While scientists can tag migrating birds, mammals, and other animals to track their movements, the precise migration patterns of butterflies and other insects too small for tagging evaded scientists’ scrutiny for decades. That changed in 1996, when Leonard Wassenaar and Keith Hobson, working at the time as isotope scientists for Environment Canada, demonstrated that isotopic techniques could be used to determine the origin of individual monarch butterflies and deduce the species’ annual migration routes. Now, the same technique is being used to study other butterfly species.
Then and now: Decades ago, Wassenaar and Hobson collected 1,200 specimens of monarch butterflies and used an International Atomic Energy Agency (IAEA) database of stable isotopes in rainwater called the Global Network of Isotopes in Precipitation (GNIP) to determine the origin of individual butterflies by measuring the deuterium content in the insects’ wings.
Wassenaar now heads the Isotope Hydrology Laboratory within the IAEA’s Isotope Hydrology Section, which maintains and operates several global isotope data networks for hydrology and climate studies, including GNIP.
Hobson is now a researcher at the University of Western Ontario in Canada, and he recently coauthored a study published in the journal Diversity that used the same stable isotope technique to determine the probable origins and migration paths of six different North American butterfly species. That research was described in a news article released by the IAEA on April 8.
Conservation is the motivation: “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,” Hobson said.
That knowledge can be used to develop strategies to protect vulnerable populations. “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 Wassenaar. “The linkage between geographic locations in the annual life cycle of butterflies cannot be established without using isotope methods.”
The method: Deuterium, or heavy hydrogen, is found in precipitation and tracked through GNIP, a worldwide isotope monitoring network of hydrogen and oxygen isotopes in precipitation with hundreds of monitoring sites in over 90 countries generating more than 130,000 monthly isotope records. GNIP was initiated in 1960 by the IAEA and the World Meteorological Organization, and decades of data have produced reliable maps of precipitation across latitudes.
The deuterium content of water that is ingested by humans and animals can be detected in metabolically inert tissues such as hair, wings, claws, feathers, or bone. In the case of butterflies, the deuterium in wing chitin (scales) can enable a probabilistic identification of the geographical area where the insect’s life began.
A migration bottleneck: The research team behind the recent study of six butterfly species collected butterflies that had been killed by passing cars on a highway through the Sierra Madre Oriental mountains, near the city of Monterrey in northeast Mexico. During the fall migration season, southbound butterflies must fly through narrow mountain valleys, and the same valleys are spanned by highway bridges. While the intersection of human and insect travel brings the journey of many individual butterflies to an abrupt end, salvaging road-killed insects can provide a large sample for study without sacrificing live individuals.
While the exact destination of the butterflies collected for the study cannot be known, the researchers were able to assume, based on the north-south movement of butterflies through the site, that individuals were migrating from points north of the site and within the species’ known distribution ranges. The study area was surveyed twice per week in September through November 2019, for a total of 13 sampling days.
The findings: The study revealed that four out of the six species studied had traveled from the northern United States or from southern Canada.
The study also revealed information on the migration style of specific species, including, for example, that the American Snout butterfly had the longest migration route—one that uses “chain migration.” American Snout butterflies born in the northern parts of the species’ range settle in Mexico for the winter after those born in the southern parts had already migrated farther south.
The researchers found that the Queen butterfly exhibits “leapfrog migration.” While individual Queen butterflies born in southern parts of North America made a short journey south, their northern-born counterparts took a much longer trip, “leapfrogging” past their southern kin to spend the winter farther south.
A third species, the Dogface butterfly, was shown to practice “panmixia,” with individual butterflies settling together during the migration path regardless of their region of origin.