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Magnetic compass orientation in amphibians is mediated by a lightdependent magnetoreception mechanism (Phillips & Borland 1992. Nature 359:142) involving extraocular photoreceptors located in or near the pineal organ (Deutschlander et al. 1999. Nature 400: 324). The functional properties of this response are consistent with a photoreceptor-based magnetoreception mechanism involving a photo-induced radical pair reaction (Ritz et al. 2000. Biophysics J. 78:707). The radical pair mechanism is predicted to be sensitive to radio frequency fields, with the most effective frequencies being those that are in resonance with the hyperfine couplings of the radical pair. Here we test the prediction that a broad band RF field overlapping the range of the Zeeman and hyperfine splittings (~1-40 MHz) should cause a change in orientation if magnetoreception is based on a radical-pair mechanism. Experimental subjects were Eastern red-spotted newts Notophthalmus viridescens that were trained in an aquatic outdoor tank with an artificial shore at one end to exhibit shoreward magnetic compass orientation (earlier references). Newts were tested in an terrestrial, visually symmetrical, indoor arena. Each newt was tested only once with equal numbers tested in each of four horizontal alignments of a static earth-strength magnetic field (magnetic North at geographic North, East, South or West). Magnetic compass orientation was compared with and without the presence of a low level, broadband oscillating field (~0.1 MHz-15 MHz). Depending on the length of training in the outdoor tank, newts tested in the absence of the oscillating field exhibited highly significant unimodal or bimodal orientation relative to the trained magnetic direction (p < 0.01, Rayleigh test). In the presence of the oscillating field, both responses were abolished. To determine whether the effects of the oscillating field were specific to the light-dependent magnetic compass, control experiments were carried out in an outdoor arena in which newts were able to orient using celestial compass cues (sun position and/or polarized light patterns). Celestial compass orientation was unaffected by exposure to the oscillating field. These findings suggest that the effects of the broadband oscillating field are specific to the magnetic compass, and provide further evidence for the involvement of a radical pair mechanism. more...