According to biologist Sönke Johnsen, understanding magnetoreception is `the last holy grail of sensory biology'. Despite years of experiments,scientists still aren't sure exactly how it works. There are two alternative theories for how animals, such as birds and turtles, detect magnetic fields. The first is that migrating animals have tiny particles of magnetite in their heads, which effectively act as `mini compasses' in response to the magnetic field. The second is that light-sensitive molecules in the eyes –photopigments – could play a role in detecting magnetic fields. As a vision biologist, Johnsen is interested in the second theory, so he teamed up with physicists Erin Mattern and Thorsten Ritz to find out if light absorption by certain molecules is associated with migratory birds being able to detect magnetic fields (p. 3171).

The team went back to the literature and collected the results from 62 experiments on light-dependent magnetoreception in songbirds that migrate at twilight, or at night. They grouped the experiments into categories depending on how the birds behaved: whether they oriented correctly in their migration direction, indicating that they were detecting a magnetic field, or incorrectly, indicating that they probably weren't.

To unravel light-dependent magnetoreception, they team needed to take into account the light entering the birds' eyes in each experiment, and what happens to it when it gets there. First they needed to calculate what the birds' photopigments are sensitive to. They calculated the quantum catch of 7 of the birds' photopigments; `this is how many photons of light are collected by a photopigment over a set amount of time,' Johnsen explains.

Having calculated how much light the birds' photopigments could pick up,the team then calculated which photopigments were stimulated in each of the experiments, which were carried out under different light conditions. They calculated the quantum catch of the light used in the experiments hitting the photopigments, and also calculated the opponency. This is where the stimulation of one photopigment is subtracted from the effect of another. They then related photopigment stimulation to the behavioural results, to see if there was a link between one photopigment, and a magnetoreceptive behaviour.

`We really wanted to find the smoking gun', says Johnsen. However, while the team's `smoking gun' remains elusive, for now, they did find that there were experimental situations that inhibited magnetoreception, where the birds oriented incorrectly. First they found that experiments with bright light levels, with a high quantum catch, inhibited magnetoreception. The boundary where this effect stopped was at light levels very similar to sunset. They also found that there was inhibition of orienting behaviour where there was long wavelength (reddish) light present, with the cut-off for this effect in the yellow/green part of the spectrum. So for these birds to detect magnetic fields, the results suggest that `it needs to be blue, and dim', says Johnsen. The photopigments that might be causing this effect are a long wavelength red cone, and the pigment semiquinone, which is a breakdown product of another photopigment, cryptochrome.

The results also highlighted that there are big gaps in the light spectrum that haven't been investigated yet, which will help others design future experiments, but Johnsen explains that more research is needed to understand if light levels really are influencing magnetoreception, or if this effect is due to the birds' motivation to migrate.

Johnsen, S., Mattern, E. and Ritz, T. (
). Light-dependent magnetoreception: quantum catches and opponency mechanisms of possible photosensitive molecules.
J. Exp. Biol.