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Moths Talk about Sex in Various Ways

Moth and bat

Moths are nocturnal, and their major enemy is the bat. To defend bats, many moths developed ears sensitive to the bat’s echolocation cries and they also developed various behaviors to avoid being captured by bats. Now it turns out that some moths are able to utilize both their hearing and their avoidance behavior in a completely different application: to communicate with partners about sex. According to a Danish/Japanese joint study, various moth species talk about sex in probably a great number of different ways. This finding sheds new light on the evolution of sound communication and behavior.

The sole purpose for moths to develop ears is to hear the approaching of their worst enemy, the bats. It has been long believed that moths were dumb, yet many of them in fact produce sounds – just so softly that bats can’t hear them. This has been learned for a few years, and now new study reveals that moths develop various ways to not only use the sense of hearing, but also their avoidance behavior that was initially developed as a defense against bats.

 “We have studied two moths and see that they use ears and behavior quite differently when they talk about sex. There is no reason to believe that others do not do it in their own ways, too. The variation in using these skills must be large,” says Annemarie Surlykke, sensory physiology researcher from Department of Biological Sciences, University of Southern Denmark (SDU).

She and her Japanese co-workers from University of Tokyo have examined two species, Asian corn borer moth (Ostrinia furnacalis) and Japanese lichen moth (Eilema japonica). Both of them are similar to other moths that they developed ears to protect them from bats, however, they have also managed to gain more from their sense of hearing. Male moths from both species have developed an approach to court females using sound – but the approaches are quite different.

The Asian corn borer moth uses a simple technique: It produces sounds similar to the echolocation cry of a hunting bat. Therefore, the male manages to fool the female to believe that there is a bat nearby. She responds by sitting still perfectly in an anti-bat freeze position to avoid bat’s attention, and now the male moths can mate with her, since it’s much easier when she sits still. The researchers also played the sound of a hunting bat and the sound of a courting male mating in the laboratory, respectively, and it was found that the females responded in both cases by freezing. Thus, the researchers conclude that female moths simply can’t distinguish the two sounds.

The male Japanese lichen moth appears to be more advanced. The male, too, emits a sound resembles a hunting bat. However, when the researchers played first the bat’s sound and then the sound of a courting male, the females could hear the difference in the details of the sounds and would only mate if she heard the sound from a courting male. This result suggests that the evolution of bat defense to sexual communication has gone a step forward with the Japanese lichen moth: It has developed a specific signal to recognize courting males, while the Asian corn borer moth is not able to distinguish between sounds from a hunting bat and a courting male.

Annemarie Surlykke says: “The acoustic communication between moths and bats is a textbook example of the interaction between prey and predator. However, our research demonstrate how such a system can evolve, so also moths employ their ability to hear and produce sounds to communicate sex and that they have developed so many ways to achieving it. This is a beautiful instance of evolutionary diversity.”

Moths’ hearing ability occurred about 50 – 60 million years ago, when bats began to fly in the night sky and used echolocation for orientation. Ever since, these two creatures have been locked in an eternal arm’s race with the moths trying to avoid the bats and the bats attempting to find the moths.

In fact, moth ears are quite simple constructions with just one, two or four sensory cells at most, but it’s sufficient to capture the bat cries. “Until several year ago, we believed that the vast majority of moths didn’t use their ears for anything else other than survival,” explains Annemarie Surlykke.

However, in the year of 2009, she and her colleagues showed that far more moths than previously thought actually produce sounds. They selected 13 species and it turned out that 70 percent of the males produced different sounds to communicate with females. Of course, almost all sounds generated by moths is ultrasound and can’t be heard by human, but for sure by bats.

At first glance, it appears to be a bad idea to make sound when your worst enemy almost exclusively use the sense of hearing to hunt with – producing a sound is equal to calling the bat. However, Annemarie Surlykke explains, moths only produce sounds when they are very close to each other, usually when they are not further apart than two centimeters. The signals of their whispering are so weak that it will be difficult for bats flying at a distance to register them.

 “Now I’m pretty convinced that there is a lot of whispering among moths, which is so quiet that it’s hard to detect and thus we mistakenly believe it doesn’t occur. Our results provide a whole new understanding of many directions, and specifically, evolution of sound communication that was originally developed for defense against an enemy, could lead to a new understanding of evolutionary processes.”

Source: ScienceDaily

Image source: fineartamerica.com