Volume 27, Issue 3 - Special Issue: Magnetoreception and navigation in vertebrates from quantum mechanics to neuroscience and behaviour; Guest Editor: Henrik Mouritsen

Every year, billions of animals leave their home range and start seasonal migrations in order to find more favorable resources and to escape harsh environmental conditions. These round trips often span thousands of kilometers. To successfully navigate along their route, animals rely on various external references. While landmarks and celestial cues like stars or the sun are easy to imagine as guidance on these journeys, using the geomagnetic field for orientation is more elusive. The geomagnetic field is an omnipresent cue, which can be sensed and relied upon by many animals, even when visual cues are sparse. How magnetic fields can be perceived seems to vary between birds and fish. While birds seem to use a mechanism based on the quantum mechanical properties of electron spins, fish may have evolved a compass similar in its function to the technical devises developed by humans. How these mechanisms work precisely and how they are integrated are research questions addressed in SFB 1372.

The remarkable ability of migratory birds to navigate accurately using the geomagnetic field for journeys of thousands of kilometres is currently thought to arise from radical pair reactions inside a protein called cryptochrome. In this article, we explain the quantum mechanical basis of the radical pair mechanism and why it is currently the dominant theory of compass magnetoreception. We also provide a brief account of two important computational simulation techniques that are used to study the mechanism in cryptochrome: spin dynamics and molecular dynamics. At the end, we provide an overview of current research on quantum mechanical processes in avian cryptochromes and the computational models for describing them.

A class of light-activated proteins in the eyes of birds, called cryptochromes, are thought to act as the primary magnetic sensors allowing night-migratory songbirds to navigate over thousands of kilometers using the earth’s magnetic field. Having evolved from DNA-repairing photolyases, cryptochromes have redirected the energy from light to fuel a variety of other functions: as photoreceptors, as regulators of the circadian clock – and, in some species, most likely as sensors of the magnetic field. While the quantum effects of magnetic fields on cryptochromes are already being studied in detail, almost nothing is known about the signaling cascade involving cryptochrome as the primary receptor protein. Two different screening methods have identified potential interaction partners that suggest an involvement of the visual phototransduction pathway, the visual cycle, potassium channels or glutamate receptors, but more pioneering research is needed to unravel the signaling cascade responsible for transducing the magnetic signal.

Night-migratory birds use the Earth’s magnetic field to determine the direction in which they want to migrate. Many studies suggest that this “magnetic compass sense” is light dependent and mediated by blue light sensors, called cryptochromes, which are expressed in the retina of night-migratory birds. In this review, we summarize the evidence that the avian retina processes not only visual information but also magnetic compass information. We also review the current knowledge on cryptochrome expression in the bird retina and highlight open questions which we aim to address within the framework of SFB 1372 Magnetoreception and Navigation in Vertebrates .

In addition to other natural orientation cues such as the stars, the sun, landmarks and olfactory cues, migrating birds possess the ability to orient by the Earth’s magnetic field. In recent years, neuroscientific research has pinpointed brain regions and connecting neuronal pathways that seem to be involved in processing magnetic information. To date, the most compelling neuroanatomical and behavioural evidence comes from the visual and trigeminal sensory systems. We expect that navigational information from both systems could be integrated in higher-order brain structures, such as the hippocampus and the “decision-making” caudolateral nidopallium. This review summarizes the current state of research on the neurosensory basis of magnetoreception in birds.