THIS STORY ORIGINALLY appeared in WIRED Italia and has been translated from Italian.
The dozen butterflies were gracefully flying over a beach in French Guiana when Gerard Talavera saw them. It only took him a moment to realize they were extraordinary. They weren’t 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 journeys to rest. To reach this beach, Talavera realized, they would have had to travel 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, in collaboration with an international research team, has shown that the insects did indeed cross the Atlantic and they believe they know how, too. The details of this long migration Has been published in Nature Communications.
To trace the butterflies’ mysterious journey and prove their origin, the team conducted a series of analyses. Although migratory insects like butterflies are numerous, it is very difficult for scientists to track them: for example, researchers cannot attach tracking devices as they would to other animals, because they are often too large and heavy for the insects to carry. Clues to the butterflies’ origin had to be obtained from other data sets.
First, the team examined weather data from the weeks before the butterflies arrived and found that wind conditions could have favoured a journey from Africa to South America. The experts also sequenced the butterflies’ genomes and found that they showed closer kinship to populations in Africa and Europe, ruling out the possibility that the creatures had flown in from North America.
Spurred to investigate further, the team analysed 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 the concentration of isotopes varies around the world, the composition of isotopes in the butterflies’ wings can act as a geographical fingerprint, indicating their likely place of origin. The closest isotopic matches were in West Africa and Europe.
Finally, using innovative molecular techniques, the team sequenced the DNA of pollen grains attached to the insects and managed to identify the flowers from which the creatures had taken nectar. The analysis showed that they were carrying pollen from two species of plants that flower only at the end of the rainy season in tropical Africa.
Taken together, all the research pointed to the butterflies having managed to cross the Atlantic, a feat never before recorded. “We tend to 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 co-author of the study.
It was a long journey for the insects, probably lasting between five and eight days, and only made possible by extremely favourable 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, which helps fertilise the soil. Amazon.
“The butterflies could only have completed this flight if they had used a strategy that alternated between active flight, which consumes a lot of energy, and gliding with the wind,” says study co-author Eric Toro-Delgado, who is doing a PhD at the Institute of Evolutionary Biology in Barcelona. “We calculated that without wind, the butterflies could have flown a maximum of 780 kilometres before consuming all their energy.”