Male crickets were being attacked by a lethal parasite. Soon, a mutation spread throughout the population that rendered the crickets silent, providing UCR researchers with important information about evolution.
Deep in the Hawaiian island of Kauai, a page-turner of a mystery is unfolding. It has all the seedy elements—sex, murder, manipulation and survival.
While most residents are unaware of the ongoing drama, UCR’s Marlene Zuk, an evolutionary biologist, is on the case.
And things are heating up.
Her task: Keep tabs on the island’s nonnative male field crickets, Teleogryllus oceanicus. A killer has been on the loose, preying on the love-struck males.
Like many crimes, this one is committed by someone close to the victim. In this case, an ungrateful guest, Ormia ochracea, a parasitoid fly, kills its host so that its own offspring may live.
The fly finds the male cricket just the same way the female cricket finds him – through his mating chirp. This little fly can hear in cricket-call frequency, an unheard-of feat for any other fly. Once she locates her host cricket, she lands on its back and deposits larvae, which burrow into the body until they are ready for their grand exit, killing the cricket in a gruesome, explosive manner.
Is this the end for our hapless hero?
Not quite. Zuk’s team did observe a dramatic decline in these field crickets but in the case of the mutating cricket, there’s a plot twist – a dramatic physical change.
The research team found that greater than 90 percent of male field crickets on Kauai shifted, in less than 20 generations, from having normal wings to mutated “flatwings” that no longer produce the cricket equivalent of Barry White to attract female crickets for mating.
Like all mutations, the wing alteration arose spontaneously, but it has been a mixed blessing to the male crickets. Keeping quiet has a big benefit (staying alive) and one potential drawback (your soul mate may never find you).
While Zuk, an expert in sexual selection, has studied crickets for a while, it was when the Society for the Study of Evolution met in Hilo, Hawaii, in 1991 that she started exploratory fieldwork.
“I had heard that the Teleogryllus were introduced to the islands,” explains Zuk, “so I collected a bunch and dissected them there. No one knew that those populations were subject to an acoustically orienting parasitoid, so that was an exciting discovery to begin with.”
During subsequent years, Zuk and her researchers discovered that the crickets-and-fly drama was taking place on three islands – Kauai, Oahu and Hawaii – as well. Her team’s quiet sleuthing discovered that noisy male crickets were dying out and those with wings that lack the file and scraper that produce the love song were surviving.
The biggest result from this year’s field work is that the proportion of flatwings on Oahu skyrocketed from only four males out of the sample population in 2005 to nearly half in 2007. The mutated males still get the girl – they stick close to the males who still produce song.
In field cricket circles, it’s not enough just to attract the female with a call. The male has to perform a special courtship song once she is nearby. But since many males have become the strong and silent type, they skip the foreplay. Kauai’s female crickets seem willing to accept this brevity, perhaps because they have evolved to be less picky due to the increased population of silent males on the island.
“It had long been proposed that natural enemies, predators and parasites might locate food by orienting to the mating calls of their prey or host,” says William Cade of the University of Lethbridge in Canada, who first researched the relationship between crickets and the Ormia fly in Texas. “Zuk’s recent discovery that Ormia has actually caused males to lose the physical structures on their wings and that this evolution has taken place in a few generations is very significant. Selection must be especially strong to cause male wings to become smooth and female-like. And this has to be one of the fastest cases of evolution ever discovered in any species.”
“Virtually everything we’ve discovered has been a surprise – really the biggest one isn’t the fact that the crickets didn’t go extinct, but the speed at which the mutation spread and evolution occurred,” says Zuk. “People usually think that evolution takes millions of years, but we saw it in the virtual blink of an eye, less than 20 generations.”
As for the next chapter, it’s full of questions. Will all singing males eventually die out? If so, how will the females find their nonmusical guys? And where will the flies go to continue their life cycle?
Zuk sees her work as having implications that go beyond studying the life cycle of a cricket and a parasitoid fly.
“It’s important to understand how evolution works, since that explains so much about the diversity of life on Earth,” she explains. “Our work adds to the understanding of how quickly organisms can change.”
To see a graphic and grisly video of how the Ormia fly larva emerges from its host, visit www.newsroom.ucr.edu/cgi-bin/display.cgi?flash=1418