Turtles, birds, and butterflies can migrate thousands of kilometers--even over vast oceans largely free of landmarks. Scientists suspect that these animals find their way by sensing Earth's magnetic field, yet the exact nature of this internal compass has remained a mystery. Now, researchers believe they have come closer to solving the puzzle: a magnetic-sensing chemical reaction within the eye.
Scientists are split over how animals sense Earth's magnetic field. Some have argued for actual magnets: They say that microscopic iron crystals in a bird's beak help it navigate by sensing the alignment of the metal relative to Earth's magnetic field. Such magnetite crystals have been found in birds' beaks, but it's not clear how, or if, they're wired up to the brain.
Another theory relies on chemical reactions affected by magnetic fields. When struck by light, a protein in the eye called cryptochrome changes into one of two states that differ in the position of an unpaired (or radical) electron. The ratio of these two states depends on the orientation of cryptochrome to magnetic fields. The "radical-pair" camp argues that birds navigate chemically with cryptochrome and visually by tracking the position of the sun and stars.
Two new studies support the radical-pair idea. In the first, a team led by Henrik Mouritsen, a biologist at the University of Oldenburg in Germany, altered the brains of robins. In one case, they snipped the nerve connecting the brain to the beak. In another, they created a lesion in a brain region called cluster N, which is thought to process the magnetic sensing of eye cells bearing cryptochrome. The results, published 29 October in Nature, showed that robins can still detect magnetic fields even without the nerve connection to their beaks. But with a lesion in cluster N, the birds not only lost their ability to navigate but also couldn't detect magnetic fields in the laboratory.
In the second study, Erin Hill and Thorsten Ritz, biophysicists at the University of California, Irvine, addressed a criticism of the radical-pair theory. Cells in a bird's eye might contain cryptochrome, say critics, but the cells will not be perfectly aligned with each other. All that disorder would introduce too much noise for the brain to translate the cryptochrome reaction into a magnetic sense, they say. Hill and Ritz calculated how much disorder would make cryptochrome useless as a biological compass. The results, published online this week in the Journal of the Royal Society Interface, indicate that a messy cellular environment isn't necessarily a deal-breaker for the radical-pair theory. According to Ritz, even using the cryptochrome in a single cell, a bird should be able to sense its orientation relative to Earth's magnetic field. "But what that cell looks like is still unknown," he says, so the real arrangement of cryptochrome remains a mystery.
The new evidence "removes a major obstacle for the [radical-pair] hypothesis," says Kenneth Lohmann, a behavioral biologist and an expert on turtle migration at the University of North Carolina, Chapel Hill. But it's still possible that both sides of the internal compass debate are right. "The weight of the evidence now seems to favor the existence of at least two different mechanisms of magnetoreception," Lohmann says, "one based on radical pairs and the other on magnetite."