Sun shows the way for monarchs on the move

By STEVE CONNOR

One of the greatest mysteries in biology - how the monarch butterfly navigates more than 3200km on its annual migration - has finally been solved.

The insect is able to marry a sophisticated biological clock with the sun's position in the sky 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 United States border with Canada using an internal biological clock that lets them use the sun as a reliable compass no matter what time of day it is.

Some monarch butterflies return to the same trees in the Mexican mountains that their great-great-grandparents used 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 have an internal clock that can accurately follow the patterns of night and day - a so-called circadian rhythm. They do.

"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. 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 on a journey that has mystified naturalists.

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

"Monarchs have a genetic programme to undergo this marvellous long-term flight in the fall [autumn]," Dr Reppert said. "They are essentially hell-bent on making it to their over-wintering grounds.

"Monarch butterfly navigation seems to involve the interaction between clock and a compass. This makes monarch navigation a bit simpler than navigation in foraging insects where each new route has to be learned."

The study, published in the journal Science, used a device for studying the direction of butterfly flight developed by entomologists Henrik Mouritsen and Barrie Frost from Queen's University, Ontario.

They showed last year that monarch butterflies use the sun as a compass for navigation - rather than, for instance, the Earth's magnetic field.

But because the sun moves in the sky, it can be used only with an internal biological clock that can compensate for the sun's ever-changing position.

In the flight simulator each butterfly was tethered harmlessly by its thorax, using beeswax glue and thin tungsten wires. A gentle breeze stimulated the tethered butterfly 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 those of autumn, when the sun rises about 7am and sets at 7pm.

Another group of monarchs was raised in light that was shifted six hours earlier - suggesting that the Sun rose at 1am and set at 1pm. And a third group was raised in 24-hour light.

The butterflies raised under normal light conditions flew in a southwesterly direction, the route they would have taken to reach Mexico from the eastern US-Canadian border.

Those raised in an early "clock-shifted" pattern also flew towards the southwest, 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 and they were incapable of using the sun as a time-compensated compass.

"The necessity for circadian control for the time-compensation component of monarch navigation shows that a functioning clock is essential for successful migration," the researchers said.

Previous research has shown that many insects judge the time of day using a genetic biological clock. Research on fruit flies, for instance, indicated that a gene called "per" is essential for the clock to work.

"Increasing knowledge of the genetic makeup of the monarch circadian clock will help tease apart the entire migratory process, a process that remains one of the great mysteries of biology," Dr Reppert said.

The findings

* Monarch butterflies flew in the right direction under normal daylight hours.

* They still flew in the right direction when an artificial laboratory "sun" rose six hours earlier - proving their navigational body clocks followed the sun.

* But butterflies in 24-hour light lost their bearings and flew straight at the sun.

- INDEPENDENT