Within just 5 years, singing has nearly died out among a population of cricket on a Hawaiian island, researchers report.
A mutation for silence has spread so fast because an invasion of deadly flies finds male crickets to attack by following their chirps, says Marlene Zuk of the University of California, Riverside.
However, female crickets also listen for chirps to find the males, so a guy who can’t sing has a problem. The mute males seem to be coping, at least temporarily, by clustering around the few remaining chirpers, Zuk and her colleagues report in the Dec. 22 Biology Letters.
“What surprises me most is that the cricket song went away so fast,” says Ron Hoy of Cornell University, who also studies crickets and the song-tracking fly. “Natural selection is coming down like a hammer,” he says.
The crickets (Teleogryllus oceanicus) that are losing their songs came to Hawaii from Australia and western Pacific islands. Their nemesis, the Ormia ochracea fly, invaded the Hawaiian Islands from North America. These flies are about the size of houseflies but have big red eyes and fly at dawn or dusk.
“You’ve never seen them,” says Zuk. “You don’t have anything they want.”
In 1975, William Cade, now of the University of Lethbridge (Alberta), reported that the female fly follows cricket chirps to deposit larvae on a male. The larvae dig in and eat the cricket from the inside.
Zuk and her colleagues found in the 1990s that Kauai had intense fly infestations, parasitizing one-third of the crickets. In her 2001 survey, she heard only one male calling.
In 2003, Zuk didn’t hear anything in her night searches. Then, she says, “suddenly, in my headlamp, I start seeing crickets.” For insect biologists, quiet, nighttime male crickets are shocking. “It’s like finding out that peacocks dropped their tails,” Zuk says.
She found that the cricket population on Kauai was higher than it had been for years, but few males still had wings with functional chirping equipment. When coauthor Robin Tinghitella, also of Riverside, bred those crickets, she concluded that the silence came from changes in only a gene or two.
When the researchers broadcast recorded cricket chirping, they found that the mutant males hopped unusually close to the speakers. Lurking near a chirping male is probably their only way to meet females, says Zuk.
Once a female arrives, a male cricket normally produces a special, soft courtship chirp to persuade her to mate. Theorists have proposed that females of island species tend not to be too choosy. As Zuk puts it, “If there are only four guys and you don’t like any of them, you die, and so do your genes.”
Zuk’s work gives “the most dramatic example yet of how flies have shaped communication in crickets,” says Cade.