A quantum compass in the robin’s eye: how birds see the magnetic field + test

Contents

• A puzzle more than 50 years old
• A compass built from a chemical reaction
• Radical pairs, or where quantum physics comes in
• The robin provides the evidence
• Equipment that did not exist before
• An eye that lives on a shortage of oxygen
• The final test: swapping carbon
• Why we need a bird’s compass
• In short
• Five questions: what kind of navigator are you? (test)

⇑A puzzle more than 50 years old

Migratory songbirds are true lightweights. They weigh less than 50 grams, yet every year they cover thousands of kilometres at night to reach their wintering grounds. On the way they rely on several navigation systems at once: they watch the stars, remember landmarks and follow their sense of smell. On top of that, they can sense the direction of Earth’s weak magnetic field.

And here the problem begins. Research into this sense has been going on for more than 50 years, and the mechanism is still unknown. We know that it works. We do not know how.

⇑A compass built from a chemical reaction

It is exactly this question that the QuantumBirds project took on. It is funded by the European Research Council and coordinated by the University of Oxford. For seven years, specialists from very different fields worked in it: quantum mechanics, spin chemistry, behavioural biology and biochemistry.

The hypothesis they tested is bold. Birds would literally see the magnetic field thanks to chemical reactions taking place in the eye. Peter Hore, one of the project’s principal investigators, does not hide that it sounds strange: “At first glance, the notion that a chemical reaction could serve as a compass seems preposterous.”

No wonder. As Hore explains, the strength of the interaction between a single molecule and Earth’s magnetic field is only a millionth of the strength of a typical chemical bond. Such a stimulus seems far too weak to change anything.

⇑Radical pairs, or where quantum physics comes in

The key lies in proteins called cryptochromes, found in birds’ retinas. According to the researchers, light triggers the formation in them of radical pairs – short-lived molecular systems in which things happen that you would expect to find in a quantum physics textbook rather than in biology.

“When light hits these proteins, the generated radical pairs exist in a quantum superposition state,” explains Peter Hore. And he adds: “Given this delicate quantum state, the yield of the chemical reaction is highly sensitive to the direction of an external magnetic field.”

In other words: how efficiently the reaction runs depends on how the bird is positioned relative to Earth’s field. And because everything happens in the eye, the information arrives right where it is needed. “This effectively encodes navigation data directly into the bird’s visual system,” says Hore.

⇑The robin provides the evidence

Theory is one thing, but proof was needed. The team showed that cryptochrome-4a found in the European robin is sensitive to weak magnetic fields. The researchers regard this as the first direct evidence that this molecule has such a property. For the quantum compass hypothesis, it is a strong foothold.

⇑Equipment that did not exist before

To look so deep into a bird’s eye, the scientists first had to build their own tools. They included:

• highly sensitive spectrometers that pick up subtle changes in the proteins;
• a high-throughput facility that purified more than 40 protein variants from different bird species;
• the world’s first non-magnetic two-photon calcium-imaging instrument, which makes it possible to record magnetic signals in retinal neurons.

The last of these is especially important, because it lets researchers observe not only the proteins themselves but also the response of nerve cells.

⇑An eye that lives on a shortage of oxygen

Something surprising came to light along the way. The inner part of a bird’s retina operates under a constant shortage of oxygen. This discovery explains why research on this part of the eye has gone so slowly over the past decades: conditions in the laboratory simply did not match those in a living organism.

⇑The final test: swapping carbon

Now the scientists are preparing an experiment meant to settle the matter. “We are on the brink of a definitive answer through conducting a sophisticated isotopic swap,” announces Henrik Mouritsen, the other principal investigator.

The idea is to replace the natural carbon-12 in the flavin molecules in a bird’s eye with heavier carbon-13. Computer simulations showed that such a swap should change how birds respond to disruption caused by radio waves. The team has already created a way to introduce flavins with carbon-13 into birds’ eyes and is carrying out behavioural tests to show whether the change affects their sense of orientation.

The stakes are high. “A successful result will provide undeniable proof of this quantum compass,” stresses Mouritsen.

⇑Why we need a bird’s compass

Although this is basic research, its results may prove useful in surprisingly practical matters. Understanding how migratory birds choose their direction could help steer them towards habitats resilient to climate change instead of degraded areas. Similar mechanisms also operate in organic light-emitting diodes, so cheap and non-toxic magnetic sensors could be built on this basis. Another idea is navigation systems that cope where the GPS signal is blocked or disrupted.

⇑In short

Songbirds may carry in their eyes a compass that works thanks to quantum physics. The QuantumBirds team showed that the robin’s cryptochrome-4a protein responds to a weak magnetic field, built new equipment for studying the retina and discovered that its inner part lives with a shortage of oxygen. The carbon-swap experiment is meant to give the final answer and, along the way, suggest ideas for nature conservation and navigation without GPS.

⇑Five questions: what kind of navigator are you? (test)