It took several years for the crickets of Kauai to fall silent. When Marlene Zuk first visited the Hawaiian island in 1991, she heard the insects chirping away, loudly and repeatedly. But every time she went back, the chirping diminished. In 2001, she only heard a single male, apparently singing into the void.
The crickets had disappeared from sight, too. But when Zuk returned to Kauai in 2003, she started seeing them again, seemingly in greater numbers than before. They were there, sitting on blades of grass, illuminated by her headlamp. They just weren’t singing.
To be a quiet cricket is to defy the essence of cricketkind. Crickets sing. Crickets are noisy. The males attract females by calling with a pair of specialized structures on their wings. One, the file, is a vein with several evenly spaced teeth. The other, the scraper, is a raised ridge. When the file rubs against the scraper, it’s like running the teeth of a comb along the edge of a table: It makes a thrrrrrrp sound. But on Kauai, 95 percent of the males had tiny files that grew at strange angles. When these crickets rubbed their flat wings together: crickets.
These changes happened because of a parasitic fly with sensitive hearing and a fondness for crickets. It finds males by eavesdropping on their calls and splatters their bodies and surroundings with larvae. These larvae then burrow into the crickets and devour them from the inside out. At its height, this fly had infested a third of the male crickets on Kauai, and seriously reduced their numbers. That’s why Zuk had such trouble finding and hearing them.
The crickets only bounced back when they acquired a mutation that altered the males’ wings and silenced them. In fewer than 20 generations, they managed to avoid the grave by becoming as silent as one. It was, as Zuk once said, “one of the fastest cases of evolution that’s happened in the wild.”
It comes at a cost. The silent males might be undetectable to flies, but they aren’t attracting any females either. They’re now reduced to hanging around the small minority of males who can still sing, on the off-chance that approaching females get distracted and mate with them instead. It’s a substantial trade-off, but one that Hawaii’s crickets have made on two separate occasions. In 2005, Zuk’s team found flat-wing males on the neighboring island of Oahu. Within a few years, half the males there had fallen silent.
They’re still trying to sing, though. By filming the crickets, Nathan Bailey and Will Schneider at the University of St Andrews, showed that the flat-wing males from Oahu are still trying to rub their reduced scrapers across their degenerate files. Their movements are indistinguishable from those of their noisy peers. They even keep to the usual rhythms. “They’re still trying to do this even though they can’t produce sound,” says Bailey (who was once a part of Zuk’s team).
It takes a lot of effort to make these movements, even when they produce no noise. Singing crickets typically burn about four times as much energy as quiet ones. The silent flat-wing crickets are still paying this energetic cost, but since they aren’t attracting any females, they’re getting none of the benefits. “From what we can tell, this isn’t serving a known function,” says Bailey. “But that doesn’t mean we can’t look for one.”
He doesn’t have any good guesses about what that function might be, though. The vibrations from the flat-wings’ efforts don’t travel very far, so it seems unlikely that they are still communicating in some inaudible way. They sing (or try to) at night, so it’s unlikely that they’re simply mimicking the form of an audible male to deceive passing females. “For now, it’s a mystery,” Bailey says.
But for Zuk, the crickets’ futile efforts aren’t surprising. “It underscores people’s bent toward anthropomorphism that they are always surprised by this, as if they think the crickets should be able to detect that they are flatwings and adjust their behavior accordingly,” she says. “We shouldn’t assume that animals can control their biology to do what seems logical to us. If we lost our voice, we wouldn’t keep moving our lips as if we were still talking. That doesn’t mean that’s what the crickets would do.”
In the future, the crickets might eventually lose their now-defunct strumming movements altogether. They could acquire mutations that reduce the cost of their efforts. They might even use those movements as the basis for some new adaptation. “I’m not sure any of us will live long enough to see how this plays out,” says Bailey.
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