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Bioelectromagnetism
but how do they convey information such as the direction of the MF to the brain? How has the simple
magnetite sensor evolved as the magnetic-based sophisticated GPS? Tat is unclear and enigmatic.
Another strong candidate hypothesis is that the mechanism is being clarifed in migratory bird
research. It is said that a blue light-sensing protein called “cryptochrome (CRY)” in the retina of the eye
plays an important role as a magnetic sensor or magnetoreceptor (frst described by Möller et al., 2004).
CRY proteins are also components of the central circadian clockwork (Yuan et al., 2007), and are closely
related to the light-dependent DNA repair enzymes, the photolyases (Cashmore, 2003). It is reported
that the CRY is fngered as the smoking gun in the exquisite magnetic reception of birds (Roberts,
2016). As a putative magnetoreceptor, four diferent isoform CRYs (CRY1a, CRY1b, CRY2, CRY4a, and
CRY4b) have been identifed in the retinae of several bird species. CRY1a (Liedvogel et al., 2007; Nießner
et al., 2011) is found from garden warblers (Sylvia borin) and European robins (Erithacus rubecula),
CRY1b (Bolte et al., 2016; Nießner et al., 2016) is from European robins, migratory northern wheatears
(Oenanthe oenanthe), and homing pigeons (Columba livia), CRY2 (Mouritsen et al., 2004) is from migra
tory garden warblers, and CRY4 (Günther et al., 2018; Pinzon-Rodriguez et al., 2018; Hochstoeger et al.,
2020; Wu et al., 2020) is from European robins. In particular, more recently, R. Wiltschko et al. (2021)
reviewed and speculated that CRY1a (termed as gwCRY1a) appears to be the most likely receptor mol
ecule for magnetic compass information due to its location in the outer segments of the UV cones with
their clear oil droplets. In the case of cockroaches, however, CRY2 mediates sensitivity to the magnetic
declination in American cockroaches, Periplaneta americana (Bazalova et al., 2016).
Tese CRYs could generate free radical pairs (RPs) for quantum-assisted magnetic sensing, which
play a key role as a kind of “quantum compass” (Hiscock et al., 2016), and are deeply involved in a
magnetic sense. Tat is, CRY-dependent magnetoreception is currently proposed to be a result of light-
initiated electron transfer chemistry in the protein, which is magnetically sensitive by virtue of the
RPM (Rodgers and Hore, 2009; Dodson et al., 2013). “Spin-correlated RPs” can undergo coherent mix
ing between singlet and triplet spin states, which have diferent reactive fates, and this mixing process
can be modulated by MFs (Woodward et al., 2009). In the principle of magnetoreception mechanism
according to the model of RPM, it is not possible to distinguish whether the directions of electron spins
are opposite or the same, so in principle, it is possible to detect the inclination of the MF lines, but which
is N or S polarity. It is impossible to obtain information on the direction itself. Terefore, it is called an
“inclination compass” or “axial compass” in the magnetic sense that only the information on the dip
angle (World Data Center for Geomagnetism, Kyoto), which is the orientation component of the mag
netic vector, can be obtained. Tus, the avian magnetic compass does not distinguish between magnetic
“N” and “S” as indicated by polarity, but between “poleward” where the MF lines point to the ground,
and “equatorward,” where they point upward (Wiltschko and Wiltschko, 2005).
Te geomagnetic variables of the intensity of the feld and the inclination of its lines, as well as “mag
netic anomalies,” are used by animals to ascertain their position. Here, the magnetic anomaly is an area
where the GMF is spatially distorted and presents opportunities to assess how the homing or migration
behavior of animals is afected by spatial variation of geomagnetic parameters (Dennis et al., 2007). Te
magnetic anomalies are mainly produced by magnetized rocks that change the feld value on the Earth’s
surface (Moskowitz et al., 2015). It can be thus assumed that animals have a biological analog of a GPS;
the diference is that it is not based on satellite signals but on the GMF (Lohmann, 2010). If local MF
intensity does play a role in their navigational map, then behavior undertaken by animals to determine
their locations relative to goals may be infuenced by the orientation of the local MF intensity. To test
this idea, Dennis et al. (2007) conducted the following experiment: pigeons were released at unfamiliar
sites and fight trajectories recorded by GPS-based tracking devices. Release sites were located in or
around the Auckland Junction Magnetic Anomaly in New Zealand (Dennis et al., 2007). In total, 92
complete fight trajectories of pigeons were obtained and 59 out of the 92 (64.1%) trajectories exhibited
signifcant alignment: 29 trajectories (31.5%) were aligned parallel to the feld, 33 (35.7%) were aligned
perpendicular to the feld, and 42 (45.7%) were aligned both parallel and perpendicular to the feld
(some individuals exhibited more than one type of response) (Dennis et al., 2007). Tus, many pigeons