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.

Researchers have long known that an array of factors can affect when eggs laid by all kinds of creatures finally hatch. Some fish eggs, for instance, hatch only at certain light or temperature levels, while fungal infections can prompt lizard eggs to crack open early. Chemical or physical signals sent by predators can prompt some frog embryos to speed up their breakouts, while others delay hatching in a bid to stay safe. In lizards and other reptiles, however, such "environmentally cued hatching" strategies aren't well understood.

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.

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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."

There may be a downside to such emergency exits, however. "[E]arly hatching skinks were significantly smaller and left behind larger residual yolks in their eggs than spontaneously hatching skinks," the authors write, potentially reducing the chances of survival. Still, it is probably better to be stunted than eaten, Doody says.

The skink study is "very cool" and "very clear—we really don't have well documented examples like this in reptiles," says biologist Karen Warkentin of Boston University. In the 1990s, she discovered a tropical frog that can hatch early in response to vibrations from predators, and has since become a prominent scholar of hatching cues. There's growing evidence, she says, that embryos are much more sensitive to the world outside their eggs than once believed. "This is not just happening in delicate skinks—I'm thinking that environmentally cued hatching is very widespread, in many groups." But exactly how embryos make the decision to stay put or bail out, she says, "is something we're still trying to understand."