Seeing Earth's Magnetic Fields: Cryptochromes and Blue Light

Birds Can See Earth's Magnetic Fields -- And We Finally Know How That's Possible

By Michelle Starr

The mystery behind how birds navigate might finally be solved: it's not the iron in their beaks providing a magnetic compass, but a newly discovered protein in their eyes that lets them "see" Earth's magnetic fields.

These findings come courtesy of two new papers -- one studying robins, the other zebra finches.

The eye protein is called "Cry4," and it's part of a class of proteins called "cryptochromes" -- photoreceptors sensitive to blue light, found in both plants and animals. These proteins play a role in regulating circadian rhythms.

There's also been evidence in recent years that, in birds, the cryptochromes in their eyes are responsible for their ability to orient themselves by detecting magnetic fields, a sense called "magneto-reception."

We know that birds can only sense magnetic fields if certain wavelengths of light are available -- specifically, studies have shown that avian magneto-reception seems dependent on blue light.

This seems to confirm that the mechanism is a visual one, based in the cryptochromes, which may be able to detect the fields because of "quantum coherence."

To find more clues on these cryptochromes, two teams of biologists set to work. Researchers from Lund University in Sweden studied zebra finches, and researchers from the Carl von Ossietzky University Oldenburg in Germany studied European robins.

The Lund team measured gene expression of three cryptochromes, Cry1, Cry2, and Cry4, in the brains, muscles and eyes of zebra finches. Their hypothesis was that the cryptochromes associated with magnetoreception should maintain constant reception over the circadian day.

They found that, as expected for circadian clock genes, Cry1 and Cry2 fluctuated daily -- but Cry4 expressed at constant levels, making it the most likely candidate for magnetoreception.

This finding was supported by the robin study, which found the same thing.

"We also found that Cry1a, Cry1b, and Cry2 mRNA display robust circadian oscillation patterns, whereas Cry4 shows only a weak circadian oscillation," the researchers wrote.

But they made a couple of other interesting findings, too. The first is that Cry4 is clustered in a region of the retina that receives a lot of light -- which makes sense for light-dependent magnetoreception.

The other is that European robins have increased Cry4 expression during the migratory season, compared to non-migratory chickens.

Both sets of researchers caution that more research is needed before Cry4 can be declared the protein responsible for magnetoreception.

The evidence is strong, but it's not definitive, and both Cry1 and Cry2 have also been implicated in magnetoreception, the former in garden warblers and the latter in fruit flies.

Observing birds with non-functioning Cry4 could help confirm the role it seems to play, while other studies will be needed to figure Cry1's role.

So what does a bird actually see? Well, we can't ever know what the world looks like through another species' eyes, but we can take a very strong guess.

According to researchers at the Theoretical and Computational Biophysics group at the University of Illinois at Urbana-Champaign, whose researcher Klaus Schulten first predicted magnetoreceptive cryptochromes in 1978, they could provide a magnetic field "filter" over the bird's field of view (like in the picture above).

The zebra finch study was published in the "Journal of the Royal Society Interface," and the robin study was published in "Current Biology."

Proust Was a Neuroscientist

Here's a fun, Gladwellian-type read by Jonah Lehrer. The chapter on the French chef Ecoffier and the discovery of the taste sensation of "umami" is especially well done.

Proust Was a Neuroscientist by Jonah Lehrer

In this technology-driven age, it’s tempting to believe that science can solve every mystery. After all, science has cured countless diseases and even sent humans into space. But as Jonah Lehrer argues in this sparkling debut, science is not the only path to knowledge. In fact, when it comes to understanding the brain, art got there first.
Taking a group of artists — a painter, a poet, a chef, a composer, and a handful of novelists — Lehrer shows how each one discovered an essential truth about the mind that science is only now rediscovering. We learn, for example, how Proust first revealed the fallibility of memory; how George Eliot discovered the brain’s malleability; how the French chef Escoffier discovered umami (the fifth taste); how Cézanne worked out the subtleties of vision; and how Gertrude Stein exposed the deep structure of language — a full half-century before the work of Noam Chomsky and other linguists. It’s the ultimate tale of art trumping science.
More broadly, Lehrer shows that there’s a cost to reducing everything to atoms and acronyms and genes. Measurement is not the same as understanding, and art knows this better than science does. An ingenious blend of biography, criticism, and first-rate science writing, Proust Was a Neuroscientist urges science and art to listen more closely to each other, for willing minds can combine the best of both, to brilliant effect.