Quantum compass may steer nighttime flight of birds

An international team of biologists, chemists and physicists has produced new evidence supporting the hypothesis that migratory birds can navigate at night using the varying quantum state of electrons in their retinas.

The work, conducted at the University of Oldenburg (Germany) and Oxford University (UK), shows how a molecule, known as cryptochrome 4, found in birds’ retinas responds to magnetic fields and might be able to turn a weak magnetic signal from the Earth’s field into a visual signal, using a quantum chemical process known as radical pairing. 

The new work is reported in the June 24 issue of Nature. 

The theory of the cryptochrome quantum compass is derived from theoretical work developed by senior authors P.J. Hore and Henrik Mouritsen, in collaboration with Ilia Solov’yov and others. It depends on a quantum chemical mechanism known as “radical pairing.” 

In radical pairing, a short-lived intermediate product in a chemical reaction can be comprised of two radicals possessing unpaired electron spins. The spins can be either anti-parallel (singlet state) or parallel (triplet state). In the singlet state the radical pairs are considered to be quantum entangled.

Electron spin, short for spin angular momentum, produces a magnetic moment for the electron. That electron magnetic moment can be affected by an external magnetic field, including the Earth’s, by a process known as Zeeman interaction. Zeeman interactions can cause the spin of one of the electrons in the radical pair to change quantum states (“flip”).

This change of quantum state can have a direct effect on a chemical reaction involving radical pairs. Depending on the spin state, the reaction products may be able to either recombine or be prevented from recombination and proceed to end products. Radical pairing has been shown to occur in cryptochromes, a class of flavoprotein found in plants and animals that is sensitive to blue light. 

Specifically, it is thought that the radical pairs are formed in cryptochromes excited by blue light by “the sequential hopping of an electron along a chain of three or four tryptophans [amino acids] to the photoexcited, non-covalently bound flavin adenine dinucleotide (FAD) chromophore,” according to the Nature paper.

The establishment of this new mechanism of magnetoreception would add to the already accepted mechanism of small iron-based magnetic domains found in organisms as small as bacteria. Such iron-mineral-based putative sensors are known to exist in the upper beak of birds, including the European robin.

Cryptochrome from the European robin

Jingjing Xu, a doctoral student in Prof. Henrik Mouritsen’s group at Oldenburg, took a decisive step in the work by first extracting the genetic code for the flavoprotein, ErCRY4 (cryptochrome 4), from the night-migratory European robin, then developing a method for producing large quantities of the substance in bacterial cell cultures. 

Detailed measurements of the magnetic sensitivity and other parameters of the substance were then carried out at Oxford University by teams working with Christiane Timmel and Stuart Mackenzie. 

The experiments so far were carried out in vitro, and the magnetic fields used were stronger than the Earth's magnetic field. 

"It therefore still needs to be shown that this is happening in the eyes of birds," Professor Mouritsen stresses in a review of the work on his University of Oldenburg website, adding that such studies are not yet technically possible.

As the authors summarize their results at the conclusion of the Nature paper: 

“In conclusion, we have demonstrated that CRY4 from the night-migratory European robin seems to be fit for purpose as a magnetic sensor, and that it is more magnetically sensitive than CRY4 from the non-migratory pigeon and chicken under identical measurement conditions in vitro.

“Our data also suggest that nature might have found ways to independently optimize the magnetic sensing and signaling properties of CRY4 through control of its photochemistry. To determine whether CRY4 acts as a magnetoreceptor molecule in vivo, direct manipulations of this protein in the eyes of night-migratory songbirds would be required. We hope that such experiments will become possible in the future.”