An Australian moth uses the Earth鈥檚 magnetic field to help find its way across the continent. While other insects have been shown to navigate using Earth鈥檚 magnetic field, the moth is the first to do so over long distances and at night.
Bogong moths (), like the famous monarch butterflies in the Americas, make an epic migration. In spring, about 2 billion of them leave their breeding grounds on the dry, flat plains of south-east Australia, and fly over 1000 kilometres to a set of around 50 caves high in the Australian Alps. There they spend the summer, dormant. In autumn, they return to the plains where they reproduce and die.
of the University of Lund, Sweden and his colleagues studied how the moths find their way. 鈥淲hen we began this study, we were convinced that the bogong moth would exclusively use celestial cues in the sky, such as the stars and the moon, for navigation during migration,鈥� he says. But that is not what they found.
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The team trapped wild moths and placed them one at a time in a flight simulator where they could watch them closely. The simulator was completely blank inside except for two simple landmarks, and it was fitted with magnetic coils so the team could manipulate the magnetic field within.
Lost moths
If the visual and magnetic cues both directed the moths to fly a particular direction, they did so. 鈥淭hey love the pretend mountain landmark, and love to fly towards it,鈥� says Warrant.
But if the cues were contradictory, the moths became disoriented. 鈥淲hen we disrupted the relationship between the magnetic and visual cue, they totally lost it, and it was as if the visual landmark had disappeared,鈥� says Warrant. 鈥淭hey flew hard, but in the wrong direction.鈥�
Warrant thinks the magnetic field is the most reliable guide for the moths, because it does not change, but that they regularly check visual landmarks as well.
It鈥檚 not yet clear how the moths do it. Warrant suspects the magnetic sensor is an eye protein called cryptochrome 4, which is sensitive to magnetic fields. 鈥淭hey鈥檝e been found in the eyes of birds,鈥� says Warrant. As a result, it鈥檚 been suggested that birds can 鈥渟ee鈥� magnetic fields.
鈥淢agnetic sense has been known in other insects for some time, such as desert ants finding their nest direction,鈥� says Warrant. 鈥淲hat we鈥檝e found is the first example of magnetic sensing in a migrating insect that travels long distances.鈥�
In 2014, another group described experiments that purported to show that monarch butterflies use Earth鈥檚 magnetic field during their long journeys (). However, Warrant says the results have been greeted with 鈥済reat scepticism鈥�.
鈥淎lmost all the fields used were much stronger than the Earth鈥檚 field,鈥� he says, and not many monarchs responded to a field on a par with Earth鈥檚. 鈥淎nother lab has tried to do the same experiments in over 400 Monarchs and could not find a single butterfly that reacted to changes in the Earth鈥檚 magnetic field.鈥�
