How some animals use the Earth's magnetic field to navigate

COME wintertime thousands of garden warblers, pied flycatchers, and bobolinks—all tiny songbirds—will cross the equator heading south for sunnier climes. It is an epic trip. For guidance they will rely on the position of the sun and stars, as well as smells and other landmarks. They may also use the Earth's magnetic field, thanks to a sense known as magnetoreception. Theories about it have long attracted quacks. Franz Anton Mesmer, a German doctor working in the late 1700s, argued that living things contain magnetic fluids, which, when out of balance, lead to disease. His idea of “animal magnetism” was debunked and similar ones viewed with scepticism. But magnetoreception has drawn more serious attention in the past half-century. A pioneering study in 1972 demonstrated that European robins respond to magnetic cues. The list of animals with a magnetic sense has since grown to include species in every vertebrate category, as well as certain insects and crustaceans. Some may use it simply to orient, such as blind mole rats. Others—salmon, spiny lobsters, thrush nightingales—may use it for migration and homing, alongside other sensory cues. How do they do it?

Think of the Earth's magnetic field as shaped by a bar magnet at the centre of the planet. From the southern hemisphere, magnetic field lines curve around the globe and re-enter the planet in the northern hemisphere. A few features of the field vary predictably across the surface of the Earth. Intensity is one variable—the Earth's magnetic field is weakest at the equator and strongest at the poles. Another is inclination. The angle between the field lines and the Earth changes with latitude, so an animal migrating northwards from the equator encounters steadily steeper inclination angles on its route.

Animals can potentially derive two types of information from the geomagnetic field: the direction in which they are facing, and where they sit relative to a goal. Directional information is the more basic, as polarity lets animals orient north or south as if using a compass. But this has limited utility over long distances. A strong ocean current can sweep turtles off track; winds can do the same for migratory birds. Determining latitude relative to an end point is more useful, and magnetic cues like intensity and inclination may help. Take loggerhead sea turtles (pictured). They swim from the coasts of Florida into the North Atlantic gyre, circling it for years before returning to their natal beaches to breed. Straying from the course can have deadly consequences. One study put hatchlings in test sites that simulated the magnetic fields at three points on the outer edge of the gyre. In all three cases, the turtles reoriented to stay within its confines. Another study, published in April, showed that turtles nesting on far-off beaches with similar magnetic properties (like two on either side of the Florida peninsula, at similar latitudes) had more in common genetically than with those nesting closer by. Turtles, it would seem, can get lost while searching for their natal beach. They may swim to one farther afield but more magnetically familiar and breed there.

Questions still abound. The evidence for a magnetic sense is mostly behavioural; researchers have yet to find receptors for it. Part of the problem is that the cells could be located anywhere inside an animal, since magnetic fields pass freely through tissue. (By contrast, cells that enable the other senses, like sight and smell, make contact with the external environment.) Two theories of magnetoreception dominate. One says animals have an intracellular compass. Another suggests that chemical reactions influenced by the geomagnetic field produce the sense. For researchers, this means more questions than answers.