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ELECTRICAL SENSES 



Although their outcome was consistent with the proposed geomagnetic 

 orientation, neither of these tests was fully conclusive. The avoidance 

 reactions proved the sharks' sensitivity to fields of geomagnetic strengths, 

 but were not of obvious biological significance. The animals' homing 

 tendency was biologically more interesting, but at the time we were not 

 technically prepared to reverse the field to randomize any remaining 

 alternative cues, and thus to verify the magnetic nature of the response. 

 Though preliminary, these early observations did encourage me to pursue 

 this new line of research. 



After moving to Massachusetts, I constructed new, specifically designed 

 magnetic facilities on the Quissett campus of the Woods Hole Oceanographic 

 Institution. To scale down the technical problems of controlling the ambient 

 magnetic field, I looked for a good experimental animal of smaller size, 

 which one of my students found in the round stingray, Urolophus halleri 

 (Figure 8). The round stingray is a hardy, alert, and very lively elasmobranch 

 of subtropical and tropical seas, reaching an average fin span of 25 cm. We 

 selected specimens of only 15 to 20 cm to fit our tanks. To establish their 

 magnetic abilities, we trained them to seek reward and avoid punishment at 

 positions predetermined by the direction of the earth's magnetic field 

 (Kalmijn 1977a,6). 



Figure 8 The two stingrays Urolophus halleri that learned to secure food from a plastic 

 tub in the magnetic east and to avoid a similar enclosure in the magnetic west of their 

 tank. The enclosures have their gates facing the wall (cf. the diagram of Figure 10). Inside 

 screens with openings of their own can be rotated to open or close the gates. 



