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due to a direct effect of light on the newts' perception of the magnetic field, we trained newts under long-wavelength light by covering the training tank with a long-wavelength-transmitting gel filter (two layers of Lee #101) 5 . Under long-wavelength light, these newts orientated themselves parallel to the shoreward axis, indicating that they had learned the direction of the shore with respect to the rotated magnetic information under long-wavelength light (Fig. 1c,f).As well as ocular photoreceptors, newts have extraocular photoreceptors in the pineal complex 8 and possibly the hypothalamus 9 . To determine which photoreceptors are involved in the magnetic compass response, we manipulated the wavelength of light reaching the extraocular photorecep- Extraocular magnetic compass in newtsGeomagnetic orientation is widespread among organisms, but the mechanism(s) of magnetoreception has not been identified convincingly in any animal 1 . In agreement with biophysical models proposing that the geomagnetic field interacts with photoreceptors 2-4 , changes in the wavelength of light have been shown to influence magnetic compass orientation in an amphibian, an insect and several species of birds (reviewed in ref. 5). We find that light-dependent magnetic orientation in the eastern red-spotted newt, Notophthalmus viridescens, is mediated by extraocular photoreceptors, probably located in the pineal complex or deeper in the brain (perhaps the hypothalamus).Experiments investigating shoreward magnetic compass orientation have demonstrated that the newt's perception of the direction of the magnetic field is rotated 90 o under long-wavelength (greater than 500 nm) light 5,6 . We recently trained newts under natural skylight to aim for the shore by placing them for 12-16 hours in waterfilled tanks with an artificial shore at one end 5,7 . The magnetic orientation of individual newts was then tested in a circular, visually symmetrical indoor arena under depolarized light. Under full-spectrum light (from a xenon arc source), they exhibited bimodal magnetic orientation parallel to the shoreward axis in the training tank (Fig. 1a,d). In contrast, under long-wavelength light, they orientated themselves perpendicular to the shoreward direction (Fig. 1b,e).To demonstrate that the 90 o shift in orientation under long-wavelength light was tors. Small round 'caps' (5 mm in diameter) were attached to the dorsal surface of the head of each newt using cyanoacrylate glue, and remained in place during both training and testing. Equal numbers of newts were capped with either a clear filter (Lee #130) or a filter that transmitted only long-wavelength light (equivalent to two layers of Lee #101). The caps were positioned to alter the spectral properties of light reaching the pineal and surrounding structures, whereas light reaching the eyes was unaffected.Clear-capped newts were tested to control for any nonspecific effects of the caps on the newts' orientation behaviour.All newts were trained outdoors under natural skylight and tested for m...

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