In a recent experiment captive sharks learned to approach an area of the tank for a food reward when a magnetic coil surrounding the tank was turned on33. Although this was interpreted as a response to the imposed magnetic field, an equally plausible explanation is that the crucial stimulus might have been the transient electric field that is generated whenever the current through a coil is turned on21. Similarly, rays have been conditioned to move towards a specific magnetic direction in an enclosure29, but whether the rays responded to the direction of the field per se, or instead to the presence of field anomalies, is debatable1, 34.
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