Red-eyed tree frogs, Agalychnis callidryas, lay their eggs on foliage at the edge of ponds; when the tadpoles hatch, they fall into the water. Normally, an egg hatches six to seven days after it is laid. The ones that [Karen] Warkentin is pointing to, judging from their size and shape, are about five days old, she says. Tiny bodies show through the clear gel-filled membrane. Under a microscope, the red hearts would just be visible.
She reaches down to wet her hand in the pond water. “They don’t really want to hatch,” she says, “but they can.” She pulls the leaf out over the water and gently runs a finger over the eggs.
Sproing! A tiny tadpole breaks out. It lands partway down the leaf, twitches and falls into the water. Another and another of its siblings follow. “It’s not something I get tired of watching,” Warkentin says.
With just a flick of her finger, Warkentin has demonstrated a phenomenon that is transforming biology. After decades of thinking of genes as a “blueprint”—the coded DNA strands dictate to our cells exactly what to do and when to do it—biologists are coming to terms with a confounding reality. Life, even an entity as seemingly simple as a frog egg, is flexible. It has options. At five days or so, red-eyed tree frog eggs, developing right on schedule, can suddenly take a different path if they detect vibrations from an attacking snake: They hatch early and try their luck in the pond below. - Continue reading at Smithsonian.com.