7.4 Birds may "see" magnetic fields

http://arstechnica.com/journals/science.ars/2007/10/02/birds-may-see-magnetic-fields

Birds may "see" magnetic fields

By John Timmer | Published: October 02, 2007 - 07:50AM CT

It's become clear over the last few decades that life, from birds to bacteria, can sense the orientation of the Earth's magnetic fields. Some of the basic proteins involved in performing this sensing have even been identified. But, in birds at least, merely recognizing the direction of the field is only a small part of the problem; the sensory input must be interpreted by the brain and integrated with other information to help guide the animal's migratory behavior. A new, Open Access publication in PLoS ONE builds on past data to help build the case that birds perform this sensory integration via a specialized adaptation of the visual system. In other words, they perceive magnetic fields using part of the visual system, so in a very real sense brids "see" the magnetic field.

The work takes advantage of the previous identification of a gene, called ZENK, that is expressed in neurons after they have been active. Using ZENK activity as a marker, a group of cells in the forebrain of birds—called "Cluster N"—was shown to be active when migratory birds performed magnetic orientation in the dark. Cluster N was located adjacent to an area of the forebrain that helps process visual information. Other research has shown that the proteins believed to detect magnetic changes are present in the retina of birds. What was lacking was a connection between the two.

The new work makes that connection. The researchers injected a green dye in Cluster N, and a red dye into the retina of birds that perform magnetic orientation. Those dyes diffused down the length of the axons that connect these structures, and wound up in the same place: a structure called the dorsolateral geniculate complex, which is involved in visual perception. Other experiments showed that the connections between these structures closely parallels the wiring of the visual system. The authors refer to it as involving "restricted subregions" of the visual pathway.

The authors conclude by saying, "our findings strongly support the hypothesis that migratory birds perceive the magnetic field as a visual pattern and that they are thus likely to 'see' the magnetic field." This seems to be how things typically happen in biology: rather than evolving something entirely new, natural selection tends to take advantage of an existing system, modifying it just enough to do something new. In this case, the ability to interpret spatial information that's present in the visual system makes a nice fit for interpreting the magnetic field.

PLoS ONE, 2007. DOI: 10.1371/journal.pone.0000937

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