It’s a love story as old as time itself: boy Asian corn borer moth (Ostrinia furnacalis) meets girl Asian corn borer moth; girl secretes sex pheromones; boy goes through his courtship ritual, a little song-and-dance routine where he rubs his wings against his thorax to produce a soft, whispering sound. It’s a sweet little love song that, it turns out, hides a vicious lie.
In 2009, this romantic tale played out in a lab in Japan so that Ryo Nakano, from the RIKEN Brain Science Institute’s Laboratory for Biolinguistics, could observe the males’ seduction technique. They knew that the buggy love song increased the moths’ mating success somehow, but its function and effect weren’t clear. Nakano and colleagues ran a series of experiments where the ability either to make or detect the mating sounds were removed so they could figure out the secrets of the song.
Nakano and his team silenced several male moths by surgically removing the sound-producing scales located on their forewings and punctured the tympanic membranes of several female moths so that they couldn’t hear males’ calls. Next they observed how deafened versus intact females reacted to male advances, and how frequently mute versus intact males convinced females to get it on with them.
Intact female-intact male, intact female-silent male and intact males-deafened female pairs were let loose in a flight tunnel to do their thing and the courtship behaviors of the males – exposing their genitalia while fanning their wings, raising the wings into position for producing courtship songs, assuming the position for attempted copulation, etc. – were observed until the females either accepted or rejected their advances. That last part is where things get tricky. For each attempt at sex, the male has to bend his abdomen based on the position of the female in order to get a grip on her genitalia with his genital claspers. It’s a difficult little maneuver, and like human males’ “yawn and put the arm around her” move, even the slightest movement of the female can screw it all up.
Females who could hear the males’ songs readily accepted them as partners and pairs of intact moths got busy 98% of the time. In all, 94 of the 96 intact males succeeded in mating with intact females, though only 15% succeeded on the first attempt and the rest kept trying until they either succeeded or, in the case of two unlucky guys, were interrupted by the escape of the female. When either partner was acoustically impaired – deaf or mute – though, the rates of sexual success fell dramatically. The females eventually accepted only 63% of the muted males after several mating attempts. The other 37% couldn’t get things right, even after repeated attempts, before the females wriggled away (results for the deafened female and intact male pairs were similar). The team also observed that, if the initial mating attempt failed, the males who could make noise pumped up the volume of their song during subsequent attempts and that the sound level strongly affected their acceptance by females.
Nakano and his colleagues noted that neither the intact nor muted males gave up easily. The interruption of the males’ mating attempts was almost always the fault of the female trying to get away from her suitor. The variation in the number of mating attempts and mating success among intact and impaired pairs didn’t seem to be a problem of tenacity, but female behavior and Nakano wondered if that’s where the males’ courting sounds worked their magic.
When the team analyzed the female response to courtship calls, they found that the females froze in their tracks during the serenade, making it easier for the males to mount them. Nakano proposed a “predator recognition” hypothesis that assumed the females recognized the males’ song as the sound of predatory bats. Staying still, of course, is a great way to small insects to “drop off” a bat’s sonar and not be recognized as prey, and bat calls do elicit freezing responses in many insects, including moths. Faking a bat call, then, is a great way for a male moth to cause a freeze response and get the girl to lie still long enough for him to score.
To test this whether this sonic date rape drug actually worked, Nakano and colleagues examined, in a separate study, whether female moths of the species Spodoptera litura, which exhibit mating behaviors similar to O. furnacalis, could discriminate between male songs and bat calls. They couldn’t, and simulated bat calls played over a loudspeaker while muted males attempted to mate with them had the same effect as the courtship song of intact males: they stopped moving and the males had a much easier time of “boring their corn.”
References: Nakano, R., Takanashi, T., Skals, N., Surlykke, A., & Ishikawa, Y. (2010). Ultrasonic courtship songs of male Asian corn borer moths assist copulation attempts by making the females motionless Physiological Entomology, 35 (1), 76-81 DOI: 10.1111/j.1365-3032.2009.00712.x
Nakano R, Takanashi T, Skals N, Surlykke A, & Ishikawa Y (2010). To females of a noctuid moth, male courtship songs are nothing more than bat echolocation calls. Biology letters, 6 (5), 582-4 PMID: 20219743
Image: Ostrinia furnacalis, courtesy of the Hannam University Natural History Museum