A monarch butterfly resting on a flower
Shutterstock/Shelly Jefferson Morton
Monarch butterflies with larger white spots on their wings are more successful at migrating long distances. It’s unclear how these spots help the butterflies, but it’s possible they cause temperature differences across the insect’s large wings, reducing drag and allowing them to fly more efficiently.
Monarchs (Danaus plexippus) have an impressive migration by any measure, but especially so for an insect that weighs barely more than a drop of rain. Some populations travel up to 4,000 kilometers from Canada and the US to the forested mountains of Mexico, where they settle for hibernation.
Migrating monarchs have white spots framing their orange-black wings, while other closely related butterflies that don’t migrate lack them altogether. That led Andy Davis at the University of Georgia in the US and his colleagues to investigate what role the spots may play. “Everyone knows [monarchs] have spots, but it has never been studied before,” says Davis. “No one knows about those places.”
The researchers analyzed images of more than 400 monarch wings at different stages of the insects’ autumn migration from north to south. They predicted that butterflies with the most black on their wings would be most successful because they absorb more of the sun’s warming rays. But to their surprise, they discovered the opposite. The most successful migrants had about 3 percent less black and 3 percent more white on their wings.
“It’s hard to see with the naked eye, but for the monarchs during the long flight, that can mean the difference between failure and success,” says Davis. “We were quite shocked by that.”
They also examined monarch specimens from museums and compared them to six other closely related species, showing that migratory monarchs have larger patches than their year-round cousins.
The researchers suspect that the white spots limit the absorption of sunlight and radiation. The resulting difference in temperature between light and dark areas of their wings could change how air flows in a way that reduces drag and improves aerodynamics, though this has yet to be explored. The spots may have emerged as the insects evolved to migrate longer distances than their ancestors.
Those ideas need to be put to the test, says Marcus Kronfort at the University of Chicago. “It opens up a whole new area in the study of butterfly color patterns,” he says.
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