Jun 18th 2025
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TWICE A YEAR the skies of south-eastern Australia fill with billions of Bogong moths. In the spring these unassuming brown critters, about an inch long, fly south from their birthplace in Queensland or New South Wales to the Australian Alps where they enter a months-long hibernation-like state in the cool mountain caves. In the autumn, they fly the 1,000km-long return leg to breed.
Decades ago Eric Warrant, an Australian zoologist based at the University of Lund, first saw the swarms and wondered how they found their way. In a paper published in Nature on June 18th he has finally revealed the answer: they navigate using the stars, a skill previously thought unique to humans and certain birds. The finding suggests that animals of all stripes may be capable of more impressive feats of navigation than previously appreciated.
To reach this conclusion, Dr Warrant and his team captured 95 moths heading to their breeding grounds. Each was then put into a custom-made moth-friendly flight simulator under the open skies: a large transparent bucket-like enclosure with satellite images projected onto the bottom. Each moth was tethered to a cable tracking its movement and the projected images changed direction accordingly, to give the moth the impression it was travelling.
The experiments found that the moths kept pointing north—even in cloudy conditions with nothing in the sky to guide them. This confirmed what Dr Warrant’s team already knew from previous research: that moths can orient themselves using only Earth’s magnetic field. But if the moths were exposed to images of landmarks that were moved about, the moths became confused. That suggested they used visual cues as well. What those cues were, however, was unclear.
Testing this required moving the flight simulators indoors, to a laboratory that could produce a magnetic field equal and opposite to Earth’s. New moths placed in the buckets with an image of the night sky projected above them were able to find their way. When Dr Warrant’s team rotated the projection, the moths’ directions rotated with it. If shown a modified sky with random star positioning, they became confused. For Dr Warrant, that suggested the moths were flying by the stars.
To find out how such simple animals were pulling off so complex a feat, the researchers peered into the moths’ tiny brains. Three areas were of particular interest: the optic lobes, which process visual information; the central complex, which deals with spatial orientation; and the lateral accessory lobes, which steer the moths’ bodies. Minuscule electrodes were inserted into a small number of brain cells in these regions while the moths were held fast and shown different orientations of the night sky. Neurons in all three were activated with significantly more intensity when the stars suggested that the moths were facing south.
Basil el Jundi, a specialist in insect navigation at the University of Oldenburg, in Germany, who was not involved with the study, says the work is “fantastic”. A similar double-compass system—in which navigation is possible using either the magnetic field or the stars—is highly likely to be present in other nocturnal insects because their brains are very similar, he says. Even species that rely on the Sun to migrate, such as the Monarch butterfly, might use a second compass when environmental conditions require.
For now, it is not clear what exact stellar features the moths use. Then there is also the question of how the moths know when to end their southward journey, as the caves are in a part of the world to which they have never been exposed.
Dr Warrant believes that the caves’ distinctive smell (noticeable even to nearby humans) may help the moths on the home stretch. One hypothesis is that the smell comes from previous generations of dead moths that cover the cave floor. Another is that a little parasitic worm, which infects and feeds on the moths while they sleep, is responsible for the odour. Resolving such niceties will take time. But for now, Dr Warrant knows he is on the right track. ■
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This article appeared in the Science & technology section of the print edition under the headline “Stars in their eyes”
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