THIS STORY ORIGINALLY appeared on WIRED Italy and was translated from Italian.
A dozen butterflies were gracefully flying over a beach in French Guiana when Gerard Talavera spotted them. It took him a moment to realize that they were unusual. These were not just any butterflies, but nymphs (Vanessa thistle) — handsome orange, white, and black insects that don’t live in South America. They regularly migrate from Europe to sub-Saharan Africa, but stop several times along the way to rest. To reach this beach, Talavera realized they would have to travel more than 4,200 kilometers, crossing the Atlantic Ocean nonstop.
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 he believes they know how to do it. Details of this long migration have been published IN Nature communication.
To track the butterflies’ mysterious journey and prove their origins, the team conducted a series of analyses. Although migratory insects such as butterflies are bountiful, scientists have a challenging time tracking them: for example, researchers cannot attach tracking devices to them as they would to other animals, because they are often too immense and weighty to be carried by insects. Clues to the butterflies’ origins had to be drawn from other data sets.
First, the team examined weather data from the weeks leading up to the butterflies’ arrival and found that wind conditions could have been conducive to the journey from Africa to South America. Experts also sequenced the butterflies’ genomes and found that they were more closely related to populations in Africa and Europe, ruling out the possibility that the creatures had flown in from North America.
Encouraged to investigate further, the team analysed the atoms of two chemical elements—hydrogen and strontium—in the butterfly wings. The elements can come in slightly different forms, known as isotopes, due to having different numbers of neutrons in their nuclei. Because the concentration of isotopes varies around the world, the isotope composition of the butterfly wings can act as a geographic fingerprint, indicating their likely place of origin. The closest isotope matches were in West Africa and Europe.
Finally, using novel 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 were taking nectar. The analysis showed that they were carrying pollen from two species of plants that bloom only at the end of the rainy season in tropical Africa.
Taken together, the studies suggested that the butterflies flew across the Atlantic Ocean, something that had never been seen before. “We tend to think of butterflies as symbols of fragile beauty, but science shows us that they can perform incredible feats. There’s still a lot to learn about their capabilities,” 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, probably lasting five to eight days, and was only possible because of exceptionally favorable wind conditions. The air currents that aided the insects, known as the Saharan Air Layer, are also responsible for transporting immense amounts of dust and sand from the Sahara to South America, helping to fertilize Amazon.
“The butterflies could only complete this flight with a strategy that alternated between active flight, which is energy-consuming, and gliding with the wind,” says study co-author Eric Toro-Delgado, who is studying for a PhD at the Institute of Evolutionary Biology in Barcelona. “We estimate that without the wind, the butterflies could fly a maximum of 780 kilometers before they used up all their energy.”