Think of a monarch butterfly and a striking image appears: black and orange wings, with white spots around the black edges. These white spots can actually help monarchs complete their long-distance migrations by changing the airflow around their wings.

Or, at least, that’s the provocative new claim from a team of scientists who have analyzed hundreds of monarch wings collected along the migration route.

“If you have bigger white spots, you’re simply better off reaching Mexico,” says Andy Davis, a researcher at the University of Georgia.

He and his colleagues believe that the color pattern may interact with sunlight to create subtle temperature differences on the wing that can alter airflow and facilitate the butterflies’ flight.

They are working on experiments that will test it by flapping real butterfly wings in a test chamber that will allow them to observe the movement of air.

Other butterfly scientists have responded to their work with both enthusiasm and skepticism.

“As far as I can tell, this is a completely new idea. And I think it’s pretty exciting,” says Marcus Kronforst, an evolutionary biologist at the University of Chicago who was not involved in this study.

“I’ve been working on butterfly color patterns my whole life, basically, and never, ever thought about this,” says Kronforst. “It never occurred to me that it could affect the way the butterflies fly.”

One of the most famous migrations

Monarch butterflies definitely need to fly well to complete their annual fall migration of up to 3,000 miles.

“This is one of the most famous insect migrations in the world. Monarchs from as far north as Canada fly south every fall all the way to some of the mountain peaks in central Mexico,” says Davis.

“One of the things that makes it so fascinating as a scientist,” he says, “is how something so small and so fragile can make such a spectacular journey.

In the spring, monarchs leave their wintering grounds and migrate north, breeding as they go. These summer monarchs only live for weeks, but their successive generations move the species steadily northward until the autumn migration.

Then one generation of kings who live much longer postpones cultivation and heads decidedly south. “These monarchs fly for about two months, fly all the way down there, or they die trying,” Davis says.

Davis previously noticed that monarch butterflies with a darker, redder color seemed to fly better in labs. “We didn’t really know why at the time,” he says.

Most research on butterfly color focuses on its use for camouflage or as a warning to predators. Since monarchs eat milkweed, which contains toxins, their bright orange color serves as a sign that they are unpalatable prey.

Researcher Andy Davis created this graphic using images from www.butterfliesofamerica.com with permission.

/ Andy Davis

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Andy Davis

Researcher Andy Davis created this graphic using images from www.butterfliesofamerica.com with permission.

So his report linking darker wing color to better flight performance caught the attention of Mostafa Hassanalian, a biologist at the New Mexico Institute of Mines and Technology.

Hassanalian had studied the black-and-white patterns of migratory birds and concluded that the color black could affect temperature and airflow in ways that could improve their flight performance.

So the researchers got together and decided to look at the black and white coloration in the edge of the king’s wings. Among them were Brenden Herkenhoff of New Mexico Tech, as well as Christina Vu and Paola Barriga of the University of Georgia.

They studied nearly 400 wild monarch wings from different locations along the migration route.

What they found is that the wings of the summer monarchs that are not migrating have less white, while those that are migrating but haven’t made it all the way to Mexico have more.

And the monarchs that reached Mexico have even more white, according to a report in the journal PLOS ONE.

The researchers also looked at white spots on closely related butterfly species that either don’t migrate or only semi-migrate.

“We compared the monarchs against all these other relatives,” says Davis. “And sure enough, the kings had slightly bigger spots than everyone else.”

Meanwhile, other research has suggested that more darkness on the monarch’s wings in the spring may be an advantage for butterflies, by increasing heat absorption on colder days, Davis says.

“It could be that there is a kind of dueling selective pressure on the monarchs in the spring versus the fall,” he says, with white dots being useful for southward migration and darker color being an advantage for generations migrating north.

Tries on the king’s wings

Hassanalian says they are working on experiments that will attach the king’s wings to a mechanism that flaps them. This allows them to experiment with flapping different colored wings in a test chamber which allows them to see airflow around the wings.

“We’ve now built the setup for bird wings with a flapping mechanism, and we’re going to do the same approach for butterflies,” he told NPR in an email.

Without demonstrating an actual effect on airflow, some researchers are skeptical.

“I would be very interested in more aerodynamic measurements,” says Mary Salcedo, a Cornell University postdoctoral researcher who studies insect wings. “I would love to see aerodynamic testing of their lift and drag coefficients.”

After all, the white spots may not have any effect, or be related to anything else.

Perhaps the white spots somehow help to avoid predation by birds, for example, and monarchs that migrate in the fall need this protection more because they live longer than summer monarchs and are more exposed to this threat.

“I find this aerodynamic explanation exciting. It is definitely different,” says Kornforst. “I just think there are possibly some other explanations that could be looked at.”

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