

524 ELECTRICAL SENSES 



Each morning, two observers conducted a series of 10 to 20 trials after 

 first turning on the lights, shutting off the flow of air, and checking the 

 direction of the magnetic field. Then they simultaneously introduced two 

 circular enclosures into the pool, one in the magnetic East, the other in the 

 magnetic West (Figure 10). The enclosures consisted of plastic tubs, 30 cm in 

 diameter, with a 20-cm-wide opening near the bottom for the stingrays to 

 enter. After positioning the enclosures at a distance of 25 cm from and with 

 the openings toward the wall of the pool, the observers stepped back to 

 watch the behavior of the animals from behind a black felt screen. When one 

 of the stingrays entered either the east or west enclosure, both observers 

 blocked off the opening by lowering a gate. If the animal chose the east, by 

 definition the correct enclosure, it was rewarded with a small piece of 

 herring; if the animal took the west or incorrect enclosure, it was gently 

 prodded with a blunt Plexiglas rod as a form of punishment. Eventually, the 

 enclosures were moved to the magnetic North and South of the pool, and 

 the animal was set free for the next trial, to start 1 to 2 min later. 



It took the animals some time before they learned to rely on the magnetic 

 field in deciding which side of the pool to avoid and where to go for food. 

 Once conditioned, however, two of the stingrays made the highly significant 

 scores of 56 and 164 correct vs 22 and 84 incorrect choices respectively (P < 

 0.001, calculated with the chi-square, corrected for continuity). A third ray 

 had to be removed because it could not compete and was actually losing 

 weight. After the initial training, the stingrays' orientational performances 



N 



W V E E \ W 



N 



Normal Reversed 



Figure 10 To receive reward and to avoid punishment, the stingray Urolophus halleri 

 ignores the enclosure in the magnetic west to enter the one in the magnetic east. The 

 magnetic nature of the animal's response becomes evident upon reversal of the field: the 

 stingray again enters the enclosure in the magnetic east, though it is now located at the 

 opposite side of the pool. (From Kalmijn 1977b.) 





