Monarch butterflies (Danaus plexippus) undertake possibly the most remarkable migration in the insect world, requiring exceptional flying ability. Females are even better fliers than males, requiring fewer stops on their epic journey, and for the first time scientists know why. 

Dr. Andy Davis and undergraduate Michael Holden of the University of Georgia compared the wings of 47 male and 45 female monarch butterflies collected for a previous study. They found that while females had smaller wings, they more than compensate for this by having four percent thicker wings that have to carry seven percent less weight per unit area.

Not surprisingly, this makes the females' wings more robust, with reports of fewer females found with damaged wings. It also makes them more efficient. "Both of these elements would play important roles in determining the outcome of the migration," Davis said in a statement.

"We expected we'd find that females have bigger flight muscles, but it was the opposite," said Holden. The males have larger flight muscles than the females.

However, while the females' lower body weight makes for an obvious advantage for an airborne creature, the discovery that extra wing thickness is worth the weight may have wider applications. "Having damaged wings is a death sentence during the migration," Davis said.

Millions of years of natural selection has perfected the thickness-to-size ratio of bird and insect wings to their specific lifestyles, but designs for airplane wings or even wing turbine blades are often carried out with much guesswork. Consequently, these findings may have applications outside the migratory realm of butterflies and inspire technological design. 

In the Journal of Insects, Holden and Davis published details on why the differences between the sexes occur. They speculate that the differences may arise because "after successful pairing, males carry the females for up to 18 hours," which may have forced the development of muscles that can do double work for a time, while being overbuilt for normal flight.

All the specimens were from a migrating population. Monarchs that live in areas where migration is not required tend to have smaller wing sizes. 

"This work will be important for improving scientific understanding of the migratory cycle," added Davis.