— What do you do when your only means of attracting members of the opposite sex also puts your life in jeopardy? For field crickets on the Hawaiian island of Kauai, shutting up seems to work.
According to a new study, rapid evolution in the Kauain population of the oceanic field cricket Teleogryllus oceanicus has rendered nine-tenths of the males there incapable of producing their iconic night-time call. The genetic mutation, which changes the shape of the males wing to make it silent, means the crickets are better adapted to avoid a deadly parasite.
The finding dumbfounded biologist Marlene Zuk, at the University of California in Riverside, US, who first thought the dwindling population of crickets she was studying had gone extinct when she no longer heard their calls.
If youre a cricket and youre a male, your life is defined by calling, Zuk explains. How are you going to find a female, and once you do, how are you going to get her to mate with you without your call?
Zuk found that the quiet crickets maintained their reproductive chances by congregating near to the few remaining male crickets that are still capable of calling.
In laboratory tests involving speakers that broadcast cricket calls, males that were physically unable to call would approach as close as 1 centimetre to the speakers. In contrast, males that could make their own calls usually settled more than a metre away.
The reason, says Zuk, for such a dramatic physical and behavioural change is the small fly, Ormia ochracea. The fly uses highly sensitive ears to hone in on the male crickets call before depositing its larvae on the crickets back. The larvae then dig into the crickets body where they eat their host alive in about a week.
In previous studies, Zuk noticed that the hopping insects behaved differently around the flies. Populations of crickets that lived in areas where the flies were present were more likely to wait until it was completely dark to start calling. They were also more reluctant to resume calling in laboratory tests after they had been disturbed by the simulated sound of a fly buzzing by.
Zuk calculates that the new wing mutation first occurred on the island of Kauai sometime in the late 1990s and spread to 91% of males there by 2004.
Loss of calling has resulted in a strong reduction in mortality for the male crickets. Out of 121 males that had the non-calling mutation, only one harboured the fly larvae compared to previous infestation rates of greater than 30% in normal-winged males.
William Cade, a researcher of cricket biology at the University of Lethbridge in Alberta, Canada, says the study substantiates prior theories: It confirms our predictions from years ago that acoustically orienting enemies, especially flies, can have a profound effect on the evolution of mating signals in crickets.
Now that just 10% of males on Kauai still have normal wings and can call, what happens next open to debate. The silent flatwing crickets depend on a certain proportion of males still being able to attract females, so extinction is a possibility, says Zuk.
Another possibility, she says, is that the proportions of flatwings and normal-wings will cycle, so that the normal-wings will be at an advantage now since they get more mates, but that would mean parasite fly populations increase, which then puts the flatwings at more of an advantage, and so forth.
This kind of oscillation has been predicted for other host-parasite or predator-prey systems, but seldom demonstrated, Zuk says.
Journal reference: Biology Letters (DOI: 10.1098/rsbl.2006.0539)