Monarchs' Mexican send-off: Scientists solve mystery of the built-in clock and compass

The mystery of how the monarch butterfly can migrate thousands of miles across North America without losing its way has been solved after decades of speculation about one of the natural world's most intriguing spectacles.

Laboratory experiments with a butterfly "flight simulator" show the monarch uses an in-built sun compass and biological clock to orient itself accurately during its 3,000-mile shuttle journey between Canada and Mexico.

Henrik Mouritsen and Barrie Frost, of Queen's University in Kingston, Ontario, found the monarch butterfly used its sophisticated biological clock to determine the correct direction of flight in relation to the sun's position in the sky.

Each autumn, millions of brightly coloured monarchs fly south from as far away as the US-Canadian border to their wintering grounds in the forests high in the mountains of Mexico. The following spring they make the return journey to their breeding grounds across America.

One of the most colourful migrations in nature has puzzled scientists because the insects did not appear to have the sophisticated direction-finding sensory organs of larger migratory animals, such as birds and whales.

Dr Mouritsen and Dr Frost captured 59 wild monarchs as they began their journey south and tethered them using a beeswax glue to filaments in a wind tunnel where they were encourage to fly with the help of a gentle updraft. (After the experiment, they were released unharmed into the wild.)

With only the sun visible, the butterflies consistently flew in the same south-westerly direction they would have flown naturally. When the butterflies were moved indoors to simulate cloudy weather they flew in random directions, showing that they were not simply using a magnetic compass.

To confirm the sun compass theory, the scientists trained some monarchs to fly in artificial daylight that had been "clock-shifted" by six hours, forward or backward. These butterflies flew at 90 degrees to either the right or left of the direction they should have been flying, depending on whether the biological clocks had been advanced or delayed.

"We are confident that the directions chosen by the butterflies in our flight simulator are not random directions, but accurately represent their intended flight directions," the scientists report in the Proceedings of the National Academy of Sciences.