If you believe bigger is better when it comes to reproduction, the females of a certain species of moths agree with you, but not for the reason you think. In fact, they agree more with Charles Darwin when he predicted that sexual selection favors males with exaggerated sensory receptor structures. Was Darwin saying women prefer guys with bigger … noses? No, but he would have been pleased with the results of a new study which show that female gum-leaf skeletoniser moths prefer males with big antennae and have evolved a way to attract the ones with the biggest ones. Can they bottle this?
But Darwin also proposed that sexual selection can favor males who are better at detecting and responding to signals from females, including chemical signals like pheromones. So males with sensory structures that can better detect female signals may have the edge in finding them in order to mate and pass on their genes.
Ah, pheromones – those magical chemicals that drive males wild. Dr. Mark Elgar, an evolutionary biology professor at the University of Melbourne’s School of Biosciences, co-authored a study published in published in the journal Science of Nature which looked at the pheromones emitted by Uraba lugens or gum-leaf skeletoniser moths whose scary name comes from what they can do to eucalyptus trees.
Female gum-leaf skeletonisers have a brief seven-day life cycle in which to mate and reproduce with males more attracted to porch lights. PhD student and study co-author Tamara Johnson set up traps in Royal Park, Melbourne, containing either one or two females. The following morning, she counted the trapped males and measured their antennae.
Johnson found that traps containing only one female attracted males with the biggest antennae, proving that bigger is better when it comes to detecting the pheromones of the females. Interestingly, the size of the antennae has no link to male body size – no “small wings means big antennae” correlation here – although the capacity to maintain those large antennae means they’re stronger than the average male moth.
The next discoveries were surprising. The team found that females were able to adjust their pheromone emissions to attract the males with the absolute biggest antennae. However, in this case, bigger wan NOT better. The females actually lowered the pheromones to a point where only the males with huge antennae could detect them. The researchers also found that the females who missed out on the first round of big antennae males INCREASED their pheromone output in order to attract more males, even though they were the less desirable, small antennae variety. Just like in humans, even wimpy males can get lucky at “last call.”
What does all of this mean, Dr. Elgar?
Our results show that females may have a significant and largely unrecognized role in the sexual selection of elaborate antennae.
Not only that, the females pass the traits of big antennae to their male offspring and pheromone control to the females. That’s good news for moths, bad news for eucalyptus trees.
Darwin would be proud. He’d probably also agree that The Skeletonisers would be great name for a metal band.
(Moth photo: Peter Marriott, Moths of Victoria)