Giselle Clarkson

Every spring, billions of bogong caterpillars metamorphose into moths on the western plains of New South Wales and Queensland. Each moth has a five-centimetre wingspan and weighs just a third of a gram, but its brand-new, speckled brown wings carry it 1000 kilometres southeast to the Australian Alps.

The moths spend the baking summers keeping cool in caves and cracks in the mountains, then fly north again in the autumn to breed. For the Aboriginal peoples of the high country, the annual coming of the bogongs has long been an important seasonal marker and a time for feasting (apparently, they taste like peanut butter). But for scientists such as Eric Warrant, an Australian entomologist at the University of Lund in Sweden, the moth’s long-distance migration is an irresistible puzzle. “The overarching question is, how the hell do they do this?”

For centuries, humans have marvelled at the mysterious arrivals and departures of migrating animals. Aristotle declared that redstarts transform into robins in winter: an intuitive suggestion, given the similar-looking redstarts migrate to Africa at a time when robins, which breed farther north, arrive to spend winter in Greece. (He also thought eels reproduced asexually, springing fully formed from the mud—an idea medieval Christians latched onto, as that meant the unsexy eels could be safely eaten during Lent.)

Other ancient European writers theorised that swallows hibernate in the mud in the bottom of lakes, and that cranes fly off to the world’s end each winter, where they do battle with people mounted on goats and rams (obviously). Widespread medieval stories held that barnacle geese either grew like fruit on trees or from goose barnacles attached to floating driftwood, and one 17th-century English naturalist even proposed that storks flew to the moon in one great flock—a journey of two months, he calculated, during which the storks were mostly asleep. Polynesians, on the other hand, were under no such illusions—they followed migrating whales, pigeons, cuckoos and godwits to find new lands, including New Zealand.

Recent advances in satellite tracking have shown with unprecedented detail what migrating animals do and where they go: the annual peregrinations of manta rays and humpback whales, the last great journey of an adult eel to its birthplace, and the record-breaking 29,000-kilometre return flights of kuaka/bar-tailed godwits from New Zealand to the Yellow Sea to Alaska and back again.

It might not be the moon, but it’s awe-inspiring nevertheless. And the question of how they do it—how so many different kinds of animals find their way with such precision across vast distances, in the dark, over the seas, tossed by storms, sometimes alone, sometimes never having made the journey before—remains one of the greatest mysteries in biology.

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For scientists, the first step is to figure out what natural cues a given species might be using as signposts. Like the Polynesian navigators, many animals look to the sky.

The sun is a common cue, but because it moves across the sky each day, to stay on track animals must combine information about the angle of the sun above the horizon—the azimuth—with their internal circadian clock. Homing pigeons, meadow voles, shearwaters, turtles, lizards, fish larvae and frogs have all been found to use a time-compensated sun compass in this way.

Monarch butterflies also rely on such a compass on their annual round trip from the northern United States and Canada to Mexico.

The butterflies’ circadian clocks are located inside their tiny antennae—when researchers daubed them with black paint or snipped them off, the monarchs could no longer navigate correctly.

Other insects use polarised light. As the sun’s rays pass through the atmosphere, particles scatter some of them, making them vibrate along a single plane. This polarised light creates a pattern in the sky—even when the sun itself is obscured by clouds or fog. Some humans can actually see highly polarised light under certain conditions—called Haidinger’s brushes, the phenomenon reportedly looks like two bowtie shapes stacked on top of each other to form a blue-and-yellow cross, with the yellow axis pointing towards the sun. (There has been speculation that Vikings used this effect to navigate in open seas on cloudy days.)

Insect eyes are much better adapted than ours to see these celestial patterns. Dung beetles use polarised sunlight to guide them as they roll dung balls away from poo piles, and when desert ants venture out to scavenge on dead animals, they use a combination of polarised light and step-counting—their own internal pedometers—to track their position so they can scurry straight back to their nests out of the heat.

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What if you’re a nocturnal bogong moth, awake when the sun is in bed? “The moon is a really hopeless cue,” says Warrant—it’s not always up, it shifts around and it varies in brightness. But there’s one source of information available to animals on even the darkest, cloudiest night: the geomagnetic field. Almost as old as the Earth itself, it’s generated in the planet’s core and extends out into space in two huge loops stretching from one magnetic pole to the other. (It also acts as a sort of blanket, protecting the planet from cosmic rays and solar wind.)

To us, the magnetic field is invisible, accessible only via a compass: the needle lines up with the magnetic north pole. Zebra finches, turtles and cockroaches are aware of it, though. The giant sea slug (also known as the orange-peel nudibranch) reportedly orients its body between magnetic north and east prior to a full moon, and many migrating animals seem to pick up on subtle changes in the magnetic field from place to place, using that information to figure out what direction to head in.

This was Warrant’s first thought when he started trying to answer the question of how bogong moths navigate. He and his students devised a series of experiments.

The scientists captured migrating moths on a remote hilltop in northwestern New South Wales, glued tungsten rods to their backs and attached them to tiny tethers inside a tabletop arena the team had carted up the mountain. They then surrounded the arena with magnetic coils to mimic and manipulate the Earth’s magnetic field, assuming that if the moths were using it as a primary navigational cue, they would switch their flight direction as the field around them was altered.

“After two years of experiments, we discovered they did not do this,” says Warrant. He wanted to kick himself—he’d spent three decades researching insect vision, yet had forgotten what excellent eyesight moths have. Subsequent experiments suggested the moths were setting their initial direction using their magnetic sense, but then, like hikers mapping a compass needle onto a distant tree, used visual beacons to guide them onwards.

It occurred to Warrant that the Milky Way “is an enormously strong visual stimulus”. To a moth, it probably looks like “a lovely stripe of light”—brightest in the south and fading in intensity as it arcs into the north. To test this theory, his team had the moths fly in a magnetic vacuum (so they could study their visual sense in isolation) and used a computer program designed for planetaria to project the night sky above them.

Inside the arena, with only the image of the natural sky for guidance, the moths knew exactly where to go, but when the researchers rotated the sky image by 180 degrees, the whole population of bogongs turned and flew in the wrong direction. When the stars’ positions were randomised, the moths were completely disoriented.

Another hint has come from the moths’ tiny brains: Warrant has found cells that seem to be responsible for measuring the rotation of the stars in the sky. “For me, the bogong moth is one of the most remarkable insects that exists,” he says. “They can determine their direction using the stars. As far as we know, there’s only us and some species of migratory songbirds that can do that.”

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One of the difficulties with explaining navigation is that different animals use different methods—and most combine more than one. There’s a lot we don’t know about tuna, New Zealand’s longfin and shortfin eels—including exactly where in the tropical Pacific they spawn, whether adults travel there in groups or alone, and how they find their way. NIWA eel expert Don Jellyman says it’s likely a combination of several cues.

Lab research in the 1980s showed that migrating adult eels are sensitive to the magnetic field. In a 2021 study, Norwegian scientists compared the routes taken by juvenile and adults of five species, including long and shortfins, to magnetic gradients—the way the intensity or direction of the magnetic field changes across the Earth’s surface. They theorised that juvenile eels might imprint magnetic information as they float on sea currents from the tropics towards New Zealand and Australia, then use other cues like salinity and smell to find a river to spend their adult life in. “Eels have a phenomenal sense of smell—they’re called the bloodhound of the sea,” says Jellyman.

Decades later, ready to spawn and die, the adult eels could retrace that gradient to find the general spawning area. It’s not a perfect method, as the Earth’s magnetic pole itself shifts by around 10-50 kilometres a year—enough to make for significant navigational error over the lifespan of a single eel. (Some longfins migrate at 25, but others live to 80 or even possibly over 100.)

But once the horny eels get close, Jellyman says they probably follow their noses to find each other in the vast open ocean.

“As soon as they start spawning, that will create a unique scent, which will be a big turn-on and a guidepost for others.”

We know even less about how humpback whales navigate from the tropics to the Southern Ocean with such spectacular accuracy each year, says Travis Horton from the University of Canterbury. (You can’t put a whale in a table-top arena or experimental tank like you can a bogong moth or eel.) They might use magnetic sense—one study showed grey whales were four times more likely to strand themselves during solar storms, which send the Earth’s magnetic field briefly haywire—or, Horton suggests, they might be able to sense gravitational gradients, which also vary across the planet.

Figuring it all out will be the work of generations of scientists, says Horton. “It’s so gobsmackingly remarkable to a human that a whale can swim 8000 kilometres across the open ocean and find the same pile of basalt year after year after year. That’s why people get so fired up about it.”

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Even once scientists think they’ve worked out what cue an animal might be employing, learning exactly how their brains and bodies sense and use it is whole other question. Magnetic sense, for instance, has been a riddle for aeons, says biologist Henrik Mouritsen of the University of Oldenburg in Germany. He’s originally from Denmark, and started birdwatching aged 10.

At 15, he observed young desert wheatears wander confused on a Danish beach; the little buff-coloured birds had come from Mongolia and were meant to be flying to India or Africa. He wondered how they had got so lost, and started researching animal navigation mechanisms. “I wasn’t very impressed with what people knew, so I thought this is something that I would like to figure out.” After three decades of research, requiring collaboration with chemists, ecologists, neurobiologists and quantum physicists, he thinks he’s finally getting close to an answer.

In the 1960s, scientists discovered that some bacteria contain tiny magnetite crystals that act to orient the single-celled organisms along the lines of the Earth’s magnetic field, just like a compass needle. (It happens automatically, even when the bacteria are dead.) Over the next few decades, various researchers (including retired University of Auckland animal-navigation expert Michael Walker) found similar magnetite crystals in the beaks of pigeons, the noses of trout and the heads of bats, and theorised that these tiny crystals could somehow influence the flow of charged atoms in and out of animals’ cells.

It’s definitely plausible, says Mouritsen, but solid evidence remains lacking—and in the tiny night-migrating songbirds he studies, he thinks something even more remarkable is going on.

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Bar-tailed godwits might be migrational megastars, but they don’t make good lab animals—they’re too big and too rare. Mouritsen’s team instead studies tiny European robins and Eurasian blackcaps, some of which migrate from Northern Europe to the Mediterranean and Africa, using the stars, the setting sun and the magnetic field to show them the way. They always fly at night, but outside of the migration season they sleep instead.

In a simple lab experiment, Mouritsen’s team put the birds in a special funnel lined with scratch-sensitive paper. At night, when the birds felt the restless call to migrate, they started jumping around in the direction of Africa, leaving scratch marks on the side of the funnel.

The researchers could then alter all sorts of variables—the light, the magnetic field—and observe how they influenced the birds’ behaviour. Another team of scientists discovered that under red and yellow light, the birds would jump around as usual, but in random directions. They could only use their magnetic compass in blue, turquoise and green light.

What could magnetic sense have to do with vision? This was a strange and significant finding that eventually led biologists to quantum mechanics—a theory of physics that describes the physical properties of nature at the scale of molecules and atoms.

“In the 1970s, there was a genius physicist called Klaus Schulten—one of those theoretical physicists that just understand things that normal human beings do not,” says Mouritsen. Schulten proposed that a migrating bird’s compass relies on unstable, spinning pairs of molecules called radical pairs, which are sensitive to both light and to minute fluctuations in the Earth’s magnetic field. These molecules, he thought, could determine the fate of chemical reactions in the bird’s cells, providing a signal that helps them to orientate.

The only molecule known to form such pairs is called cryptochrome—a protein located in the eyes of birds (and fish and reptiles) that helps control the circadian clock. Mouritsen spent 14 years trying to make a complete cryptochrome molecule from the DNA of blackcaps in the lab—and when at last he did, he and colleagues at the University of Oxford showed it behaved just as the quantum theory predicted it would, and was in fact magnetically sensitive.

The team also looked at the birds’ brains, and found that the visual part lit up when the animals were using their magnetic sense. When they were blindfolded with tiny black cloths, they could neither see nor navigate. Further experiments are needed, but Mouritsen’s convinced that these night-migrating songbirds, at least, are detecting the Earth’s magnetic field with the cryptochromes in their eyes—and can maybe even see it.

It’s hard to picture what these magnetic maps might look like. Birds already see the world in many more colours than we do. The magnetic sense might appear like a kind of shading over top of the visual field, or it could be something completely different that’s beyond our limited human imagination, says Mouritsen. “We have no idea exactly how they see it, and we never will, I think, because we will not be a bird.”

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Unsolved questions drive science onwards. The radical-pair findings may have implications for quantum computing and also for conservation—for instance, people doing translocations of migratory birds must take their magnetic and celestial compasses into account, says Mouritsen, and either move them as babies before they imprint, or prevent them from sensing the magnetic field or seeing the sky at the wrong location.

Still, what we know about how animals navigate remains a mere fraction of the complex, detailed whole—like a glittering fragment of stars glimpsed through a clouded sky.

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