egg.
What exactly is it that the embryos are paying attention to? We know that the embryos use vibration to assess risk in the context of snake attacks. So a snake will come along, start biting them, moving them, jiggling them. Those vibrations are enough to make the embryos hatch.
By recording the vibrations from those attacks and playing them back in various modified ways, we can see what are the things that make them hatch, and what do they just ignore.
The embryos have to be able to discriminate what’s a dangerous jiggle from what’s just the wind or the rain or some benign thing that’s jiggling them. And they do, and they’re very good at it. They don’t respond to frequencies that are not present in predator attacks.
Also, we know that the embryos will hatch prematurely if they’re about to suffocate. So we put an individual egg in a little glass cup. We flood them, we videotape them, and then we can see: How good are they at hatching? Does it take them a really long time to get out of the egg, or can they just pop right out?
In their egg masses, normally they have this orientation with their face and their gills to the front, where the oxygen is coming in. And if they turn around, they’ll move again. And they keep moving until they’re oriented back to where the oxygen is.
When they’re flooded, their first response is, “Oh, I must facing the back. Turn around. Turn around.” In order to make a good decision when they need to hatch or not in flooding, they need to first know that they’re not just facing the wrong way.
So they’re really as embryos gathering information about their environment and making decisions that are improving their survival.
Adaptive embryo behavior: It opens up a whole world of questions that we can ask now about eggs.