Ten years ago male crickets on the Hawaiian islands of Oahu and Kauai began to fall mysteriously silent, and now scientists have discovered why.
The culprit is a parasitic fly, relatively new to the region, that’s attracted to the sound of the male field cricket’s song. After the fly’s arrival, crickets on both islands underwent a genetic mutation on their wings that prevents them from singing, according to study leader Nathan Bailey, an evolutionary biologist at the University of St. Andrews in Scotland. By keeping silent, the crickets avoid becoming dinner.
These silencing mutations occurred separately on Oahu and Kauai at almost exactly the same time, creating a rare case of rapid, convergent evolution, according to a study published recently in the journal Current Biology.
“It’s not often you get to watch these things happening in the field,” said Robin Tinghitella, an evolutionary biologist at the University of Denver who studies field crickets.
“We are finding evidence of evolution in action happening right in front of us,” said Tinghitella, who was not involved in the new study.
When the ancient Polynesians began their diaspora through the South Pacific—including Hawaii—they brought the field cricket with them.
Males sing by rubbing a pointed scraper on the end of one wing against a file on the other, which is a modified wing vein that contains several teeth. (Also see “World’s Loudest Animals—’Power Saw’ Cricket, More.”)
The sound resonates through special structures in the wing and lures the females toward an attractive male. The males’ song also serves several other functions, including a show of aggression.
This singing scheme has worked effectively for thousands of years on islands across the Pacific Ocean, but at some point after the crickets arrived in Hawaii—scientists still aren’t sure exactly when—the parasitic fly Ormia ochracea arrived. (See Hawaii pictures.)
The flies’ larvae use the song to find a male cricket and then burrow inside. The larvae then consume the male from the inside out, using the nutritious cricket to aid their development. Within a week, the cricket is dead and the larvae are developed and ready to move on.
Not Over Till the Male Cricket Sings
Bailey and colleagues studied changes to the wing shape of crickets from both islands. Because the changes are slightly different in each population and the mutations occurred in different parts of the genome, Bailey is confident that the mutations happened separately on each island.
“Losing the ability to sing means a lot of behavioral dynamics will be changed,” he noted. For instance, these quiet males may have a tougher time finding females. (See “Cricket Has World’s Biggest Testicles (But Puny Output).”
Interestingly, though, the silent males have a strategy—mooching off males that can still sing.
“On Kauai, there are still roughly 5 percent [of] normal males, whereas about half of the males we found on Oahu are normal,” Bailey said.
The silent males depend on their more vocal counterparts to sing to attract females: When the females come to check out the singing males, the silent males pounce.
Just call them the strong, silent types.