As you know, every fall, monarch butterflies embark on one of nature’s greatest journeys. The insects flap more than 2,000 miles from their summer homes in the Pacific Northwest and Canada south to the mountains of central Mexico. But did you know that only about 30 percent survive the trip? According to a recent report in the New York Times, the tiniest of traits might explain those survivors' success -- the edges of their wings tend to be slightly spottier.
The report quotes Andy Davis, a biologist at the University of Georgia and an author of "How the monarch got its spots: Long-distance migration selects for larger white spots on monarch butterfly wings," which was recently published in the journal PLOS ONE. “No one even knew what these spots were for in monarchs,” Davis said. “All of a sudden, it seems like they’re really important.”
As the Times reported: Monarch wings are mostly orange, but their edges are black, punctuated with tiny white spots. Dr. Davis was curious if those black edges contributed to monarchs’ migratory capabilities. Dark colors absorb more heat, and studies of seabirds have suggested that the temperature difference between dark and light feathers can change air flow patterns, enabling birds with blacktopped wings to “enhance their flight efficiency — basically increasing the lift and decreasing the drag,” said Mostafa Hassanalian, a mechanical engineer at the New Mexico Institute of Mining and Technology.
To test the hypothesis, Dr. Davis enlisted Tina Vu, then an undergraduate at the University of Georgia, to undertake a tedious task: going through photographs of 400 monarch butterflies and measuring the amount of black and white on the edges of their wings.
Ms. Vu spent hours tracing the white spots and discovered, instead, that the monarchs who survived the trip to Mexico tended to have more white spots, rather than the extra-dark wings the researchers had predicted. She also analyzed some of the butterflies’ nonmigratory relatives, who had fewer spots than the ones making the big migration. Conclusion? Maybe extra white spots have a function.
According to the Times: The researchers’ working theory is that the spots reduce drag by creating pockets of heating and cooling on the wing edge, which could create tiny eddies of rising air. The difference is subtle: The butterflies that completed the migration were only about 3 percent spottier than the ones at the starting line. But the scientists suspect that even a small reduction of drag could make a tangible difference in flight capabilities.
Next step -- test this hypothesis with wind tunnel experiments on artificial monarch wings.
Naturally, there are skeptics. The Times quoted Ayse Tenger-Trolander, a biologist at the University of Chicago who was not involved with the initial study. “There’s a little part of me that wonders if some of these differences are due just to the way that the wing physically develops,” Dr. Tenger-Trolander said.
But other researchers are heartened that the follow-up research will be a collaboration between biologists and engineers. "Such a cross-disciplinary collaboration might yield not only a better understanding of monarchs’ abilities to complete their migration in the face of habitat destruction and climate change, but also inspiration for more efficient aircraft. 'If we’re right, and if the monarchs really are utilizing this simple design technique, imagine how simple it would be to just get some paint and throw it on an airplane wing,' ” Dr. Davis told the Times.