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Hawai'i Crickets Evolved "Silent" Wings to Evade Parasites, Study Finds

Scott Norris
for National Geographic News
October 3, 2006
 
Male field crickets on the Hawaiian island of Kauai have given up
singing in order to survive (Hawa-
i'i map
).

The crickets can no longer trill or chirp because of a recent evolutionary adaptation that has changed the structure of their wings over the course of just a dozen or so cricket generations.

The silent lifestyle protects male crickets from a deadly parasitic fly that locates its victims by sound, researchers say.

But the muted males can no longer perform the nightly serenades thought to be essential for attracting female crickets and ensuring reproduction.

So the insects employ a sneaky technique: Hang around males who can chirp to attract females and then intercept a mate.

Robin Tinghitella is a graduate student who participated in the research, which was led by Marlene Zuk of the University of California, Riverside.

Tinghitella says that the mutation seems to have been accompanied by looser standards on the part of the female crickets.

In other cricket populations, she notes, silent males are rejected.

"Without some sort of relaxation in how choosy females are, the mutation never would have taken off," Tinghitella said.

The team's research, which was partially funded by the National Geographic Society, appears in the online edition of the journal Biology Letters.

(National Geographic News is part of the National Geographic Society.)

Fatal Attraction

Normally male crickets call females by rubbing specialized protruding wing parts together, creating an effect similar to running a thumbnail down the teeth of a comb.

But Zuk's team knew that something had changed when they arrived on Kauai in 2001 and encountered silence (related feature: Kauai travel guide).

Surveys that year showed that the island's oceanic field cricket population had plummeted.

On Kauai, the team knew, the crickets' courtship signals entice both females and an invasive parasitic fly that originally hails from the North American mainland.

The fly deposits burrowing larvae in singers' bodies. Host crickets are killed as the larvae develop.

In previous studies, Zuk and others had documented changes in the male crickets' call structures and activity patterns that made them less vulnerable to the sound-orienting fly.

Lab work done after the 2001 visit revealed that pressure from the parasitic fly had triggered rapid proliferation of males with what the researchers call the flatwing mutation.

This genetic change causes male crickets to develop flat, femalelike wings that can't create the insects' characteristic noise.

Male crickets could no longer make the signal call used to attract mates, nor could they make a softer courtship call required to induce mating when females are in close range.

Despite these disadvantages, by 2003 the field cricket population was thriving again—even though 90 percent of the males had lost the ability to solicit mates.

"We thought it was possible the flatwings were intercepting females attracted to other male callers," Tinghitella said.

Field experiments using recorded cricket calls confirmed this hypothesis.

Classic Example

The development in Hawai'i has provided Zuk with a kind of natural experiment for examining an important issue in evolution: the costs and benefits of sexual signaling.

Sexual selection favors the development of conspicuous features, like the peacock's tail, in males competing to attract mates (related photo: male peacock displays its tail).

But features like bright feathers or loud songs can also put males at risk.

Loss of the male crickets' sound-producing apparatus to avoid the parasitic fly was a dramatic and unexpected development, the researchers say, akin to the rapid evolution of a tailless peacock.

"It's a unique finding, to see in a relatively short period of time the complete loss of a signal that seems to be so important," said Eileen Hebets, a biologist at the University of Nebraska-Lincoln.

"I think this will become a really highly cited, classic example of tradeoffs between natural selection and sexual selection," Hebets added.

For the crickets, the change seems to be a successful strategy, at least so far. The population continues to increase, and only one percent of the male crickets harbor the parasitoid larvae.

But a key question remains unsolved: Why don't females refuse to mate with the males unable to issue courtship calls?

The researchers are currently conducting experiments to see if female crickets from island populations tend to be generally less choosy in general than their mainland counterparts.

They are also watching the cricket population on the neighboring island of Oahu, where four mutant males were recently discovered.

"The good thing about Oahu," Tinghitella said, "is that we can watch this whole process again from the beginning."

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