The Secret in the Spots on Monarch Butterflies’ Wings

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

“No one even knew what these spots were for in monarchs,” said Andy Davis, a biologist at the University of Georgia and an author of a study published on Wednesday in the journal PLOS ONE. He added, “All of a sudden, it seems like they’re really important.”

Monarch wings are mostly orange, but their edges are black, punctuated with tiny white spots. Dr. Davis was curious if those black edges contributed to monarchs’ migratory capabilities. Dark colors absorb more heat, and studies of seabirds have suggested that the temperature difference between dark and light feathers can change air flow patterns, enabling birds with blacktopped wings to “enhance their flight efficiency — basically increasing the lift and decreasing the drag,” said Mostafa Hassanalian, a mechanical engineer at the New Mexico Institute of Mining and Technology.

He and Dr. Davis teamed up to see if the same held true in monarchs, with darker-winged butterflies having an aerodynamic advantage, making them more likely to survive the long migration than spottier peers.

To test this hypothesis, Dr. Davis enlisted Tina Vu, then an undergraduate at the University of Georgia, to undertake a potentially tedious task: going through photographs of 400 monarch butterflies and measuring the amount of black and white on the edges of their wings.

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

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 that survived the trip to Mexico tended to have more white spots, rather than the extra-dark wings the researchers had predicted. Additionally, she analyzed some of the butterflies’ nonmigratory relatives, which also had fewer spots than the ones making the big migration.

“When we saw that there were more white spots, we were like, ‘OK, so those have to have a function,’” said Ms. Vu, who plans to pursue graduate studies in biology.

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

The next step for the researchers will be testing their hypothesis with wind tunnel experiments on artificial monarch wings.

Ayse Tenger-Trolander, a biologist at the University of Chicago who was not involved with the study, said that while a correlation between migratory success and spots is present, she is 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 due just to the way that the wing physically develops,” Dr. Tenger-Trolander said.

Micah Freedman, a biologist at the University of British Columbia who was also not involved with the study, agreed that follow-up studies were needed, but said, “I really like the idea that this paper involves both biologists and engineers.”

Such a cross-disciplinary collaboration might yield 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 really are utilizing this simple design technique, imagine how simple it would be to just get some paint and throw it on an airplane wing,” Dr. Davis said.

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