How a Group of Butterflies Managed to Fly 4,200 Kilometers Without Stopping

THIS STORY ORIGINALLY appeared on WIRED Italia and has been translated from Italian.

The dozen butterflies were flying gracefully over a beach in French Guiana when Gerard Talavera spotted them. It only took a moment to see they were extraordinary. These were not just any butterflies, he saw, but painted ladies (Vanessa cardui)—beautiful orange, white, and black insects that do not live in South America. They migrate regularly from Europe to sub-Saharan Africa, but stop several times during their travels to rest. To reach this beach, Talavera realized, they would have to have traveled more than 4,200 kilometers, crossing the Atlantic Ocean without a break.

That was back in 2013. Now, after 10 years of research, Talavera—an entomologist at the Barcelona Botanical Institute—working with an international research team has proven that the insects did in fact cross the Atlantic, and they think they know how, too. The details of this long migration have been published in Nature Communications.

To trace the butterflies’ mysterious journey and prove their origin, the team carried out a number of analyses. Although migratory insects like butterflies are numerous, it’s very difficult for scientists to track them: Researchers cannot, for example, attach tracking devices as they would with other animals, because these are often too large and heavy to be carried by the insects. Clues as to the butterflies origins had to be gleaned from other datasets.

First, the team examined the meteorological data for the weeks leading up to the butterflies’ arrival, and found that wind conditions could have supported a journey from Africa to South America. The experts also sequenced the genomes of the butterflies, and found that they showed a closer kinship with populations from Africa and Europe, thus ruling out the possibility that the creatures had flown down from North America.

Encouraged to delve deeper, the team then analyzed atoms of two chemical elements—hydrogen and strontium—in the butterflies’ wings. Elements can exist in slightly different forms, known as isotopes, as a result of having different numbers of neutrons in their nuclei. Because the concentration of isotopes varies around the world, the makeup of isotopes in the butterflies’ wings can act like a geographical fingerprint, indicating their likely place of origin. The closest isotope matches were for West Africa and Europe.

Finally, using innovative molecular techniques, the team sequenced the DNA of pollen grains attached to the insects, and were able to identify the flowers from which the creatures had taken nectar. Analysis showed that they were carrying pollen from two species of plant that bloom only at the end of the rainy season in tropical Africa.

Taken together, all the investigations suggested that the butterflies flew across the Atlantic Ocean, a feat never recorded before. “We usually see butterflies as symbols of the fragility of beauty, but science shows us that they can perform incredible feats. There is still much to discover about their capabilities,” says Roger Vila, a biologist at the Institute of Evolutionary Biology in Barcelona and a coauthor of the study.

It was a long journey that the insects made, probably lasting five to eight days, and was only possible thanks to extremely favorable wind conditions. The air currents that assisted the insects, known as the Saharan Air Layer, are also responsible for transporting large amounts of dust and sand from the Sahara Desert to South America, helping to fertilize the Amazon.

“The butterflies could have completed this flight only by using a strategy that alternated between active flight, which is energy-costly, and gliding with the wind,” says study coauthor Eric Toro-Delgado, who is studying for a PhD at Barcelona’s Institute of Evolutionary Biology. “We estimate that without wind, the butterflies could have flown a maximum of 780 kilometers before consuming all their energy.”