No. 104: Mar-Apr 1996
To find the "magnetic mountain," you must venture out into the Gulf of California about 15 miles east of the Baja Peninsula. Out there, beneath the boat, you can find a basaltic mountain named Espiritu Santo. Next, you don your face mask and descend toward the submerged peak. At about 70 feet, you will likely find yourself surrounded by scores, possibly hundreds, of scalloped hammerheads, some as long as 13 feet. They will ignore you and the teeming fish as they slowly wheel passively around the submerged mountain. Why do these big sharks congregate in this spot? Marine biologists have been asking this for years. (SF#20)
A.P. Klimley and his colleagues decided to find the answer. First, by direct observation, they determined that the sharks' main purpose was not pro-creation, although some mating did occur. Mainly, the hammerheads just idled away the daylight hours. At dusk, they disappeared. Klimley et al next implanted some sharks with transmitters and followed them at night. This was their feeding time, they swam 10-15 miles to deep waters where they gorged on squid. At daybreak, they were back drifting around Espiritu Santo. Apparently, the mountain was just a place to rest. But how did the hammerheads find their way back so unerringly? Furthermore, by tracking the tagged fish, the researchers found the sharks often precisely followed the same paths in what seemed a featureless ocean. How did they do it?
It was quickly ruled out that they were following specific ocean currents or the bottom topography. The hammerheads seemed to possess some sort of unrecognized navigation sense. Suspecting they might be sensing the geomagnetic field, Klimley began towing a magne tometer behind the boat. Sure enough, the hammerheads were following paths coincident with lines of high magnetic gradient. And Espiritu Santo itself turned out to be a sort of magnetic beacon from which radiated these magnetic "paths" that the hammerheads followed so exactly. Now the question became: How do hammerheads -- and perhaps other animals -- sense such exceedingly small changes in the geomagnetic field?
Some birds and mammals do have small particles of magnetite in their bodies, but no one knows how they might be incorporated into a sensory organ. On the other hand, hammerheads and many other sharks are extraordinarily sensitive to electrical fields, responding to fields as low as 10-8 volt/centimeter. Possibly the sharks' forward motion cuts the magnetic lines of force generating an electrical navigational signal. No one knows as yet. And so one mystery leads to another.
(Klimley, A. Peter; "Hammerhead City," Natural History, 104:33, October 1995)
Comment. Some marine biologists suspect that some deep-water whales and dolphins inadvertently strand themselves while following magnetic "paths" like those radiating from Espiritu Santo. (See SF#38)
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