Light-dependent magnetoreception in birds: the behaviour of European robins,Erithacus rubecula, under monochromatic light of various wavelengths and intensities

Activation of Cryptochrome 4 from Atlantic Herring

Marine fish migrate long distances up to hundreds or even thousands of kilometers for various reasons that include seasonal dependencies, feeding, or reproduction. The ability to perceive the geomagnetic field, called magnetoreception, is one of the many mechanisms allowing some fish to navigate reliably in the aquatic realm. While it is believed that the photoreceptor protein cryptochrome 4 (Cry4) is the key component for the radical pair-based magnetoreception mechanism in night migratory songbirds, the Cry4 mechanism in fish is still largely unexplored. The present study aims to investigate properties of the fish Cry4 protein in order to understand the potential involvement in a radical pair-based magnetoreception. Specifically, a computationally reconstructed atomistic model of Cry4 from the Atlantic herring (Clupea harengus) was studied employing classical molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) methods to investigate internal electron transfers and the radical pair formation. The QM/MM simulations reveal that electron transfers occur similarly to those found experimentally and computationally in Cry4 from European robin (Erithacus rubecula). It is therefore plausible that the investigated Atlantic herring Cry4 has the physical and chemical properties to form radical pairs that in turn could provide fish with a radical pair-based magnetic field compass sensor.

Interactions between electromagnetic radiation and biological systems

Even though the bioeffects of electromagnetic radiation (EMR) have been extensively investigated during the past several decades, our understandings of the bioeffects of EMR and the mechanisms of the interactions between the biological systems and the EMRs are still far from satisfactory. In this article, we introduce and summarize the consensus, controversy, limitations, and unsolved issues. The published works have investigated the EMR effects on different biological systems including humans, animals, cells, and biochemical reactions. Alternative methodologies also include dielectric spectroscopy, detection of bioelectromagnetic emissions, and theoretical predictions. In many studies, the thermal effects of the EMR are not properly controlled or considered. The frequency of the EMR investigated is limited to the commonly used bands, particularly the frequencies of the power line and the wireless communications; far fewer studies were performed for other EMR frequencies. In addition, the bioeffects of the complex EM environment were rarely discussed. In summary, our understanding of the bioeffects of the EMR is quite restrictive and further investigations are needed to answer the unsolved questions.

Migratory birds are able to choose the appropriate migratory direction under dim yellow narrowband light

Currently, it is generally assumed that migratory birds are oriented in the appropriate migratory direction under UV, blue and green light (short-wavelength) and are unable to use their magnetic compass in total darkness and under yellow and red light (long-wavelength). However, it has also been suggested that the magnetic compass has two sensitivity peaks: in the short and long wavelengths, but with different intensities. In this project, we aimed to study the orientation of long-distance migrants, pied flycatchers (Ficedula hypoleuca), under different narrowband light conditions during autumn and spring migrations. The birds were tested in the natural magnetic field (NMF) and a changed magnetic field (CMF) rotated counterclockwise by 120° under dim green (autumn) and yellow (spring and autumn) light, which are on the 'threshold' between the short-wavelength and long-wavelength light. We showed that pied flycatchers (i) were completely disoriented under green light both in the NMF and CMF but (ii) showed the migratory direction in the NMF and the appropriate response to CMF under yellow light. Our data contradict the results of previous experiments under narrowband green and yellow light and raise doubts about the existence of only short-wavelength magnetoreception. The parameters of natural light change dramatically in spectral composition and intensity after local sunset, and the avian magnetic compass should be adapted to function properly under such constantly changing light conditions.

Effects of low-level RF fields reveal complex pattern of magnetic input to the avian magnetic compass

The avian magnetic compass can be disrupted by weak narrow-band and broadband radio-frequency (RF) fields in the lower MHz range. However, it is unclear whether disruption of the magnetic compass results from the elimination of the perception pattern produced by the magnetic field or from qualitative changes that make the pattern unrecognizable. We show that zebra finches trained in a 4-arm maze to orient relative to the magnetic field are disoriented when tested in the presence of low-level (~ 10 nT) Larmor-frequency RF fields. However, they are able to orient when tested in such RF fields if trained under this condition, indicating that the RF field alters, but does not eliminate, the magnetic input. Larmor-frequency RF fields of higher intensities, with or without harmonics, dramatically alter the magnetic compass response. In contrast, exposure to broadband RF fields in training, in testing, or in both training and testing eliminates magnetic compass information. These findings demonstrate that low-level RF fields at intensities found in many laboratory and field experiments may have very different effects on the perception of the magnetic field in birds, depending on the type and intensity of the RF field, and the birds' familiarity with the RF-generated pattern.

On the anisotropic weak magnetic field effect in radical-pair reactions

For more than 60 years, scientists have been fascinated by the fact that magnetic fields even weaker than internal hyperfine fields can markedly affect spin-selective radical-pair reactions. This weak magnetic field effect has been found to arise from the removal of degeneracies in the zero-field spin Hamiltonian. Here, I investigated the anisotropic effect of a weak magnetic field on a model radical pair with an axially symmetric hyperfine interaction. I found that S-T± and T0-T± interconversions driven by the smaller x and y-components of the hyperfine interaction can be hindered or enhanced by a weak external magnetic field, depending on its direction. Additional isotropically hyperfine-coupled nuclear spins preserve this conclusion, although the S → T± and T0 → T± transitions become asymmetric. These results are supported by simulating reaction yields of a more biologically plausible, flavin-based radical pair.

Quantum-mechanical insights into the anisotropic response of the cryptochrome radical pair to a weak magnetic field

Cryptochrome photoreceptors contain a photochemically generated radical pair, which is thought to mediate sensing of the geomagnetic field direction in many living organisms. To gain insight into the response of the cryptochrome to a weak magnetic field, we have studied the quantum-mechanical hyperfine spin states of the radical pair. We identify quantum states responsible for the precise detection of the magnetic field direction, taking into account the strongly axial hyperfine interactions of each radical in the radical pair. The contribution of these states to the formation of the cryptochrome signaling state sharply increases when the magnetic field becomes orthogonal to the hyperfine axis of either radical. Due to such a response, the radical pair may be able to detect the particular field direction normal to the plane containing the hyperfine axes of the radicals.

The amphibian magnetic sense(s)

Sensitivity to the earth's magnetic field is the least understood of the major sensory systems, despite being virtually ubiquitous in animals and of widespread interest to investigators in a wide range of fields from behavioral ecology to quantum physics. Although research on the use of magnetic cues by migratory birds, fish, and sea turtles is more widely known, much of our current understanding of the functional properties of vertebrate magnetoreception has come from research on amphibians. Studies of amphibians established the presence of a light-dependent magnetic compass, a second non-light-dependent mechanism involving particles of magnetite and/or maghemite, and an interaction between these two magnetoreception mechanisms that underlies the "map" component of homing. Simulated magnetic displacement experiments demonstrated the use of a high-resolution magnetic map for short-range homing to breeding ponds requiring a sampling strategy to detect weak spatial gradients in the magnetic field despite daily temporal variation at least an order of magnitude greater. Overall, reliance on a magnetic map for short-range homing places greater demands on the underlying sensory detection, processing, and memory mechanisms than comparable mechanisms used by long-distance migrants. Moreover, unlike sea turtles and migratory birds, amphibians are exceptionally well suited to serve as model organisms in which to characterize the molecular and biophysical mechanisms underlying the light-dependent 'quantum compass'.

Myths in magnetosensation

The ability to detect magnetic fields is a sensory modality that is used by many animals to navigate. While first postulated in the 1800s, for decades, it was considered a biological myth. A series of elegant behavioral experiments in the 1960s and 1970s showed conclusively that the sense is real; however, the underlying mechanism(s) remained unresolved. Consequently, this has given rise to a series of beliefs that are critically analyzed in this manuscript. We address six assertions: (1) Magnetoreception does not exist; (2) It has to be magnetite; (3) Birds have a conserved six loci magnetic sense system in their upper beak; (4) It has to be cryptochrome; (5) MagR is a protein biocompass; and (6) The electromagnetic induction hypothesis is dead. In advancing counter-arguments for these beliefs, we hope to stimulate debate, new ideas, and the design of well-controlled experiments that can aid our understanding of this fascinating biological phenomenon.

Why is it so difficult to study magnetic compass orientation in murine rodents?

A magnetic compass sense has been demonstrated in all major classes of vertebrates, as well as in many invertebrates. In mammals, controlled laboratory studies of mice have provided evidence for a robust magnetic compass that is comparable to, or exceeds, the performance of that in other animals. Nevertheless, the vast majority of laboratory studies of spatial behavior and cognition in murine rodents have failed to produce evidence of sensitivity to magnetic cues. Given the central role that a magnetic compass sense plays in the spatial ecology and cognition of non-mammalian vertebrates, and the potential utility that a global/universal reference frame derived from the magnetic field would have in mammals, the question of why responses to magnetic cues have been so difficult to demonstrate reliably is of considerable importance. In this paper, we review evidence that the magnetic compass of murine rodents shares a number of properties with light-dependent compasses in a wide variety of other animals generally believed to be mediated by a radical pair mechanism (RPM) or related quantum process. Consistent with the RPM, we summarize both published and previously unpublished findings suggesting that the murine rodent compass is sensitive to low-level radio frequency (RF) fields. Finally, we argue that the presence of anthropogenic RF fields in laboratory settings, may be an important source of variability in responses of murine rodents to magnetic cues.

A magnet attached to the forehead disrupts magnetic compass orientation in a migratory songbird

For studies on magnetic compass orientation and navigation performance in small bird species, controlled experiments with orientation cages inside an electromagnetic coil system are the most prominent methodological paradigm. These are, however, not applicable when studying larger bird species and/or orientation behaviour during free flight. For this, researchers have followed a very different approach, attaching small magnets to birds, with the intention of depriving them of access to meaningful magnetic information. Unfortunately, results from studies using this approach appear rather inconsistent. As these are based on experiments with birds under free-flight conditions, which usually do not allow exclusion of other potential orientation cues, an assessment of the overall efficacy of this approach is difficult to conduct. Here, we directly tested the efficacy of small magnets for temporarily disrupting magnetic compass orientation in small migratory songbirds using orientation cages under controlled experimental conditions. We found that birds which have access to the Earth's magnetic field as their sole orientation cue show a general orientation towards their seasonally appropriate migratory direction. When carrying magnets on their forehead under these conditions, the same birds become disoriented. However, under changed conditions that allow birds access to other (i.e. celestial) orientation cues, any disruptive effect of the magnets they carry appears obscured. Our results provide clear evidence for the efficacy of the magnet approach for temporarily disrupting magnetic compass orientation in birds, but also reveal its limitations for application in experiments under free-flight conditions.

Nuclear polarization effects in cryptochrome-based magnetoreception

The mechanism of the magnetic compass sense of migratory songbirds is thought to involve magnetically sensitive chemical reactions of light-induced radical pairs in cryptochrome proteins located in the birds' eyes. However, it is not yet clear whether this mechanism would be sensitive enough to form the basis of a viable compass. In the present work, we report spin dynamics simulations of models of cryptochrome-based radical pairs to assess whether accumulation of nuclear spin polarization in multiple photocycles could lead to significant enhancements in the sensitivity with which the proteins respond to the direction of the geomagnetic field. Although buildup of nuclear polarization appears to offer sensitivity advantages in the more idealized model systems studied, we find that these enhancements do not carry over to conditions that more closely resemble the situation thought to exist in vivo. On the basis of these simulations, we conclude that buildup of nuclear polarization seems unlikely to be a source of significant improvements in the performance of cryptochrome-based radical pair magnetoreceptors.

Neuronal circuits and the magnetic sense: central questions

Magnetoreception is the ability to sense the Earth's magnetic field, which is used for orientation and navigation. Behavioural experiments have shown that it is employed by many species across all vertebrate classes; however, our understanding of how magnetic information is processed and integrated within the central nervous system is limited. In this Commentary, we review the progress in birds and rodents, highlighting the role of the vestibular and trigeminal systems as well as that of the hippocampus. We reflect on the strengths and weaknesses of the methodologies currently at our disposal, the utility of emerging technologies and identify questions that we feel are critical for the advancement of the field. We expect that magnetic circuits are likely to share anatomical motifs with other senses, which culminates in the formation of spatial maps in telencephalic areas of the brain. Specifically, we predict the existence of spatial cells that encode defined components of the Earth's magnetic field.

Rapid Oxidation Following Photoreduction in the Avian Cryptochrome4 Photocycle

Avian magnetoreception is assumed to occur in the retina. Although its molecular mechanism is unclear, magnetic field-dependent formation and the stability of radical-containing photointermediate(s) are suggested to play key roles in a hypothesis called the radical pair mechanism. Chicken cryptochrome4 (cCRY4) has been identified as a candidate magnetoreceptive molecule due to its expression in the retina and its ability to form stable flavin neutral radicals (FADH^●) upon blue light absorption. Herein, we used millisecond flash photolysis to investigate the cCRY4 photocycle, in both the presence and absence of dithiothreitol (DTT); detecting the anion radical form of FAD (FAD^●-) under both conditions. Using spectral data obtained during flash photolysis and UV-visible photospectroscopy, we estimated the absolute absorbance spectra of the photointermediates, thus allowing us to decompose each spectrum into its individual components. Notably, in the absence of DTT, approximately 37% and 63% of FAD^●- was oxidized to FAD_OX and protonated to form FADH^●, respectively. Singular value decomposition analysis suggested the presence of two FAD^●- molecular species, each of which was destined to be oxidized to FAD_OX or protonated to FADH^●. A tyrosine neutral radical was also detected; however, it likely decayed concomitantly with the oxidation of FAD^●-. On the basis of these results, we considered the occurrence of bifurcation prior to FAD^●- generation, or during FAD^●- oxidization, and discussed the potential role played by the tyrosine radical in the radical pair mechanism.

The biophysical, molecular, and anatomical landscape of pigeon CRY4: A candidate light-based quantal magnetosensor

The biophysical and molecular mechanisms that enable animals to detect magnetic fields are unknown. It has been proposed that birds have a light-dependent magnetic compass that relies on the formation of radical pairs within cryptochrome molecules. Using spectroscopic methods, we show that pigeon cryptochrome clCRY4 is photoreduced efficiently and forms long-lived spin-correlated radical pairs via a tetrad of tryptophan residues. We report that clCRY4 is broadly and stably expressed within the retina but enriched at synapses in the outer plexiform layer in a repetitive manner. A proteomic survey for retinal-specific clCRY4 interactors identified molecules that are involved in receptor signaling, including glutamate receptor-interacting protein 2, which colocalizes with clCRY4. Our data support a model whereby clCRY4 acts as an ultraviolet-blue photoreceptor and/or a light-dependent magnetosensor by modulating glutamatergic synapses between horizontal cells and cones.

How quantum is radical pair magnetoreception?

Semiclassical methods cannot accurately simulate magnetic field effects relevant to avian magnetoreception, which may therefore deserve the label “quantum biology”.

The rotating magnetocaloric effect as a potential mechanism for natural magnetic senses

Many animals are able to sense the earth’s magnetic field, including varieties of arthropods and members of all major vertebrate groups. While the existence of this magnetic sense is widely accepted, the mechanism of action remains unknown. Building from recent work on synthetic magnetoreceptors, we propose a new model for natural magnetosensation based on the rotating magnetocaloric effect (RME), which predicts that heat generated by magnetic nanoparticles may allow animals to detect features of the earth’s magnetic field. Using this model, we identify the conditions for the RME to produce physiological signals in response to the earth’s magnetic field and suggest experiments to distinguish between candidate mechanisms of magnetoreception.

Chemical compass behaviour at microtesla magnetic fields strengthens the radical pair hypothesis of avian magnetoreception

The fact that many animals, including migratory birds, use the Earth's magnetic field for orientation and compass-navigation is fascinating and puzzling in equal measure. The physical origin of these phenomena has not yet been fully understood, but arguably the most likely hypothesis is based on the radical pair mechanism (RPM). Whilst the theoretical framework of the RPM is well-established, most experimental investigations have been conducted at fields several orders of magnitude stronger than the Earth's. Here we use transient absorption spectroscopy to demonstrate a pronounced orientation-dependence of the magnetic field response of a molecular triad system in the field region relevant to avian magnetoreception. The chemical compass response exhibits the properties of an inclination compass as found in migratory birds. The results underline the feasibility of a radical pair based avian compass and also provide further guidelines for the design and operation of exploitable chemical compass systems.

A light-dependent magnetoreception mechanism insensitive to light intensity and polarization

Billions of migratory birds navigate thousands of kilometres every year aided by a magnetic compass sense, the biophysical mechanism of which is unclear. One leading hypothesis is that absorption of light by specialized photoreceptors in the retina produces short-lived chemical intermediates known as radical pairs whose chemistry is sensitive to tiny magnetic interactions. A potentially serious but largely ignored obstacle to this theory is how directional information derived from the Earth's magnetic field can be separated from the much stronger variations in the intensity and polarization of the incident light. Here we propose a simple solution in which these extraneous effects are cancelled by taking the ratio of the signals from two neighbouring populations of magnetoreceptors. Geometric and biological arguments are used to derive a set of conditions that make this possible. We argue that one likely location of the magnetoreceptor molecules would be in association with ordered opsin dimers in the membrane discs of the outer segments of double-cone photoreceptor cells.

Disruption of Magnetic Compass Orientation in Migratory Birds by Radiofrequency Electromagnetic Fields

The radical-pair mechanism has been put forward as the basis of the magnetic compass sense of migratory birds. Some of the strongest supporting evidence has come from behavioral experiments in which birds exposed to weak time-dependent magnetic fields lose their ability to orient in the geomagnetic field. However, conflicting results and skepticism about the requirement for abnormally long quantum coherence lifetimes have cast a shroud of uncertainty over these potentially pivotal studies. Using a recently developed computational approach, we explore the effects of various radiofrequency magnetic fields on biologically plausible radicals within the theoretical framework of radical-pair magnetoreception. We conclude that the current model of radical-pair magnetoreception is unable to explain the findings of the reported behavioral experiments. Assuming that an unknown mechanism amplifies the predicted effects, we suggest experimental conditions that have the potential to distinguish convincingly between the two distinct families of radical pairs currently postulated as magnetic compass sensors. We end by making recommendations for experimental protocols that we hope will increase the chance that future experiments can be independently replicated.

Magnetoreception-A sense without a receptor

Evolution has equipped life on our planet with an array of extraordinary senses, but perhaps the least understood is magnetoreception. Despite compelling behavioral evidence that this sense exists, the cells, molecules, and mechanisms that mediate sensory transduction remain unknown. So how could animals detect magnetic fields? We introduce and discuss 3 concepts that attempt to address this question: (1) a mechanically sensitive magnetite-based magnetoreceptor, (2) a light-sensitive chemical-based mechanism, and (3) electromagnetic induction within accessory structures. In discussing the merits and issues with each of these ideas, we draw on existing precepts in sensory biology. We argue that solving this scientific mystery will require the development of new genetic tools in magnetosensitive species, coupled with an interdisciplinary approach that bridges physics, behavior, anatomy, physiology, molecular biology, and genetics.

Magnetic field effect in natural cryptochrome explored with model compound

Many animals sense the Earth's magnetic-field and use it for navigation. It is proposed that a light-dependent quantum effect in cryptochrome proteins, residing in the retina, allows for such an iron-free spin-chemical compass. The photochemical processes, spin-dynamics and its magnetic field dependence in natural cryptochrome are not fully understood by the in vivo and in vitro studies. For a deeper insight into these biophysical mechanisms in cryptochrome, we had introduced a flavin-tryptophan dyad (F10T). Here we present the magnetic field dependence of 1H photo-CIDNP NMR on F10T and a theoretical model for low-field photo-CIDNP of F10T. This model provides mixing mechanism of energy-levels and spin-dynamics at low magnetic fields. Photo-CIDNP has been observed even at Earth's magnetic field (~0.05 mT). These experiments prove F10T to be an excellent model compound establishing the key mechanism of avian-magnetoreception and provide insight into the optimal behaviour of cryptochrome at Earth's magnetic field.

Radical-pair-based magnetoreception in birds: radio-frequency experiments and the role of cryptochrome

The radical-pair hypothesis of magnetoreception has gained a lot of momentum, since the flavoprotein cryptochrome was postulated as a structural candidate to host magnetically sensitive chemical reactions. Here, we first discuss behavioral tests using radio-frequency magnetic fields (0.1-10 MHz) to specifically disturb a radical-pair-based avian magnetic compass sense. While disorienting effects of broadband RF magnetic fields have been replicated independently in two competing labs, the effects of monochromatic RF magnetic fields administered at the electronic Larmor frequency (~1.3 MHz) are disparate. We give technical recommendations for future RF experiments. We then focus on two candidate magnetoreceptor proteins in birds, Cry1a and Cry1b, two splice variants of the same gene (Cry1). Immunohistochemical studies have identified Cry1a in the outer segments of the ultraviolet/violet-sensitive cone photoreceptors and Cry1b in the cytosol of retinal ganglion cells. The identification of the host neurons of these cryptochromes and their subcellular expression patterns presents an important advance, but much work lies ahead to gain some functional understanding. In particular, interaction partners of cryptochrome Cry1a and Cry1b remain to be identified. A candidate partner for Cry4 was previously suggested, but awaits independent replication.

Zebra finches have a light-dependent magnetic compass similar to migratory birds

Birds have a light-dependent magnetic compass that provides information about the spatial alignment of the geomagnetic field. It is proposed to be located in the avian retina and mediated by a light-induced, radical-pair mechanism involving cryptochromes as sensory receptor molecules. To investigate how the behavioural responses of birds under different light spectra match with cryptochromes as the primary magnetoreceptor, we examined the spectral properties of the magnetic compass in zebra finches. We trained birds to relocate a food reward in a spatial orientation task using magnetic compass cues. The birds were well oriented along the trained magnetic compass axis when trained and tested under low-irradiance 521 nm green light. In the presence of a 1.4 MHz radio-frequency electromagnetic (RF)-field, the birds were disoriented, which supports the involvement of radical-pair reactions in the primary magnetoreception process. Birds trained and tested under 638 nm red light showed a weak tendency to orient ∼45 deg clockwise of the trained magnetic direction. Under low-irradiance 460 nm blue light, they tended to orient along the trained magnetic compass axis, but were disoriented under higher irradiance light. Zebra finches trained and tested under high-irradiance 430 nm indigo light were well oriented along the trained magnetic compass axis, but disoriented in the presence of a RF-field. We conclude that magnetic compass responses of zebra finches are similar to those observed in nocturnally migrating birds and agree with cryptochromes as the primary magnetoreceptor, suggesting that light-dependent, radical-pair-mediated magnetoreception is a common property for all birds, including non-migratory species.

Localisation of the Putative Magnetoreceptive Protein Cryptochrome 1b in the Retinae of Migratory Birds and Homing Pigeons

Cryptochromes are ubiquitously expressed in various animal tissues including the retina. Some cryptochromes are involved in regulating circadian activity. Cryptochrome proteins have also been suggested to mediate the primary mechanism in light-dependent magnetic compass orientation in birds. Cryptochrome 1b (Cry1b) exhibits a unique carboxy terminus exclusively found in birds so far, which might be indicative for a specialised function. Cryptochrome 1a (Cry1a) is so far the only cryptochrome protein that has been localised to specific cell types within the retina of migratory birds. Here we show that Cry1b, an alternative splice variant of Cry1a, is also expressed in the retina of migratory birds, but it is primarily located in other cell types than Cry1a. This could suggest different functions for the two splice products. Using diagnostic bird-specific antibodies (that allow for a precise discrimination between both proteins), we show that Cry1b protein is found in the retinae of migratory European robins (Erithacus rubecula), migratory Northern Wheatears (Oenanthe oenanthe) and pigeons (Columba livia). In all three species, retinal Cry1b is localised in cell types which have been discussed as potentially well suited locations for magnetoreception: Cry1b is observed in the cytosol of ganglion cells, displaced ganglion cells, and in photoreceptor inner segments. The cytosolic rather than nucleic location of Cry1b in the retina reported here speaks against a circadian clock regulatory function of Cry1b and it allows for the possible involvement of Cry1b in a radical-pair-based magnetoreception mechanism.

The Effect of Extremely Low Frequency Alternating Magnetic Field on the Behavior of Animals in the Presence of the Geomagnetic Field

It is known that the geomagnetic field can influence animal migration and homing. The magnetic field detection by animals is known as magnetoreception and it is possible due to two different transduction mechanisms: the first one through magnetic nanoparticles able to respond to the geomagnetic field and the second one through chemical reactions influenced by magnetic fields. Another behavior is the magnetic alignment where animals align their bodies to the geomagnetic field. It has been observed that magnetic alignment of cattle can be disrupted near electric power lines around the world. Experimentally, it is known that alternating magnetic fields can influence living beings, but the exact mechanism is unknown. The parametric resonance model proposes a mechanism to explain that effect on living beings and establishes that, in the presence of a constant magnetic field, molecules associated with biochemical reactions inside cells can absorb resonantly alternating magnetic fields with specific frequencies. In the present paper, a review is made about animal magnetoreception and the effects of alternating magnetic fields in living beings. It is suggested how alternating magnetic fields can interfere in the magnetic alignment of animals and a general conclusion is obtained: alternating magnetic field pollution can affect the magnetic sensibility of animals.

Extracellular recordings reveal absence of magneto sensitive units in the avian optic tectum

There is a consensus that birds detect the earth's magnetic field and use some of its features for orientation and homing purposes. Since the late 1960s, when the first solid behavioral evidence of magnetoreception was obtained, much research has been devoted to describing the ethological aspects of this behavior. The neurophysiological basis of magnetoreception has been much less studied, although a frequently cited 1986 report described a high prevalence (70 %) of magneto-sensitive neurons in the pigeon optic tectum with high signal-to-noise ratios (Semm and Demaine, J Comp Physiol A 159:619-625, 1986). Here, we repeated these neurophysiological experiments using anesthetized as well as awake pigeons and new recording techniques. Our data indicate that magneto-sensitive units do not exist in the avian tectum.

Cryptochrome expression in the eye of migratory birds depends on their migratory status

Most passerine birds are nocturnal migrants. When kept in captivity during the migratory periods, these species show a migratory restlessness, or Zugunruhe. Recent studies on Sylvia warblers have shown that Zugunruhe is an excellent proxy of migratory disposition. Passerine birds can use the Earth's geomagnetic field as a compass to keep their course during their migratory flight. Among the candidate magnetoreceptive mechanisms are the cryptochromes, flavoproteins located in the retina that are supposed to perceive the magnetic field through a light-mediated process. Previous work has suggested that expression of Cryptochrome 1 (Cry1) is increased in migratory birds compared with non-migratory species. Here we tested the hypothesis that Cry1 expression depends on migratory status. Blackcaps Sylvia atricapilla were caught before fall migration and held in registration cages. When the birds were showing robust Zugunruhe, we applied a food deprivation protocol that simulates a long migratory flight. When the birds were refed after 2 days, their Zugunruhe decreased substantially, as is expected from birds that would interrupt migration for a refuelling stopover. We found that Cry1 expression was higher at night than during daytime in birds showing Zugunruhe, whereas in birds that underwent the fasting-and-refeeding protocol and reduced their levels of Zugunruhe, night Cry1 expression decreased to daytime levels. Our work shows that Cry1 expression is dependent on the presence of Zugunruhe and not on species-specific or seasonal factors, or on the birds being active versus inactive. These results support the hypothesis that cryptochromes underlie magnetoreceptive mechanisms in birds.

Compass magnetoreception in birds arising from photo-induced radical pairs in rotationally disordered cryptochromes

According to the radical pair model, the magnetic compass sense of migratory birds relies on photochemical transformations in the eye to detect the direction of the geomagnetic field. Magnetically sensitive radical pairs are thought to be generated in cryptochrome proteins contained in magnetoreceptor cells in the retina. A prerequisite of the current model is for some degree of rotational ordering of both the cryptochromes within the cells and of the cells within the retina so that the directional responses of individual molecules do not average to zero. Here, it is argued that anisotropic distributions of radical pairs can be generated by the photoselection effects that arise from the directionality of the light entering the eye. Light-induced rotational order among the transient radical pairs rather than intrinsic ordering of their molecular precursors is seen as the fundamental condition for a magnetoreceptor cell to exhibit an anisotropic response. A theoretical analysis shows that a viable compass magnetoreceptor could result from randomly oriented cryptochromes contained in randomly oriented cells distributed around the retina.

Magnetic field perception in the rainbow trout Oncorynchus mykiss: magnetite mediated, light dependent or both?

In the present study, we demonstrate the role of the trigeminal system in the perception process of different magnetic field parameters by heartbeat conditioning, i.e. a significantly longer interval between two consecutive heartbeats after magnetic stimulus onset in the salmonid fish Oncorhynchus mykiss. The electrocardiogram was recorded with subcutaneous silver wire electrodes in freely swimming fish. Inactivation of the ophthalmic branch of the trigeminal nerve by local anaesthesia revealed its role in the perception of intensity/inclination of the magnetic field by abolishing the conditioned response (CR). In contrast, experiments with 90° direction shifts clearly showed the normal conditioning effect during trigeminal inactivation. In experiments under red light and in darkness, CR occurred in case of both the intensity/inclination stimulation and 90° direction shifts, respectively. With regard to the data obtained, we propose the trigeminal system to perceive the intensity/inclination of the magnetic field in rainbow trouts and suggest the existence of another light-independent sensory structure that enables fish to detect the magnetic field direction.

Reaction kinetics and mechanism of magnetic field effects in cryptochrome

Creatures as varied as mammals, fish, insects, reptiles, and birds have an intriguing sixth sense that allows them to orient themselves in the Earth's magnetic field. Despite decades of study, the physical basis of this magnetic sense remains elusive. A likely mechanism is furnished by magnetically sensitive radical pair reactions occurring in the retina, the light-sensitive part of animal eyes. A photoreceptor, cryptochrome, has been suggested to endow birds with magnetoreceptive abilities as the protein has been shown to exhibit the biophysical properties required for an animal magnetoreceptor to operate properly. Here, we propose a theoretical analysis method for identifying cryptochrome's signaling reactions involving comparison of measured and calculated reaction kinetics in cryptochrome. Application of the method yields an exemplary light-driven reaction cycle, supported through transient absorption and electron-spin-resonance observations together with known facts on avian magnetoreception. The reaction cycle permits one to predict magnetic field effects on cryptochrome activation and deactivation. The suggested analysis method gives insight into structural and dynamic design features required for optimal detection of the geomagnetic field by cryptochrome and suggests further experimental and theoretical studies.

'Fixed direction'-responses of birds in the geomagnetic field

IN A RECENT PAPER, WE DESCRIBED THE ORIENTATION BEHAVIOR OF TWO PASSERINE MIGRANTS UNDER DIM RED LIGHT: the birds headed westward in spring as well as in autumn, displaying a 'fixed direction'-response. 'Fixed direction'-responses in other directions were observed under abnormal light regimes. Here, we point out the characteristic features of the 'fixed direction'-responses, in particular their differences to normal compass orientation, and discuss their implications. The conditions under which they are observed suggest complex interactions between magnetoreception and the visual system.

Quantum probe and design for a chemical compass with magnetic nanostructures

Magnetic fields as weak as Earth's may affect the outcome of certain photochemical reactions that go through a radical pair intermediate. When the reaction environment is anisotropic, this phenomenon can form the basis of a chemical compass and has been proposed as a mechanism for animal magnetoreception. Here, we demonstrate how to optimize the design of a chemical compass with a much better directional sensitivity simply by a gradient field, e.g., from a magnetic nanostructure. We propose an experimental test of these predictions, and suggest design principles for a hybrid metallic-organic chemical compass. In addition to the practical interest in designing a biomimetic weak magnetic field sensor, our result shows that gradient fields can serve as powerful tools to probe spin correlations in radical pair reactions.

Acuity of a cryptochrome and vision-based magnetoreception system in birds

The magnetic compass of birds is embedded in the visual system and it has been hypothesized that the primary sensory mechanism is based on a radical pair reaction. Previous models of magnetoreception have assumed that the radical pair-forming molecules are rigidly fixed in space, and this assumption has been a major objection to the suggested hypothesis. In this article, we investigate theoretically how much disorder is permitted for the radical pair-forming, protein-based magnetic compass in the eye to remain functional. Our study shows that only one rotational degree of freedom of the radical pair-forming protein needs to be partially constrained, while the other two rotational degrees of freedom do not impact the magnetoreceptive properties of the protein. The result implies that any membrane-associated protein is sufficiently restricted in its motion to function as a radical pair-based magnetoreceptor. We relate our theoretical findings to the cryptochromes, currently considered the likeliest candidate to furnish radical pair-based magnetoreception.

Night-time neuronal activation of Cluster N in a day- and night-migrating songbird

Magnetic compass orientation in a night-migratory songbird requires that Cluster N, a cluster of forebrain regions, is functional. Cluster N, which receives input from the eyes via the thalamofugal pathway, shows high neuronal activity in night-migrants performing magnetic compass-guided behaviour at night, whereas no activation is observed during the day, and covering up the birds’ eyes strongly reduces neuronal activation. These findings suggest that Cluster N processes light-dependent magnetic compass information in night-migrating songbirds. The aim of this study was to test if Cluster N is active during daytime migration. We used behavioural molecular mapping based on ZENK activation to investigate if Cluster N is active in the meadow pipit (Anthus pratensis), a day- and night-migratory species. We found that Cluster N of meadow pipits shows high neuronal activity under dim-light at night, but not under full room-light conditions during the day. These data suggest that, in day- and night-migratory meadow pipits, the light-dependent magnetic compass, which requires an active Cluster N, may only be used during night-time, whereas another magnetosensory mechanism and/or other reference system(s), like the sun or polarized light, may be used as primary orientation cues during the day.

A sky polarization compass in lizards: the central role of the parietal eye

The present study first examined whether ruin lizards Podarcis sicula are able to orientate using the e-vector direction of polarized light. Ruin lizards were trained and tested indoors, inside a hexagonal Morris water maze, positioned under an artificial light source producing plane polarized light with a single e-vector, which provided an axial cue. Lizards were subjected to axial training by positioning two identical goals in contact with the centre of two opposite side walls of the Morris water maze. Goals were invisible because they were placed just beneath the water surface, and water was rendered opaque. The results showed that the directional choices of lizards meeting learning criteria were bimodally distributed along the training axis, and that after 90 deg rotation of the e-vector direction of polarized light the lizards directional choices rotated correspondingly, producing a bimodal distribution which was perpendicular to the training axis. The present results confirm in ruin lizards results previously obtained in other lizard species showing that these reptiles can use the e-vector direction of polarized light in the form of a sky polarization compass. The second step of the study aimed at answering the still open question of whether functioning of a sky polarization compass would be mediated by the lizard parietal eye. To test this, ruin lizards meeting learning criteria were tested inside the Morris water maze under polarized light after their parietal eyes were painted black. Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.

Quantum control and entanglement in a chemical compass

The radical-pair mechanism is one of the two main hypotheses to explain the navigability of animals in weak magnetic fields, enabling, e.g., birds to see Earth's magnetic field. It also plays an essential role in spin chemistry. Here, we show how quantum control can be used to either enhance or reduce the performance of such a chemical compass, providing a new route to further study the radical-pair mechanism and its applications. We study the role of radical-pair entanglement in this mechanism, and demonstrate its intriguing connections with the magnetic-field sensitivity of the compass. Beyond their immediate application to the radical-pair mechanism, these results also demonstrate how state-of-the-art quantum technologies could potentially be used to probe and control biological functions.

Cryptochromes--a potential magnetoreceptor: what do we know and what do we want to know?

Cryptochromes have been suggested to be the primary magnetoreceptor molecules underlying light-dependent magnetic compass detection in migratory birds. Here we review and evaluate (i) what is known about these candidate magnetoreceptor molecules, (ii) what characteristics cryptochrome molecules must fulfil to possibly underlie light-dependent, radical pair based magnetoreception, (iii) what evidence supports the involvement of cryptochromes in magnetoreception, and (iv) what needs to be addressed in future research. The review focuses primarily on our knowledge of cryptochromes in the context of magnetoreception.

Directional orientation of birds by the magnetic field under different light conditions

This paper reviews the directional orientation of birds with the help of the geomagnetic field under various light conditions. Two fundamentally different types of response can be distinguished. (i) Compass orientation controlled by the inclination compass that allows birds to locate courses of different origin. This is restricted to a narrow functional window around the total intensity of the local geomagnetic field and requires light from the short-wavelength part of the spectrum. The compass is based on radical-pair processes in the right eye; magnetite-based receptors in the beak are not involved. Compass orientation is observed under 'white' and low-level monochromatic light from ultraviolet (UV) to about 565 nm green light. (ii) 'Fixed direction' responses occur under artificial light conditions such as more intense monochromatic light, when 590 nm yellow light is added to short-wavelength light, and in total darkness. The manifestation of these responses depends on the ambient light regime and is 'fixed' in the sense of not showing the normal change between spring and autumn; their biological significance is unclear. In contrast to compass orientation, fixed-direction responses are polar magnetic responses and occur within a wide range of magnetic intensities. They are disrupted by local anaesthesia of the upper beak, which indicates that the respective magnetic information is mediated by iron-based receptors located there. The influence of light conditions on the two types of response suggests complex interactions between magnetoreceptors in the right eye, those in the upper beak and the visual system.

Effects of disorder and motion in a radical pair magnetoreceptor

A critical requirement in the proposed chemical model of the avian magnetic compass is that the molecules that play host to the magnetically sensitive radical pair intermediates must be immobilized and rotationally ordered within receptor cells. Rotational disorder would cause the anisotropic responses of differently oriented radical pairs within the same cell to interfere destructively, while rapid molecular rotation would tend to average the crucial anisotropic magnetic interactions and induce electron spin relaxation, reducing the sensitivity to the direction of the geomagnetic field. So far, experimental studies have been able to shed little light on the required degree of ordering and immobilization. To address this question, computer simulations have been performed on a collection of radical pairs undergoing restricted rigid-body rotation, coherent anisotropic spin evolution, electron spin relaxation and spin-selective recombination reactions. It is shown that the ordering and motional constraints necessary for efficient magnetoreception can be simultaneously satisfied if the radical pairs are uniaxially ordered with a moderate order parameter and if their motional correlation time is longer than about a quarter of their lifetime.

Avian magnetite-based magnetoreception: a physiologist's perspective

It is now well established that animals use the Earth's magnetic field to perform long-distance migration and other navigational tasks. However, the transduction mechanisms that allow the conversion of magnetic field variations into an electric signal by specialized sensory cells remain largely unknown. Among the species that have been shown to sense Earth-strength magnetic fields, birds have been a model of choice since behavioural tests show that their direction-finding abilities are strongly influenced by magnetic fields. Magnetite, a ferromagnetic mineral, has been found in a wide range of organisms, from bacteria to vertebrates. In birds, both superparamagnetic (SPM) and single-domain magnetite have been found to be associated with the trigeminal nerve. Electrophysiological recordings from cells in the trigeminal ganglion have shown an increase in action potential firing in response to magnetic field changes. More recently, histological evidence has demonstrated the presence of SPM magnetite in the subcutis of the pigeon's upper beak. The aims of the present review are to review the evidence for a magnetite-based mechanism in birds and to introduce physiological concepts in order to refine the proposed models.