October 18, 1943 - allied forces were advancing through Nazi-held Italy.   In a desperate attempt to weaken German forces occupying the village of Calvi Veccia, British infantry called in for aerial bombardment.  In the last moments, British ground forces were able to capture the town, but were unable to establish radio contact to cancel the bombing.  In a panic, a message was tied to the leg of a carrier pigeon named G.I. Joe.  As soon as he was let loose from his cage, Joe shot into the air and bee-lined directly for HQ.  He flew 20 miles in 20 minutes, successfully delivering the note just as the bombers were taxiing on the runway, saving more than a thousand British and Italian lives.


G.I. Joe receiving his medal for gallantry.

Similar stories of avian navigational prowess are common, including the tale of the intrepid Cher Ami, a WWI carrier pigeon who saved a battalion of trapped soldiers.  Cher Ami flew 25 miles in 25 minutes, despite being shot by enemy soldiers (blinded in one eye, wounded in the left breast, with a left leg that was blown to pieces), delivering a critical request for reinforcements that saved almost 200 men.  These inadvertently brave birds were prized for their ability to rapidly return to their roost, even when they were taken many miles away from home in a small, dark cage. 

If all this sounds familiar, then you may have heard it first on Radiolab in their Bird’s-Eye View episode.  These accounts invoke a mysterious power held by our fine-feathered friends, who are able to voyage vast distances without the aid of maps or GPS.  This brings us to a recent suite of scientific discoveries involving avian extrasensory perception (hereafter referred to as magnetoreception). 


Magneto - master of magnetism.

The word magnetoreception conjures super-human mutant powers from X-Men comics, but it refers to the subtle ability of birds (and other animals, including sea turtles, salmon, and fruit flies) to perceive magnetic fields (Wiltschko and Wiltschko, 1996).  There are two competing theoretical mechanisms for how this sense functions: magnetite particles (tiny magnets) in the bird’s beak and light-induced chemical reactions in the eye (Edmonds, 1996; Ritz et al., 2000).  Ritz et al. (2000) proposed a viable mechanistic pathway for the latter hypothesis, by which the product yields of quantum chemical reactions are altered by weak magnetic fields (i.e. the Earth’s magnetic field).  This mechanism has been empirically supported by several recent studies (Solov'yov et al., 2007; Gegear et al., 2010; Gauger et al., 2011).

Cryptochrome, showing the flavin residue in red and the tryptophan residues in yellow.

In a nutshell, a light-sensitive protein called cryptochrome is thought to mediate bird magneto-vision via a magnetically-sensitive chemical reaction.  Cryptochrome contains a flavin (FAD) molecule that is excited by blue/green light.  Excited FAD passes an electron off to a chain of tryptophan residues, which carry the electron away, giving rise to a spatially separated quantum-entangled radical electron pair (turning the protein to its ‘on’, or signaling state).  As one electron experiences a slightly different magnetic field than its partner, the pair oscillates between singlet and triplet spin states via hyperfine coupling.  The back-transfer of the electron to the FADH radical (see below) returns it to its ‘off’ (non-signaling) state, but this process is spin-dependent (requiring a singlet state).


FAD light cycle.

Because the magnetic field influences the proportion of radical pairs that are in the singlet vs. triplet state, the field determines how long the molecule will be turned on or off.  As these proteins are embedded in the retina, they create a mosaic of long-lived ‘on’ cryptochromes and cryptochromes in their ‘off’ position (depending on how the field changes) across the surface of the eye.  Due to their presence in the retina, some have speculated (Ritz et al., 2000) that cryptochrome signaling influences photoreceptor cells.  If this is the case, then the bird might distinguish the geomagnetic field as patterns of light and dark, which would change depending on the angle of the bird’s view relative to the field lines (see below).

Quantum Bird Vision - A panorama of Frankfurt, Germany.  This figure shows the cryptochrome signal alone (grey-scale), and if it were layered over the visual signal (color).

Until recently, it was assumed that living organisms were too ‘warm and wet’ to allow for delicate and transitory quantum entanglements to persist for any length of time.  A paper published this year in Physical Review Letters (Gauger et al., 2011) demonstrated that the entangled electron pairs produced in avian magnetoreception are able to persist in an entangled state for up to 100 microseconds.  This is incredible, as experimental physicists find it difficult to maintain an entangled quantum state for more than 80 microseconds under cryogenic (very low-temperature) laboratory conditions.  Evolution uses all tools that the physical universe lays at its doorstep, so we shouldn’t be surprised to observe biological adaptations that make use of quantum-level phenomena.  If we could learn how to mimic the physiochemical conditions that shield these fragile quantum states over microsecond timescales at ambient temperatures, then we could make huge technological advances in the development of quantum technologies (e.g. quantum computation).

D-Wave - the first commercial quantum computer?

It is fascinating to ponder sensations that we are unable to feel.  For example, how does it feel when a rattlesnake sees a rodent with its infrared heat-sensors?  What is the sensation that a shark gets when it feels the electromagnetic signature of a fish swimming past?  Perhaps we can better identify with magnetoreception, because it seems to be closely linked to vision, although we’ll never quite know the exact impression that a robin gets from seeing the Earth’s rippling electromagnetic field, like light playing across an underwater landscape…or will we?  A recent article, published in Nature Communications (Foley et al., 2011), shows that human cryptochrome proteins (also located in the eye) are able to restore magnetoreception in Drosophila (fruit flies).  These little insects, like birds, are able to orient themselves relative to magnetic fields.  Foley et al. (2011) genetically altered their flies by removing the Drosophila cryptochrome gene, rendering them blind to magnetism.  Incredibly, when they inserted the human cryptochrome gene into these mutant flies, their magnetic sight was restored.  Humans, therefore, possess functional cryptochromes in their retinas, but we may have lost the capability to translate this sensory input into a downstream biological response.  These findings could lead us towards a sixth human sense!  Imagine how art, culture, and science would evolve in some future magnetoperceptive world.


Teenage Mutant Magnetoreceptive Turtles!

References

Edmonds, D. (1996) A sensitive optically detected magnetic compass for animals. Proceedings of the Royal Society B. 263:295-298.

Foley, L.E., Gegear, R.J., Reppert, S.M. (2011) Human cryptochrome exhibits light-dependent magnetosensitivity. Nature Communications. DOI: 10.1038/ncomms1364

Gauger, E.M., Rieper, E., Morton, J.J.L., Benjamin, S.C., Vedral, V. (2011) Sustained quantum coherence and entanglement in the avian compass. Physical Review Letters. 106(4):1-4.

Gegear, R., Foley, L.E., Casselman, A., Reppert, S.M. (2010) Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism. Nature. 463:804-808.

Ritz, T., Adem, S., Schulten, K. (2000) A model for photoreceptor-based magnetoreception in birds. Biophysical Journal. 78:707-718.

Solov’yov, I.A., Chandler, D.E., Schulten, K. (2007) Magnetic field effects in Arabidopsis thaliana cryptochrome-1. Biophysical Journal. 92:2711-2726.

Wiltschko, W., Wiltschko, R. (1996) Magnetic orientation in birds.  Journal of Experimental Biology. 199:29-38.