Vision is often almost useless at night so many nocturnal species, such as gleaning bats, resort to acoustic methods to perceive their environment. They broadcast ultrasonic echolocation calls to locate insect prey. Yet many prey species have evolved ultrasonic hearing in a bid to outsmart their hunters, and some prey species have even gone a stage further. They have appropriated bat calls into their own repertoire for sexual communication. Borrowing from their predator, male corn borer moths use ultrasonic songs resembling bat calls to increase their copulation success. Wondering how mimicking bats helps the courting males, Ryo Nakano and colleagues decided to find out precisely how female moths respond to the males' advances and publish their results in Physiological Entomology. It seems likely that male calls induce female moths to behave as if they are under attack, rendering them motionless and suppressing female interference during the act of mating.
The team used an experimental approach to listen in to the inner workings of the private world of moth sexual relations. First they silenced the males by surgically removing the sound-producing scales located on their forewings, while puncturing both of the females' tympanic membranes so that they could not hear males' calls. Next they observed how deafened versus intact females reacted to male advances, and how frequently mute or intact males convinced females to copulate with them.
Nakano and his team confirmed that when both partners were intact the females readily accepted the males as partners and they copulated successfully 98% of the time. However, when either partner was acoustically impaired (through losing either their hearing or the ability to call), their success fell to roughly 60%: courtship songs are instrumental in corn borer mating. And when the team analyzed the female's responses to the male's ultrasonic courtship calls they found that the females reacted as if they were under attack and froze, making it easier for the males to mount them.
Focusing on call function, the team recorded male songs to see whether they increased the sound level of subsequent mating attempts if the initial attempt failed. Nakano and his colleagues found that the moths did get louder during subsequent mating attempts, suggesting that males can pump up the volume as desperation kicks in. Finally, the team played synthesized calls to intact females at various sound levels and found that sound level impacted strongly on whether the males were accepted as mates. They found that the louder males were more successful, so increasing the volume after a jilting makes perfect sense.
In summary, females' responses to predators and amorous males are identical. They are immobilized by the males' calls, which trigger their anti-predator freezing behaviour. In effect, the male exploits a female sensory bias against calls heralding dangerous predators, causing her to act in a way amenable to mating attempts. However, the calls do not really suit classic notions of chivalrous males wooing females. Here, courtship seems instead to carry decidedly manipulative undertones. After all, males are pulling the wool over their mates' ears by profiting from an existing anti-predator response in order to mate. As males seem to be selected to immobilize females effectively to ensure successful copulation, it would be interesting to investigate existing variation across males in call quality or ability to vary sound levels. Equally, it would be exciting to see whether females can resist manipulation by reliably recognizing differences between bat sonar and male calls.
