Australia's bogong moths are the first nocturnal insects discovered to use the Earth's magnetic field in long-distance migration, according to new research.

Key points: Bogong moths use the Earth's magnetic field and visual landmarks to stay on track during their long-distance migration

Bogong moths use the Earth's magnetic field and visual landmarks to stay on track during their long-distance migration Tethered in 'flight arenas' the moths changed direction when the magnetic and visual cues were aligned, and became confused when they were in conflict or only exposed to one cue.

Tethered in 'flight arenas' the moths changed direction when the magnetic and visual cues were aligned, and became confused when they were in conflict or only exposed to one cue. How animals detect magnetic fields is still unknown, but current theories include magnetic particles in cells, or a cryptochrome protein compass

Each Spring, bogong moths emerge from beneath the soil of south-eastern Australia's plains and take to the air on the first leg of their annual migration, after spending the winter gorging themselves on plant roots to fuel up.

For a long time, scientists wondered how these moths — just a few centimetres long — made the incredible migration of over 1,000 kilometres to alpine caves in New South Wales and Victoria, and then back to their birthplace, to mate and reproduce in winter.

Now a team of scientists from Australia and Sweden have found that bogong moths can sense the Earth's magnetic field and use it as a compass to keep them on the right flight path during the journey, as reported today in the journal Current Biology.

Magnetic compass and visual landmarks set moths straight

Billions of bogong moths line the walls of alpine caves during summer — there's about 17,000 moths per square metre. ( Supplied: Eric Warrant )

The bogong moth is one of only two insects — the other being the monarch butterfly — known to make long migrations from large geographical areas to very specific locations, said study co-author Eric Warrant, sensory physiologist from the University of Lund, Sweden.

"All the different populations of bogong moths around south-eastern Australia have inherited their own migratory direction [to get to the mountains]," Professor Warrant said.

Bogong moths fly for over 1,000km to reach the cosy caves of Mount Kosciuszko and nearby alpine areas. ( Supplied: Eric Warrant )

Just like an adventurer navigating remote terrain, the moths first get their bearings, then follow landmarks, and recalibrate their route often, he said.

"When they take flight, I suspect the first thing they do is use the Earth's magnetic field as a kind of compass to work out the direction they need to fly in to reach the mountains around Mount Kosciuszko," he said.

"Once they've worked that out they look for something in that direction, like a mountain top, and fly towards that."

If the moths lose sight of their original landmark, they can recalibrate their direction using the Earth's magnetic field, and pick a new visual landmark to head for.

"If they do that in a sequential way from the beginning of the migration until they reach the mountains then they should reach them without much error," Professor Warrant said.

'Flight arenas' reveal moths' superpower

Professor Warrant and his colleagues conducted their experiments in a (nearly) Magneto-proof lab, made largely from wood and plastic, at Adaminaby in the New South Wales alpine region.

Here they were able to investigate whether moths have a magnetic sense, without any interference from metallic materials and equipment.

The moths spread their wings in a flight arena, which is a cylinder surrounded by magnetic coils that the scientists can manipulate to change the direction of the magnetic field from the natural north-south polarity.

Individual moths were tethered in cylindrical "flight arenas". ( Supplied: Eric Warrant )

"We turned the magnetic field by 120 degrees towards the east and if they have a magnetic sense you might expect that these moths would also turn in the field," Professor Warrant said.

But the moths didn't turn in response to the magnetic field on its own, and the first experiments were "total failures", he said.

"They were all over the place, and it suddenly occurred to us — after two whole seasons of trying to make them cooperate by just turning the magnetic field — that many nocturnal insects have unbelievably good vision."

A black triangle on the white background of the flight arena represents a mountain-top landmark on the horizon for the moths to use in navigation. ( Supplied: Eric Warrant )

The team introduced visual cues in the form of a black triangle on the horizon, and found that the moths responded as expected when the visual and magnetic cues were aligned in the same direction.

But when they left the visual cue in place, and changed the direction of the magnetic field, the moths became very confused.

"If they were only interested in the visual landmark, they would have kept flying towards that unperturbed, but just by turning the field, we made them completely disoriented," Professor Warrant said.

"That was strong proof they had a magnetic sense."

The monarch and the moth

Professor Warrant and colleagues were not the first team to investigate the influence of Earth's magnetic field on migratory insects.

In 2014, University of Cincinnati sensory ecologist Patrick Guerra was part of a team that claimed the monarch butterfly used the Earth's magnetic field to assist its long-distance migration in the daylight hours, after observing that they could stay on track even on cloudy days when their main cue, the sun, wasn't visible.

Unlike nocturnal bogong moths, the monarch butterfly is active during the day.

"Monarchs don't use the polarity of the magnetic field, but they can use the inclination angle to determine if they are flying towards the equator or the pole," Dr Guerra said.

"They also need to be exposed to certain wavelengths of light to have this magnetic sense activated.

Every year monarch butterflies migrate from northern United States and southern Canada to a few mountains in central Mexico, where they huddle in the branches of oyamel fir trees over winter. ( Unsplash: Alex Guilluame )

"We found that their magneto-sensors are possibly located in the antennae, because when we prevented light from entering the antennae the butterflies weren't able to navigate using a magnetic compass."

However, not everyone agrees that monarchs use magnetic cues to migrate, according to Professor Warrant.

"[The scientists] used strong magnetic fields and got results, then used a natural-strength magnetic field and got some kind of reaction in a very tiny fraction of the butterflies," he said.

"Other people have tried exactly the same kind of thing, with hundreds of butterflies, and have never seen a single reaction to magnetic fields."

The concept of magneto-reception in animals in general is still controversial, Dr Guerra said.

"It's not intuitive how to sense [magnetic fields], because humans don't rely on that sense, or even have that sense," he said.

Finding the 'holy grail' of sensory physiology

Exactly how animals sense the Earth's magnetic field remains a mystery, but there are two leading theories narrowing in on the mysterious magnetic sensory mechanism.

The first theory is that there are tiny crystals of magnetite physically linked to ion channels in a neuron, somewhere in the nervous system.

"These little magnetite particles are always lined up with the magnetic field," Professor Warrant said.

"So, if the animal turns it will create a shearing force which literally drags these ion channels open or closed and creates an electrical signal."

The small, unassuming bogong moth makes a journey of over 1,000 kilometres each Spring. ( Supplied: Ajay Narendra )

The second theory involves a special kind of protein molecule called cryptochrome — and this is where things start to get weird.

The light-sensitive cryptochrome molecules could be used as a magneto-sensor, at least in some invertebrates, according to Professor Warrant.

"When [bluish] light is present these molecules have the ability to change their quantum mechanical spin state," he said.

"It's a quantum mechanical effect that could generate enough energy to cause a neuron to open and close ion channels, creating the electrical signal which is used to sense and perceive things."

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Fish and sea turtles seem to use the magnetite particle mechanism, whereas birds rely mostly on cryptochromes, according to Professor Warrant.

"It's likely that in birds the cryptochromes are in a certain type of photoreceptors in the eye," he said.

"We think this could be the case for [bogong] moths as well."

However, it remains unclear whether the nocturnal moths fly in enough light for the cryptochrome mechanism to be activated, said Dr Guerra.

"It's hard to say whether the light outside would be intense enough or the right wavelengths to activate a light-dependent system," he said.

It's possible that some animals, including birds and moths, could have more than one mechanism so that if one cue can no longer work — if there's no light for example — they don't get lost or have to stop their migration.

However, the real needle in the haystack is finding exactly where the sensory organ, or cells, or cell, is located in an animal, said Professor Warrant.

Dr Guerra said it was important to study the sensory mechanisms of migration in a nocturnal insect like the bogong moth — and that Professor Warrant and his team had a made a good start.

"Now they've established the bogong moth does have a magnetic sense, they could find out how they might navigate outdoors or under more naturalistic conditions," Dr Guerra said.

"They should look at what the mechanism is that a nocturnal animal uses to sense the magnetic field, and start teasing out whether it's the light-dependent [cryptochrome] mechanism or the presence of magnetite in the cells."