Everywhere around us, Earth's magnetic field is passing through our bodies. We are immersed in it. Yet we go through our entire lives without feeling it. But some animals can feel the Earth's field. A turtle crossing the Pacific, a bird flying through the night, a salmon approaching its river of origin, or a shark moving through the vast ocean, are travelling through a world of information that we cannot perceive. That is one of the strange things about life on Earth: the planet emits signals everywhere, but evolution has given different animals the ability to perceive different facets of it.
Image above: Earth's magnetic field on the surface in June 2014
A planet-sized compass
Earth behaves somewhat like a giant magnet. Its magnetic field, generated by its spinning core, extends far out into space and surrounds the planet. At the surface, the field varies in strength and direction from place to place. This creates a remarkable possibility: if an animal can detect the magnetic field, it can use it as a map.
The magnetic field is not identical everywhere. Its strength, inclination, and other characteristics change geographically, so an animal capable of detecting these differences can use them to navigate.
Sea turtles and their invisible maps
Few marine travellers are more mysterious than sea turtles. Young turtles leave their birthplace and travel vast distances across the ocean without landmarks to guide them. Yet they find their way.
Experiments have provided some of the most intriguing evidence for magnetic navigation in animals. Young sea turtles exposed to magnetic fields resembling those found at different locations respond by changing their swimming direction.
They do not just seem to know which way is north. They respond differently to the magnetic signature associated with different geographical locations. This has led scientists to propose that sea turtles possess a magnetic map. It helps explain how a turtle can navigate far through a featureless ocean. It follows the magnetic environment that guides it across the planet the way we follow landmarks.
Salmon: finding the way home
Salmon face an equally remarkable navigational challenge. Many species spend years of their lives in the ocean before returning to their home stream to reproduce. Their journeys can take them hundreds or thousands of kilometres.
How do they find their way?
Smell is extremely important once salmon approach their natal rivers. They know the scents associated with their home stream and use those cues during the final stages of their migration. But smell cannot explain how a salmon finds its way home across the vastness of the ocean.
Research has shown that salmon respond to magnetic fields and that their movements are associated with the magnetic characteristics of different regions. Researchers think that salmon learn the magnetic signature of the area where they enter the sea as juveniles then, years later, they use the changing magnetic landscape to navigate back to the same region. Once back in their home region, they can also use their sense of smell to locate the very stream where they originated.
Birds that see the magnetic field
Bird migration provides some of the most remarkable examples of animals using Earth's magnetic field.
Migratory birds travel astonishing distances, crossing continents and oceans, sometimes at night, when landmarks are difficult to see. Experiments have shown that many birds orient themselves using Earth's magnetic field.
But here the mystery becomes even more intriguing. One proposed mechanism involves light-sensitive proteins called cryptochromes, found in the eyes of birds.
According to the leading hypothesis, light entering the eye initiates chemical reactions in Cryptochrome proteins, which act as light-sensitive, quantum sensors allowing birds to detect the direction of Earth’s magnetic field. Since these cryptochrome proteins are inside the photoreceptor cells of the retina, it appears that the changing chemical reactions subtly modulate what the bird seas. Scientists believe that, superimposed on the bird’s field of vision, is a visual representation of the magnetic field.
Magnetite
Another possible mechanism involves magnetite. Magnetite is a naturally magnetic mineral composed of iron and oxygen. Tiny magnetite particles have been found in various animals, including some species known to navigate using magnetic information. Because magnetite responds physically to magnetic fields, it has long been considered a candidate for biological magnetic sensing.
A microscopic magnetic particle could move or exert force when Earth's magnetic field changes. The sensory system could potentially detect that movement and convert it into information conveyed through the nervous system. But finding magnetite in an animal does not, by itself, prove that the animal uses it as a magnetic sensor. This is one of the great challenges in studying magneto-reception. Scientists need to demonstrate not simply that magnetic material is present, but how it connects to the animal's sensory systems and behaviour.
Sharks
Sharks bring the mystery into the ocean in a particularly interesting way. They already possess an extraordinary sensory system for detecting electricity: the ampullae of Lorenzini. These tiny sensory organs allow sharks and their relatives to detect extremely weak electrical fields in seawater. And seawater is moving through Earth's magnetic field.
That creates a remarkable physical possibility. When a conductor moves through a magnetic field, it can generate an electrical potential. A large animal swimming through seawater therefore produces electrical signals associated with its movement through Earth's magnetic field.
Moving seawater itself can generate electrical signals. Could sharks use their electro-receptive system to detect magnetic information indirectly? Laboratory experiments have demonstrated magnetic orientation in some sharks and rays, and electro-reception provides a plausible sensory pathway. But the precise mechanisms involved remain an active area of research. Their electrical sense may give them access to information about magnetism, but exactly how different species use that information is still being investigated.
Rays, skates and other ocean travellers
Rays and skates also possess electro-receptive organs and have demonstrated magnetic responses in experimental studies. For animals living over sandy bottoms, continental shelves, and open water, magnetic information could provide a useful navigational reference when visual landmarks are limited.
Some marine animals travel through environments that change dramatically with depth and distance. Others migrate seasonally between feeding and breeding areas. A magnetic sense could complement other information from currents, temperature, smell, sound, light, and the Earth's physical landscape. Animals likely integrate information from all their senses to navigate.
Whales and dolphins: Another mystery
Large marine mammals have also attracted interest because many species undertake extraordinary migrations. Humpback whales, for example, travel between high-latitude feeding grounds and tropical breeding areas.
There is intriguing evidence and considerable speculation that they use the earth’s magnetic field for navigation, but the case is not as firmly established as it is for some migratory birds, sea turtles, and other animals.
Whales have other ways of navigating. They can detect sound over enormous distances, respond to coastlines and seafloor features, and may use changes in water temperature and currents. The scientific mystery is not simply "Which animals have a magnetic sense?" It is also: What information does each animal actually obtain from the magnetic field, and how does it combine that information with its other senses?
Insects see Earth's magnetic field
Magneto-reception is not restricted to large migratory animals. Some insects appear capable of detecting Earth's magnetic field. Honeybees, for example, have been studied for magnetic sensitivity, while evidence from ants suggests that magnetic information may contribute to orientation in some species.
The discovery that tiny animals can respond to a planetary-scale physical field changes the way we think about navigation. An animal does not need to see a mountain, follow a river, or recognise a coastline. It can potentially carry a directional reference with it wherever it goes.
Young animals sense and learn about the magnetic field
One of the most intriguing questions is whether magnetic navigation is entirely inherited or partly learned. Some young animals seem to possess an innate response to magnetic direction, for example, young birds migrating alone for the first time. But they also appear to learn features of the magnetic environment over time.
A map written in the Earth
What is extraordinary is that these animals are not merely detecting Earth's magnetic field: they are reading its geography. Across the planet, magnetic intensity and inclination vary. These variations create patterns that an animal capable of detecting them can use to distinguish one place from another.
For a sea turtle crossing thousands of kilometres of ocean, this could provide a form of navigation that requires no visible landmark. For a bird crossing an ocean or a continent at night, it could provide a directional reference. For a salmon returning to its home river, it provides a guide through the vast, featureless ocean. For a shark moving through a changing seascape, magnetic information might supplement its electrical sense.
The same planet that provides living things with an intricate physical habitat, also provides them with an invisible coordinate system.
Ethologist Ila France Porcher, author of Yes, Fish Feel Pain, The True Nature of Sharks, and six other books on wildlife behaviour, spent fifteen years closely observing fish and shark behaviour in Tahiti, resulting in several scientific papers. Her writings are based on decades of first-hand observations of wildlife and focus on the individuality and intelligence of individuals, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates, and international media for its unique blend of field observation, art, and science.
