Agalychnis callidryas lays gelatinous clutches on vegetation overhanging still water. Undisturbed embryos hatch at around six to seven days and fall into the pond below. If a snake begins eating the clutch, or a wasp starts removing embryos, siblings still inside their capsules can hatch in well under a minute and drop clear.
The embryos discriminate rather than panicking at any movement. Experimental work by Karen Warkentin and colleagues showed they respond to particular patterns of vibration, and largely ignore the shaking produced by rain or wind. Hatching is achieved by releasing enzymes from glands on the snout that open the capsule.
Escaping early is not free. Premature hatchlings enter the water smaller and less developed, and are more vulnerable to aquatic predators such as dragonfly larvae and fish. The embryo is therefore trading one risk for another, and the decision point shifts with age: very young embryos are less able to hatch on demand than those close to their normal hatching day.