Table 1.

Support and criticism for the “radical-pair” and “magnetite-based” magnetoreception hypotheses

	Support	Criticism
Radical–Pair	Experiments showing that birds can only sense the magnetic field under illumination with relative short wavelength, as opposed to longer wavelengths [41]	The effect of the magnetic field on the spin-state of the molecule has not been demonstrated, either in vitro or in vivo, under the Earth’s weak magnetic field, but only under a field orders of magnitude stronger [20, 47]
		The activation mechanism is missing, meaning how the signal transduces to initiate a neural response.
Magnetite-Based	Magnetite crystals have been detected in magnetic sensing fish, reptiles and birds [18, 28, 48]	The magnetites found in some magnetic-sensing animals are not associated to the animals’ neuronal, or other tissue, but rather located in macrophages [30] or as contaminants [32]
	Magnetotactic-bacteria (MTB) can act upon the field via similar magnetite crystals [2, 22]	No one has seen magnetite crystals serving as a ‘magnetic-sensor’ except in bacteria [20] The activation mechanism is missing, meaning how the signal transduces to initiate a neural response.