The idea that animals have a "sixth sense" to navigate has long been regarded as a folk notion more akin to crystal balls than hard science. But current research indicates that a range of organisms--from bacteria to large mammals--may use internal compasses to get around.
Organisms as diverse as bacteria, butterflies, salamanders, whales and humans all contain magnetically aligned material, some of which is known to be magnetite (iron oxide). The function of this magnetite may vary greatly among species, but the most interesting speculation centers on magnetoreception, the theory that some organisms use biological bar magnets to enhance their sense of direction.
In many creatures, chains of magnetite crystals form "compasses" that aid migration over long distances and the location of food close to home. These internal magnets are made through the process of biomineralization--the extraction of minerals from the environment and their conversion into a usable form.
Results of some recent research shed light on, among other things, why whales and dolphins sometimes beach themselves at certain points. And preliminary work with humans suggests to some scientists that man might have biological compasses as well, an idea that is met by others with skepticism.
The field was once considered "a romping ground for charlatans," according to Caltech geophysicist Joseph Kirschvink. But the skepticism surrounding magnetoreception subsided in 1975 when then-graduate student Richard Blakemore discovered bacteria containing chains of magnetite crystals encapsulated in organic membranes.
Each of these crystals is a magnet unto itself, in many respects similar to, but much smaller than, the bar magnets used by schoolchildren. A bacterium's membrane keeps these individual crystals literally "in line," preventing them from lumping together. Held together by the membrane, the individual magnets can then work as a team to form a single magnet called a magnetosome.
Bacteria that contain magnetosomes have a great advantage over those that do not. While the latter move about in a random fashion, frequently traveling in circles, the former are able to swim in straight lines, which allows them to cover more new territory in their search for food.
The 20 or more magnetite crystals that make up a magnetosome measure from 400 to 1,000 angstroms. (An angstrom is equivalent to three-billionths of a foot.) At the proper size, these crystals form a magnet that is locked into a specific magnetic orientation, such as north-south.
Since Blakemore's discovery, magnetotactic (magnet-containing) microbes have been found in oceans and freshwater lakes throughout the world. In addition, many higher animals show evidence of using the Earth's magnetic field in navigation.
In 1989, Michael Walker and M. E. Bitterman, both of the University of Hawaii, trained honeybees to respond to very small changes in local magnetic field intensity, thereby augmenting a previous discovery that the bees carry magnetosomes within their abdomens. In a study to be published in September of this year, Kirschvink and his wife, Atsuko, replicated Walker and Bitterman's results.
Work with yellowfin tuna led to the discovery of magnetosomes in tuna skulls, and researchers have also located structures in salmon that are identical to those found in magnetotactic bacteria. Salmon use odor as a means of navigation, and magnetoreception may serve as a backup system.
Work with whales and dolphins by Margaret Klinowska of Cambridge University and similar studies by Kirschvink have investigated the possible link between magnetoreception and strandings, the inexplicable beaching of migrating cetaceans.
To appreciate their results, it is helpful to visualize the Earth as a gigantic magnet with its magnetic poles roughly located at its geographic poles. The magnetic field over the continents is fairly constant. Not so with the field over the ocean floor. Underwater volcanoes along the Pacific and Atlantic mid-oceanic ridges spew forth magnetite-containing lava. As the lava cools, the poles of the magnetic crystals align in the direction of the Earth's magnetic field.
Periodically throughout geological time, the Earth's field has changed directions (from a north-south configuration, as we now have, to a south-north configuration). When the Earth's field reverses, the orientation of each newly formed crystal of magnetite also changes, though the magnetite crystals in already-cooled lava do not change their orientation.
Long bands of alternating magnetic pull (north-south or south-north), which run parallel to the mid-oceanic ridges are thereby formed. As a result, the ocean floor is "zebra-striped" with bands of magnetite--one stripe oriented north-south, the next south-north. Bands of strong magnetic pull (maxima) are formed when the Earth's magnetic field is strong in either direction. Much thinner bands of weak magnetic pull (minima) are formed as the Earth's magnetic field is changing its orientation.
As the continental plates are pushed away from the mid-oceanic ridges in opposite directions--in the continual process of plate tectonics--new magnetically banded ocean floor is created. Whales and dolphins generally ride the minima--the narrow band between the north-south and south-north configurations.
Provided that an animal possesses a natural compass, it could use this magnetic road map for charting a course through otherwise featureless seas by following "roads" of magnetic maxima or "center lines" of magnetic minima.
Using detailed maps of the Geological Society of America's East Coast magnetic survey and exact positions of cetacean strandings from Smithsonian Institution compilations, Kirschvink has just completed an update of a study done in 1986. Results from both studies, as well as Klinowska's studies of strandings in British waters, indicate that whales and dolphins tend to beach at coastal points of magnetic minima.
The researchers hypothesize that cetaceans follow bands of magnetic minima during long-distance migrations. Strandings may occur when the animals, for reasons not yet known, follow these bands onto shore at coastal magnetic irregulari
Some dolphins and toothed whales, however, tend to strand near magnetic maxima, which indicates that a cetacean's choice as to whether to follow magnetic minima or maxima may depend on a variety of factors. For example, during migration it might be more practical to follow magnetic minima, while during feeding a creature might follow magnetic maxima around seamounts, since these are usually areas of great biological productivity and sources of food.
Psychologist Gordon Bauer of the University of Hawaii has analyzed the head regions of humpback whales and several species of dolphins. He found high concentrations of magnetite in the cetaceans' skulls in approximately the same region in which magnetite has been identified in the skulls of homing pigeons and tuna.
Kirschvink's work with human cadavers has so far been inconclusive, but he adds, "We know what the (magnetite) structure looks like in lower animals and it should be similar in higher animals, including humans."
Humans certainly biomineralize iron--probably as we do calcium--from the foods we eat. Additionally, magnetic material is detectable in human tissue. Indeed, the older the person, the higher the iron content of the brain. What is not known is whether this iron is in the form of magnetite and, if it is, whether it is used in magnetoreception.
Kirschvink, for one, is not yet ready to embrace the possibility of human magnetoreception. Rather, he speculates that magnetite in humans, if it does exist, may act as "a trace-metal dump," sequestering harmful trace metals from the rest of the body in much the same way that trees deposit wastes in their bark. "Once they're (the trace metals) inside a magnetite crystal, they're inert," he explained.
R. Robin Baker of the University of Manchester is far less skeptical. He has conducted experiments designed to replicate, with blindfolded humans, tests that have demonstrated a magnetic sense in pigeons. Baker says his findings indicate that humans appear to possess such a sense of direction, which he hypothesizes is impaired when the subject is allowed to use other senses or cues, such as sight or topography.
The fact that we are unaware of any "sixth sense" is not in itself reason to discount human magnetoreception. Baker's subjects tended to abandon the hypothetical magnetic sense as soon as other information was made available, such as the warmth of the sun or the direction of the wind on their blindfolded faces.
As with most species that have been studied, humans prefer some cues to others and organize their available senses in a hierarchical way. Pigeons, for example, prefer landmarks and cues from the sun and stars when these are available and relegate their magnetic sense to a subordinate position.
Further biochemical and behavioral research into magnetoreception may someday remove the shroud of mystery that surrounds the notion of a sixth sense in humans, while establishing a common chain of magnetite between the lowly bacterium and the Earth's highest animal.
The Theory of Animal Migration
Some researchers believe that the theory of magnetoreception may unlock the secret of animal migration. Within the tissue of all animals lie magnetic particles, which, when bunched together, form a bar magnet. This magnet then relates to a set of invisible magnetic "bar codes" on the ocean floor that define north and south.
Animals ingest iron oxide particles (magnetite) that form a chain of crystals in the tissues, creating an internal magnetic compass.
Underwater volcanoes along the mid-ocean ridges spew forth magnetite-containing lava. As the lava cools, the magnetic crystals align with the Earth's magnetic field. Periodically throughout geological time, the Earth's field has changed directions. When it reverses, the orientation of the newly formed crystal of magnetite also changes. As a result, the ocean floor is striped with bands of magnetite, some oriented north-south, others south-north. Migrating animals use the bands as invisible landmarks.