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Head for the hills! A male giant water bug (and tag-along eggs) will flee its stream when flash floods threaten.

Water Bugs Flee the Flood

KNOXVILLE, TENNESSEE--Giant water bugs living in Arizona streams flee flash floods by climbing uphill when it rains, researchers have found. They suspect the bugs are taking advantage of an ancient behavior that once served another purpose: Related species migrate to temporary breeding pools when it rains. If so, the find is a prime example of a species co-opting an ancestral trait.

Two decades ago, evolutionary biologists Stephen Jay Gould and Elisabeth Vrba coined a new term, "exaptation," to denote a trait that had evolved for one purpose and then later in evolutionary history was co-opted for another. For example, the feathered tails that birds evolved for flight have come in handy as sexual ornaments for some species, such as peacocks.

Now entomologists David Lytle and Robert Smith of the University of Arizona in Tucson say they've found a prime example of exaptation, this time in the realm of behavior. Giant water bugs, a family of aquatic insects that can grow up to 6 centimeters long, usually live in ponds and use rainfall as a cue to fly to upland temporary pools to breed. Two species of giant water bug in Arizona's Chiricahua Mountains live in streams where flash floods can kill 90% of aquatic insects. But the giant water bugs suffer only 15% mortality, Lytle found, because they flee streams at the onset of heavy rain (or simulated rain from a fire hose) and return the next day.

Examining a family tree of giant water bugs, the researchers mapped which species seek temporary water holes for mating. They deduced that it's a 150-million-year-old ancestral trait shared by most members of the group and exapted twice by river-bound species that need to flee frequent floods. Lytle presented the findings here on 28 June at the annual joint meeting of the Society for the Study of Evolution, the Society of Systematic Biologists, and the American Society of Naturalists.

"It's a good sign" that researchers are combining behavioral studies with evolutionary thinking, says cladistic ethologist John Wenzel of Ohio State University in Columbus. "Up until recently, behavioral ecology operated without a time axis." In this case, the authors show "the adaptive behavior is clearly quite ancient," Wenzel says.

Related sites

Watch, in QuickTime, a giant water bug fleeing a sudden (fire-hose-induced) downpour
Robert Smith's Web page
More info on giant water bugs
The American Museum of Natural History's Southwestern Research Station, where the research was conducted