That many animals sense and respond to Earth’s magnetic field is no longer in doubt, and people, too, may have a magnetic sense. But how this sixth sense might work remains a mystery. Some researchers say it relies on an iron mineral, magnetite; others invoke a protein in the retina called cryptochrome.
Magnetite has turned up in bird beaks and fish noses and even in the human brain, as Joe Kirschvink of the California Institute for Technology in Pasadena reported in 1992, and it is extremely sensitive to magnetic fields. As a result, Kirschvink and other fans say, it can tell an animal not only which way it is heading (compass sense) but also where it is. “A compass cannot explain how a sea turtle can migrate all the way around the ocean and return to the same specific stretch of beach where it started out,” says neurobiologist Kenneth Lohmann of the University of North Carolina, Chapel Hill. A compass sense is enough for an animal to figure out latitude, based on changes in the inclination of magnetic field lines (flat at the equator, plunging into the earth at the poles). But longitude requires detecting subtle variations in field strength from place to place—an extra map or signpost sense that magnetite could supply, Lohmann says.
Except in bacteria, however, no one has seen magnetite crystals serving as a magnetic sensor. The crystals could be something else—say, waste products of iron metabolism, or a way for the body to sequester carcinogenic heavy metals. In the early 2000s, scientists found magnetite-bearing cells in the beaks of pigeons. But a follow-up study found that the supposed magnetoreceptors were in fact scavenger immune cells that had nothing to do with the neural system. And because there is no unique stain or marker for magnetite, false sightings are easy to make.
Making sense of it all
Scientists studying magnetoreception are zooming in on two possible mechanisms: a mechanical sensor based on the magnetic mineral magnetite and a biochemical sensor based on the protein cryptochrome.
Cryptochrome, too, offers much to like. When short-wavelength light strikes it, it becomes what chemists call a “radical pair”: a molecule containing two unpaired electrons whose spins can be either aligned or not. A magnetic field can flip the spins back and forth between aligned and nonaligned states, changing the chemical behavior of the molecule. In 1978, Klaus Schulten, a physicist at the University of Illinois, Urbana-Champaign, had suggested that animals could use radical-pair reactions for magnetoreception. But he didn’t have a molecule that could support those reactions until the late 1990s, when researchers discovered cryptochrome serving as a light sensor in mammalian retinas. Most researchers focused on cryptochrome’s control over circadian clocks, but Schulten knew that the molecule could form a radical pair. “This was my day,” Schulten says. “Finally now I had a really good candidate.” In 2000, he published a study showing how magnetic fields could influence cryptochrome reactions to create light and dark patches in the visual fields of birds.
A retinal cryptochrome sensor could explain why blue or green light apears to activate birds’ compasses but red light jams them, or why birds seem to tell north from south by measuring changes in the field’s inclination instead of reading the magnetic field directly. (Crypto-chrome can’t “feel” magnetic polarity.) As with magnetite, however, scientists haven’t yet seen the molecule in action and don’t know exactly how it might alter neural circuitry. Worse, laboratory experiments show that it takes magnetic fields orders of magnitude stronger than Earth’s to trip a cryptochrome sensor.
So who’s right? It doesn’t have to be either-or, says Peter Hore, a physical chemist at the University of Oxford in the United Kingdom who likes the idea that nature evolved two different magnetoreception systems. “The map sense could be magnetite, the compass sense could be radical pairs,” he says. It would be the best of both worlds—or at least the best way to navigate this one.