In 1855 Russian zoologist Alexander von Middendorff was one of the first scientists to postulate that animals could sense the Earth’s magnetic field and use it for navigation.1 Versions of a magnetic compass are now known to exist in all vertebrate classes (birds, fish, reptiles, amphibians, and mammals), as well as insects and some bacteria. Despite a significant amount of research and discoveries about the use of the magnetic field by animals, as stated in a paper just published in the journal Science, “While behavioral studies have provided compelling evidence for the existence of a magnetic sense, the molecules, cells, and central circuits involved in magnetoreception are unknown.”2
Technical Challenge
There are several challenges associated with organisms being able to detect the geomagnetic field. One is that the geomagnetic field intensity in on the order of 50 microTesla. The intensity varies with latitude, being the strongest at the poles, and is only 30 microTesla near the equator. By comparison, the geomagnetic field intensity is slightly larger than the magnetic field from a microwave oven, but also about 100 times weaker than the field of a refrigerator magnet.
Theorized Mechanisms
Three different mechanisms have been theorized for animal magnetoreception, all of which have supporting evidence. One is that the magnetic field is detected using a mechanically sensitive compass formed by iron oxide magnetite. Second is the electromagnetic induction theory that movement by an animal through the magnetic field induces a current in an electroreceptor circuit. Third is the so-called “radical pair theory” that “light induces the formation of radical pairs, and that the spin state of these electrons is influenced by the magnetic field.”3 The radical pairs are formed in a photoreceptive cryptochrome flavoprotein, which is sensitive to blue light.
A recent paper by University of Crete physicists Iannis Kominis and Efthimis Gkoudinakis assessed the theoretical performance of three magnetoreception sensor methods.4 The analysis is based on a theoretical model of how close magnetic sensing can come to a fundamental energy resolution limit bounded by Planck’s constant. Magnetic sensing in man-made magnetometers has been improved significantly through the use of quantum technology. That provides a baseline to compare how well animal magnetoreception sensors perform. The results indicated that radical pair magnetoreception could be a quantum process. The other two methods would appear to be based more on classical physics.
Magnetite Mechanism
Magnetite (Fe3O4) is usually found as single domain crystals approximately 20-50 nanometers in diameter, which makes it very difficult to detect in organisms. The crystals are permanent magnets that align with Earth’s magnetic field. Several animal species have been found to form biogenic magnetite, including magnetotactic bacteria and trout fish.5 The bacteria use the compass to guide their swimming in finding favorable conditions.6
Electromagnetic Induction
The physics behind this mechanism is that when a magnetic field moves through a conductor, an electric current is induced. This can occur in organisms when moving through a magnetic field and the current is induced in an organ that is a conductor. It has been theorized that a good candidate for this would be fish since they have electroreceptors and the ocean is a good conductive medium.7 Until recently there has been very little scientific evidence to validate this mechanism in animals.8
Radical Pair Theory
A radical is a molecule that contains an odd number of electrons. A radical pair consists of two radicals that have been created simultaneously, usually by a chemical reaction. Radicals are magnetic because the electron (in common with the proton and the neutron) has a property known as spin or, more accurately, spin angular momentum. This mechanism has been proposed to exist in birds, Monarch butterflies, and fruit flies.
There has been “very strong evidence that the magnetic compass is light dependent, that birds perceive magnetic compass information as a visual impression, and that the primary sensor must be located in both eyes.”9 Oxford University chemist P. J. Hore further explains, “Cryptochrome-4a has been proposed as the receptor for the magnetic compass sense of migratory songbirds. Radical pairs are formed by blue-light excitation of the flavin group of the FAD (Flavin Adenoside Dinucleotide) chromophore followed by quantum based sequential electron transfers along a chain of four tryptophan residues.”10
While fruit flies (Drosophila melanogaster) do not migrate or navigate as birds do, there is evidence that they have a magnetic compass, and that it could be based on the radical pair mechanism. The literature indicates that there are at least 15 papers over the past 50 years reporting the existence of a fly magnetic sense, and several of these suggest a Cryptochrome based mechanism.11 A proposed model in fruit flies is that, “A light-induced electron-transfer reaction whereby radical pairs (RPs) are formed, the spin-states of which are sensitive to magnetic fields as small as 50 microTesla. This so-called RP mechanism canonically requires the flavoprotein CRY, which is best known for its role as a circadian blue light photoreceptor in flies and as a light-insensitive transcriptional regulator in the circadian clock of mammals.”12
Research has also found the possibility for a radical pair mechanism in Monarch butterflies. One study determined that there is evidence the “CRY1 protein is involved in the detection of vector direction, supporting its role in a geomagnetic compass.”13 The same study also determined that the Monarch’s antennae and the compound eyes are necessary for Monarch light-dependent magnetoreception.
Latest Research on Bird Magnetoreception
A paper just published in Science by a group of researchers provides strong evidence that the mechanism in birds is based on electromagnetic induction.14 The experiment involved pigeons and attempted to determine which of the theorized mechanisms is responsible for magnetoreception. A mechanism based on magnetite was ruled out as it has not been found in pigeons. The experiment focused on determining brain neuronal activity associated with bird’s semi-circular canals, which are part of the vestibular system. Vertebrates are designed to use this system to maintain balance. Some describe the vestibular system as a “biological gyroscope,” since it functions in a similar way to detect movement and acceleration in three dimensions. In the paper the authors observe that it seems logical that there is a connection between detection of the magnetic vector information and the vestibular system since that would enable the mechanism for the determination of the desired navigation path.
The results of the study indicated that a changing (not static) magnetic field was required to generate neuronal activity, and that this occurs without light. This result ruled out the radical pair quantum effect mechanism since that relies on activation by light. Therefore, the conclusion of the paper is that the mechanism in birds is based on electromagnetic induction.
While this latest research is significant and potentially answers a key question on magnetoreception in birds, it does not answer it for all animals. For example, the mechanism found in pigeons cannot exist in insects since they have completely different hearing and balance mechanisms. There is also good evidence for the magnetite mechanism in some animals. Therefore, it appears likely that different animals are designed to detect the geomagnetic field using different mechanisms.
It must be kept in mind that simply detecting the geomagnetic field is only one element of a compass and navigation system. Other elements include: (1) transferring the sensor information and processing it in a brain neural network; (2) having a desired navigation goal (either heading or map location); and (3) comparing the difference between the compass sensor information and desired goal to compute the navigation route. All of these elements must be designed to function as a coherent system, and thus have the appearance of engineering design. All three mechanisms of magnetoreception exhibit varying degrees of complexity.
Notes
- G. C. Nordmann, et al., “Magnetoreception – A sense without a receptor,” PLOS Biology, October 23, 2017.
- G. C. Nordmann, et al., “A global screen for magnetically induced neuronal activity in the pigeon brain,” Science, 20 November 2025.
- Malkemper, et al., “Neuronal circuits and the magnetic sense: central questions,” Journal of Experimental Biology (2020) 223.
- Iannis Kominis and Efthimis Gkoudinakis, “Approaching the Quantum Limit of Energy Resolution in Animal Magnetoreception,” PRX LIFE 3, 013004, 16 January 2025.
- Perez-Huerta, et al., “Biogeochemical fingerprinting of magnetotactic bacterial magnetite,” PNAS, 2022, Vol. 119, No. 31.
- Nordmann, et al., “Magnetoreception – A sense without a receptor.”
- Bellono, et al., “Molecular basis of ancestral vertebrate electroreception,” Nature, Vol. 543, 16 March 2017.
- Nordmann, et al., “Magnetoreception – A sense without a receptor.”
- P.J. Hore and Henrik Mouritsen, “The Radical-Pair Mechanism of Magnetoreception,” Annual Review Biophysics, 2016. 45:299-344.
- P.J. Hore, “Spin Chemistry in Living Systems,” National Science Review, 28 March 2024.
- Steven M. Reppert, “Magnetic field effects on behaviour in Drosophila,” Nature: Matters Arising, 1 May 2024.
- Bradlaugh, et al., “Essential elements of radical pair magnetosensitivity in Drosophila,” Nature, Vol. 615, 2 March 2023.
- Wan, et al., “Cryptochrome 1 mediates light-dependent inclination magnetosensing in monarch butterflies,” Nature Communications (2021) 12:771.
- G. C. Nordmann, et al., “A global screen for magnetically induced neuronal activity in the pigeon brain.”