Scientists have shown that the annual migration of the monarch butterfly is achieved with just an in-built “compass” – based on the position of the sun – which tells the insect which direction it should fly at the appropriate time of the year.

Researchers have long speculated on how the insect, which weighs about half a gram, is able to make the return journey to the mountain forests of Mexico for winter, especially as those born in late summer, would not have made the journey before.

Experts thought they used an internal, genetically encoded “map” to locate their position, as well as a built-in compass to tell them where to fly. But now a study has shown that the butterflies manage with just a compass alone.

“To be a true navigator, you need both a compass and a map. We’ve known for some time that monarchs use external cues, such as the sun and magnetic field, as a built-in compass that can indicate their latitude. But having an internal map requires knowledge of both latitude and longitude,” said butterfly expert  Professor Ryan Norris of the University of Guelph in Ontario. “Given the challenge of this migratory journey and the fact that these insects weigh less than a gram, it is a remarkably simple system they use to travel thousands of kilometres to a site they have never seen,” Professor Norris said.

The scientists took monarch butterflies from the Ontario region of eastern Canada and tested their migratory flight patterns in experiments set up 2,500km away to the west in Calgary. They found that the monarch continued to try to fly in the same direction and did not compensate for the geographic displacement.

“The monarchs we tested in Guelph flew south-west, in the general direction of Mexico. When we tested them in Calgary, they flew in the same general direction as if they were in Ontario, suggesting they did not know they had been displaced 2,500km,” said Rachel Derbyshire, who carried out the work published in the Proceeding of the National Academy of Sciences.

It is likely the butterflies also use landmarks, such as mountain ranges, to help them find their way, and possibly scent when they are near to their final goal, the oyamel trees of the Mexican highlands where they clump together in their thousand to spend the winter season.