From the New Yorker website, the beginning of an answer to a question we ponder every so often:
By M. R. O’Connor
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…