Hundreds of Monarch butterflies line a tree trunk in the El Rosario butterfly sanctuary in the Mexican state of Michoacan in a file photo. REUTERS/Andrew Winning

LONDON (Reuters) - Scientists have uncovered a group of 40 genes that appear to make North America’s monarch butterflies fly thousands of miles south each autumn.

It is the first time that researchers have honed in on the exact genes driving migratory behavior in any animal.

Monarchs are famous for their epic 4,000 km (2,500 miles) overland migrations from Canada to Mexico, but what drives them has been a mystery.

Now neurobiologist Steven Reppert at the University of Massachusetts Medical School and colleagues have found a suite of genes that are activated when the insects migrate.

“Our data are the first to provide a link between gene expression profiles in the brain and migratory state in any animal that undergoes long-distance migration,” they wrote on Tuesday in the British-based open access journal BMC Biology.

Hundreds of millions of monarchs fly to ancestral feeding grounds in Mexico every year in a journey which is unique in the butterfly world. They use an internal clock and a “sun compass,” which senses the angle of the sun, to orientate themselves on the journey.