Every year, millions of monarch butterflies fly up to 3,000 miles from southern Canada and the upper reaches of the United States to their wintering grounds in mountain forests outside Mexico City.
“Everybody knows about the monarchs’ migration,” says Andy Davis, an animal ecologist at the University of Georgia. “But one of the things that we still don’t understand very well is how they’re capable of making such a tremendous flight while being such a small animal with limited energy.”
Amazingly, some of the monarch’s continent-spanning magic may be owed to the size of its itty bitty wing spots, according to a study published today in PLOS One.
By studying photos of wing patterns of nearly 400 monarchs collected at various locations on their migration path, Davis and his co-authors discovered an interesting pattern—butterflies that make the journey all the way to Mexico tend to have white wing spots that are three percent larger than those of monarchs found in places such as Georgia or Minnesota. At the same time, the Mexican animals have three percent less black coloration on their wings than monarchs collected during earlier phases of the multi-generational migration. (Read how monarch butterflies migrate 3,000 miles.)
Now, here’s why this is interesting.
Migrating monarchs soar at heights of up to 1,200 feet. As sunlight hits those wings, it heats them up, but unevenly. Black areas get hotter, while white areas stay cooler. The scientists believe that when these forces are alternated, as they are with a monarch’s white spots set against black bands on the wings’ edges, it seems to create micro-vortices of air that reduce drag—making flight more efficient.
Similar drag-reducing characteristics have been discovered in shark skin and the coloration of seabird wings. All of it adds up to some serious potential for the future of human technologies.
“If you want to develop drones that are flying for longer time and harnessing energy from sunlight, this is the best thing that we can look at,” says study co-author Mostafa Hassanalian, an associate professor of mechanical engineering at New Mexico Tech.
How the monarch butterfly got its spots
The second part of the study evaluated size differences in white spots between the monarch and six of its closest cousins in the genus Danaus.
When study co-author Christina Vu, then a student of Davis’, quantified the size of all those butterflies’ spots, she found the monarchs had by far the largest white markings. Next were the southern monarchs (D. erippus), which are semi-migratory, followed by the other five species—including the Jamaican monarch (D. cleophile), the solider or tropical queen (D. erisimus thetys), the queen (D. gilippus Berenice), and striated queen (D. g. strigose)—none of which migrate.
Indeed, it seems spot size—and, the scientists theorize, its ability to reduce drag—might be linked to migration itself.
“I think that the fall migration is already a major selective event each year,” says Davis. “It ensures that only the fittest individuals reach the finish line. The ones without any infections; or disease; or with the biggest, most robust wings.” (See National Geographic's amazing photos of monarchs.)
“And in this case, the best spot patterns,” he says.
The butterfly effect
“A butterfly can flap its wings in Peking and in New York, you get rain instead of sunshine,” quips Jeff Goldblum’s character in the original Jurassic Park.
From Havana to Donnie Darko, many films have introduced audiences to the so-called “butterfly effect,” or the general idea that small changes can sometimes create large consequences. And while many references misinterpret the idea originally laid out by MIT meteorology professor Edward Lorenz in 1963, it’s tempting to use the metaphor as a way to understand how a monarch’s wing patterns could be of any consequence.
After all, we’re talking about spots the size of a pencil eraser on an insect that weighs about as much as a kernel of corn.
“But the sheer distance that they have to travel, they’re spending 10 hours a day in the air, for 60 days in total, to get to their destination,” says Davis. (Read how monarchs may be doing better than we thought.)
So a minor difference—in this case, spot size—"is compounded on a daily basis. And so that could be the difference between life and death in during the migration.”
The next step would be to test whether such a small difference in coloration could have observable effects on drag, Michaël Nicolaï, a biologist at Ghent University in Belgium, said in an email.
For instance, the new research found just a three percent difference in coloration, whereas a study he conducted on seabirds revealed a 20 percent increase in efficiency for darkly colored wing feathers.
Nicolaï does agree that even tiny differences could be beneficial over such large distances, however, until real experimental measurements are provided, he will remain “very optimistic” but “not convinced.”
Davis hopes his study inspires others to research that question.
“No one in the world of butterflies is looking at their wings in this way,” says Davis. “People are going to be looking at every other butterfly species out there now thinking, ‘Oh my God, now what does this color do for its flight?’”
“I think this is going to be revolutionary.”