GRONINGEN, THE NETHERLANDS—For insects such as the tobacco budworm moth, beauty is actually in the “nose” of the beholder, as females use chemical scents called pheromones to lure in potential mates. And—as in people—some moths are attractive. Others … well, not so much. Now, evolutionary biologists have learned that these unattractive female moths better their odds of mating by hanging out with their more attractive counterparts.
“We often think of mate choice as a perfect and entirely binary process—you are attractive or you are not—but this is clearly not the case,” says Therésa Jones, a behavioral and evolutionary ecologist at the University of Melbourne in Australia, who was not involved with the work. Wouter Halfwerk, a behavioral ecologist at the University of Amsterdam, adds that the results, reported this week here at the XIV Congress of the European Society of Evolutionary Biology, “provide an answer of how unattractiveness can evolve, which challenges our notion of beauty.”
The new work was done by Astrid Groot, an evolutionary biologist at the University of Amsterdam who studies the evolution of sexual signals. She specializes in the tobacco budworm moth (Heliothis virescens) because so much is already known about its caterpillar, a widespread crop pest in the United States often caught by farmers with pheromone-scented traps. In field studies, she and other researchers noticed that some females never seem to attract males. But how could they reproduce enough to pass along their less-than-sexy scent? To find out, she and colleagues raised multiple generations of the budworm in the lab, testing each generation’s females for how quickly males home in on their scents. By separately breeding the most and least attractive females, the researchers gradually created two strains, one of supersexy smellers and one of, for lack of a better word, stinkers.
They then paired off females of both strains in different combinations, and let them play the moth equivalent of The Dating Game. Pairs of attractive females almost always snagged a mate. Pairs of unattractive females never did. But when an unattractive female was paired with an attractive one, she was able to mate an average 17% of the time, Groot reported. Males make a beeline to the attractive scent, but then apparently miss their target once they get close in, she explains.
Attractive females may tolerate being cheated out of a mating because they, too, benefit from having a less attractive pal nearby, Groot and her colleagues discovered. When accompanied by another attractive female, each female has a 50-50 chance of mating. But with an unattractive pal, the beautiful moth’s chances improve. What’s more, the male is much quicker to make his choice, Groot said.
The results “demonstrate the importance of the social environment,” Halfwerk says. “One form does not attract males on its own, only in close proximity of the other form.” That result also parallels what’s been found in humans: that an attractive woman in a crowd of less attractive women also seems to attract more attention. But pinning down exactly why this happens should be much easier in moths than people, she notes. “That’s the nice thing about insects.”