a, A nanoscale Cry/MagR magnetosensor complex with intrinsic magnetic polarity acts as a light-dependent biocompass. Linear polymerization of Fe–S cluster-containing magnetoreceptors (MagR) leads to the formation of a rod-like biocompass at the centre (core, yellow), surrounded by photoreceptive cryptochromes (Cry; outer layer, cyan). b, Cross-section of a, indicating that electron transportation from the FAD group in Cry to the Fe–S cluster in MagR upon light stimulation may be possible. c, The biocompass model of magnetoreception. In animal navigation systems, the Cry/MagR magnetosensor complex may act as a biological compass that perceives information from the Earth’s geomagnetic field, such as polarity (as with a conventional compass), intensity and inclination. The surface representation of the Cry/MagR structure (cyan and yellow) has been validated by EM in this study (Figs 2 and 3). The intrinsic magnetic moment of the magnetosensor may form a polarity compass for the sensing of directional information from the Earth’s geomagnetic field. The capability to detect the intensity and the spontaneous alignment of the magnetosensor in magnetic fields (as shown on the left-hand side, and further elucidated in Fig. 5a, b), may form the basis of an intensity sensor and inclination compass. Earth’s magnetic poles (black arrows) are offset from the axis of rotation (black line). The inclination angle (labelled as ‘I’) and intensity of the field are indicated by the direction and length of the arrows (red in the Northern Hemisphere and blue in the Southern Hemisphere). MagR and Cry/MagR magnetosensors from two species, monarch butterfly (Danaus plexippus, upper right) and pigeon (Columba livia, lower right), were tested in this study, highlighting the evolutionarily conserved biocompass model.