Scientists discover how the monarch butterfly navigates 2,000-mile migration without a map

Scientists have solved one of natures most enduring mysteries - how the monarch butterfly is able to navigate more than 2,000 miles on its annual migration route.

A series of experiments has revealed how this tiny insect is able to marry a sophisticated biological clock with the sun's position so that it can fly across the North American continent without losing its way.

Monarch butterflies migrate between their wintering roosts in central Mexico to their summer breeding grounds as far north as the US-Canadian border. Scientists have now discovered that they employ an internal biological clock that enables them to use the sun as a reliable compass no matter what time of day it is.

This form of navigation is so accurate that it allows some monarch butterflies to return to the same trees in the Mexican mountains that were used by their great, great grandparents as roosting sites the previous winter.

Scientists, led by Steven Reppert of the University of Massachusetts Medical School, used a butterfly flight simulator to discover whether the insects had an internal clock that could accurately follow the patterns of night and day - a so-called circadian rhythm. "We have shown the requirement of the circadian clock for monarch butterfly migration. When the clock is disrupted, monarchs are unable to orient toward Mexico," Dr Reppert said. "Without proper navigation, their migration to the south wouldn't occur and that generation of butterflies would not survive."

Each autumn, millions of monarchs fly south to Mexico where they accumulate in the oyamel trees growing in the mountains to form vivid orange-and-black "hanging baskets". In spring, the monarchs fly north to their summer breeding grounds and at the end of the summer, four or five generations later, the butterflies begin to fly south again.

Unlike many other navigating animals - such as foraging bees - monarch butterflies do not learn the route they should follow because the ones that migrate south have never been to Mexico, Dr Reppert said.

"Monarchs have a genetic programme to undergo this marvellous long-term flight in the fall [autumn]. They are essentially hell-bent on making it to their over-wintering grounds," Dr Reppert said. "Monarch butterfly navigation seems to involve the interaction between clock and a compass. This makes navigation a bit simpler than navigation in foraging insects, where each new route has to be learnt."

The study, published in the journal Science, used a device for studying the direction of butterfly flight developed by Henrik Mouritsen and Barrie Frost, entomologists from Queen's University, Ontario. They demonstrated last year that the sun was the compass that monarch butterflies used for navigation - rather than using the earth's magnetic field. But because the sun moves in the sky, it can only be used in conjunction with an internal biological clock.

The flight simulator allowed each butterfly to be tethered harmlessly by its thorax using beeswax glue and thin tungsten wires. A gentle breeze stimulated the tethered insect to fly and the apparatus permitted it to choose any direction.

In Dr Reppert's experiment, one group of butterflies was kept in laboratory conditions where the periods of light and dark matched that for autumn - when the sun rises at about 7.00am and sets at 7.00pm.

Another group of monarchs was raised in light that was shifted six hours earlier - suggesting that the sun rose at 1.00am and set at 1.00pm. And a third group was raised in conditions of 24-hour light.

The scientists found that the butterflies raised under normal light conditions flew south-west, the route they would have taken to reach Mexico from the eastern US-Canadian border.

Those raised in an early "clock-shifted" pattern flew towards the south-east, showing that their clocks were giving them wrong information about the position of the sun - but in a way that the clock-compass theory would predict.

Those raised in 24-hour light flew directly towards the sun no matter what time of day it was, indicating that their circadian rhythm had been completely upset. "The necessity for circadian control for the time-compensation component of monarch navigation shows that a functioning clock is essential for migration," the scientists say.

Dr Reppert said: "Knowledge of the genetic make-up of the monarch circadian clock will help tease apart the entire migratory process, a process that remains one of the great mysteries of biology."

Seasonal travellers

Wandering albatross

With wingspans of up to 3.5 metres, these birds fly for weeks over the southern oceans. One was recorded as having travelled 3,700 miles in 12 days.

Sea turtles

Many species are great travellers. The green turtle migrates from Brazil to Ascension Island, in the Atlantic, to lay their eggs. It is believed they may be able to smell the land. They are thought not to eat while they travel, for up to six months.

Whales

Humpback whales travel 7,500 miles a year from summer feeding grounds off Antarctica to their northern breeding grounds. The journey takes about six months and they live on blubber reserves.

Swallows

Swallows fly south to winter in southern Africa. They use all sorts of means to find their way, including the stars. Some have made the journey in five weeks.