Monday, August 3, 2009

The Elusive Sense of Magnetism

Pigeons were my constant companions in the early years of academic study. They descended on the university buildings in flocks, settling on the railings of the balconies that ran along the stories outside. They cooed and bantered outside, watching us study hard inside. At times, their habits were annoying. But I always liked to watch them take off in bunches, swaying from one side to the other and then lifting off up and away. The burghers of our city commonly considered pigeons a pest. Pest control was often called in to get rid of them.

However, this did not happen in our case. Our birds were homing pigeons. Wolfgang Wilschko, the most enthusiastic zoology professor I ever met, his wife and students were studying the birds' ability to use Earth's magnetic field for flight orientation. Other migratory birds have demonstrated this capability. However, how the magnetic sense actually works was little understood. The team transported marked birds to various locations in about a sixty mile radius in a battered, mouse-gray VW microbus on loan from the Deutsche Forschungsgemeinschaft, the German equivalent of the National Science Foundation. They recorded the time the pigeons needed to find back to their loft. Their travel time was compared to the geography they had to navigate, in particular whether magnetic field perturbations produced by intersecting power lines distracted them on their path, delaying their return.

Exposing the pigeons to artificial magnetic fields in the laboratory left them temporarily disoriented. Regardless, the birds commonly found their way back home. For this, they appeared to rely on the inclination, rather than the polarity, of geomagnetic field vectors. That is, the birds could discriminate between equatorial and polar directions of the magnetic field vectors for compass orientation and proved profoundly sensitive to disturbances in the field. Fine-grain local field aberrations may provide additional cues for the birds' navigation (Wilschko and Wilschko, 2005). Sensitivity to the geomagnetic field has been observed in a number of vertebrate animals other than birds (Lohmann and Johnsen, 2000). Begall and others (2008) showed that domestic cattle as well as free roaming deer align their bodies according to the magnetic poles.

Despite the formidable evidence for a magnetic sense, conclusive evidence for magnetoreceptor cells remains elusive to date. An early hypothesis posited that migrating birds could virtually see magnetic field polarity, detecting in the clear sky streaks of light of differing intensity polarized by the geomagnetic field.

Another hypothesis suggested that nerve cells containing minute deposits of biogenic magnetic minerals may detect magnetism. For example, the iron oxides magnetite (Fe3O4) and maghemite (Fe2O3) have been identified in trigeminal nerve fibers innervating the skin on pigeons' bills (Fleissner and others, 2007). In accord, preliminary evidence shows that trigeminal nerve fibers respond to changes in magnetic field strength of geomagnetic magnitude (Semm and Beason, 1990).

In a separate line of investigation, external magnetic fields of strengths comparable to those found in nature proved in principle capable of temporarily altering the concentration of short-lived pairs of radicals intermediate to photo-chemical reactions (Maeda and others, 2006). The interconversion of the pairs' spin states eventually determines the type of reaction product, the yield of which is proportionate to magnetic field strength and direction. Cryptochromes, that is blue light-sensitive flavoproteins, spatially-oriented in the photoreceptor cells of the avian retina are suggested as likely candidates to sustain such reaction (Rodgers and Hore, 2009). The magneto-sensitive process may modulate the birds' perception of light not unlike that originally proposed for magnetic field-related polarized light.

Taken together, much evidence suggests that migratory animals possess a keen sense for magnetic fields, using a "built-in" biological magnetic compass to orient themselves in their environments. Two fundamentally different molecular mechanisms are presently suggested to underlie this ability. One involves magnetic matter embedded in nerve cell fibers of the peripheral sense of touch. The other involves the formation of radical pairs in photoreceptors of the visual system. With either proposal, the precise mechanisms of stimulus reception, stimulus transduction and sensory pathway processing need yet to be uncovered.

  • The text of the post is available for download in pdf-format from the scribd store.
  • Perhaps this incident was caused by sudden strong local disturbances of the earth's magnetic field that the blackbirds use for navigation (01/05/11):
  • Putman and others (2011) provide behavioral evidence that also loggerhead turtle hatchlings use the Earth's magnetic field to determine latitude as much as longitude for navigation on their migrations. Listen to Joe Palca's segment with the title "For Turtles, Earth's Magnetism Is A Built-In GPS" broadcast today on National Public Radio's Morning Edition (03/02/11).
  • In a blow to the hypothesis that magnetic sensory receptor cells invest the avian beak, Treiber and others (2012) provide histological evidence that the previously implicated magnetite containing cells are iron-rich macrophages. Moreover, Wu and Dickman (2012) showed that nerve cell electrical spiking in the pigeon's vestibular brainstem known for processing information on balance and spatial orientation (equilibrioception) encodes magnetic field direction, intensity, and polarity, suggesting that magnetic receptor cells are located in the inner ear (05/31/2012).
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  1. I would love to sit and read your page, but the background is offensive to the eyes, and the font is definitely too narrow for this color, making it difficult to read

  2. I am sorry you could not read my post. The text printed in Times New Roman, 14, on white background is now available for download in pdf-format with hyperlinks on Scribd for a small fee (see link in footnote). I hope that helps, Peter.

  3. The design of the blog is really heavy on the eyes.