“Knowing where butterflies come from during migration helps to inform conservation strategies that may be needed to protect the resources in their breeding areas. Similarly, knowing where they go in winter helps to protect those habitats during the time they are there,” said Leonard Wassenaar, Head of the IAEA’s Isotope Hydrology Laboratory. “The linkage between geographic locations in the annual life cycle of butterflies cannot be established without using isotope methods.”
The research is based on measuring deuterium — a rare isotope of hydrogen — in rainwater, which is directly ingested by animals and humans. As rainwater and its deuterium composition are unique to the area where it rains, rainwater deuterium content serves as a direct marker that scientists can use to identify the origin of individual animals that were grown in different areas by measuring the amount of deuterium in hair, wings, claws, feathers or bones. For butterflies, the deuterium contents are measured in the wings and show the area where the insect was born.
While isotopes have been used for decades to establish with precision the migration paths of some insects, such as the Monarch butterfly (Danaus plexippus), the migration patterns and paths of dozens of other species of butterflies, insects and other migratory animals are yet to be established in Mexico and abroad.
Butterfly migration patterns: chain, leapfrog and panmixia
The study revealed that, to survive the winter, four out of the six butterfly species travelled from the North of the United States or from Southern Canada to Mexico. The study also revealed further information on the migration style of the butterfly species.
It found that the American Snout butterfly (Libytheana carinenta) had the longest migration route and that its migration was a “chain migration”. This means that the American Snout butterflies born in the northern parts of the subcontinent were found to settle in Mexico for the winter only after those born in the southern parts had already migrated further south.
The Queen butterfly (Danaus gilippus), on the other hand, was observed to have performed “leapfrog migration” within the subcontinent. This means that individual Queen butterflies born in southern parts of the subcontinent made a short journey further south. Their northern-born counterparts, however, were found to have travelled further south than the southern-born butterflies for the winter, in other words “leapfrogging” past those born in more southern areas.
A third species, the Dogface butterfly (Zerene cesonia), was shown to practice “panmixia”, meaning that the individual butterflies would mix with one another and settle together during the migration path, regardless of their region of origin.
Determining butterflies’ birthplaces from their wings
To study the migration paths of the six species, scientists collected samples of butterflies that had been killed by passing cars on a specific mountain valley road popular with several types of migrating butterflies. The samples were collected between September and November 2019. To establish the migration path, the scientists determined their place of birth by analysing the deuterium in their wings and by comparing it to the data on rain isotopes present in an IAEA database (click here for more information on the technique).
“This type of research is important because, on the one hand, it helps us understand the evolution of the patterns in animals, and on the other hand, from a conservation perspective, it helps us to predict which populations may be more vulnerable to events along the migration route, such as climate events, car collisions and habitat loss,” said Keith Hobson, researcher of the University of Western Ontario in Canada and co-author of the study.
Pic caption :Scientists studied the migration of six butterflies with isotopes:American Snout butterfly (Libytheana carinenta), Queen butterfly (Danaus gilippus), Cloudless Sulphur butterfly (Phoebis sennae), Empress Leilia butterfly (Asterocampa leilia), Variegated Fritillary butterfly (Euptoieta claudia), and Southern Dogface butterfly (Zerene cesonia). (Photo credits: S. Bright, V. Charny, J. Gallagher, J. Green)
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