The monarch butterfly covers a distance of more than 3,500 kilometres to reach its winter quarters. How does it manage this impressive continental flight? Oldenburg biologist Henrik Mouritsen and his team have been looking for answers - and have found them.
They weigh barely a gram and embark on a journey of more than 3,500 kilometres every year. We are talking about "Danaus plexippus", the "monarch butterfly" - the butterfly that is one of the most famous migratory butterflies. In fixed seasonal rhythms, it travels thousands of kilometres through North America to its winter quarters in central Mexico. Huge swarms then invade the area - just a few hectares in size - within the mountain forest of Michoacán. But how exactly do the moths manage their impressive continental flight? Prof Dr Henrik Mouritsen, a biologist and expert in bird navigation at the University of Oldenburg, has tackled this question together with his German-Danish-Canadian team. The research results have just been published in the renowned scientific journal PNAS (Proceedings of the National Academy of Sciences of the United States of America).
The researchers exposed the moths to flight simulators.
"Real navigators" are not at work when the monarch butterflies migrate, explains Mouritsen. They have neither a magnetic sense nor anything like an "internal map". In order to find out how the butterflies navigate unerringly to their winter quarters, the scientists travelled to eastern North America, to Ontario (Canada). This is home to a population of monarch butterflies that fly to the mountain forest of Michoacán in central Mexico to hibernate.
The researchers initially placed the butterflies in flight simulators - at the point where they set off on their long-distance flight. A breeze made the butterflies fly on the spot without giving them a direction. "The butterflies consistently took a south-westerly course. They orientated themselves to the position of the sun," explains Mouritsen. As soon as the flight simulators were covered, the animals were unable to orientate themselves.
But what happens when the butterflies are moved to a different location? Do they adapt their orientation to the new conditions, do they take the right route to their winter quarters? To test this, the scientists transported the flight simulators and monarch butterflies around 2,500 kilometres to the west, near Calgary in the Canadian province of Alberta. Here, too, the butterflies took a south-westerly course. This makes it clear: "They were unable to compensate for the geographical displacement," says Mouritsen. The results show that the butterflies cannot determine their own position in relation to their destination.
The butterflies use a solar compass.
In addition, the scientists analysed the distribution of around 400 monarch butterflies that were tagged and recaptured between 1952 and 2004. "All experiments and data analyses show that the butterflies use a solar compass, but are not 'true navigators'. They don't have a map to correct orientation errors," emphasises Mouritsen. But how do they still arrive in Mexico with pinpoint accuracy? Geographical barriers such as mountains or water help the butterflies to find their way. "The Rocky Mountains in the west, the Atlantic Ocean in the east and south-east and the Gulf of Mexico in the south act as a kind of funnel that funnels the monarch butterflies into their wintering grounds," says Mouritsen.
However, one puzzle remains unsolved so far: the scientists do not believe that the direction finding with the solar compass alone and the "funnel function" are sufficient to find the right trees in the mountain forest of Michoacán. "Perhaps olfactory information plays an important role here," Mouritsen surmises.
Henrik Mouritsen, Rachael Derbyshire, Julia Stalleicken, Ole Mouritsen, Barrie J. Frost, and D. Ryan Norris: "Monarch butterflies are not true navigators: evidence from an experimental displacement and over 50 years of tag-recoveries", in: PNAS, April 2013.