Somewhere in Hawaii, a population of crickets has grown eerily silent. The culprit? Evolution. To avoid detection by a parasitic fly, the insects have--over only 20 generations--developed wings that don't chirp. But the silence has its downside: The muted males can no longer catch the attention of potential mates.
The black field cricket (Teleogryllus oceanicus) hails from Fiji, Tahiti, and Australia, but at some point it made its way to the Hawaiian Islands. Here, it met another island invader: a parasitic fly known as Ormia ochracea, which is native to North America. The fly quickly learned to home in on the cricket's mating call, a chirping the cricket makes by dragging its grooved wings against each other.
Behavioral ecologist Marlene Zuk of the University of California, Riverside, has witnessed the consequences firsthand. Since 1991, she's studied a population of black field crickets on the island of Kauai, a third of which harbor the parasitic flies. And each year, she's heard less and less chirping at night. Clearly, the cricket population was on the decline.
Or was it? In 2003, the lawns surrounding Zuk's research station were silent, yet Zuk was astonished to see her headlamp lighting up crickets. "They were way more abundant then they had been [in preceding years]," she says. Back in the lab, graduate student Robin Tinghitella discovered that the wings of these silent crickets were flat, lacking the grooved ridges needed to make noise. More than 90% of the Kauai crickets the researchers examined had this aberration.
So how do these flatwings find mates? Additional experiments suggested that they need a little help from the handful of their brethren still able to raise a ruckus. When researchers played recorded mating calls in the center of a patch of grass, 108 silent crickets came to within 125 centimeters of the speaker--up to three times the number of crickets that gathered in similar experiments on the big island of Hawaii and on Oahu, where the parasitic flies attack a lower percentage of crickets. That suggests that flatwing males position themselves close to normal males from their local population and intercept females attracted to the call of the normal males, the researchers postulate in tomorrow's issue of Biology Letters. Although such interloping behavior has been seen in other species, this is the first time it's been required for the males to find a mate, says Zuk. She estimates that the flatwing aberration developed over 20 generations, or about 4 or 5 years.
That's an amazingly fast change, says evolutionary biologist William Cade of the University of Lethbridge in Alberta, Canada. Still, whether this struggle for survival will allow the flies and crickets to coexist peacefully or end in mutual annihilation is anyone's guess. The flies have grown to depend on the crickets, but the male crickets are having a harder and harder time finding a mate. Evolutionary biologist Darryl Gwynne of the University of Toronto at Mississauga says he's waiting "with bated breath to see what evolution is going to give us."
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