Lemon Shark (Negaprion brevirostris)
Magnets are familiar strangers. Most of us have at least one around the house - in our telephone receivers, stereo and TV speakers, or used to turn our refrigerators into bulletin boards and art galleries. The earliest magnets (circa 500 BC) were naturally-occurring lumps of an iron oxide mineral known as 'magnetite' (Fe3O4) and were imputed with all manner of mystical properties. Today, magnets come in all shapes and sizes: from simple bars and 'classic' horseshoes to various business logos and cartoon characters to food, fishes, cows, and even baby-blue pigs with 'Think Thin' written across their bellies - thereby holding up both our best intentions and our kids' baseball schedules. Magnets are everywhere. But few of us understand how they work.
Magnetism and electricity are fundamentally interconnected. Danish scientist Hans Christian Oersted showed in 1820 that an electric current flowing in a wire deflects a compass nearby. Whenever an electric current flows - whether from cloud to ground in the form of lightning or through a contracting muscle in the body - a magnetic field is created. The unification of electric and magnetic principles under a comprehensive mathematical theory was first achieved by Scottish physicist James Clerk Maxwell in 1864. It is now known that magnetism is a property of the atom itself. Ultimately, the magnetic properties of matter are determined by the collective behavior of the negatively charged electrons that orbit the nuclei of atoms. The magnetic dipole moment (or magnetic field) of an individual electron has two components, one resulting from the spin of the electron about its own axis, the other from its orbital motion about the nucleus. Both kinds of motion may be considered as tiny circular currents (moving charges), thus linking electricity and magnetism at an atomic level.
In most atoms and molecules, these electronic magnets are oriented in random directions and the sum total magnetic moment of all the orbital electrons is zero. However, in some highly ordered crystalline materials - such as iron, nickel, and cobalt - the spins of some orbital electrons in adjacent atoms become coupled (I will spare you the quantum physics), creating local magnetic 'domains' in which magnetization is unidirectional. Adjacent domains are magnetized in different directions, so that there is no bulk magnetization. When an external magnetic field is applied, those domains aligned with the field grow at the expense of others, resulting in a very strong type of permanent magnetization known as 'ferromagnetism'. So from where does this 'external field' come? For the answer, we must look to the Earth's core.
The Earth's liquid outer core is 2,200 km thick and flows between the mantle and the solid inner core. It is a circulating mass of nickel-iron alloy, but is not intrinsically magnetic because no material could retain parallel magnetic domains at the temperature of the Earth's core (about 4,000 °C). Instead, the Earth's magnetic field is generated by the circulation of electric charges in the molten mass. The magma moves due to convection currents that are driven by differences in heat between the inner and outer core (about 2,500 °C ). The coriolis forces that arise from the Earth's rotation twist these currents into 'rollers' of fluid. The rollers - no one is sure how many are aligned along the Earth's axis of rotation at any given time - act like the coil of wire in a dynamo to generate a dipole magnetic field. The Earth itself behaves like a gigantic magnet.
This was first proposed by William Gilbert, personal physician to Queen Elizabeth I, in his epic work Epistola de Magnete (1600). After years of experiments, Gilbert concluded that magnetic materials oriented themselves north-south and dipped downward because the Earth acts as a bar magnet whose attractive poles correspond closely to the geographical ones. The Chinese were probably the first to realize the directional properties of magnetic materials, which they used as an aid to fortune-telling. The application of a magnetized needle to navigation also originated in China, about 1000 AD. By 1200, simple compasses - consisting of a magnetized needle afloat on a piece of cork or straw - were used by European explorers. The magnetic compass suddenly enabled sailors to find an absolute direction anywhere on the globe without the complicated astronomical calculations required for celestial navigation. Using his magnetic compass, Columbus could take his bearings direct for Cipangu (Japan) and stay on the same latitude ... until he bumped into the New World. Needles whose direction-finding qualities had grown weak could be remagnetized by a precious piece of magnetite guarded by the captain. No wonder magnetite was better known as lodestone - 'the stone which leads'.
Just as the mechanical clock freed mankind from the daily need to measure time by the sun and the stars, the magnetic compass oriented mankind anew in space. This technology extended the times and seasons of seafaring, opening the globe to human exploration. But it is generally true that whenever a principle of physics exists that is applicable within the constraints of organic life, at least one species or taxon has developed a way to utilize that law of nature to survive. Since Gilbert's classical work there have been numerous reports of organisms responding or orienting to Earth's magnetic field. Until recently, however, some ambiguity has surrounded many of these studies and no mechanism of interaction between organism and field had been established. Just when it seems we have mastered almost everything there is to know about bacteria, some new and totally unexpected phenomenon comes along to renew our respect for the subtlety of living processes - even at the level of these tiny prokaryotes.
Studies of bacteria have provided definite evidence of the interaction of organism and magnetic field. In 1975, Richard Blakemore discovered marine bacteria which preferentially swim north along the lines of Earth's magnetic field. The swimming motion of these 'magnetotactic bacteria' is linear compared with that of most microorganisms, which periodically tumble and change direction. Each magnetic bacterium is propelled by flagella. With respect to orientation, these cells act as single magnetic dipoles - that is, their behavior is the same as compass needles or bar magnets. If an external magnetic field is applied, the long axis of the cell parallels the lines of force. If the field is rotated, the axis follows. When examined by electron microscopy, magnetic bacteria show one or two rows of small (0.1 micron), dense granules of magnetite running along the cell axis. Mössbaur spectroscopy, a technique especially sensitive to the chemical state of certain metallic crystals, showed that each of these granules are single-domain magnetic crystals.
The first magnetic bacteria to be studied came from the Northern Hemisphere and uniformly swam toward the north magnetic pole. Southern Hemisphere magnetic bacteria swim toward the south magnetic pole. This symmetry provides a clue to the biological raison d'être of magnetotaxis. In the Northern Hemisphere, the geomagnetic field is directed downward so that a north-swimming bacterium has a component of motion directed down into the sediment. If such a bacterium were taken to the Southern Hemisphere, it would swim upward - away from its food and anaerobic habitat. In the Southern Hemisphere the polarity is reversed, and indigenous south-seeking bacteria are also directed to their optimal habitat. Therefore, the major biological role of magnetotaxis appears to be keeping these bacteria in or near the bottom sediment. Bacteria clearly discovered the principle of the compass eons before their distant human relatives managed to accomplish that feat.
Geomagnetic phenomena are varied and widespread, so it is hardly surprising that many types of creature are known to use - or suspected of using - magnetic orientation. These include a great many migratory marine animals, including: the aforementioned bacteria, nudibranchs, lampreys, sharks, skates, rays, salmon, tunas, eels, sea turtles, seabirds, dolphins and other toothed whales. Complex electroreceptors have been found in some of these creatures (notably the elasmobranchs) and particles of magnetite have been found in most of the others. However, the evidence for actual geomagnetic orientation is inconclusive or contradictory.
The best documented case for a magnetic compass involving a known sense organ is in elasmobranchs. Adrianus Kalmijn of the Woods Hole Oceanographic Institute is the pre-eminent researcher of electric and magnetic reception in these fishes. In 1982, to test whether the Round Stingray (Urolophus halleri ) can orient to magnetic fields, Kalmijn set up weak magnetic fields in an aquarium to simulate the Earth's field in the fish's natural habitat. The rays were trained to feed in the Earth's magnetic and geographic east. When the simulated field was rotated through various degrees, the rays sought food in the 'east' defined by that imposed shift. In field experiments conducted during 1987, Kalmijn set up a ring of electromagnets in shallow waters off Bimini, Bahama Islands, along the known daily route of a Lemon Shark (Negaprion brevirostris). In a series of 'control' trials, Kalmijn observed the shark's path while the electromagnets were off. On the next day, Kalmijn switched on the pair of electromagnets which aligned with Earth's magnetic field at that location; when the shark swam by it maintained its course as though nothing had changed. Next day, as the shark swam through the ring of electromagnets, Kalmijn switched the pair of electromagnets which rotated the artificial field 90° to the weaker natural field. The shark altered its course by 90°C in the same direction as the field shift. Here, at last, is a clear demonstration of magnetic orientation by a marine creature in the wild.
So how do migrating creatures detect geomagnetism? Earth's magnetic field causes an electric current to flow through anything moving across it which can conduct electricity. This effect is used in the construction of dynamos. In a simple dynamo, an electric current is produced in a coil of wire by rotating the coil between opposite magnetic poles. A living organism may be thought of as a bag of water throughout which various ions are distributed. When an organism moves across the Earth's magnetic field, these ions move and tiny electric currents flow in its body. However, when an organism moves along the magnetic field, no current flows. The ocean is a visually concealing medium, making line-of-sight piloting and celestial navigation of little use to marine creatures. If a marine creature is able to detect the tiny currents induced in its body by geomagnetism, they would provide magnetic landmarks to help it navigate through the local environment.
Earth's magnetic field may provide several types of cues for navigation. Because the magnetic poles are displaced from the geographic rotational poles, there is usually a difference between the true north-south meridian and magnetic north. This 'magnetic variation', which may be large in some places and times, changes slowly over a period of years as the magnetic poles drift. In addition to these horizontal and vertical directional components, the geomagnetic field also has a third dimension, intensity. Geomagnetic field strength increases with latitude, from 30,000 nanoTeslas (a Tesla is the magnetic induction equal to 1 volt-second per square metre) at the equator to less than 70,000 nanoTeslas at the poles. The field intensity also decreases, as might be expected, with distance from the Earth's surface. In addition to these global features, there are low-amplitude, localized magnetic irregularities in both space and time called 'geomagnetic anomalies'. Theoretically, all these features could provide magnetotaxic creatures with a richly textured magnetic 'map'.
But even the most detailed map is subject to the navigator's skill in reading it. Large groups of toothed whales occasionally swim ashore and lie stranded on the beach, unable to move their huge bulks back into the sea. Such 'mass strandings' tend to recur at certain sites, notably New England, northwest Britain, Tasmania, and parts of South and Western Australia. Accounts of mass standings date back centuries, yet until recently there was no explanation for this bizarre behavior. In 1988, Margaret Klinowska of Cambridge University plotted records of mass strandings in Britain and the United States against geomagnetic maps. These maps plot variations in intensity of the magnetic field at the Earth's surface caused by differences in the underlying rocks. The variations are represented as contour lines so that areas of high magnetism appear as 'hills' and low magnetism show up as 'valleys'. Klinowska's analysis revealed that most mass strandings and virtually all repeated strandings occurred where the magnetic 'valleys' were oriented perpendicular to the shore.
This sensational finding suggests that at least some whales navigate by following a magnetic map of the ocean floor. On land, magnetic variations are very irregular and there are many visual cues to guide navigation. There are no such landmarks in the vast, dark ocean. But there are regular magnetic variations. Magnetic hills and valleys stretch for huge distances across the ocean floor, and toothed whales seem to use the magnetic contour lines as invisible 'roads'. These magnetic freeways often follow continental margins, but not always. Klinowska theorises that whales may strand when they follow these magnetic roads onto shore. Klinowska has also suggested that the daily pattern of variation in the total geomagnetic field may function as a biological 'travel clock' for whales; solar activity can affect this pattern, possibly causing irregular fluctuations which disturb the clock. Therefore, whale mass strandings may be the magnetic equivalent of traffic accidents.
How do these whales sense Earth's magnetic field? Evidence is accumulating from Germany suggesting that cetacean retinas, which contain magnetite, are sensitive to magnetic fields of an intensity consistent with geomagnetism. Whether toothed whales actually navigate by geomagnetic cues is not yet clear. Mass strandings usually involve toothed whales which migrate over long distances. The more sedentary dolphins and porpoises seldom run into these difficulties. They presumably become familiar with all the small local anomalies and so build up a much more detailed magnetic map of the area in which they live. How toothed whales detect and respond to such minute local variations in geomagnetism remains a mystery, but Klinowska's magnetic highway theory is the best model we have to explain why these whales mass strand. Who knew magnetism could be so dangerous?
.... I don't know about you, but I'm getting rid of those silly plastic fruits on my refrigerator door!