Evidence has been mounting that female monarch butterflies are better at flying and more successful at migration than males, and researchers from the University of Georgia have now come up with an explanation—but not one they expected. In a study comparing physical traits of female and male monarchs, they found that although female monarchs have smaller wings and smaller flight muscles than males, their wings are thicker and also bear less weight per square inch, making them both sturdier and more efficient in flight.
“Both of these elements would play important roles in determining the outcome of the migration,” said lead author Andy Davis, a research scientist at the Odum School of Ecology. “Until now, we had no idea why females were better flyers than males, but this study definitely helps to answer that question.”
Davis and coauthor Michael Holden, an undergraduate ecology student, measured the wings and body parts of 47 male and 45 female monarchs, specifically targeting those characteristics that are important for flight, such as the ratio of wing size to body size, the size of flight muscles, and wing thickness.
“We expected we’d find that females have bigger flight muscles, but it was the opposite,” said Holden. “Males had the largest muscles.”
Their analysis revealed, however, that female bodies tend to be lighter in relation to their wing size, meaning their wings have to carry less weight overall. This would make their flight more efficient.
“The way I think about it is that per flap of their wing, females use less energy to move their bodies relative to males,” Holden said.
In addition, the wings of females were significantly thicker than those of males, making them less likely to break or tear during migration.
“Having damaged wings is a death sentence during the migration,” said Davis, “so if females have thicker wings it would give them an edge over males in that regard.”
By the end of the study, the researchers had accumulated a substantial data set of flight-related measurements, the first of its kind for monarchs.
“We believe this work will be important for improving scientific understanding of the migratory cycle, and it will also serve as a reference point for future studies aimed at flight characteristics of monarch butterflies,” said Davis.
For Holden, the research experience has had a more immediate impact.
“I hadn’t ever really done anything in a lab before, outside of classes,” he said. “I was in Dr. Davis’s physiological ecology class and I discovered that I’m really interested in that aspect of ecology. I approached him about doing a project, and he said yes.”
Holden, of Athens, who will graduate this spring with a Bachelor of Science in ecology, has already lined up an internship that will take him to Costa Rica next year to document moth species, and he plans to pursue graduate studies after that.
“Experiences like Michael’s are exactly why our faculty are so committed to providing opportunities for our undergraduate students to participate in serious hands-on research and other forms of experiential learning,” said John Gittleman, dean of the Odum School. “We know it opens up worlds of possibility for them, in whatever fields they choose to pursue.”