New research has shown that monarch butterflies can follow the Earth’s magnetic field to make their annual migration.
Each autumn, millions of monarch butterflies make an epic journey from Canada to the Michoacan mountains in Mexico. Despite making this 2500 mile journey for the first and only time, the butterflies find their way to the same place as earlier generations. Researchers have long known that the butterflies follow the sun to head south, but this does not explain how the butterflies can still find their way on overcast days. This has led to suspicion that the butterflies could be using an inbuilt magnetic compass, like other migratory species.
A new experiment, published in Nature Communications, is the first to show convincing evidence of the butterflies relying on a magnetic field to choose direction. During the 2012 and 2013 autumn migration seasons, researchers placed butterflies in a flight simulator surrounded by magnets. As expected, the butterflies headed ‘south’, following the inclination of the magnetic field. When the researchers flipped the inclination of the magnetic field, the butterflies started to fly in the opposite direction.
Crucially, this internal compass is dependent on violet light, which was not considered in past experiments. Previous attempts to study the butterflies have used light sources or filters that may not let through enough violet light. In those studies, the researchers could not see any response from the butterflies when they altered the magnetic field.
This light dependence strongly suggests that the butterflies use the same mechanism as fruit flies and European robins for detecting magnetic fields. This relies on proteins known as cryptochromes, which are present in the antennae or eyes. When these proteins are hit by light, they trigger a chain reaction that is influenced by weak magnetic fields.
Last month, a different group of scientists showed that even extremely weak radio waves can disrupt the magnetic compass of European robins. The strengths and frequencies of the electromagnetic noise studied were the equivalent of being within five kilometres of an AM radio station. If this can affect the birds’ ability to migrate, it could also have implications for the monarch butterfly and other migratory species.