Wasps drive frog eggs to (escape) hatch.

Tree frog embryos plop out of their eggs in moments of danger, and now a researcher has found that their responses are proportional to the threat.

That an embryo can respond at all to predators represents a recent rethinking of the powers of eggdom. One of the first pushes for this view came from studies of red-eyed tree frogs (Agalychnis callidryas), which lay their eggs in plants dangling over water. In 1995, Karen Warkentin, now of the University of Kentucky in Lexington, reported that a snake attack can prompt a whole clutch of the embryos to pop out of their eggs early.

"It's dramatic," says Warkentin. "An egg can hatch in a second. You have eggs hatching and larvae jumping out of the corner of the snake's mouth."

Now, she's presenting a more sophisticated embryo strategy. If a wasp attacks an egg a day or two before normal hatching, only that embryo and perhaps near neighbors usually try a quick escape, Warkentin reports in the October ANIMAL BEHAVIOR. The small-scale response makes sense, she argues. Wasps take only one embryo away at each visit, but a snake can quickly devour a clutch.

In an earlier study, Andrew Sih, also of the University of Kentucky, found the reverse response--a hatching delay--in streamside salamander eggs exposed to chemical traces of predatory sunfish or marauding gangs of flatworms. Since then, several other researchers have found evidence of danger-related hatching changes.

"I wouldn't say it was common yet," says Douglas Chivers of the University of Saskatchewan in Saskatoon, Canada. He predicts that animals facing greatly varying risks will be most likely to have emergency hatching. Chivers adds that as far as he knows, Warkentin's report of graded responses is a first.

Warkentin documented the frog embryos' response to wasps by monitoring egg clutches at a Panamanian pond. Nearly half of 122 clutches showed distinctive signs of wasp attacks.

In sessions of close watching, Warkentin saw wasps land on clutches and start ripping at an egg. Embryos that were at least 4 days old often burst out and sometimes got away. Although some premature hatching occurred spontaneously, most of the time, wasps were the cause, she says.

Warkentin also found that jiggling a clutch would trigger a mass hatching. However, she points out that tropical storms do not cause embryos to hatch early. "Perhaps it's something about the pattern of vibrations," Warkentin speculates. She adds that she doesn't know how the eggs can tell the difference.