How do monarch butterflies make their incredible 3,000-mile mass migrations from Canada down to the coast of California and into Mexico each autumn? It's down to genetics, a study has found; in fact, researchers say, it's all due to one single gene.
After sequencing the monarch genome, they report finding a single gene -- linked to the efficiency of the creatures' flight muscles -- that allows them to fly so far and for so long.
Monarch butterflies are mostly a North American species, although populations also exist in Central and South America. Monarchs outside of North America do not migrate.
The single gene identified in the study, named Collagen IV alpha-1, was linked to a "stand-out difference" between the migratory and non-migratory species of monarchs, the researchers said.
The gene is involved in generating muscle tissue, and migratory monarchs were found to have a significantly more efficient muscle metabolism than those that don't migrate, the researchers said.
Flight muscle performance appears to be key, since differences between species that migrate and those that don't are minimal when they're not in flight, they said.
An analogy, they said, might be the difference between marathon runners -- in this case the migrating butterflies -- and sprinters, those that don't migrate.
"I find it amazing that these little butterflies live for months and fly thousands of miles to perform this annual migration," University of Chicago ecology and evolution Professor Marcus Kronforst says.
In North America, butterflies living east of the Rocky Mountains spend winters in Mexico to escape cold weather, while those west of the Rockies spend winters on the California coast before returning home in the spring.
"Our study shows that [migratory] monarchs have been doing this every year for millions of years," Kronforst says. "There is nothing else like this on the planet."
The finding comes amidst a drastic decline in the populations of the migrating butterflies, the scientists point out.
Whereas around a billion monarchs migrated south to Mexico in 1996, this past winter saw just an estimated 35 million make the trip, with the declining numbers blamed on habitat loss caused by human activity, pesticides killing off their main food source of milkweed, and ongoing climate change, experts explain.
"We are seeing possibly the tail end of this phenomenon of migration," Kronforst says. "To me, it adds a sad exclamation point to the story."
When the species first appeared in North American around 1 million to 2 million years ago, they likely migrated over much shorter distances than they do today, the researchers propose.
The migration routes probably began to lengthen around 20,000 years ago, as the last Ice Age ended, when the butterflies could find more extensive regions of milkweed plants available in the U.S. Midwest, they said.
