For a migrating monarch butterfly, the key to success might be in its spots, according to a new study published Wednesday in the journal PLOS One.
Scientists suspected, based on past research about seabirds, that monarchs with dark wings would have the most successful flights. But their findings revealed the opposite.
Among 400 photographed pairs of monarch butterfly wings that the team examined, the ones with the most white spots belonged to the most successful migrants, which made it all the way to Mexico. The scientists say that having whiter wings could unevenly heat the wing surface in a way that reduces drag. This might allow the insects to move more efficiently on their impressive migratory journeys.
“No one even knew what these spots were for in monarchs,” co-author Andy Davis, an animal ecologist at the University of Georgia, tells the New York Times’ Kate Golembiewski. “All of a sudden, it seems like they’re really important.”
The monarch’s migration is one of the most well-known marvels of the insect world. While other butterfly species may migrate in a single direction, often in search of food, the monarch is the only one that completes a death-defying, two-way migration, like birds do. Each fall, monarch butterflies will take flight en masse, moving more than 2,000 miles from the United States and Canada to their wintering grounds in Mexico. And in the summer, they repeat the process, traveling north to breed.
Other species of butterflies that did not migrate had smaller white spots than the monarchs, the researchers found. And the wings of short-lived, non-migratory generations of monarchs—which are born in the summer and do not survive to see migration time in the fall—also had smaller spots.
According to the scientists’ theory, larger white spots give migrating monarchs a boost by creating a patchwork of heating and cooling along the edges of their wings. As darker wing areas grow warm and lighter areas stay cool, it could influence tiny, swirling pockets of air to form around the spots. These eddies might affect the way air flows past the butterfly, reducing drag on its wings. Maybe, the researchers write, the spots evolved to be larger and more plentiful as the monarchs evolved to migrate longer distances.
Still, some scientists say more research is needed to confirm this link. “I’d be really interested in more aerodynamic measurements,” Mary Salcedo, a biomechanist at Cornell University who was not involved in the study, tells NPR’s Nell Greenfieldboyce. “I’d love to see aerodynamic tests on their lift and drag coefficients.”
To try and get this data, the team hopes to build and operate artificial monarch wings in a test chamber that resembles a wind tunnel. Further study could rule out some other possible explanations for the white spot trend, such as predator defense or bleaching from the sun during migration, according to the paper.
But if the finding holds true, it could have lessons for engineers. Applying the butterflies’ white spots to drones could improve their efficiency, the researchers say. “Your drone would be able to carry more, because this coloration helps them gain extra lift,” co-author Mostafa Hassanalian, a mechanical engineer at New Mexico Tech who has created drones from taxidermy birds, tells Popular Science’s Zayna Syed.
Overall, the difference in the insects’ spot size was small—those that made it to Mexico had white spots just 3 percent larger than monarchs that ended up at sites in the U.S. “It’s hard to see with the naked eye,” Davis tells New Scientist’s Corryn Wetzel—only image processing software can reveal the subtle variations in spot size.
But these very small differences are not inconsequential, Davis says to National Geographic’s Jason Bittel. “That could be the difference between life and death during the migration.”