THIS STORY ORIGINAL appeared in Italy by phone and is translated from the Italian.
Dozens of butterflies were flying gracefully over a beach in French Guiana when Gerard Talavera spotted them. It only took a moment to see that they were extraordinary. These were not just any butterflies, but painted ladies (Vanessa Cardui)-beautiful orange, white and black insects that do not live in South America. They regularly migrate from Europe to sub-Saharan Africa, but stop several times during their journeys to rest. To reach this beach, Talavera realized, they must have traveled more than 4,200 kilometers, crossing the Atlantic Ocean without stopping.
That was in 2013. Now, after 10 years of research, Talavera – an entomologist at the Botanical Institute of Barcelona – working with an international research team has proven that the insects did indeed cross the Atlantic, and they think they also know how. The details of this long migration are published in Nature Communications.
To trace the mysterious journey 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: Researchers cannot, for example, attach tracking devices as they would other animals because they are often too large and heavy to be carried by insects. Data on the origin of the butterflies had to be gathered from other data sets.
First, the team examined meteorological records for the weeks before the butterflies arrived 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 relationship to populations from Africa and Europe, thus ruling out the possibility that the creatures had flown from North America.
Encouraged to dig 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 different numbers of neutrons in their nuclei. Because the concentration of isotopes varies around the world, the composition of isotopes in butterfly wings can act as a geographic 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 the pollen grains attached to the insects and was able to identify the flowers from which the creatures had taken the nectar. Analysis showed that they carried pollen from two types of plants 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 before recorded. “We usually see butterflies as symbols of the fragility of beauty, but science shows us that they can perform extraordinary feats. There is still much to discover about their abilities,” says Roger Vila, a biologist at the Institute of Evolutionary Biology in Barcelona and a co-author of the study.
It was a long journey that the insects made, likely lasting five to eight days, and was only possible thanks to extremely 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 have completed this flight only by using a strategy that alternated between active flight, which costs energy, and gliding with the wind,” says study co-author Eric Toro-Delgado, who is studying for a PhD at the Barcelona Institute. Evolutionary biology. “We estimate that without wind, the butterflies could have flown a maximum of 780 kilometers before using up all their energy.”