Lots of creatures, from bees to whales, perceive magnetic fields and use them to orient. However, we’re still not certain how animals sense the fields. Detecting them with sensitive electronic instruments is one thing. But “sensing magnetic fields as weak as that of the Earth is not easy using only biological materials,” says Henrik Mouritsen, a biologist who studies the mechanisms underlying animal navigation at the University of Oldenburg in Germany. Birds possess no obvious sense organ devoted to the task. But because the field can pervade tissue, the sensors may be hidden deep within their
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