The great migration of monarch butterflies from Canada to Mexico is down to white spots on the edge of their wings, according to new research.
It solves the mystery of one of nature’s most famous journeys, spanning 3,000 miles, say scientists.
How the monarch got its spots sounds like one of Rudyard Kipling’s Just So Stories.
But the more it has, the better it is at reaching its long-distance wintering destination.
It’s believed the spots change airflow patterns around the wings. Monarchs are the only insects to attempt such a massive trek.
Lead author Dr. Andy Davis, of Georgia University, said: “We undertook this project to learn how such a small animal can make such a successful long-distance flight.
“We actually went into this thinking that monarchs with more dark wings would be more successful at migrating because dark surfaces can improve flight efficiency. But we found the opposite.”
The orange-winged wonder travels more than 3,000 miles from all over North America to spend the winter hanging from oyamel fir trees in mountain forests.
It has been performing the feat for thousands of years. How an animal with a brain the size of a poppy seed navigates to this one special place has baffled ecologists for decades.
The monarchs with less black on their wings and more white spots were the ones that made it to their ultimate destination in south and central Mexico.
Davis said: “It’s the white spots that seem to be the difference maker.”
The US team analyzed nearly 400 wild monarch wings collected at different stages of their journey, measuring color proportions.
The study in PLOS ONE found successful migrant monarchs had about three percent less black and three percent more white on their wings.
An additional analysis of museum specimens that included monarchs and six other butterfly species showed that the monarchs had significantly larger white spots than their non-migratory cousins.
The only other species that came close to having the same proportion of white spots on its wing was its semi-migratory relative, the southern monarch.
Butterflies’ coloring could be related to the amount of radiation they receive during their journey. The monarchs’ longer journey means they’re exposed to more sunlight. As a result, they have evolved to have more white spots.
Davis said: “The amount of solar energy monarchs are receiving along their journey is extreme, especially since they fly with their wings spread open most of the time.
“After making this migration for thousands of years, they figured out a way to capitalize on that solar energy to improve their aerial efficiency.”
But as temperatures continue to rise and alter the solar radiation reaching Earth’s surface, monarchs will likely have to adapt to survive.
Co-author professor Mostafa Hassanalian, of the New Mexico Institute of Mining and Technology, said: “With greater solar intensity, some of that aerial efficiency could go away.
“That would be yet one more thing that is hindering the species’ autumn migration to Mexico.”
Previous work showed summer populations of monarchs have remained relatively stable over the past 25 years.
It suggests that the species’ population growth during the summer compensates for butterfly losses due to migration, winter weather and changing environmental factors.
Davis said: “The breeding population of monarchs seems fairly stable, so the biggest hurdles that the monarch population faces are in reaching their winter destination.
“This study allows us to further understand how monarchs are successful in reaching their destination.”
Produced in association with SWNS Talker
Edited by Daisy Atino and Saman Rizwan