A Most Kinky Moth

A retired entomologist discovers a world of behavioral diversity in the courtship rituals of a well-studied moth species.

May 1, 2015
Carrie Arnold

ANDRZEJ KRAUZESporting brown wings flecked with white, the gold swift moth looks pretty drab. Its sex life, however, is anything but. Mating in most moth species occurs when the alluring scent of a female causes a male to fly in search of a potential partner. But male gold swift moths that are ready to mate form a large group called a lek from which the females choose their partners.

In 2007, at a summer holiday cottage outside the town of Inverness in Scotland, newly retired University of Leeds entomologist John Turner was washing up the supper dishes as the sun was setting. Looking out into the small clearing behind the house, Turner noticed gold swift moths, a species he had studied previously, flying and mating in the twilight. Abandoning his chore, he rushed outside to watch the moths. He noted the lekking behavior, but he also observed any number of variations: males approaching large groups of females and even two moths meeting and copulating midair.

“They seemed to display every conceivable mating procedure,” Turner says.

Each day thereafter, he returned to the stand of woodrush and low-lying plants an hour or two before sunset to watch the moths perform their mating dances, the entomologist armed with little more than a notebook in which he frantically scribbled his observations in the rapidly disappearing daylight. Turner observed the males hovering in midair, releasing a pheromone that smelled of ripe pineapple, as females flocked nearby. When a female selected a male, the two would retreat to a nearby branch to mate. Even as his retirement continued, Turner was able to get the funds needed to purchase an infrared camera to document the behaviors in greater detail.

In addition to the lekking behavior that he had previously documented, he observed females hanging from leaves and emitting pheromones to attract males. On other occasions, males formed a swarm and followed an individual female until she landed on a leaf, where she proceeded to mate with one of the males. Males and females also appeared to attract each other and dance in midair before mating. A few times, males even tried to mate with each other. The moths’ sexual positions were just as varied.

Turner returned each summer for seven years to document the moths’ mating behavior. His discoveries, published earlier this year in the Biological Journal of the Linnean Society, show that gold swift moths could teach even the authors of the Kama Sutra a thing or two about sexual behaviors.

“Sex is obviously very exciting in the gold swift world,” says James Mallet, an entomologist at Harvard University. “We’ve overestimated the uniformity of sexual behavior when obviously it’s quite diverse.”

Gold swift moths are common across continental Europe and Great Britain, and naturalists have been studying these nocturnal insects for more than a century. Nearly 40 years ago, one of Turner’s earliest papers determined that, while female moths might hover around the males, they didn’t knock them out of the air during the mating process, as moth collectors had claimed. But because most insects have fairly stereotypical sexual behaviors that don’t vary much, Turner didn’t bother to investigate the question much further—until his vacation in the summer of 2007.

Thomas Simonsen, an entomologist at the Natural History Museum, London, says that the varied behaviors of the gold swift moth also contain hints as to how these behaviors might have evolved and how some groups of moths may have made the transition from “typical” moth mating behaviors to lekking, which has only been observed in a few families.

“In moths like the gold swift, the females are actually attracted to the male, but for most other moths and butterflies, it’s the other way around,” Simonsen says. “By showing how this one moth species displays all of these different mating behaviors in one place at the same time, you can see how this could have evolved.” The question then remains; Why didn’t scientists notice the gold swift moth’s impressive mating repertoire sooner? To Turner, the answer is obvious. Mating occurs at twilight, when daytime predators have finished hunting but bats and other nocturnal threats haven’t yet emerged. But it means that scientists aren’t out looking for the moths, either.

“At twilight, none of the moth people are out yet, and all the butterfly people have gone to the pub,” Turner says.