Monarch butterflies navigate with compass but no map


Every autumn, millions of monarch butterflies (Danaus plexippus) converge on a small cluster of Mexican mountains to spend the winter. They have journeyed for up to 4,000 kilometres from breeding grounds across eastern North America. And according to a study, they accomplish this prodigious migration without ever knowing where they are relative to their destination.  

The monarchs can use the position of the Sun as a compass, but when Henrik Mouritsen, a biologist at the University of Oldenburg in Germany, displaced them by 2,500 kilometres, he found that they did not correct their heading. “People seemed to assume that they had some kind of a map that allowed them to narrow in on a site a few kilometres across after travelling several thousands of kilometres,” he says. Now, “it is clear that they don’t”. His results are published in the Proceedings of the National Academy of Sciences1.

For more than five decades, scientists have teamed up with amateurs to tag and monitor free-flying monarchs, creating a database of their migrations. When Mouritsen analysed these records, he realized that the monarchs tend to spread out over the course of their migration. Their distribution was a good fit with the predictions of a mathematical model that assumed that the monarchs were flying with just a compass, rather than a compass and a map. 

Mouritsen also captured 76 southwesterly flying monarchs from fields near Guelph in Ontario, Canada, and transported them 2,500 kilometres to the west, to Calgary in the Canadian province of Alberta. He placed the butterflies in a “flight simulator” — a plastic cylinder that kept them from seeing any landmarks except the sky — and tethered them to a rod that let them point in any direction without actually flying away.

Written By: Ed Yong
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  1. the Rocky Mountains

    Wonder if the same idea applies to monarchs in the west that migrate to southern California. If displaced, would they keep flying south.

    So sad about dwindling monarch numbers – I remember 20 years ago watching a steady flow of them heading south. Rested in the middle of the city during migration, too.

    Logging of the sanctuary area, dramatic loss of milkweed in the plains, hard frosts in Mexico, all taking its toll. Monarch groups encourage folks to plant milkweed. The small town in Mexico near the wintering grounds depend on the monarchs for tourist income.

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