Magnetite, the iron-bearing mineral that was the lodestone of the ancients, appears increasingly to be at least part of the basis for a magnetic "sixth" sense found in many living organisms.
Magnetite has been discovered in bacteria, honeybees, and pigeons. Now three California scientists report finding it in the dolphin.
Describing their discovery in the forthcoming (Aug. 21) issue of the journal Science, J. Zoeger of Los Angeles Harbor College and J. R. Dunn and M. Fuller of the University of California at Santa Barbara say, "This appears to be the first reported occurrence of magnetite in a mammal." It remains to be shown that dolphins do, in fact, sense magnetic fields and that they use Earth's field as a navigational aid.
Be that as it may, this is not the first indication of a magnetic sense in mammals. Last May, Robin R. Baker and Janice G. Mather of the University of Manchester, England, described experiments with wood mice that showed unambiguously that the mice sense Earth's magnetic field and use it to help find their way.
Mr. Baker also has experimented with students and thinks he has found a similar sense in humans -- a sense that humans are unaware ofbut still can use.
Thus there is little doubt that the ability to detect weak magnetic fields is widespread, constituting what was until recently an unsuspected "sixth sense."
Commenting on this, James L. Gould of Princeton University, one of the discoverers of magnetite in honeybees and pigeons, has observed: "Before the 1970s, the notion of magnetic-field sensitivity was relatively easy to dismiss. The evidence was either anecdotal . . . or weak and unconvincing. . . . In the last ten years a more substantial and compelling body of evidence has been accumulated . . ."
There are two lines of such evidence -- behavioral, showing the existence and use of the magnetic sense, and physiological, in which scientists are finding what amounts to biological magnets.
In behavioral studies, scientists not only show that a life form can sense magnetic force, they show it uses Earth's magnetic field for orientation and guidance and that it can be confused by extraneous fields. This is what was done with the British wood mice and with the students Baker has studied.
The students, blindfolded and transported in buses, were found repeatedly to have a good sense of their direction from home base. But this was destroyed when students wore magnetic helmets that masked or confused Earth's field. Baker has said he thinks it has been "proved beyond doubt that man really does have a magnetic sense of direction."
However, this conclusion is not yet generally accepted by other investigators. In particular, Gould at Princeton, working with Kenneth P. Able of State University of New York, has failed to replicate Baker's results. They do not dismiss these, but they do urge further careful experimentation.
There is no such uncertainty with a number of other life forms, especially those in which magnetic material has been found. Honeybees and pigeons show clear magnetic navigational ability. Studies by Gould and others have uncovered grains of magnetite in bees and needles of this material embedded in tissue with strong magnetic properties in the heads of pigeons.
The link between magnetite and navigation is especially clear in several species of fresh- and saltwater bacteria, which are known to orient and swim along Earth's magnetic field in a preferred direction. Under the microscope these bacteria show lines of tiny magnets in their body.
Furthermore, R. P. Blakemore of the University of New Hampshire and R. B. Frankel of the Massachusetts Institute of Technology and several colleagues, have shown that these magnets have the correct polarity for bacteria to find their way in the Northern and Southern Hemispheres, changing polarity at the equator.
Now Zoeger, Dunn, and Fuller have found magnetic material in the head of the common Pacific dolphin. Their paper in Science elaborates on an earlier report thay gave at the spring meeting of the American Geophysical Union a year ago.
Dissecting four dolphins that had died of natural causes, they found magnetic tissue in all of them. However, in one, they also found a flake of material that is at least partly magnetite. Attached to this thin flake is a tissue structure that could be a nerve assembly. The scientists speculate that at least this species of dolphin can sense the torque Earth's field would exert on the flake as the animal moves. Or perhaps the flake flexes under the magnetic torque and this flexing is sensed.
Although the dolphin has not yet been shown to have and use a magnetic sense, the scientists note that "the association of apparent nerve fibers with the magnetite suggests that the magnetite . . . has a sensory function."