Talk about hatching an escape plan. Unborn lizards can erupt from their eggs days early if vibrations hint at a threat from a hungry predator, new research shows. The premature hatchlings literally "hit the ground running—they hatch and launch into a sprint at the same time," says behavioral ecologist J. Sean Doody, who is now at the University of Tennessee, Knoxville.

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That curtain began to lift a bit a few years ago, when Doody and student Philip Paull of Monash University in Australia began studying a population of delicate skinks (Lampropholis delicata) in a park near Sydney. There, the common lizards laid white, leathery eggs the size of aspirin capsules in rock crevices. The eggs generally incubate for 4 to 8 weeks before hatching, but Doody got a surprise in 2010, when he and Paull were plucking eggs from the crevices to make measurements. "They started hatching in our hands, at just a touch—it shocked us," Doody recalls. "It turned into a real mess, they were just hatching everywhere."

Soon, Doody launched a more systematic study of the phenomenon. In two lab experiments, the researchers compared the hatching dates for skink eggs exposed to vibrations with those of eggs that weren't shaken. And in three field experiments, they poked and prodded eggs with a small stick, or squeezed them gently with their fingers to measure how sensitive the eggs were to the kinds of disturbances a predator, such as a snake, might cause. They also measured how far the premature hatchlings could dash.

Together, the experiments offer "compelling evidence" that embryonic skinks can detect and respond to predator-like signals, the authors write in the March 2013 issue of Copeia. The vibrated laboratory eggs, for instance, hatched an average of 3.4 days earlier than the unshaken controls. And in the field, the hatching of disturbed eggs was "explosive," they note; the newborns often broke out of the egg and then sprinted more than one-half meter to nearby cover in just a few seconds. "It's amazing," Doody says. "It can be hard to see because it happens so quick."