Monarch butterflies undertake a perilous migration each year, which seems to be becoming even more dangerous, since fewer monarchs are successfully arriving at their overwintering quarters. Five new studies have just been published in a themed collection that answer some ongoing questions about their migration, and raise new questions in the process
By Andy Davis
Arguably, one of the world’s most famous insects, the monarch butterfly, is currently experiencing dire problems with its migration in eastern North America. Fewer and fewer monarchs are successfully reaching their overwintering destinations, and scientists aren’t sure why. Because of this, the need for research on their migration has never been more urgent. A new collection of research studies has just been published in the online journal, Animal Migration, which scientists hope will help in this effort.
The studies cover a wide range of topics relating to different components of the migration, and each of them makes new discoveries or in some cases, makes us question old assumptions about the migration.
Survival of the largest
Scientists have known that migratory monarchs tend to be larger than those that do not migrate. In one study in the collection, Andy Davis, researcher from the University of Georgia, photographed archived specimens of monarchs in museums, and used advanced computer software to measure the size of their wings. They were amazed to discover that North American monarchs have been gradually getting larger over the last 100 years, possibly because of a shift in the types of milkweeds available. Their data also point to the fact that the migration itself is the reason that monarchs keep getting larger, because only large-winged monarchs survive the journey, and pass on their genes to the next generation.
A surprise change of winter destination
Another study led by Dr. Hannah Vander Zanden, from the University of Florida, examined monarchs that wintered in South Florida using a special analytical technique that can pinpoint where the monarchs came from based on a sample of their wings. Amazingly, they discovered that half of the monarchs sampled appeared to originate from the American Midwest, which means that these monarchs chose not to fly to Mexico, where the majority of monarchs go for the winter. This discovery could be relevant to the shrinking overwintering population, especially if more of these “alternative” overwintering locations are found.
Parasite causes brittle wings
Since monarchs are naturally prone to infections with a tiny protozoan parasite, one team of scientists examined how this parasite affects the development of their wings. Since the infection leads to considerable losses of monarchs during the migration, the scientists wondered if the parasite made their wings weaker. They used an ingenious device to measure the tensile strength of preserved wings and found that infected monarchs have more brittle wings. This means they would suffer damage during the migration, which would slow them down or force them to drop out of the migration altogether.
Other studies in the collection included a review and analyses of the impact of hurricanes on monarch migration, as well as work that examined physical features of spring and fall monarchs in Michigan.
One step closer in saving the monarchs
Collectively, these studies each highlight a different element of the migration that needs to be considered for conservation efforts. With the knowledge gained from each new study, scientists can come one step closer to preserving the magical migration of this charismatic insect.
Read the original article here:
Hannah B. Vander Zanden, Carol L. Chaffee, Antonio González-Rodríguez, D.T. Tyler Flockhart, D. Ryan Norris, Marta L. Wayne: Alternate migration strategies of eastern monarch butterflies revealed by stable isotopes, 13.12.2019.