The Secret in the Spots on the Wings of Monarch Butterflies

Every fall, monarch butterflies embark on one of nature’s greatest journeys. These insects flutter more than 2,000 miles from their summer homes in Canada to the mountains of central Mexico. Only about 30 percent survive the journey. Researchers have found a common trait in the butterflies that make it: The edges of their wings tend to be a bit more blotchy.

“Nobody even knew what these places were for monarchs,” says Andy Davis, a biologist at the University of Georgia and an author of a study published Wednesday in the journal PLOS ONE. He added: “Suddenly it seems like they are really important.”

Monarch wings are usually orange, but their edges are black, interrupted by small white spots. Dr. Davis wondered if those black borders contributed to monarchs’ migratory abilities. Dark colors absorb more heat, and seabird studies have suggested that the temperature difference between dark and light feathers can change airflow patterns, allowing black-winged birds “to improve their flight efficiency — essentially increasing lift and reducing drag,” said Mostafa Hassanalian, a mechanical engineer at the New Mexico Institute for Mining and Technology.

He and dr. Davis teamed up to see if the same was true for monarchs, with dark-winged butterflies having an aerodynamic advantage that makes them more likely to survive the long migration than blotchy counterparts.

To test this hypothesis, Dr. Davis enlisted Tina Vu, then a student at the University of Georgia, to perform a potentially tedious task: looking at photos of 400 monarch butterflies and measuring the amount of black and white on the edges of their wings.

“I had to keep encouraging her because it sounded so stupid at the time,” said Dr. Davis.

The hours Ms. Vu spent tracing the white spots in Photoshop while listening to true-crime podcasts paid off in a startling discovery: The monarchs who survived the trip to Mexico tended to have more white spots than the extra dark wings the researchers had predicted. In addition, she analyzed some of the butterflies’ non-migratory relatives, which also had fewer spots than those that made the great migration.

“When we saw there were more white spots, we thought, ‘Okay, so those must have a function,'” says Ms Vu, who plans to pursue a doctoral degree in biology.

The researchers’ working theory is that the spots reduce drag by creating heating and cooling at the wing edge, which can create tiny vortices of rising air. The difference is subtle: The butterflies that completed the migration were only about 3 percent more blotchy than those at the starting line. But the scientists suspect that even a small reduction in drag can make a tangible difference in flight capabilities.

The next step for the researchers is to test their hypothesis with wind tunnel experiments on artificial monarch wings.

Ayse Tenger-Trolander, a biologist at the University of Chicago who was not involved in the study, said that while there is a link between migratory success and spots, she’s interested to see if these follow-up studies show that the spots actually help. “There’s a little part of me that wonders if some of these differences are just due to the way the wing develops physically,” said Dr. Petite Trolander.

Micah Freedman, a biologist at the University of British Columbia who was also not involved in the study, agreed that further research was needed, but said, “I like the idea that this article includes both biologists and engineers.”

Such interdisciplinary collaboration could provide not only a better understanding of monarchs’ abilities to complete their migration in the face of habitat destruction and climate change, but also inspiration for more efficient aircraft. “If we’re right, and if the monarchs actually use this simple design technique, imagine how simple it would be to just throw some paint on an airplane wing,” said Dr. Davis.