5 days ago
16
To the seasoned ear, the trilling of chiffchaffs and wheatears is as sure a sign of spring as the first defiant crocuses. By March, these birds have started to return from their winter breaks, navigating their way home to breeding grounds thousands of kilometres away – some species returning to home territory with centimetre precision. Although the idea of migration often conjures up striking visions of vast flocks of geese and murmurations of starlings, “the majority,” says Miriam Liedvogel, director of the Institute of Avian Research (IAR) in Germany, “migrate at night and by themselves, so they have no one to follow.”
Liedvogel has had a fascination with birds since childhood, and often wondered how they navigate these lengthy migrations. She is not alone, with even Aristotle pondering the mystery and mistakenly concluding that redstarts change into robins over the winter. As Liedvogel points out, migration behaviour is varied and much remains unknown, but we now have enough data on bird behaviour to rule out species transmogrification, among other theories. Studies have revealed that 95% of migrating birds travel at night, alone, and without parental guidance, so the behaviour must be partly inherited. These birds use the Earth’s magnetic field to find their way, and it is likely that at least part of the biological mechanism that allows them to do this can be explained through quantum mechanics.
Biophysicist Klaus Schulten came up with the idea currently favoured for explaining magnetic field-sensing in birds in 1978, alongside his colleagues Charles Swenberg and Albert Weller at the Max Planck Institute for Biophysical Chemistry in Germany. The idea hinges on what happens when electrons gain energy. A more familiar response might be the generation of a current, as in a photovoltaic device (or solar cell) when the sun is out, but other effects can take place too. Electrons favour hanging out in pairs, but absorbing energy can lead to an electron moving from one molecule to another. At this point, both the molecule gaining and the molecule donating an electron have unpaired electrons, earning those molecules the hippy-sounding term “free radical”.
Electrons have a quantum property described as “spin”, and when two free radicals are formed in this way, the spins take on a particular arrangement that is sensitive to magnetic fields. This means that any change in biochemistry the molecule undergoes during the bird’s natural bodily processes – and the rates of these reactions – will be affected by the presence of a magnetic field. So this “radical pair effect” could allow birds to sense a magnetic field. However, researchers have also suggested that a little bit of magnetic iron oxide incorporated into a bird’s beak could do much the same thing, acting as a tiny compass needle. Particles of magnetite get everywhere: they’re inside the food we eat and are carried on the air that we breathe, so it’s inevitable that they end up in our bodily tissues. People have reported finding them inside the heads of birds, although not where they can enter the brain for it to sense magnetic fields. What is more, a bit of magnetite alone could not explain the navigating behaviour people observe in birds.
Peter Hore is a professor of chemistry at Oxford University, who has been studying the possible mechanisms allowing birds to sense magnetic fields for more than 20 years, and he lists some of the evidence in favour of the radical pair effect. One finding is that birds do not seem to be able to sense the difference between north and south poles in a magnetic field, so much as the direction towards a pole or an equator. “That suggested it [the magnetic sensing] was not based on magnetic minerals, which behave like compass needles,” says Hore. Put a bird in the opposite hemisphere and it will still aim its flight towards the equator for warmer weather in the winter.
Another compelling finding is that it seems light is needed for birds such as robins to sense the magnetic field, and it is needed to kickstart the radical pair effect too. At the quantum level, all measurements are accompanied by an exchange of energy. Looking in detail at the wavelengths of light absorbed in the radical pair effect has also pointed towards a particular protein: cryptochrome 4. Schulten suggested in 2000 that a cryptochrome protein might be a plausible candidate for hosting the radical pair mechanism in birds, although at the time cryptochromes had only just been discovered and only one was known. Liedvogel began her PhD just a few years later and began looking into the cryptochromes known in birds at that time, a study she describes as being “crazy hard”, before then adding: “Now, with much more failure and much more knowledge also, we understand why that was so.” It turns out that while cryptochromes 1, 2 and 4 are found in the eyes of these migrating birds, the binding between cryptochromes 1 and 2 and the crucial light-absorbing pigment in the radical pair effect is not nearly as strong as it is for the later discovered cryptochrome 4.
Careful study of the wavelengths of light absorbed by cryptochrome 4 revealed exactly which radicals were forming. In 2021, Hore and his colleagues were able to test the magnetic field sensitivity of cryptochrome 4 from a robin, which migrates, and compare it with that of a chicken, which doesn’t. They showed that the robin’s protein had a greater sensitivity to magnetic fields than the chicken’s. Furthermore, when they mutated the candidate parts of the protein flagged as forming the radicals, no magnetic field sensitivity effects were detected. All this strongly favoured the radical pair mechanism in cryptochrome 4 proteins as the basis for magnetic field-sensing in birds, despite the tiny values of the energies involved. The Earth’s magnetic field strength at the surface is a mere 50 microtesla – a standard medical MRI uses fields at least 20,000 times higher – so the energies these interactions amount to are tiny: a million times smaller than the thermal energy of the molecules just jigging around at body temperature.
With the discovery of cryptochrome 4 and evidence in its favour for the radical pair effect, Liedvogel took another tack and set out to look for signs of adaptive selection optimising the protein in the evolution of birds that migrate seasonally. She and Corinna Langebrake, who was then completing a PhD with Liedvogel at IAR, looked at all the known bird genome sequences and compared regions relevant for the production of cryptochromes in birds that migrate versus those that do not. They found that in cryptochromes 1 and 2 there was very little variation across species, which can be indicative that those proteins are universally essential, as changes would then be hazardous to survival. As Liedvogel points out, these proteins are responsible for maintaining circadian rhythms – the “body clock” – so that much all adds up. What is more, they found higher levels of variation not just in the cryptochrome 4 of migrating birds versus non-migrating birds, but in the region of cryptochrome 4 that produces radicals in the radical pair effect. There are also other regions of apparently high selectivity in cryptochrome 4, possibly pointing towards an additional but as yet unknown function of the protein. Perhaps harder to explain, however, is the absence of cryptochrome 4 in a group of birds that includes solitary long-distance nocturnal migrants, the tyranni. Behavioural experiments are under way to test whether these birds can sense the Earth’s magnetic field in the way that the cryptochrome 4 radical pair mechanism model seeks to explain, but the jury is still out.
Meanwhile, a paper published earlier this year suggests that there may be a limit to how far evolution can go in terms of improving a bird’s sensitivity to magnetic fields thanks to a fundamental principle in quantum mechanics. “There are trade-offs in all aspects of physical reality,” explains Iannis Kominis, an associate professor at the University of Crete in Greece, whose research has focused on quantum sensing and quantum biology in parallel for 15 years. A key trade-off in quantum mechanics is Heisenberg’s uncertainty principle, which limits how precisely you can pin down two variables, such as energy and time – the better the handle you have on one, the less you have on the other. If you follow this line of maths and logic to its natural conclusions, and given the time it takes for a physical process to happen, then you eventually land on a tiny but finite quantum of energy below which you cannot go. Since a measurement must be accompanied by an exchange of energy, this places a fundamental limit on the achievable sensitivity – whether that comes from a quantum device operating in a lab at cryogenic temperatures or a protein in a bird’s eye. Earlier this year, Kominis, together with an undergraduate student at the university, Efthymios Gkoudinakis, showed that the limit is respected in the animal kingdom, although the sensitivity achieved with the radical pair mechanism could get very close to this limit.
“It seems that nature has devised quantum technology before us, and that doesn’t sound that crazy, right?” says Kominis. “The opposite would mean that we are smarter than nature.” He also suggests that where the sensitivity achieved is not so close to the limit, there is room to advance quantum sensing technology using “the IP from mother nature to try to make a better product”.
Quantum calculations have also highlighted potential evidence in favour of the radical pair effect through the response of birds to magnetic “noise”. Calculations show that the radical pair electrons are actually swapping between “spin” states at specific frequencies, which means that the birds could become disoriented if exposed to a magnetic field fluctuating at those frequencies. “It’s not often that you can sit at your computer and do some quantum mechanical calculations and predict how an animal is going to behave,” adds Hore.
It was 10 years ago that Hore collaborated with Henrik Mouritsen and his colleagues at Germany’s University of Oldenburg, showing that robins are disoriented in the presence of urban electromagnetic noise – although the birds were sensitive to levels of noise below those the calculations had suggested. Mouritsen and his colleagues are now testing the frequencies at which a bird becomes disoriented. So far, these tally rather well with what has been calculated for the radical pair mechanism, but Hore explains such behavioural tests are time consuming, partly since they can only be conducted during migration season.
Although Hore and Liedvogel are cautious of suggesting the case is closed, the accruing data on bird behaviour, their proteins and the radical pair effect do seem to be converging on an explanation for birds’ sensitivity to magnetic fields. If correct, it amounts to an incredible feat of quantum sensing achieved not in some hi-tech cryogenic lab but in the messy organic environment of a bird’s eye. “I certainly do look at birds in a different light,” says Hore. “The term ‘bird brain’ is normally an insult – I now think of it as a compliment.”
