Every three years, the Royal Institute of Navigation organizes a conference focussed solely on animals. This April, the event was held southwest of London, at Royal Holloway College, whose ornate Victorian-era campus has appeared in “Downton Abbey.” For several days, the world’s foremost animal-navigation researchers presented their data and findings in a small amphitheatre. Most of the talks dealt with magnetoreception—the ability to sense Earth’s weak but ever-present magnetic field—in organisms as varied as mice, salmon, pigeons, frogs, and cockroaches. This marked a change from previous years, Richard Nissen, a member of the Institute, told me, when a range of other navigation aids were part of the discussion: landmarks, olfactory cues, memory, genetics, polarized light, celestial objects. “Everyone now seems completely sold on the idea that animal navigation is based on magnetism,” Nissen said. Human-centric as it sounds, most of the conference’s attendees believe that animals possess a kind of compass.

Scientists have sought for centuries to explain how animals, particularly migratory species, find their way with awesome precision across the globe. Examples of these powers abound. Bar-tailed godwits depart from the coastal mudflats of northern Alaska in autumn and set out across the Pacific Ocean, flying for eight days and nights over featureless water before arriving in New Zealand, seven thousand miles away. If the birds misjudge their direction by even a few degrees, they can miss their target. Arctic terns travel about forty thousand miles each year, from the Arctic to the Antarctic and back again. And odysseys of this sort are not limited to the feathered tribes. Some leatherback turtles leave the coast of Indonesia and swim to California, more than eight thousand miles away, then return to the very beaches where they hatched. Dragonflies and monarch butterflies follow routes so long that they die along the way; their great-grandchildren complete the journey.

Although the notion of a biocompass was widely disparaged in the first half of the twentieth century, the evidence in favor of it has since become quite strong. In the early nineteen-sixties, a German graduate student named Wolfgang Wiltschko began conducting experiments with European robins, which he thought might find their way by picking up radio waves that emanated from the stars. Instead, Wiltschko discovered that if he put the robins in cages equipped with a Helmholtz coil—a device for creating a uniform magnetic field—the birds would change their orientation when he switched the direction of north. By the start of this century, seventeen other species of migratory bird, as well as honeybees, sharks, skates, rays, snails, and cave salamanders, had been shown to possess a magnetic sense. In fact, practically every animal studied by scientists today demonstrates some capacity to read the geomagnetic field. Red foxes almost always pounce on mice from the northeast. Carp floating in tubs at fish markets in Prague spontaneously align themselves in a north-south axis. So do dogs when they crouch to relieve themselves, and horses, cattle, and deer when they graze—except if they are under high-voltage power lines, which have a disruptive influence.

The only problem is that no one can seem to locate the compass. “We are still crying out for how do they do this,” Joseph Kirschvink, a geobiologist at the California Institute of Technology, said. “It’s a needle in the haystack.” Kirschvink meant this almost literally. In 1981, as a Ph.D. student at Princeton University, he proposed that magnetite, a naturally occurring oxide of iron that he had found in honeybees and homing pigeons, was the basis of the biocompass. Even a handful of magnetite crystals, he wrote at the time, could do the trick. “One equivalent of a magnetic bacteria can give a whale a compass—one cell,” he told me. “Good luck finding it.” Even in animals smaller than a whale, this is no easy task. Throughout the two-thousands, researchers pointed to the presence of iron particles in the olfactory cells of rainbow trout, the brains of mole rats, and the upper beaks of homing pigeons. But when scientists at the Research Institute of Molecular Pathology, in Vienna, took a closer look, slicing and examining the beaks of hundreds of pigeons, they found that the iron-rich cells were likely the product of an immune response—nothing to do with the biocompass. The study’s lead researcher, David Keays, has since turned his focus to iron-containing neurons inside the pigeons’ ears.

The search for the biocompass has extended to even smaller scales, too. In 1978, the German biophysicist Klaus Schulten proposed that birds’ innate sense of direction was chemical in nature. According to his theory, incoming light would hit some sort of sensory mechanism, which Schulten hadn’t yet pinpointed, and induce a transfer of electrons, triggering the creation of a radical pair—two molecules, each with an extra electron. The electrons, though slightly separated, would spin in synchrony. As the bird moved through the magnetic field, the orientation of the spinning electrons would be modulated, providing the animal with feedback about its direction. For the next twenty years, it remained unclear which molecules could be responsible for such a reaction. Then, in 2000, Schulten suggested an answer—cryptochromes, a newly discovered class of proteins that respond to blue light. Cryptochromes have since been found in the retinas of monarch butterflies, fruit flies, frogs, birds, and even humans. They are the only candidate so far with the right properties to satisfy Schulten’s theory. But the weakest magnetic field that affects cryptochromes in the laboratory is still twenty times stronger than Earth’s magnetic field. Peter Hore, a chemist at Oxford University, told me that establishing cryptochromes as the biocompass will require at least another five years of research.

At the conference, the magnetite and cryptochrome researchers made up distinct camps, each one quick to point out the opposing theory’s deficiencies. One person stood alone: Xie Can, a biophysicist at Peking University. Xie spent six years developing a kind of unified model of magnetic animal navigation. Last year, he published a paper in Nature Materials describing a protein complex that he dubbed MagR, which consists of iron crystals enveloped in a double helix of cryptochrome—the two main theories rolled into one. Xie has yet to win over other researchers, some of whom believe that his findings are the result of iron oxide contaminating his lab experiments. (Keays has said that he will eat his hat if MagR is proved to be the real magnetoreceptor.) But at the end of the conference, with one mystery of animal navigation after another left unanswered, Xie told me that he felt more confident than ever of his and his colleagues’ model. “If we are right, we can explain everything,” he said. Michael Walker, a biologist at the University of Auckland, was more circumspect. If history is any indication, he said, many of the current hypotheses about how the biocompass works will turn out to be wrong.