Fig. 2. Hypothesized response on the hybrid magnetoreception mechanism under full-spectrum and long-wavelength light (Phillips and Borland, 1994). (A) In the proposed hybrid magnetoreception system, the magnetic compass (double-headed solid arrow) is used to align the map detector (single-headed open arrow) with respect to the axis of the magnetic field (north at top of figure) and, thus, to obtain more accurate measurements of one or more magnetic field components used for the map component of homing. In turn, the map detector, which is sensitive to the polarity of the magnetic field, is used to distinguish between the two ends of the magnetic axis when the newt is carrying out the compass component of homing, replacing the inclination (`dip angle'), which newts use when exhibiting shoreward magnetic compass orientation (Phillips, 1986a). (B,C) Under long-wavelength light, the directional response of the magnetic compass is rotated by 90° (Phillips and Borland, 1992a). (B) When newts are carrying out the compass component of homing, the 90° rotation of the magnetic compass' response would cause the axis indicated by the magnetic compass to be perpendicular to the polarity of the magnetic field indicated by the map detector, preventing newts from using the hybrid system to determine compass direction. [Previous homing studies have shown that newts held in the outdoor tanks under full-spectrum light and tested under long-wavelength light are disoriented, suggesting that they do not fall back on the inclination compass for the compass component of homing when polarity information is ambiguous (Fig. 1B; and see Phillips and Borland, 1994)]. (C) When newts are carrying out the map component of homing, the 90° rotation of the magnetic compass' response under long-wavelength light would cause newts to position the map detector perpendicular to the alignment in which it is normally positioned to take map readings, and, therefore, prevent them from obtaining map information.

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