On hot summer nights male crickets chirp constantly in their attempts to attract mates, rubbing a toothed ‘file' on one forewing over a ridge on the other forewing to produce their song. But this can be a risky business, as it might not be only females who are drawn by the males' calls. Predators and parasites may turn up too, using the same auditory cues to find a quick meal. Researchers on the Hawaiian island of Kauai have found that pressure from parasites has led to the rapid evolution of males who cannot sing at all (Bretman & Tregenza, 2007).
Crickets (Teleogryllus oceanicus) on Kauai are parasitized by a fly (Ormia ochracea) that uses cues from the male song to find its prey. The female flies lay larvae on or near male crickets – the larvae burrow into the crickets, killing them when they emerge. So for the male crickets, mating is a very risky business indeed.
Scientists began studying the Kauai crickets in 1991, and noticed that in subsequent years they were finding fewer and fewer crickets. In 2003 the number of crickets around increased sharply – but almost none of them could sing. Their forewings were effectively flat, and this feature was heritable, perhaps caused by a single mutation. What's more, the silent males were free of parasitic Ormia larvae.
But this seemed counter-productive – how could the males mate successfully if they couldn't sing to attract females? Apparently the silent, flat-winged males respond very strongly to the songs of the few males who are able to chirp. They gather closely around the singers and so have a good chance of meeting females who are also drawn to the song. Of course, this mechanism does require that some singing males remain in the population. Perhaps the singers are preferred by females and so are more successful at passing their genes on, albeit at the cost of losing some of their sons to parasites in the next generation.
The outcome seems good for the crickets, but not so for the flies – if they are unable to find somewhere to lay their larvae, the Ormia population on Kauai may become extinct.
References: A. Bretman & T. Tregenza (2007) Strong, silent types: the rapid, adaptive disappearance of a sexual signal. Trends in Ecology and Evolution 22(5): 226-228