26 September 2006

Stealth And Sacrifice Show Evolution At Work

by Kate Melville

Male crickets on the Hawaiian island of Kauai have undergone a sudden heritable change that has rendered most of them incapable of using song, their sexual signal, to attract female crickets. However, according to a new study by University of California - Riverside evolutionary biologists, the crickets have devised an ingenious behavioral change to allow them to continue meeting females. The researchers say that such changes in behavior can help what may seem like a harmful mutation spread throughout a population.

Led by Marlene Zuk, the research team found that greater than 90 percent of male field crickets (Teleogryllus oceanicus) on Kauai shifted in less than 20 generations from having normal wings to mutated "flat wings," that inhibit the crickets' sound-making abilities.

The mutation occurred, the researchers explain, to protect male crickets from a deadly parasitic fly (Ormia ochracea) that uses the cricket song to locate crickets as hosts. Once found by the parasitic fly, the cricket has larvae deposited onto it. The larvae then burrow into the cricket, develop inside, and subsequently kill the cricket when they emerge.

Zuk explained that she observed fewer crickets with each visit to the island during the 1990s. "In 2001, we heard only one calling male. But then in 2003, although we heard none of the male crickets calling, we found they were not only in high abundance, but nearly all of them also had female-like wings [flatwings], lacking the fine structures needed to produce song."

Zuk hypothesized that the loss of calling was about protecting the male crickets from the deadly fly. "But this protection has a heavy price: the loss of its sexual signal. This is obviously a huge loss for the cricket, [so] how then do female crickets locate silent males?" she wondered.

The answer it seems, is that the silent males linger around the few remaining male crickets that can call. By congregating near the callers, the flatwings enable females to find and mate with them. To test their hypothesis, the biologists performed a field experiment that demonstrated that the flatwings do indeed use the callers as female attractors.

"While we were surprised by the extraordinary speed at which the mutation spread, what is more interesting is that, ordinarily, you would expect such a change in wing morphology to quickly disappear, because males couldn't attract mates," Zuk explained. "Instead, the behavior of the flatwings allows them to capitalize on the few callers that remain, and thus escape the fly and still reproduce. This is seeing evolution at work."

Source: University of California - Riverside