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Monarch butterflies' white spots may help them fly farther, scientists say

A MARTÍNEZ, HOST:

Monarch butterflies have distinctive orange-and-black wings with small white spots. And it's those spots that recently caught the attention of a group of scientists. As NPR's Nell Greenfieldboyce reports, the researchers believe the spots could help monarchs fly during their long, annual migrations.

NELL GREENFIELDBOYCE, BYLINE: Monarch butterflies are world-class fliers. Andy Davis studies them at the University of Georgia. He says their annual migration covers thousands of miles.

ANDY DAVIS: One of the things that makes it so captivating for - as a scientist is how something so small and so delicate seeming can make such a tremendous journey.

GREENFIELDBOYCE: He says each spring, monarchs leave their overwintering grounds in Mexico. Then they head north, laying eggs as they go. Each generation of butterflies moves the species steadily northward.

DAVIS: In the spring, it's more of a progression of successive generations.

GREENFIELDBOYCE: The fall migration is different and more dramatic.

DAVIS: The fall migration is that magical one where one single generation attempts to make that long-distance, you know, 3,000-mile journey all the way to central Mexico.

GREENFIELDBOYCE: These monarchs travel for about two months.

DAVIS: Flying all the way down there or they die trying.

GREENFIELDBOYCE: It's a grueling trip, and Davis and some colleagues wondered if it might be affected by the pattern of color on monarchs' wings. They've analyzed hundreds of monarch wings from butterflies all up and down its range. And they noticed something about the monarchs that successfully completed the long-distance flight.

DAVIS: Those monarchs that reached Mexico tended to have slightly less black on their wings and slightly more white on their wings.

GREENFIELDBOYCE: To see if white spots might really be somehow related to migration, they looked at closely related butterfly species - ones that don't migrate and ones that were only semimigratory.

DAVIS: And so we figured if the migration has selected for white spots in the monarchs, then we would see larger spots in the monarchs compared to everybody else. And that's exactly what we found.

GREENFIELDBOYCE: He and his colleagues describe their findings in a science journal called PLOS One. They believe the pattern of black and white on the edge of a monarch's wing could potentially affect airflow because the dark and light patches would be hotter or cooler in the sun. Other butterfly researchers say it's a wild idea. Marcus Kronforst is an evolutionary biologist with the University of Chicago. He's studied wing color for basically his whole career.

MARCUS KRONFORST: It's never crossed my mind that it might influence how the butterflies fly, that it would influence their sort of aerodynamic efficiency. It is - as far as I can tell, it's a totally new idea.

GREENFIELDBOYCE: He says most research on wing color has studied how it can be used for camouflage or as a warning to birds that might be looking for a snack.

KRONFORST: The reason monarchs have those bright color patterns is to warn predators that the butterfly is toxic.

GREENFIELDBOYCE: He finds the idea of color affecting flight intriguing but thinks there needs to be more evidence. That's also the view of Mary Salcedo, who studies insect wings at Cornell University. She'd be interested in seeing flight-related experiments done with wings that have different color patterns.

MARY SALCEDO: I'd love to see aerodynamic tests on their lift and drag coefficients.

GREENFIELDBOYCE: That's the kind of work that Davis and his colleagues are focused on now. They're planning to use a mechanism that will flap real butterfly wings in a testing chamber that tracks the movement of air.

Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF OKAMI'S "UNDERGROWTH") Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Nell Greenfieldboyce is a NPR science correspondent.