Vulture s-eye view. The Rüppell s griffon vulture of central Africa can cover up to 200km a day in its quest for food. The Sun s position and familiar landmarks may help it navigate, but it s highly likely that, as other birds and animals do, it also uses magnetic sensing to find its way. Animal magnetism Can birds see the Earth s magnetic field? The latest research on navigation suggests they can and even hints that humans may be able to detect it too. STORY BY PETER MEREDITH Rob Pilley / John Downer Productions Ltd / Gyps rueppellii 38 Australian Geographic January February 2013 39
All creatures great and small Migrating animals make special use of the Earth s magnetic field, but many other animals are now known to use it too. Can all animals detect magnetism? The more scientists probe this question, the more it looks as though the answer is yes. Already they know of dozens of creatures, from butterflies and birds to frogs and fish, that have this ability and use it to navigate. When you think about it, it s logical. The Earth s magnetic field is constant and allpervasive. Added to vision, hearing or sense of smell, the ability to perceive it would maximise a creature s capacity to relate to the environment and therefore to survive. TURTLES After they ve hatched on the north Florida coast, young loggerhead turtles set off on a migration that lasts several years. Joining the current that circulates clockwise around the north Atlantic, they may cover 14,000km (right) before returning to the beaches of their birth. Dr Kenneth Lohmann, a biologist at the University of North Carolina, USA, has found that the hatchlings always swim in directions that keep them in the current, wherever they may be in the Atlantic. He believes loggerheads have both a magnetic compass that reacts to the Earth s magnetic field and a mineral-based magnetometer that means they can map magnetic anomalies on the sea floor (red) the more they travel, the better their map. W HY CHICKENS? That was the question. To biologist Dr Ursula Munro, a fellow scientist s suggestion they investigate whether chickens could sense the Earth s magnetic field seemed nonsensical. I ridiculed the idea, says Ursula, an ecologist and expert on animal behaviour at the University of Technology, Sydney. A built-in magnetic compass makes sense in migratory birds, which must find their way across huge distances. But chickens? They don t move far; why would they need it? Since the 1960s, scientists have confirmed that more than 20 migratory species of bird use the Earth s magnetic field to help them find their way. In the early 2000s, Ursula herself found that the tiny Tasmanian silvereye navigates magnetically during its annual migrations up the Australian coast (see page 46). It seemed that species that took part in long-haul travel were uniquely equipped for this task. Which was why, in 2004, Ursula was dismissive when Dr Raf Freire, then at the University of New England, NSW, suggested testing chickens to see if they too had the knack. Nevertheless, she agreed to collaborate with Raf on a series of experiments using young chickens. What emerged from that research astonished not only Ursula but also the wider scientific community. I was gobsmacked, she says. O NE OF THE FIRST people to suggest that inanimate objects could exert a force on living creatures was Franz Mesmer, an 18th-century Austrian physician, although his theory of animal magnetism soon fell out of favour. Ancient skill. Biologist Raf Freire has proved that chickens have mechanisms for detecting magnetism in both the beak and the eye. The fact a bird with a long pedigree possesses these skills may mean all modern birds do too. Then, in the 1950s, scientists noted that caged European robins, which normally migrate southward in autumn, would assemble at the southern end of their cages in the appropriate season. In the following decade, Wolfgang Wiltschko, a student at Germany s J.W. Goethe University, in Frankfurt and now the world s leading authority on avian navigation placed electromagnetic coils around the cages of robins to alter the magnetic field, causing the birds to gather at a different compass point. This and subsequent investigations by him and his wife, Roswitha, proved beyond doubt that birds can detect magnetic fields and use them to orient themselves. These days we know that birds are not the only creatures that can do this. Snails, fruit flies, bees, butterflies, salamanders, newts, lobsters, frogs, bats, salmon, trout, whales, sea turtles and the mole rat of East Africa can, too (see All creatures great and small, opposite). While the evidence that these creatures can sense and use the Earth s magnetic field is pretty conclusive, how exactly they do it and what organs they use to do so are the subjects of fiercely competitive research around the world. Raf, who now lectures in animal behaviour at Charles Sturt University in Wagga Wagga, NSW, says this makes it an exciting field to be working in. A lot of important discoveries are happening very quickly, he says. You can feel the race to try and nail down what the mechanisms are. Continued page 44 Dean Golja / Gallus gallus domesticus Illustration: Jeff Goertzen; termite mounds: Getty Images WHALES Humpback whales can swim thousands of kilometres across open ocean without deviating from a straight line by more than one degree. Some scientists believe they navigate using TERMITES Magnetic termites of the NT build tombstone-like mounds aligned north south, with the flat sides facing east and west. To find out what role magnetism plays in nest-building, biologists Dr Ursula Munro and Dr Peter Jacklyn sliced the tops off several mounds and left the termites to repair them, some under natural conditions, others with bar magnets to alter the magnetic field. Checking the nests five months later, the scientists found that although externally the mounds looked similar, the arrangement of the interior chambers was different in the mounds with the magnets. The scientists theorise that Earth s magnetic field determines interior design as well as the mound s orientation. both the Sun s position and information from a built-in magnetic inclination compass. Others think the whales also possess a builtin magnetometer. Since the sea floor is highly active magnetically, this mechanism may enable them to identify magnetic features (right, in red) beneath them. This may explain whale stranding: a pod of migrating whales may misread a magnetic signpost, take the wrong turn and end up on a beach. Some whales can map the ocean floor using their calls as sonar, but humpbacks can t do this. Energy efficient. Magnetic termites in the NT use the Earth s magnetic field to orient their mounds so that the sun warms both sides equally and keeps internal temperatures stable. 40 Australian Geographic
Field of vision. Northern gannets migrate thousands of kilometres annually, probably using magnetism as well as the Sun, stars and smell to navigate. Equipped with an eye-based magnetic compass, they may be able to actually see the Earth s magnetic field. Rob Pilley / John Downer Productions Ltd / Gyps rueppellii 42 Australian Geographic January February 2013 43
Nose about. Snow geese fly more than 5000km south from North America annually. These birds may use both a visual magnetic compass and a mineral-based magnetic receptor in their beaks to identify magnetic features in the landscape. The Earth is one vast magnet, with its magnetic poles situated close to the planet s geographic poles. Matthew Gordon / John Downer Productions Ltd. / Chen caerulescens Jeff Goertzen Navigating at the local level Research shows birds can detect magnetism in a number of different forms. 1 As well as being able to sense the Earth s magnetic field through its eye, a bird has a magneto-receptor in its bill that responds to magnetic intensity on a more local level. This built-in natural magnetometer relies on bundles of magnetically sensitive iron oxides (maghemite and magnetite) in the upper beak: particles align with magnetic field lines, affecting the cells around them. An array of nerve endings relays the information to the brain. Many animals perhaps all have these magnetically sensitive iron oxides in their bodies and are believed to use them to help with navigation. Biologist Dr Raf Freire, at Charles Sturt University in Wagga Wagga, NSW, has shown that both chickens and ducks can detect and use small magnetic fields and that they may be using the magnetometer in their beaks to do so. 2 magnetic anomaly map of Australia Rocks on the Earth s surface have different magnetic intensities. These features are known as magnetic anomalies. By detecting them with its built-in magnetometer while on the move, a bird can compile a map of landmarks. Humans use magnetic anomaly maps to search for minerals. This one shows magnetic anomalies in Australia, with red indicating the most intense spots. These magnetic features are so well-defined and create such a distinctive pattern that it s easy to understand how being able to perceive them might open up a whole new navigational dimension. Scientists speculate that animals have two distinct magnetic sensing mechanisms in their bodies, each with a different function. In birds, both of these magneto-receptors are believed to be in the head. It s not known if they work simultaneously, independently or jointly. However, as the Wiltschkos suggested in a 2005 paper, one of the receptors probably acts as a magnetometer which measures magnetic intensity and the other as a compass. In animals, this action could be based on two magnetically sensitive forms of iron oxide magnetite and maghemite. Particles of these minerals in an animal s body align themselves with a magnetic field, affecting cells around them and thus firing off signals to the brain. Many bird species have tiny bundles of these minerals in the upper beak region, and Raf has found that anaesthetising the nerves in this region impairs a bird s capacity to sense a magnetic field. Different geographic features and zones have differing magnetic intensities. As a bird flies over them during a migration, its magnetometer may detect these anomalies and enable it to compile a mental map of magnetic signposts for future use. The second mechanism, the compass, may indicate direction. But unlike a conventional compass, it doesn t distinguish between north and south; it tells the bird only where a pole is and where the equator is. As Ursula says: It doesn t matter whether a bird is in the Southern or Northern hemisphere, in autumn it knows it needs to go equator-wards. HERE S HOW SCIENTISTS think the avian compass works. The organs for this mechanism are believed to be in the right eye, but perhaps in the left eye also. Research indicates this magneto-receptor may be based on pigment proteins in the retina known as cryptochromes. The Earth s magnetic field seems to induce a chemical reaction in these proteins when certain light wavelengths (mostly blue) strike the retina. This results in signals being sent from the eye to the brain via the optic nerve. Some scientists believe this may mean a bird can actually see the magnetic field. The Earth is one vast magnet, with its magnetic poles situated close to the geographic poles. Magnetic field lines extend away from the Earth at the South Magnetic Pole, travel north and plunge back into the planet at the North Magnetic Pole. So at the poles, the lines appear vertical, at the Equator they appear horizontal, and in-between they align at varying inclinations (see page 46). The precise nature of the chemical reaction in cryptochromes is believed to vary according to the angle of the magnetic field lines their inclination as they pass through the eye. Inclination, therefore, is a strong pointer to direction. Angled lines may indicate that a bird is close to a pole; horizontal lines may mean the bird is at the Equator. For the moment, what the magnetic field looks like to a bird is anybody s guess. The magnetic compass is a side-function of the eye, and magnetic information is primarily mediated by the visual system, Roswitha Wiltschko says. Yet birds must separate the magnetic from the visual information somehow. How birds perceive this information is impossible to tell. Illustrators have tried different techniques to depict what they imagine a bird may see. Some have shaded parts of the visual image in grey. Others have varied the colour intensity, with some parts of the image being brighter than others (see page 46). Raf, on the other hand, suggests the information might appear on the image as dots or blotches. Overall, the effect might be like the heads-up display projected onto the windscreen of a jet fighter to give the pilot vital information. There s a theory that by swinging its head from side to side and thus changing the angle between the magnetic field lines and its eye a bird generates a moving visual impression of the magnetic field. Some migratory birds scan the horizon in just this way before setting out on a long journey. They appear to do it more at dusk, when the dim blue-green of the sky may be of the right wavelength for maximum sensitivity to magnetism. In fact, movement, either from head-scanning or from flight itself, may be important for seeing field lines. If the inclination compass depends on light, logic says it shouldn t work in the dark. Yet some migratory birds seem to be able to orient themselves while flying at night. Ursula attributes this to the fact there is always some light available. Yes, it s light dependent, but birds can navigate at night using it because there s always light from the Moon and stars. It s never pitch black. R AF IS FOND OF chickens. Much of his work on them has been aimed at improving their welfare. Back in 2004 he was at the University of New England, where he and a team that included the initially sceptical Ursula and the Wiltschkos launched into a series of tests to find out whether the birds were equipped to sense magnetism. In initial tests, the scientists trained chicks to find a red pingpong ball hidden behind a small screen. We trained them to always search for the ball in a certain direction, say always north, and then we used copper coils to shift the magnetic field by 90 and, hey presto, instead of going north the chickens went east, Raf says. This meant they were using the Earth s magnetic field to navigate by. It was neat and straightforward. Subsequent tests proved conclusively that chickens possess not 44 Australian Geographic January February 2013 45
Bird s field of vision The magnetic field: a bird s-eye view Experts think that birds may actually be able to visualise the magnetic field. 1 How a bird sees Earth s magnetic field lines is not known for sure but research suggests it may involve dark or bright patches or shading (see above) in the field of view. The location of the bright patch or shading would indicate the direction of the pole or the equator. Sweeping the head from side to side may cause these reference points to shift around the image and thus stand out better. Birds sometimes do this when they re about to migrate. Even the movement of flight may help to clarify the image in a bird s eye, such as the Tasmanian silvereye (below), which completes a remarkable migration across Bass Strait and up the east coast of Australia to Queensland. Powerful electric currents in the Earth s core make the planet act 2 like a giant magnet. Invisible magnetic field lines rise from the South Magnetic Pole, circle the planet and dive into the North Magnetic Pole. At the poles the lines are vertical; at the Equator they are horizontal; elsewhere they are found at varying inclinations. 3 When light of a certain wavelength mostly blue enters a bird s eye and strikes its retina (left), it s thought to energise the molecules of pigment proteins called cryptochromes. This makes cryptochromes sensitive to the magnetic field. How this affects a bird s vision depends on where it is on the Earth s surface and what the inclination of the magnetic field lines (shown here in green) is at that point. This is an inclination compass, not a polarity compass, so it can t differentiate between north and south, only between equatorwards and pole-wards. Illustration: Jeff Goertzen; bird: Zosterops lateralis; Brisbane and globe: Getty Images Getty Images What about you? Magnetic sensing may exist in all creatures, including people. In animals it has been found to have various uses. In humans it perhaps manifests simply as a sense of direction. only a magnetic compass, but also an iron-ore-based magnetoreceptor in the upper beak area. They were dramatic discoveries. Although magnetic sensing was known in more recently evolved branches of the bird family, this was the first time it had been seen in an archaic lineage, such as the chicken s. What s more, it had survived thousands of years of domestication. Our thinking is that it originated in an avian ancestor, before the chicken line broke off, so it s quite an ancient skill, Raf says. An implication of this is that all birds, and perhaps most animals, may sense magnetic fields. Raf has no doubt about this. Unlike vision, which requires huge amounts of neural computing power, magnetic sensing is simple and economical. The Earth s magnetic field is omnipresent and receptors can monitor it all the time, providing constant background information. Jungle-dwelling chicken ancestors would have used it on their home range about a kilometre square, Raf says. It would be hard for them to distinguish trees in the jungle visually, so they would just use something as simple as the magnetic compass to navigate. Ursula agrees. Magnetic sensing might simply be one component among the large number of cues available to aid navigation... I believe now that it is everywhere. It makes sense. I f it s common in nature, do humans have it? Some scientists believe so, though they stress it s only a guess. In the 1970s zoologist Dr Robin Baker of the University of Manchester, UK, claimed to have shown that humans could orient themselves magnetically. However, no other scientist has been able to reproduce his results. In 1992, US scientists published a paper confirming that humans have small amounts of naturally occurring magnetite in their bodies. As for cryptochrome, scientists have known since the 1990s that humans have this pigment protein in the retina. Until recently, though, it seemed only to play a role in setting the biological clock. Then, in 2011, Professor Steve Reppert, a neuroscientist at the University of Massachusetts, USA, experimented with fruit flies that had been genetically engineered to be cryptochrome-deficient and were poor magnetic navigators. By splicing the human cryptochrome gene into the flies, he restored their navigational capacity. He d made a link between human cryptochrome and magnetic sensing. Based on our studies, I believe humans have magnetosensing abilities, Steven says. As for how we might perceive and use the Earth s magnetic field, he adds: We believe that human magnetosensing may aid visual spatial perception rather than acting as a compass for directional information. So maybe you can t tell north from south on a cloudy day. But if you re blessed with a good sense of direction, perhaps it s all down to your cryptochromes. AG 46 Australian Geographic January February 2013 47