The monarch butterfly uses a magnetic compass to direct its migration towards the equator, reports a study published this week in Nature Communications.

The North American monarch butterfly (Danaus plexippus) is well known for its extraordinary migration, from the United States and southern Canada, thousands of miles southwards to the mountains of Michoacan in central Mexico, where it spends the winter. Monarchs are known to use a time-compensated sun compass to direct their flight. However, when weather conditions preclude this (overcast skies etc.), they remain flying in the expected southern direction.

Steven Reppert and colleagues used a flight simulator, surrounded by a magnetic coil system, to show that, in addition to a sun compass, the monarchs use a magnetic compass that helps direct their flight towards the equator. They also found that this magnetic compass is light-dependent-with relevant light-sensitive magnetosensors appearing to reside in the antennae-and is an essential orientation mechanism to aid migration.

Following recent reports showing that electromagnetic noise disrupts magnetic compass orientation in birds, this study now raises the concern that human-induced magnetic noise might also be a potential hazard for monarchs.