THIS STORY ORIGINALLY appeared on WIRED Italy and was translated from the Italian.
A dozen butterflies fluttered gracefully over a beach in French Guiana when Gerard Talavera spotted them. It only took a moment to see that they were special. They weren’t just any butterflies, he saw, but painted ladies (Vanessa cardui) — a beautiful orange, white and black insect that does not live in South America. They regularly migrate from Europe to sub-Saharan Africa, but make several stops to rest during their journeys. To reach this beach, Talavera realized, they would have to travel more than 4,200 kilometers and cross the Atlantic Ocean non-stop.
That was in 2013. Now, after 10 years of research, Talavera – an entomologist at the Botanical Institute of Barcelona – has, in collaboration with an international research team, proven that insects have actually crossed the Atlantic, and they think they know how too. . Details of this long migration were published in The nature of communication.
To trace the mysterious path of the butterflies and prove their origin, the team performed a series of analyses. Although migratory insects such as butterflies are abundant, they are very difficult for scientists to track: For example, researchers cannot attach tracking devices as they can with other animals because they are often too large and heavy for the insects to carry. Clues to the origin of the butterflies had to be obtained from other data sets.
First, the team examined meteorological data for the weeks leading up to the butterflies’ arrival and found that windy conditions may have favored the journey from Africa to South America. The experts also sequenced the genomes of the butterflies and found that they were more closely related to populations from Africa and Europe, ruling out the possibility that the creatures had flown in from North America.
Encouraged to delve deeper, the team then analyzed the 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 isotope concentrations vary around the world, the isotope composition of butterfly wings can act as a geographic fingerprint, indicating their likely place of origin. The closest isotopic matches were for West Africa and Europe.
Finally, using innovative molecular techniques, the team sequenced the DNA of the pollen grains attached to the insects and were able to identify the flowers from which the creatures took nectar. The analysis showed that they carried pollen from two species of plants that flower only at the end of the rainy season in tropical Africa.
Taken together, all the investigations indicated that the butterflies had flown across the Atlantic Ocean, a feat never before recorded. “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 abilities,” says Roger Vila, a biologist from the Institute of Evolutionary Biology in Barcelona and co-author of the study.
It was a long journey for the insects to make, probably lasting five to eight days, and was only possible due to exceptionally favorable wind conditions. The air currents that helped 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.
“Butterflies could only complete this flight using a strategy that alternated between active flight, which is energy-expensive, and gliding downwind,” says study co-author Eric Toro-Delgado, a PhD student at the Barcelona institute. Evolutionary Biology. “We estimate that without wind the butterflies could fly a maximum of 780 kilometers before using up all their energy.”