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Bioelectromagnetism

In the case of RPM in oscillating magnetic felds, the singlet-triplet (S-T) interconversion rate can

be signifcantly afected by oscillating magnetic felds of specifc frequencies in the “MegaHertz range”

(1–100 MHz) (Ritz et al., 2000). Te radical pair model predicts that an oscillating magnetic feld in the

megahertz range can disrupt the magnetic compass due to the electron paramagnetic resonance (EPR)

efect (Timmel and Hore, 1996). Te intensities required for the resonance efect are so low that they

would not afect any of the magnetite-based mechanisms currently considered as explained below, so

that disruption of magnetic orientation would be diagnostic for the involvement of a radical pair mecha­

nism (RPM) underlying the magnetic compass (Ritz et al., 2000). Such felds with frequencies of 0.64

MHz and above led to disorientation (Ritz et al., 2004, 2009; Kavokin et al., 2014).

Teoretical considerations and in vitro studies indicate that they are to be expected in the 0.1–10­

MHz range. Te efect of the oscillating magnetic felds should depend on their orientation with respect

to the static background feld (Cranfeld et al., 1994). Tese resonances are generally very broad and

might therefore lead to disturbing efects at virtually all frequencies within this range, provided the

intensity of the oscillating magnetic feld is sufciently strong (Henbest et al., 2004). However, a special

resonance occurs when the frequency of the oscillating magnetic feld matches the energetic splitting

induced by the static GMF; here, one expects a marked efect regardless of the structure of the molecules

forming the radical pairs.

Wiltschko et al. (2011) presented the orientation of European robins in the GMF (Control, C), and

in high-frequency felds added to the GMF in two diferent orientations (Figure 4.6, compiled from

Wiltschko and Wiltschko, 2005; data from Talau et al., 2005). First tests with a weak broadband noise

feld of frequencies from 0.1 to 10 MHz added to the GMF indeed showed that this disrupted the orienta­

tion of migratory birds (Ritz et al., 2004). Further tests used the single frequencies of 1.315 and 7.0 MHz

with an intensity of about 480 nT. When these felds were presented parallel to the geomagnetic vector,

the birds were oriented in their migratory direction, whereas they were disoriented when the same felds

were presented at an angle of 24° or 48° to the GMF (Wiltschko et al., 2011, compiled from Wiltschko

and Wiltschko, 2005, data from Talau et al., 2005). Tis is in agreement with the radical pair model and

clearly shows that the observed efect of the high-frequency feld is a specifc one. Together, these fnd­

ings indicate that the primary process of magnetoreception in birds involves an RPM.

From the perspective of biophysical theory, Ritz et al. (2000) frstly explained that the birds can

perceive even a weak magnetic feld at the GMF level on the basis of the RPM model. Schematics of

the light-dependent RPM model for quantum-assisted magnetic sensing are shown by Wiltschko and

Wiltschko (2006), modifed from Ritz et al. (2000). Here, a donor molecule (D) exists in an inactive

ground state. A donor molecule (D) absorbs a photon. By electron transfer to an acceptor molecule (A),

FIGURE 4.6 Orientation of European robins in the GMF (Control, C), and in high-frequency felds added to the

GMF in two diferent orientations (Wiltschko et al., 2011, compiled from Wiltschko and Wiltschko, 2005, data from

Talau et al., 2005). Te upper part of the diagram illustrates the orientation of the GMF and the high-frequency

feld in the three test conditions. (Reproduced with permission from Wiltschko et al., 2011, Copyright 2011, Elsevier.)