Magnetic Sense of Animals
Chemical Compass Model



Ritz Group
Theoretical and Computational Biophysics: 
Understanding the Molecular Design of Life
UC Irvine UC Irvine
Department of Physics and Astronomy
Thorsten Ritz Research Publications

The introductory figure summarizes our model for a photoreceptor-based magnetic compass in birds. The geomagnetic field can affect radical-pair reaction yields as governed by the stochastic Liouville equation depicted in the thinking bubble of the bird. These effects result in a modulation of visual information as shown in the right figure. The changes of visual modulation patterns with different orientations of the bird can explain the magnetic compass orientation observed in behavioral experiments.

The Physics Problem

In understanding the mechanism of magnetoreception, one is immediately faced with the puzzle that the geomagnetic field is very weak (ca. 50 microTesla). Any suggestion for a magnetoreceptor mechanism needs to address the question whether a field as weak as the geomagnetic field can be detected by the proposed mechanism under conditions as can be found in animals. Of the many mechanisms proposed only two have been supported by theoretical analysis and experimental evidence, namely the use of ferromagnetic material (magnetite) and the influence of magnetic fields on chemical reactions .

Chemical reactions that involve transitions between different spin states can be influenced by magnetic fields, so that one of the possible products is favored due to the influence of the magnetic field. Usually, magnetic fields much stronger than the geomagnetic field are necessary to see a significant change in products. For a particular type of chemical reaction,  radical-pair reactions, 50 microTesla (geomagnetic field strength) magnetic fields produce a small, but measurable change in product ratios. A radical pair reaction is initiated by absorption of light through a photopigment and a subsequent electron transfer to a nearby acceptor molecule. After the electron transfer, donor and acceptor each have one unpaired electron, which possess a magnetic moment. An external magnetic field can change the spin state of the radical pair system and thus alter its chemistry. In other words, the radical pair system acts as a switch that can be triggered by an external magnetic field. 

A Possible Realization

Provided that radical-pair reactions form the basis of magnetoreception in animals, how could a magnetosensory organ look like? Three components are necessary: 1. An ordered and fixed system of radical pairs with anisotropic hyperfine couplings, 2. A photoreceptor that is part of the radical pair and whose signals to the neural system are modulated through the magnetic field effects on the radical pairs. 3. A mechanism by which the nervous system of an animal can identify whether a change in signal is due to an increase in ambient light levels or due to magnetic field effects.

In a simple radical-pair model we have calculated through the use of quantum physics how the response of  a photoreceptor is changed depending on the angle of the magnetic field lines with the radical-pair system. Assuming that the radical-pair/photoreceptor system is distributed equally over the whole eye, one can evaluate how vision of an animal, e.g., a bird, is modulated. To the left, the visual modulation patterns are shown for a bird flying parallel to the horizon (geomagnetic field lines at an angle of 68 degrees with the horizon) and looking towards N,NE,E,SE,S,SW,W, and NW. The patterns are projected into the plane.