The results provide clear evidence for a magnetic sense in wood mice. When building a nest in an otherwise featureless environment, the animals exhibited a spontaneous preference for the magnetic north-northeast and south-southwest axis. These findings are consistent with the earlier report of a magnetic compass sense in this species by Mather and Baker10. A magnetic compass would be highly beneficial for nocturnal wood mice that occupy comparatively large home ranges of 1–2 ha41, perform regular foraging bouts over distances of more than 200 m, and show remarkable homing ability from unfamiliar locations after displacements of up to 350 m42,43.

It is unclear whether the observed preference is part of a homing response or a spontaneous directional preference. However, even though many animals used in this study were caught north of the testing site, none were caught south of it. This renders it unlikely that the observed axial preference is a homing response. However, the observed directional preference is consistent with the spontaneous bimodal magnetic alignment observed in other vertebrates (reviewed in44). Concordantly, laboratory mice, in addition to showing learned compass orientation relative to the magnetic field, also exhibit a weak, presumably innate (i.e., independent of any learned direction) axial preference along the magnetic north-south axis18. Recently, such a preference was also revealed in the semi-fossorial bank vole (Myodes glareolus), which the authors suggested is likely to be innate15. Consequently, although there is insufficient information to determine if the observed preference in wood mice is innate or learned, it appears likely that at least some component of the response is innate. Although the adaptive significance of spontaneous magnetic alignment remains an open question8,44,45,46, the widespread occurrence in epigeic mammals makes this response ideal for initial studies of the mechanism(s) of magnetoreception.

The available evidence indicates that subterranean, microphthalmic mole-rats rely on a light-independent and RF-insensitive magnetic particle based mechanism of magnetoreception16. On the other hand, the properties of learned magnetic compass orientation by epigeic rodents are consistent with the involvement of a radical pair mechanism40. This suggests that the visual ecology/physiology (adaptation to light levels available to diurnal and nocturnal animals active above ground, rather than to the absence of light in the subterranean ecotope), rather than phylogenetic relatedness (membership in the class Mammalia), may be the principle set of factors influencing the type of magnetoreception mechanism used to obtain directional (i.e., compass) information. To determine if macrophthalmic, epigeic wood mice indeed have a radical pair mechanism, we tested the sensitivity of wood mice to low level radio frequency fields, using the types of stimuli used in earlier studies of birds and mole-rats16,35,47, i.e., both the Larmor frequency and wideband-FM (comparable to broadband-RF used in other studies) oscillating magnetic fields. Contrary to earlier studies of migratory birds35,47, wood mice exhibited non-random directional preferences in both conditions: The distribution of bearings obtained from mice tested under the Larmor frequency condition was indistinguishable from controls (i.e., nests were bimodally distributed approximately along the north-south magnetic axis; Figure 3a), while that of mice exposed to the wideband frequency sweep was rotated by roughly 90° (Figure 3b). As the angle between the static field and the RF fields was the same in both RF conditions (see Figure 4c), the only differences were the overall intensity, the temporal pattern, and the frequency spectrum. The intensities in the Larmor frequency condition had minimum values of 785 nT in the centre of the arena and therefore greatly exceeded those shown to affect the inclination compass of birds39,47. In the wideband-FM condition the RF intensities were lower, due to the frequency response characteristics of the coil. They varied across the frequency range between 25–50 nT in the centre of the coil to twice these values at the periphery of the arena. These field strengths are comparable to the ones used in Engels et al.39, who were able to disrupt the magnetic compass orientation of European robins by broadband electromagnetic noise with a spectral intensity of 0.1–0.2 nT per 10 kHz in the range of 600 kHz–3 MHz, which upon integration over the frequency domain translates into a RF magnetic field amplitude of 30–35 nT in the time domain.

Figure 4: Overview of the testing site, the coil-setup and the nest-building arena. a. Top view of the empty horse stable that consisted of two compartments. One compartment contained the wooden enclosure, the other one the testing setup and coil systems. b. Production of the artificial static magnetic field (top view). In the control condition (mN ambient ) the horizontal of the ambient magnetic field (H ambient ) was left unchanged. To shift magnetic north by 90° counterclockwise (mN west ), an artificial magnetic field was added with a 135° clockwise aligned Helmholtz-coil pair (H artificial ) to produce a 90° shifted resultant field of the same inclination and total intensity as the ambient magnetic field. c. Profile of the test arena and the loop coil used to produce the oscillating magnetic fields in the RF range. The oscillating fields were aligned vertically at an angle of 25° to the static field lines. The scale only applies to part a. of the figure. Full size image

The results are in accordance with a radical pair mechanism of magnetoreception, providing for the first time positive evidence for such a mechanism in a mammal. Even though we cannot rule out completely that the mice were affected by both RF treatments and oriented topographically in the Larmor frequency field, the similarity between nest distribution in the latter and the orientation in the unchanged geomagnetic field suggests that the wood mice magnetic receptors were unaffected. Future experiments will hopefully verify this. For now, the fact that we observed an effect on nest positioning under a low intensity wideband-FM field, but no (behavioural) effect under the Larmor frequency field at effectively a 15–30 times higher intensity leads us to speculate about the underlying mechanism. The response is consistent with a radical pair mechanism in which both electron spins have an anisotropic coupling to their respective host molecules, due either to nuclear hyperfine interactions29,48,49 or, less likely, to spin-orbit coupling50. If we make the assumption that cryptochrome 151,52 is the molecule responsible for the radical pair mechanism also in mammals, the radical partner of the flavin adenine dinucleotide (FAD) cofactor thus could not be a reactive oxygen species (superoxide), as has been suggested for birds28,53, because superoxide is free of hyperfine interactions. The findings are rather compatible with the radical partner being tryptophan as in the original radical pair mechanism model29 or ascorbyl as recently proposed by Lee et al.54. Of course, the results do not exclude that the host molecule in mammals and perhaps other taxa might differ from that in birds. In either case, a large number of resonance frequencies, both below and above the Larmor frequency, are only possible in a radical pair mechanism in which both members of the radical pair have hyperfine interactions, which would then have been excited by the frequencies in the wideband-FM condition.

Interestingly, while RF magnetic fields so far have been found to cause disorientation in birds, in the present experiments the wideband-FM field caused re-orientation in mice, with nest-building positions shifted by approximately 90° relative to the axis of orientation observed in the ambient magnetic field. There are two possible explanations for this re-orientation. First, the wideband-RF might have disrupted input from the radical pair mechanism, causing the mice to rely on an alternative source of directional information (e.g., non-magnetic, or magnetic particle mechanism-based comparable to the fixed direction response of birds tested in unnatural light conditions37,55). Alternatively, the wideband-RF may have altered, rather than eliminated, the pattern of radical pair mechanism response, as it has recently been proposed as an explanation for an effect of a Larmor frequency magnetic field on spontaneous magnetic alignment in turtles56. To our knowledge, the possibility of this type of RF effect has not yet been addressed by the available models of the radical pair mechanism. For simple reference-probe radical pair mechanism models it is the lifetime of the spin-correlated radical pair (“spin correlation time”) that determines the magnitude of the effect of a weak RF magnetic field on the radical pair dynamics: long correlation times, in the order of 100 microseconds allow weak RF magnetic fields to fully perturb the singlet-triplet interconversion, leading to a flattened angular response (suppression of the directional dependence required for a compass). Shorter correlation times alter the absolute values of the yield without flattening the angular response50,57. Theoretically, it is possible that the radical pair mechanism in rodents is based on a radical pair with shorter spin coherence time than the ones in migratory birds, so that the effect of a RF magnetic field could be different in the two taxa. The altered response in the short-lived radical pair would be equivalent to the response produced by an intensity shift in the static magnetic field57, which could produce a change in the magnetic visual pattern.

It has been proposed that vertebrates might exploit the visual pattern of response produced by the radical pair mechanism as a global reference that could function as a simple spherical grid or coordinate system fixed in alignment relative to the magnetic field that appears as a visual pattern superimposed on the animal’s surroundings58. Such a reference system would be useful in a variety of daily challenges from integrating spatial information from multiple sensory modalities, in novel surroundings, to improving 3-dimensional path stabilization59 and course control60, to placing multiple locales into register to form a global map of familiar space46,58. If the magnetic field is perceived in this way in epigeic rodents, mice might position themselves and/or their nests in a specific alignment with respect to the pattern generated by the radical pair mechanism. Consequently, a change in the ‘visual’ pattern caused by RF treatment could result in a corresponding change in the distribution of nest positions.

It is widely believed that RF magnetic fields influence exclusively a radical pair mechanism, not a magnetic particle mechanism. This is certainly true for single-domain magnetite, where the inertia of the particles surrounded by the viscous cytoplasm is generally believed to hinder motion and thus transduction of oscillating fields in the radio frequency range36,61. However, according to Shcherbakov & Winklhofer25, a magnetic particle mechanism based on magnetic susceptibility, such as the maghemite-superparamagnetic magnetite hybrid magnetoreceptor proposed by Fleissner et al.62 would convert the radiation into thermal agitation. As with the putative effect on a radical pair mechanism, it is not clear why such a heating effect would cause re-orientation, rather than disorientation. Importantly, however, due to the higher intensity of the Larmor frequency stimulus compared to the wideband stimulus, any heating effect would have been more pronounced for the Larmor frequency condition. Consequently, the finding of an effect of the wideband RF stimulus, but not of the higher intensity Larmor frequency stimulus, argues against a nonspecific (i.e., thermal) effect on a mechanism or process other than the radical pair mechanism.