The alluring tail of the Luna moth is surprisingly useless for finding a mate

Plenty of species have traits evolved for more than one purpose. Deer antlers are built-in weapons as well as seductive doe-magnets. Octopus suckers can trap prey in their suction but also taste and smell. Bright colors in frogs signal danger to predators while flaunting reproductive viability to potential mates. The Luna moth has uniquely shaped wings that thwart predation from bats, but what else might they be good for? How does one determine the evolutionary role of a trait? 

In two recent complementary studies published in Behavioral Ecology and Biology Letters earlier this year, researchers expanded our understanding of the adaptation by testing the role of wing tails against sexual selection and bird predation.

Luna moths are native to the Eastern half of North America. Like all silk moths, they have distinctive long, trailing tails on their hindwings, or “twisted, cupped paddles” as lead author of both studies and doctoral student at the Florida Museum of Natural history Juliette Rubin said in a statement. Bats use echolocation to detect the position of objects with reflected sound, but the moth’s wing shape reflects sound waves in a way that makes the flying mammals aim for the ends of their wings. In a flap of a wing, the moth just barely dodges their predators. 

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First, the researchers wanted to see if the wing tails also played a role in sexual selection. When female Luna moths are ready to mate, they perch in one spot and release pheromones. Males, with extremely sensitive antennae, can detect and follow a pheromone trail, according to the University of Florida’s entomology department. Then, the female has her pick of suitors. 

In the first experiment, researchers placed a female moth in a flight box with two males: one with intact wings and one with the wing tails removed. Initial data suggested that females preferred tails over no-tails, but further trials demonstrated otherwise. When researchers removed tails by clipping them, the resulting damage may have hindered these males’ performance in the first trial, allowing the intact males to mate successfully.

They recreated the tail/no-tail experiment by removing tails from both males, and re-gluing them to one male, while placing glue only on the hindwings of the other. Researchers found no significant difference in mating success between them. 

To ensure the glue did not confound the results, researchers conducted an additional experiment with two intact males, one with glue on the hindwings. Similarly, they had equal mating success.

Though their elegance is attractive to us humans, the experiment revealed that Luna moth wing tails aren’t the result of sexual selection. 

Then, researchers wanted to see if the moths’ tails had any obvious drawbacks. They help moths to survive bats, a species that relies on echolocation, but what about visually-oriented predators? 

Luna moths sit still during the day, since flying in broad daylight with their large bright green wings would make them easy targets. To test whether or not their tails would have any impact on daytime predation, researchers wrapped pastry dough around mealworms and molded them to the size and shape of real Luna moths. They attached full wings and wings without tails to each half. They placed the replicas around branches and leaves in an aviary, and introduced Carolina wrens. 

The wrens ate the fake moths at the same rate regardless of wing type, indicating that the tails had no effect on whether or not birds could locate them. Some research suggests that birds rely on search images, mental representations of objects, when they are searching for prey. They use visual cues, such as the shape of moth wings, to distinguish between the prey from patterns in the background. So, the wrens may ignore the hindwing tails, using the overall shape of Luna moths to identify food, according to the press release.

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These experiments show that despite being a noteworthy feature to humans, the Luna moths’ tails do not play a role in attracting a mate, nor do they affect predation by birds.

“When we see these really obvious physical features in animals, we’re often drawn into stories we’ve heard about them,” Rubin said in the statement. “A trait that’s obvious to us, as visual creatures, might not stand out to the predators that hunt them, and the traits that we think are dynamic and alluring might not seem that way to a potential mate.”