Monarch butterflies with larger white spots on their wings are more successful in migrating long distances. It’s not clear how these points help the butterflies, but it’s possible they create temperature differences in the insect’s large wings, reducing drag and helping them fly more efficiently.
monarchs (danaus plexippus) have an impressive migration by any standard, but especially for an insect that barely weighs more than a raindrop. Some populations travel up to 4,000 kilometers from Canada and the US to the forested mountains of Mexico, where they settle for winter hibernation.
Migratory monarchs have white spots framing their orange and black wings, while other closely related non-migrating butterflies lack them entirely. that took Andy Davis at the University of Georgia in the US and colleagues to investigate what role ads may be playing. “Everybody knows [monarchs] it has spots, but it has never been studied before,” says Davis. “Nobody knows anything 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 the butterflies with the blackest wings would be more successful because they soak up more of the sun’s warm rays. But to their surprise, they found the opposite. The most successful immigrants 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 monarchs, that could mean the difference between failure and success on the long flight,” Davis says. “We were pretty surprised by that.”
They also examined museum specimens of monarchs and compared them to six other closely related species, revealing that migratory monarchs have larger spots than their year-round cousins.
The researchers suspect that the white spots limit the absorption of sunlight and radiation. The resulting temperature difference between the light and dark areas of its wings could change the way air flows in a way that reduces drag and increases aerodynamics, although this has yet to be investigated. The spots may have arisen as insects evolved to migrate longer distances than their ancestors.
These ideas need to be put to the test, he says. marcus kronford at the University of Chicago. “It opens up a whole new field in the study of butterfly color patterns,” she says.
