Recent studies have confirmed that migratory birds rely on their eyes to "see" the earth's magnetic field, giving them, in effect, a sixth sense that aids them in their migratory journeys. Swanlund Professor of Physics Klaus Schulten has spent many years exploring the biochemical mysteries of bird migration. His work has led to many of the current advances in understanding bird navigation systems. He was interviewed by News Bureau life sciences editor Diana Yates.
What is it about the earth's magnetic field that is useful to birds?
We humans cannot detect the earth's magnetic field without a compass, but birds can orient themselves to it and use it to help them migrate. Researchers had struggled, however, to find a biochemical compass in birds and other animals.
Interestingly, birds cannot differentiate between north and south, but have only an axial compass that "sees" the orientation of the earth's magnetic field, not only horizontally but also vertically.
In the northern hemisphere, the earth's magnetic field points into the ground. The birds know that where the field lines go down, it's north and where the field lines go up, it's south. If you change the vertical angle of the magnetic field in the laboratory, the birds go in the opposite direction.
What drew your research team to this problem?
We had discovered that magnetic fields can influence chemical reactions. Prior to our work, this was thought impossible because magnetic fields interact so weakly with molecules. But we found that a magnetic field can influence a chemical reaction if the reaction proceeds quickly enough to be governed by pure quantum mechanics.
An entire research field came from this discovery, called spin chemistry because the reactions involve electron transfer that produces freely tumbling spins of electrons. The spins behave like an axial compass.
So magnetic fields can influence chemistry in living organisms?
Yes. Electron transfer reactions are actually quite common in our bodies. Not all of them are candidates for magnetic field effects, but a few are.
It is natural to assume that these reactions occur in an animal that can sense a magnetic field. So we suggested that maybe the birds' compass is actually a biochemical electron transfer reaction.
Your lab was the first to suggest that cryptochrome, a protein found in the eyes of birds and other animals, might be a key to understanding how birds sense the earth's magnetic field.
Our magnetic field dependent studies had relied on the use of lasers. Once, after I had given a lecture at Harvard, the Nobel Laureate Dudley Herschbach asked me where in the bird's head is the laser. I couldn't answer that question right away but later I realized that perhaps a bird's magnetic sense was light-dependent, and occurring in the eye. I communicated this to the ornithologist Wolfgang Witschko, who worked on animal navigation. He and his colleagues altered the ambient light of navigating birds, and to their amazement saw an effect. If you filter out the blue light in the ambient light, then the birds can no longer sense the magnetic field.
But one piece of the puzzle was still missing: What was the molecular basis for this reaction in the eye? In 1999, scientists discovered that cryptochrome, a protein that plays a part in regulating the circadian rhythm, was present in the eyes of some animals. I quickly realized that cryptochromes involve electron transfer for their function and in 2000 we suggested that cryptochromes in the eyes of birds sense the earth's magnetic fields. Many experiments since then have reinforced this notion. The compass in birds seems to be finally found.
Can birds use the geomagnetic field at night?
Many migratory birds fly at night. It is always thought that they have no light at night, but that's not true. There is light there, and apparently even very dim light is sufficient for the birds to orient.
