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Painted Lady Butterfly. Photo: Gerard Talavera and Roger Vila, CSIC

Previously known to migrate from Europe to the Afrotropics during the autumn, the fate of this butterfly species and its offspring remained unknown.

A British Ecological Society funded study found that painted lady butterflies return from the Afrotropical region to recolonize in the Mediterranean in early spring, traveling an annual distance of 12,000 km across the Sahara Desert.

While the Palearctic-African migratory circuit is typically associated with birds, scientists from the Institute of Evolutionary Biology, a joint research center of the Spanish National Research Council and Pompeu Fabra University, in Barcelona, Spain, found that a butterfly species, the Painted Lady (Vanessa cardui), endures annual trans-Saharan circuits like some birds do.

Their results were published in the journal Biology Letters.

This butterfly species travels 12,000 km and crosses the Sahara Desert twice to seasonally exploit resources and favorable climates on both sides of the desert.

Few species are known to perform annual, long-range trans-Saharan circuits.

The flight of the painted lady is the longest migratory flight known in butterflies to date.

In a previously published study, researchers demonstrated that painted lady butterflies migrate from Europe to tropical Africa by the end of summer, crossing the Mediterranean Sea and Sahara Desert.

The fate of these migrants and that of their offspring remained unknown.

“Our hypothesis was that the species initiates a reverse northward migration towards Europe in spring, thus completing a regular migratory cycle," Roger Vila, one of the authors, said.

 

The answer is in the wings

With the aim of confirming this hypothesis, the researchers studied the natal origin of the butterflies that reached the Mediterranean region in early spring.

To do so, the team analyzed the stable hydrogen isotopes of the butterflies sampled in Morocco, Andalusia and Catalonia in Spain, Crete, Egypt and Israel. An isotope is a form of a chemical element whose atomic nucleus contains a different number of neutrons compared to protons in the nucleus. In water, the proportion of hydrogen and its stable isotope depends on the geographical location. When absorbing water, this proportion is maintained in plants; it later remains in the caterpillars that feed on these plants, and, eventually, in adult butterflies.

By analyzing the hydrogen stable isotopes found in the wings of adult butterflies, the researchers could determine where they had developed as caterpillars.

“It is difficult to study the movement of insects by means of observations, marking or radio tracking, since there are millions of individuals and they are very small," Gerard Talavera, who led the research, said. “This is why finding out where a butterfly grew up before undergoing the metamorphosis by means of stable isotope analysis turns out to be extremely useful. It feels like magic."

The results show that a major proportion of specimens stay in the Afrotropics during winter, and that those recolonizing the Mediterranean are most probably their offspring.

This scenario closes the loop for the Palearctic-African migratory system of Vanessa cardui, and shows that the annual distance traveled by the successive generations may reach about 12,000 km, including crossing of the Sahara Desert twice.

Whether the painted lady does regular migratory circuits similar to those of the monarch butterfly in North America, was a matter of scientific debate.

This research reveals the parallelisms in such a unique evolutionary adaptation.

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