Distle buttocks are globetrotter and sometimes cover long distances. Every year, some of the colorful butterflies buzz from Africa to Sweden and back again, where they settle north and south of the Sahara. Other copies of this kind, on the other hand, only cover short distances and stay on the Mediterranean. Why the insects behave so differently is unclear. Unlike previously assumed, the migration behavior is at least not stored in the DNA of the butterfly, as biologists have now found out. Instead, the moth trip probably depends on the seasonal and regional environmental conditions.
Distel flags (Vanessa Cardui) are bright orange butterflies with an artful black and white pattern. The colorful moths are known to be persistent long -haul flyers: Every year, the thistles travel up to 10,000 kilometers from the warm northwest of Africa to the Arctic tundra in Sweden and back again to find the perfect environmental conditions for reproduction, breeding and survival. In the spring, the insects initially fly to Europe via the Mediterranean. The following generations then make their way to Great Britain and Sweden to spend the summer there. In autumn, the thistles then return to warmer regions.
“Every individual travels the trip in a section of the annual migration cycle and its descendants,” explains co-author Daria Shipilina from the University of Uppsala. All thistles follow this relay principle, but the individuals fly differently: While some butterflies stay in the Mediterranean area, others continue to north and later back to Africa and even cross the Sahara. So far, why this is so has been a mystery. However, researchers suspected that the migration movements of the moths are genetically controlled and are reflected in a different look – similar to which some migratory birds were demonstrated.
DNA comparison of the butterflies
A team led by first author Megan Reich from the University of Ottawa has now examined the different travel routes of the thistle buttocks together with citizen scientists. For this they traveled in regions north and south of the Sahara in 2018 and 2019 – from Benin, Senegal and Morocco to Spain, Portugal and Malta – and collected 40 thistles with nets. With the help of isotope geolocalization, they then determined the geographical origin of the individual butterflies. “The principle of this method is that the composition of the stable isotopes from the wings of an adult butterfly reflects the isotope signature of the plants that he has eaten as a caterpillar,” explains Shipilina. “By analyzing hydrogen and strontium isotopes, we were able to trace the origin of these butterflies back and estimate how far they had traveled,” adds senior author Clement Bataille from the University of Ottawa. In addition, the biologists seized the DNA of the insects and compared their genes.
The geo -analysis confirmed the different travel behavior of the thistle buttocks: 13 individuals flew at least 500 kilometers, some even up to 4000 kilometers from Scandinavia to the south and crossed both Mediterranean and the Sahara. Of the other examined copies, most of them probably only flew between 140 and 240 kilometers, from a starting point in Central Europe, and remained north of the desert in the Mediterranean. Contrary to expectations, the DNA analysis showed that there is no genetic difference between short and long-distance butterflies. The biologists also found no connection between travel patterns with other physical factors such as gender, the size of the wing or the wing shape.
Do environmental stimuli decide on short or long distances?
But if the behavior is not due to the genes, how is it controlled? A possible explanation for the different migration styles could be that thistle butterfly adapts the route length in response to environmental conditions. In the Swedish late summer, for example, the rapid change of the daily length could be the starting signal for the return to the south of the Sahara, the team suspects. In contrast, the moths in the south of France, where the days are longer, do not take these stimuli and therefore only fly short distances and remain in the Mediterranean. “It is also possible that the differences in migration distance observed here can be explained in whole or in part by external mechanisms, including differences in the parasite load, energy storage and metabolism, wind support and weather events or even personality,” writes the team .
Follow -up studies should now clarify whether the migration of the thistle feast actually decreases to environmental influences and which are. It should also be investigated whether thistle butterfly in other parts of the world, other types of butterfly and other insect groups behave as well or whether the phenomenon is unique for European thistle butterfly. Previous research has already shown that Distelfalter can master a 4,200 km long crossing of the Atlantic from West Africa to French Guayana in South America. “Understanding the travel patterns of insects is of crucial importance for nature conservation. It enables to predict how environmental changes caused by humans could affect insect migration in the future, ”explains Reich.
Source: Institute of Science and Technology Austria (ISTA); Specialist articles: Pas Nexus, DOI: 10.1093/PASSNEXUS/PGAE586