Monarch butterflies that migrate vast distances have grown larger bodies and wings, researchers have discovered.
These "super sized" butterflies have evolved to cope with the demands of long-distance flight.
In contrast, monarchs that live in one place all year have wings that are up to 20% smaller, report scientists in the journal Evolution.
Monarch butterflies undergo the longest recorded two-way migration of any insect.
Up to four generations of monarch butterfly complete a round trip of up to 8,000km, flying between northern US and Canada and Mexico in search of warmer temperatures.
Numerous studies have shown that species that migrate tend to evolve longer wings than those that do not.
But the new study is one of the first to show that populations within the same species can also evolve differently-shaped wings depending on their lifestyle.
"We were surprised that average wing size differences between migratory and non-migratory monarchs were so striking and consistent," says Professor Sonia Altizer of the University of Georgia, in Athens, US.
"Our findings indicate that large, elongated wings are better for monarchs that undertake long distance flights."
Despite being famous for flying such long distances, many monarch butterflies do not migrate at all.
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"When most people think about monarchs, they associate them with this incredible migration in North America, but a lot of people don't realise that monarchs also inhabit tropical locations such as Central America, the Caribbean islands, and Pacific islands, where they can breed year-round," explains Professor Altizer.
She and her colleague and husband Mr Andrew Davis, a PhD student at the University of Georgia, used digital imaging techniques to photograph and analyse the size of monarch butterflies from either wild migratory or non-migratory populations.
As well as width, the photographic technique allowed the researchers to precisely measure the surface area of each wing.
"Comparing the largest to the smallest population, wings of monarchs from eastern North America are 20% larger than those from Puerto Rico. Averaging across all populations, migratory monarchs have wings that are 14% larger than non-migratory monarchs," says Professor Altizer.
The researchers also reared monarch butterflies in the lab from different populations, to show that large wings were inherited, having a genetic basis, rather than being produced by butterflies in response to environmental factors such as warmer temperatures.
 Taking flight from a wintering colony in Mexico |
"That further supports the idea that differences between wild monarch populations are caused by evolution," says Professor Altizer.
"What we don't know is [exactly how much] this improves the flight performance of migratory monarchs; that would be an important next step."
Another surprise is that, although migratory monarchs from both the east and west coasts have large wings, the bodies of western monarchs are around 8% smaller than eastern monarchs.
This indicates that they might be better adapted to soaring or gliding flight as opposed to powered flight and speed, says Professor Altizer.
"A final surprise was that monarchs from Hawaii were small, but had the most angular wings," she adds.
"It would be interesting to know why that is. For example, do some monarchs in Hawaii cross the ocean to move around between islands?"
Professor Altizer and Mr Davis hope to measure differences in flight performance between individual monarchs with different wing traits, testing their speed and endurance.
