More than 40 years ago, Wolfgang Wiltschko discovered that migrating birds use the Earth’s magnetic field for orientation. Further research has shown that the bird compass responds to the inclination of the magnetic field (i.e. its angle against the horizontal) rather than the north-south direction as such, and that it is likely based on chemical reactions involving molecules with unpaired electrons, i.e. radicals. This chemical compass is believed to reside in the eyes. In 2002, Wiltschko and coworkers have shown that the birds need the right eye but not the left eye for orientation – suggesting that the left hemisphere of the brain does the calculations on the magnetic measurements coming in from the right eye. Careful with the conclusion that the compass is located in the right eye only -- my guess would be that the left eye has the same compass but the right hemisphere of the brain just doesn’t know what to do with the signals.
Now Wiltschko’s group at the university of Frankfurt has reported that getting light into the right eye is necessary but not sufficient for the compass function (1). The right eye, the researchers find, actually has to be able to see properly. The experiment involves goggles that let through the same amount of light on both sides but blur the vision on one side. If the birds have a blurred view on the right eye, they lose the orientation, if it’s on the left, they orient just fine.
Wiltschko et al. discuss two possible explanations:
1) compass orientation may rely on computational connections between compass data and visual input. The case against this interpretation is that, in the 2002 experiment that first showed the dominance of the right eye in orientation, there was practically no visual input that could have served such a role. Still, the authors seem to prefer this version, as they go to great length to defend it.
2) the normal process of shutting down the input from the eye with the blurred vision may shut out the compass data as well. To me this is the more plausible interpretation (if only because it’s easier to understand).
Some fairly elementary questions remain, however, for instance, if one tests a reasonable number of birds, will there be a small population of “lefthanders” that has the compass processing on the other side ? Also, if a young bird lost the right eye in a fight, would the brain find a way to process compass data from the left eye? And lest we forget, the precise molecular details of the molecular compass also remain to be uncovered, see my pieces on the bird compass from last year, in Oxford Today (as a sidebar to the ESR feature) and, in German, in Chemie in unserer Zeit. A significant 2009 paper that I missed when I researched those articles comes from Stefan Weber's group at Freiburg and reports the observation that light can stimulate the formation of radical pairs in the most promising candidate molecules believed to host the chemical compass, namely cryptochromes (2). This adds to the growing body of evidence suggesting that the bird compass really is a radical pair mechanism residing in cryptochrome proteins.
1. K. Stapput et al., Curr. Biol. 2010, doi:10.1016/j.cub.2010.05.070
2. T. Biskup et al, Angew. Chem. Int. Ed. 2009, 48, 404.