Searching for the butterfly’s compass

A monarch butterfly being tested inside a flight simulator. (Inset)

IN A storage shed beneath a cloudless Texas sky last November, Robin Grob performed open-brain surgery on a monarch butterfly.

Strips of tape pinned the butterfly’s black-and-orange wings open beneath a microscope, holding its fuzzy, white-spotted body still.

Through the lens, its brain appeared as a pin-sized, yellowish mass.

Into it, Grob – a neurobiologist at the Norwegian University of Science and Technology – had inserted a tetrode: four electrodes, each thinner than a human hair.

Carefully, he sealed the butterfly’s head with silicone to prevent the electrodes from shifting. The insect struggled against the tape.

“Calm down,” Grob murmured. “Stop moving.”

The most delicate part was yet to come.

A scientist placing an electrode inside a butterfly’s brain. — Mark Felix/The New York Times

Grob needed to carry the butterfly outdoors to a flight simulator – an open-ended metal cylinder about the size of a coffee urn – without dislodging the electrodes. A single jolt would ruin hours of work.

If the butterfly survived the move, and if the tetrode stayed in place, Grob might capture something scientists have been chasing for decades: the precise moment when a brain senses the magnetic field that guides monarchs across a continent.

Every year, monarch butterflies travel thousands of kilometres from Canada to the oyamel fir forests of central Mexico.

They do so using a set of internal compasses, yet how an insect with a brain smaller than a grain of rice accomplishes this feat remains one of biology’s enduring puzzles.

Animals – humans included – navigate using a mix of cues: the sun and stars, polarised light, remembered landmarks. But the most mysterious of all is magnetoreception – the ability to sense Earth’s magnetic field.

“We understand how we can smell, how we can see, how we can hear,” said David Dreyer, a neuroscientist at Lund University in Sweden. “We do not understand how animals sense the magnetic field. It is the last sense that is not really understood.”

Scientists distinguish between two navigational tools: a map sense and a compass sense. A map sense allows animals to determine where they are relative to a destination; a compass sense lets them maintain direction using external cues.

Loggerhead sea turtles possess both. They can use Earth’s magnetic field not only to orient themselves, but also to determine their position – enabling them to return home from unfamiliar waters.

Migratory insects appear to operate differently. They have a compass sense but no proven map sense, meaning they can travel in a particular direction without knowing exactly where they are.

That limitation makes insects less impressive navigators – and far more useful research subjects.

“Insects offer an advantage,” said Kenneth Lohmann, a biologist at the University of North Carolina at Chapel Hill. “Their brains and nervous systems are smaller, so the neural circuits are relatively simpler.”

A researcher feeds butterflies in a lab in College Station, Texas. — Mark Felix/The New York Times

Monarchs, he added, could be key to understanding how migration works at a fundamental level.

Not everyone is convinced they even possess a magnetic sense.

“I don’t think they have one,” said Henrik Mouritsen, a biologist at Germany’s University of Oldenburg.

Two decades ago, Mouritsen published work on monarch orientation that found no evidence of magnetoreception.

“I would like to see it with my own eyes,” he said. “I tried, and I couldn’t get them to do it.”

Back in the shed, Grob watched as the first neural signals of the day flickered across his screen.

He had implanted electrodes into four distinct neurons within the monarch’s central complex – the brain region associated with spatial orientation. Monarchs have roughly 100 million neurons in total. Whether Grob had chosen the right four was anyone’s guess.

“It’s like going in blind,” he said.

The experiment relies on persistence and failure. After identifying four promising neurons, the researchers take the monarch outdoors and allow it to attempt flight while brain signals are monitored.

The goal is to spot neural activity that corresponds to changes in magnetic fields. If nothing appears, the process begins again with another butterfly.

Despite the invasive nature of the work, butterflies do not experience pain; their nervous systems lack pain receptors.

For years, Grob has pushed to conduct these recordings outdoors rather than in a tightly controlled laboratory. That is partly because monarchs need to believe they are truly migrating.

“If we want to understand migration, we need the animal in the right behavioural state,” said Basil el Jundi, a neuroscientist at Oldenburg University whose lab designed the experiment. “You have to record from a flying animal while it’s actually navigating.”

Once the silicone hardened, Grob lifted the butterfly and walked carefully towards the simulator. He attached it to a fine tether inside the cylinder and gently tapped its body. The wings began to beat.

On the monitor, two sine waves appeared, punctuated by spikes. As the butterfly adjusted its flight angle, Grob leaned closer to the screen.

Somewhere inside that minute brain, signals were firing in response to an invisible force – one that guides creatures across oceans, continents and generations.

“This,” Grob said quietly, “is beyond just a monarch.” — ©2026 The New York Times Company

This article originally appeared in The New York Times