A field that many animals see.
European robins navigate by it. Sea turtles imprint their natal beach into it. Salmon find their home river through it. Two mechanisms compete to explain how — and in humans a single, contested finding suggests the brain might process it without us knowing.
Polewards / equatorwards. Wiltschko 1972: robins read the inclination angle, not polarity. At the magnetic equator the compass fails.
How a robin might see the magnetic field.
Cryptochrome 4 sits in the robin's photoreceptors. Blue light (424-565 nm) excites its FAD cofactor, briefly creating a radical pair whose singlet/triplet ratio depends on the angle of the surrounding magnetic field. The yield modulates a chemical signal that — according to current theory — overlays the visual field as a faint brightness pattern that shifts with head orientation.
Engels et al. (Nature 2014) showed that ambient electromagnetic noise between 50 kHz and 5 MHz — far below WHO safety limits — collapses the robin's compass entirely. A grounded aluminium shield around the test hut restored orientation. Strong evidence the mechanism is quantum.
European robin
Erithacus rubecula
- Mech.
- Radical pair · Cryptochrome 4 in retina
- Signal
- Inclination angle, 424-565 nm light required
Wiltschko & Wiltschko 1972 — the classic discovery. RF noise from 50 kHz to 5 MHz disables it (Engels 2014). ErCRY4 in vitro confirmed magnetic sensitivity (Xu 2021).
Loggerhead turtle
Caretta caretta
- Mech.
- Two mechanisms · map and compass separated
- Signal
- Inclination × intensity as 2D coordinate
Imprints the magnetic signature of its natal beach. Returns decades later. Lohmann 2025: map and compass run on different biophysical channels.
Pacific salmon
Oncorhynchus nerka
- Mech.
- Magnetite · single-domain particles, 25-60 nm
- Signal
- Innate magnetic map, lateral line connection
Walker 1997 found magnetite chains in the ethmoid region. 56 years of fishery data show salmon homing routes track the drift of Earth's field (Putman 2013).
Honey bee
Apis mellifera
- Mech.
- Magnetite · ferromagnetic material in abdomen
- Signal
- Sub-microtesla resolution in conditioning trials
Liang et al. 2016 demonstrated proboscis-extension conditioning to magnetic stimuli. The waggle dance contains a residual error correlated with magnetic field direction.
Domestic dog
Canis familiaris
- Mech.
- Mechanism unknown
- Signal
- Body axis aligns N-S under calm field
Hart et al. 2013 logged 7475 defecations and urinations from 70 dogs across 37 breeds. North-south alignment emerges only when the geomagnetic field is quiet — vanishes during storms.
Human
Homo sapiens
- Mech.
- Unconfirmed · CRY2 in retina, status open
- Signal
- Alpha-band EEG drop in shielded room
Foley 2011: human CRY2 restores magnetoreception in CRY-deficient flies. Wang/Kirschvink 2019: alpha-ERD in Faraday cage when field rotates. No independent high-powered replication as of 2026.
The inclination map is computed from a centred axial-dipole approximation of Earth's field: inclination I = atan(2·tan(λ)) and total intensity F ≈ 30 µT × √(1 + 3·sin²(λ)), where λ is the geographic latitude. This is a textbook simplification — the real field includes secular variation, a tilted dipole and a 7% offset from Earth's centre — but it captures the structure that compass-using animals actually exploit. Coastline data is Natural Earth (1:110m). The radical-pair animation is illustrative; the singlet-yield curve roughly matches the orientation dependence reported by Hore & Mouritsen 2016.
From an invisible field to a sense we may share.
A field nobody feels.
Earth's magnetic field measures 25 to 65 microtesla across its surface, roughly a hundred times weaker than the magnet stuck to your fridge. The inclination angle — the tilt of the field lines — runs from 0° at the magnetic equator to 90° at the poles. Total intensity grows with latitude. Combined, the two values give nearly every point on Earth a unique fingerprint. Animals that read it have a coordinate system the rest of us have to draw on paper.
Two hypotheses, one sense.
Radical-pair theory says the compass lives in cryptochrome proteins in the retina: blue light triggers a quantum-spin reaction whose outcome depends on field orientation. Magnetite theory says biogenic magnetic particles (25-60 nm) sit in tissue and rotate or pull on nerve endings. For decades the two were rivals. Lohmann et al. (Nature 2025) showed loggerhead turtles use both — map and compass separated onto different physical channels.
Compass or map.
Wiltschko & Wiltschko 1972: European robins respond not to polarity but to the inclination angle. Tilt the field and the bird tilts its bearing. Lohmann 2001 / 2015: loggerhead turtles do something different — they read inclination and intensity as two axes of a magnetic map and imprint their birthplace into it. When the field drifts decades later, nesting density shifts with it along the Florida coast.
And the human?
Robin Baker's 1980s claims of human homing did not survive replication. In 2011, Foley, Gegear and Reppert showed human CRY2 placed into CRY-deficient fruit flies fully restores their magnetic compass — the molecule is functional. In 2019, Wang, Kirschvink and Shimojo measured a reproducible alpha-band drop in human EEG when the surrounding field rotated within a shielded chamber, only for specific orientations. As of 2026 no independent high-powered replication exists and no behavioural compass has been demonstrated. The conservative reading: unconscious neural processing of weak magnetic fields is possible, conscious magnetoreception is not established.
- Wiltschko, W. & Wiltschko, R. (1972) — Magnetic Compass of European Robins. Science 176.
- Walker, M. M. et al. (1997) — Structure and function of the vertebrate magnetic sense. Nature 390.
- Lohmann, K. J. et al. (2001) — Regional magnetic fields as navigational markers for sea turtles. Science 294.
- Begall, S. et al. (2008) — Magnetic alignment in grazing and resting cattle and deer. PNAS 105.
- Foley, L. E., Gegear, R. J. & Reppert, S. M. (2011) — Human cryptochrome exhibits light-dependent magnetosensitivity. Nature Communications 2.
- Treiber, C. D. et al. (2012) — Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons. Nature 484.
- Putman, N. F. et al. (2013) — Evidence for Geomagnetic Imprinting as a Homing Mechanism in Pacific Salmon. Current Biology 23.
- Hart, V. et al. (2013) — Dogs are sensitive to small variations of the Earth's magnetic field. Frontiers in Zoology 10.
- Engels, S. et al. (2014) — Anthropogenic electromagnetic noise disrupts magnetic compass orientation in a migratory bird. Nature 509.
- Brothers, J. R. & Lohmann, K. J. (2015) — Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles. Current Biology 25.
- Hore, P. J. & Mouritsen, H. (2016) — The Radical-Pair Mechanism of Magnetoreception. Annual Review of Biophysics 45.
- Liang, C.-H. et al. (2016) — Magnetic Sensing through the Abdomen of the Honey bee. Scientific Reports 6.
- Mouritsen, H. (2018) — Long-distance navigation and magnetoreception in migratory animals. Nature 558.
- Wang, C. X. et al. (2019) — Transduction of the Geomagnetic Field as Evidenced from alpha-Band Activity in the Human Brain. eNeuro 6.
- Xu, J. et al. (2021) — Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature 594.
- Lohmann, K. J. et al. (2025) — Learned magnetic map cues and two mechanisms of magnetoreception in turtles. Nature 638.