Why the monarch rules the skies

For years scientists have been puzzled by the extraordinary migratory ability of the monarch butterfly. Steve Connor on new findings that solve a mystery of the natural world

It is one of the most extraordinary sights in the natural world. Each spring tens of millions of brilliantly coloured monarch butterflies surge out of their wintering grounds in the high mountain forests of central Mexico on their long migration to their breeding sites across the north American continent. Some will make it as far north as the Great Lakes on the US-Canadian border, more than 2,000 miles away. The mystery of how such a fragile insect with a tiny nervous system is able to find its way on this immense round trip has now been solved by scientists in a unique series of laboratory experiments.

It is one of the most extraordinary sights in the natural world. Each spring tens of millions of brilliantly coloured monarch butterflies surge out of their wintering grounds in the high mountain forests of central Mexico on their long migration to their breeding sites across the north American continent. Some will make it as far north as the Great Lakes on the US-Canadian border, more than 2,000 miles away. The mystery of how such a fragile insect with a tiny nervous system is able to find its way on this immense round trip has now been solved by scientists in a unique series of laboratory experiments.

Each autumn, from Colorado to New England, the monarchs fly south on their return journey to their traditional roosts in Mexico. For decades scientists have speculated on how they find their way. Do they use the Sun as a compass, in which case how do they accommodate its movements in the sky? Are they guided by the Earth's magnetic field or do they follow structural features of the landscape such as rivers or mountains? Perhaps they might recognise scents or smells floating on prevailing winds coming from a particular direction, or do migrating monarchs use some other direction-finding mechanism new to science?

For an insect that weighs no more than half a gram, the effort of flying thousands miles is in itself truly stupendous. Scientists are used to recording remarkable feats in the migratory behaviour of animals, but for its size the yearly migration of the monarch is about as difficult as anyone can imagine it to be. What is equally amazing is that the return journey to Mexico is made by adults who were born that summer in the US and Canada, making them completely naive travellers who make the return leg of the journey unaccompanied by other experienced individuals. It is clear, therefore, that whatever the mechanism mon-archs use to migrate so far without losing their way, it must be genetically imprinted.

Some other species of Lepidoptera – butterflies and moths – can make similar long journeys in search of food but they only travel one way. (Strictly speaking this is emigration rather than migration.) What makes monarchs so unique is that they not only return to the wintering roosts used by their great, great grandparents the year before, but often they return to precisely the same location, even to the tree used by their ancestors. The countdown for migration begins around September. As the days begin to shorten in late summer and the nights become cooler, changes are triggered in the monarchs emerging from their chrysalides at that time of the year. Although these individuals look just like other early and mid summer adults, they will not mate or lay eggs until the following spring. Being unable to survive the cold winter, they have to fly south before daytime temperatures fall too low for the flight muscles of their cold-blooded bodies.

Although they have fat stored in their abdomen, monarchs help to fuel their trip by feeding on the nectar of flowers they pass on their journey south. Small groups of monarchs – which are usually solitary – begin to congregate at night. Spectacular orange and black bunches of monarchs eventually arrive to roost, hanging like vivid flowering baskets from the branches of the oyamel trees which flourish in a narrow belt across a few mountain tops of central Mexico.

Henrik Mouritsen, a researcher at Queen's University, Ontario, has come closer than anyone else to explaining how the monarch manages this amazing journey. Working with his colleague, Barrie Frost, they have found that monarchs do indeed use a Sun compass to find their way, and that they compensate for the movements of the Sun during the day with an accurate body clock for telling the time.

Mouritsen and Frost captured 59 wild monarchs as they began their journey south and tethered them by their thoraces using a beeswax glue to thin tungsten wires. This enabled the insects to be suspended in a gentle wind tunnel – a sort of butterfly flight simulator. (The procedure was quite harmless and all the butterflies were released into the wild at the end of an experiment.) With the help of a gentle updraft, the monarchs could be encouraged to fly in any direction of their choice – a low-friction bearing in a joint in their tether allowed the butterflies to turn easily whenever they wanted.

When the butterflies were taken outside in opaque tubes with only the Sun visible, they consistently flew in the same south-westerly direction that they would have flown naturally in the region of Ontario where they were captured. Even when the two scientists tried to force the butterflies to move in a different direction, by turning them around, the flying insects quickly corrected the change to resume their original path. "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, because in hundreds of forced-turn experiments, all butterflies immediately returned to their previous geographical heading," the scientists write in the current issue of the journal Proceedings of the National Academy of Sciences. "No matter which part of the set-up was turned while a butterfly was actively migrating in the set-up all the butterflies kept their geographical heading," the researchers say.

The average direction in which the butterflies in the flight simulator flew – a mean orientation of 225 degrees – was in close agreement to the average 220-degree orientation monitored in wild monarchs in Ontario. It was also close to the 226 degree direction from the laboratory to the Mexican wintering grounds as calculated from map bearings. The scientists were confident that the flight simulator was indeed a true reflection on wild migratory behaviour, but how to prove that it is the Sun that is the key to the monarch's uncanny ability to find its way south?

To confirm the Sun-compass theory, the scientists needed to know how the insects coped with an object that moved across the sky during daylight hours. They therefore trained some of the monarchs to fly in artificial daylight that had been "clock shifted" by six hours either forward or backwards. So some of these butterflies would view the Sun's position at midday as being where it should be at 6am and the other, late-shifted monarchs should misread the midday Sun as being in its 6pm position. If so, then each group should fly in completely different directions. Sure enough, each set of these clock-shifted 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.

"The clear and predicted directional shifts produced by clockshifting the butterflies provide strong evidence that migratory monarchs use a time-compensated Sun compass," say Mouritsen and Frost. A final issue, though, still had to be addressed. Some scientists in a previous study had found that monarchs seemed to be able to use a compass that was sensitive to the invisible magnetic fields of the earth. However, this experiment proved to have a large flaw. It now appears that the butterflies used in this study had become confused by light reflecting off the clothing of the researchers. The findings were therefore retracted.

But just to make sure that there was not something else influencing the monarchs' flight, Mouritsen and Frost moved their experiments indoors under dispersed artificial light to simulate very cloudy weather. They subjected the insects to artificial magnetic fields that were rotated in different direction. Yet the monarchs flew in random directions showing that there was no evidence for a magnetic compass. "Because the lack of magnetic orientation occurred in exactly the same apparatus where the butterflies showed clear time-compensated Sun compass orientation, this negative result becomes particularly important. Recent research showing that polarised patterns [of light] are still available even under cloudy conditions provides a plausible explanation for reports of monarchs migratory orientation under cloudy skies," they say.

So the mystery of the monarch appears to be solved. They use a time-compensated Sun compass and not a magnetic compass for finding their way to and from Mexico. Only a true pedant would now want to know the exact nature of the monarch's internal clock. And, oh yes, the same pedant may also want to know how monarchs find the precise tree used by their great, great grandparents.