Monarch butterfly wing length affected by migration behavior, according to UC Davis study

Monarch butterfly wing length affected by migration behavior, according to UC Davis study

A graduate student discovered this pattern with the help of museums

The size and length of a monarch butterfly can indicate whether it is part of a migrating population. Micah Freedman, who received his  Ph.D. in population biology from UC Davis, is the main author of this study that has been published in the Proceedings of the National Academy of Sciences. 

The project started in his first year of graduate school while living and working in Guam. During his stay there, Freedman noticed the prevalence of monarch butterflies on the island and became curious as to how and when they arrived there. This question was the root of his five-year Ph.D. project. 

Accompanying him in Guam was Hugh Dingle, an emeritus professor in the department of entomology and nematology at UC Davis. He helped introduce Freedman to these ideas that comprised the project. 

Freedman’s study looked into the massive, seasonal migration of the North American monarch butterfly. In the summer they can be found throughout the U.S. and in some parts of Canada until fall—when they migrate south to spend the winter in Mexico. This is the best studied insect migration of all time as these small bugs travel thousands of miles in their lifetime. 

There are other populations of monarch butterflies that stay put and breed in the same place, one example being those found in Guam. This is where Freedman started to measure and study the size and shape of wings in respect to migration behavior. He then discovered that they were not much different from the monarch butterflies that do migrate in North America. 

Freedman wondered why this was the case. One theory he had was that they could be related to North American butterflies and could have arrived in Guam recently. To explore this question further, he started going to museums and looking through their natural history collections. 

The Bohart Museum at UC Davis provided pin specimens of butterflies that included information on the date and location they were collected. Freedman described them as snapshots in time of what past butterflies looked like.

He visited multiple museums such as the British Museum of Natural History in London, the American Museum of Natural History in New York, the Smithsonian in Washington DC and the California Academy of Sciences in San Francisco. He also viewed university collections from UC Davis, UC Berkeley, Harvard and Cornell. These specimens were important in recreating history, using details like when butterflies got to a certain location and how they have changed over time. It was found that the wings of new non-migratory populations slowly became smaller with time due to natural selection.

“The entire reason that the study was possible was because of all the specimens in museum collections,” Freedman said. 

Museums and curators that take care of these preserved specimens have roles that commonly go unnoticed, but this study highlights the resources they preserve and how important their jobs are to science.

“The study shows very nicely how valuable using specimens can be,” Dingle said. 

Dingle also mentioned that the specimens were helpful in seeing changes over time in an evolutionary sense throughout different conditions.

Freedman also conducted an experiment where he collected live butterflies from a variety of locations around the world and brought them to UC Davis to raise together in a greenhouse under common conditions. 

“The reason we wanted to do that was to determine whether the differences that we were seeing in the wild were actually due to differences in the genetics of butterflies as opposed to differences in the environments that they experience,” Freedman said. 

The butterflies that he raised were from Hawaii, Guam, Australia and Puerto Rico. He found that the morphology of the wing size was due to genetics since the non-migrating butterflies kept their small wings.

Along with Dingle, his Ph.D. advisors Sharon Strauss and Santiago Ramirez, both currently teaching at UC Davis, assisted Freedman with this study. They helped him think through how to do some of the experiments, how to optimize the sampling he was doing, how to do analysis of the specimen and provided some of the funding for the project. The National Geographic Society also funded the study. 

At the conclusion of the study, Freedman discovered that working with a short timescale of 150 years was still enough time to witness evolution in action. Dingle agrees that the study proves, “evolution takes place and can take place very rapidly.” 

“The study demonstrated how migration or loss of migration can be this really big life-history switch that can change all sorts of aspects of the organism’s morphology and their behavior and everything about them,” Freedman said. “You can actually see a real difference between these populations that migrate versus ones that don’t, and I think that’s pretty cool.” 

Written by: Francheska Torres — science@theaggie.org