Weak radio waves in the medium-wave band are sufficient to disrupt geomagnetic orientation in migratory birds, according to a particularly well-controlled study. But the underlying biophysics remains a puzzle. See Letter p.353
Magnetobiology has largely been viewed as a stamping ground for charlatans since the followers of physician Franz Anton Mesmer failed to cure patients using a 'magnetized' tree in the eighteenth century. Numerous discoveries have begun to change that perspective, although the road has been rocky. For example, early studies suggesting that migrating animals use geomagnetic cues for navigation were plagued by variability, but it is now clear that many microorganisms and animals use a magnetic compass for part of their orientation1.
On the fringe of this fringe field were claims that radio-frequency radiation could have biological effects at levels too weak to act through the understood mechanisms of tissue heating or shock, but the experiments usually lacked proper controls and blinding techniques2,3,4. Now, however, on page 353 of this issue, Engels et al.5 demonstrate convincingly that migrating European robins stop using their magnetic compasses in the presence of extraordinarily weak, radio-frequency electromagnetic 'noise'.
Using rigorous, double-blinded experiments, the authors found that birds housed in huts screened from background electromagnetic noise were able to use their magnetic compass to orient themselves appropriately, but that their orientation was disrupted following the introduction of electromagnetic noise ranging from 20 kilohertz to 5 megahertz, at intensities similar to that measured for background anthropogenic noise in the environment. To put it into perspective, this is in the medium-wave band used for AM radio transmissions (not, for example, mobile phones), and the strength is about equivalent to what a bird in flight might experience 5 kilometres away from a 50-kilowatt AM radio station.
Two results flag this study as particularly noteworthy, and puzzling. First, the levels of radio-frequency radiation that affected the birds' orientation are substantially below anything previously thought to be biophysically plausible, and far below levels recognized as affecting human health. Second, the authors detect no trace of a sharply enhanced effect at the Larmor frequency (the natural period at which single electrons wobble around the geomagnetic-field direction), which flatly contradicts experiments on the same species performed using a similar protocol6. This failure to replicate that effect perhaps underscores previously suggested2,3 flaws in the blinding of earlier studies.
So what might be going on in these birds? Several other external stimuli that stop animals from responding to geomagnetic cues have been identified. Early studies of animal navigation noted that cues from the Sun or stars would take precedence over magnetic cues, leading to the idea that magnetism is the compass of last resort. It was then noticed that robins would ignore the magnetic field when the background intensity was shifted 20–30% outside the normal value1, and that pigeons raced poorly during geomagnetic storms. From an evolutionary perspective, ignoring geomagnetic cues at such times makes sense, because anomalies in the background field are often associated with iron deposits or lightning strikes. Some animals also stop using their magnetic compass in the presence of red-only light, but such light is present only at sunrise and sunset, when the Sun compass is most reliable3.
Hence, radio-frequency noise might be just another cue that tells migrating animals to ignore their magnetic sense, but the puzzle is why this might have evolved. Surprisingly, there is a natural source of the radio-frequency electromagnetic noise identified as disruptive by Engels and colleagues — that produced by solar storms. Coronal mass ejection (CME) events from the Sun slam plasma into Earth's magnetosphere every now and then, causing it to 'sing' at frequencies from as low as around 20 kHz up to the MHz range7, some of which even leaks through Earth's normally radio-opaque ionosphere; the lower end of this range is remarkably close to that identified by the authors. These CME events generate the beautiful polar auroras, disrupt our use of the medium-wave radio band, and sometimes perturb the background geomagnetic field at Earth's surface enough to disturb animal navigation.
All known sensory systems in animals are based on cells specialized to convert the stimulus of interest into a coded stream of action potentials that are sent to the brain8. If the effects of radio-frequency radiation are real, such cells must exist, but the mystery is in the biophysics. The lack of an enhanced effect at the Larmor frequency, and the low levels of radiation concerned, make it unlikely that a previously proposed mechanism6 for radio-sensing, based on light activation of a cellular protein called cryptochrome, is involved. But some magnetic effects on animals (such as that of a short, sharp magnetic pulse1) function through biological magnetite (Fe3O4) in tissue — might this also be the radio-wave detector?
If it is, how could such a detection mechanism have arisen? Early animals that had a simple compass patterned along the lines of magnetotactic bacteria would have needed to survive geomagnetic excursions or reversals — periods in which Earth's magnetic field weakened — and natural selection would have favoured individuals with higher cellular volumes of magnetite3,9. When the field recovered, animals would have been left with cells that have surprisingly large magnetic moments9 (Fig. 1). Such cells might then have evolved to serve other functions, such as intensity-based magnetic navigation systems, increasing the amount of magnetite further. With large enough volumes of metallically conductive magnetite in these cells, direct detection of the small electric and magnetic vectors of radio-frequency radiation might have emerged, as Engels and colleagues suggest.
Do the authors' findings have implications for humans? It seems that geomagnetic sensitivity dates back to an early ancestor of animals, and it is clearly present in many extant mammalian species. Human tissues also contain biological magnetite10. Many people claim to be bothered by radio transmissions, and some have even moved to live in radio-frequency 'quiet zones' around radio telescopes. Modern-day charlatans will undoubtedly seize on this study as an argument for banning the use of mobile phones, despite the different frequency bands involved. However, if the effect reported by the authors stands the acid test of reproducibility, we might consider gradually abandoning our use of this portion of the electromagnetic spectrum and implementing engineering approaches to minimize incidental low-frequency noise, to help migratory birds find their way.
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