Robins spark a pigment of the imagination

Evidence suggests that radical-pair theory guides migratory birds on their way.

It is now widely accepted that migratory birds use the Earth’s geomagnetic field to guide them to their dream destination. However, experts still dispute how the birds actually pick up the field and there are currently two competing theories: one involving magnetite and the other radical-pair reactions. Now an international team of researchers led by Thorsten Ritz at the University of California, Irvine, US, claims to have found new evidence to back the radical-pair mechanism.

Researchers proposed the mechanism in the late 1970s, but it has been difficult to test in vivo. The theory is that a biochemical electron transfer reaction in the birds’ brains generates intermediate radical pairs in which external magnetic fields induce a change in the spin states. As Ritz explains, ’different spin states are chemically different, ie they react differently and lead to different products ... The intermediate radical pair acts as a chemical switch that can be triggered by an external magnetic field resulting in a change of reaction rates or product ratios’. When it comes to animals, ’the most intriguing possibility is that photopigments are involved in a radical-pair reaction. Thus, the magnetic field can have an indirect effect on photoreception’.

As part of the project, Wolfgang Wiltschko’s group at the J W Goethe-Universität, Frankfurt am Main, Germany, collected robins in a botanical garden. They kept the birds in cages for a few months under artificial local light conditions before gradually increasing the ratio of the number of hours of light to dark to spark a migratory response in the birds.

The robins flew through funnel-shaped PVC cages lined with correction fluid in which scratches appeared when the birds changed course. As the birds flew, the researchers applied an oscillating magnetic field. By analysing the scratches, they found that the birds exhibited standard migratory responses when a single frequency field was aligned parallel to the geomagnetic field, but that when the field was at a 24° or 48° angle, the birds became disorientated. The researchers claim that ’these results are consistent with a resonance effect on singlet-triplet transitions and suggest a magnetic compass based on a radical-pair mechanism’.

Ritz told Chemistry World that his long-term goal is to identify and locate the radical pair involved in magnetoreception. ’Now that we have shown what to look for - a pair of molecules presumably involved in photoreception that can undergo radical-pair reactions - we also expect that many other researchers will bring their expertise in,’ he said, noting the potential of biochemical and neurophysiological techniques. ’We expect this to be a very exciting time in magnetoreception and it could well be that in the not-too-distant future we may be investigating magnetic field effects on candidate molecules extracted from animals directly.’

Klaus Schulten, professor of physics at the University of Illinois at Urbana-Champaign, described the work as ’a definite milestone on the way towards explaining the magnetic sense ... Building on previous theoretical suggestions, the authors did a magnificent job in merging physical measurement and behavioural biology’.

Emma Davies