A Sense of Magnetism

The Summer 2012 issue of I, Science took a look at perception, discussing senses and their limitations. However, one sense that was not touched upon was that of magnetism, called ‘magnetoception’. The way magnets attract and repel things, or even levitate them (most notably frogs), gives magnetism the aura of the artificial, or the quality of a neat trick; it seems outside the realm of the living.

However, living things do detect and react to magnetic fields, particularly that created by this planet, which turns a compass needle towards north. Migratory birds, social insects, fish and marine reptiles are all thought to have this talent, along with a few mammals, including rodents and possibly some confused cattle. Most of them use it to find their way home, across annual migration routes or back to a breeding site. Surprisingly, we humans probably have what it takes to sense magnetism ourselves. But despite decades of searching, the mechanism of magnetoception in animals, an internal compass, has never been definitively found.

A new technique has just been used to discover magnetic cells in trout olfactory epithelium, the tissue better known for detecting smells. The authors of the paper in PNAS refer to their unusual experimental setup as a “magnetoscope”, and they looked at trout snouts because they send signals to the trout brain in response to magnetic fields. They set up a microscope looking at “dissociated” trout olfactory epithelium – cells which were gently split up and separated so that they could move freely. Around these cells they rotated a magnetic field and looked for cells that spun along with it.

A few cells – between one and four in every 10,000 – rotated exactly in synchrony with the magnetic field, moving just like a compass needle would. How this motion might send information to the brain remains a mystery, but finding cells so precisely responsive to a magnetic field is a major advance.

These rare cells include clusters of highly reflective, iron-rich objects, which are probably magnetite crystals. Magnetite is a magnetic mineral found in many organisms, so scientists have suspected it as a candidate “needle” for any biological compass. The magnetite in the trout cells seemed to be attached to the cell membrane, allowing it to turn the whole cell round with it. The researchers measured the “magnetic moment” of the cells – that is, how strongly they react to magnetic fields.

Their result was far greater than that previously estimated, meaning that trout magnetoception may be much more sensitive than expected. Not only is it enough to find magnetic north, but also to work out position from local variations in magnetism. The “magnetoscope” could be used to find similar cells in other animals that use a similar compass-like mechanism. The authors want to test other migratory fish and birds, particularly their inner ears and upper beaks, because those tissues also contain concentrated iron compounds.

However, magnetoception does not have to depend on magnetite. Another theory proposes that a light-sensitive molecule called cryptochrome changes in sensitivity under different magnetic fields. Cryptochrome recently turned out to be essential for magnetic sense in fruit flies, and it’s involved in magnetoception in birds. Both social insects (such as ants or honeybees) and birds use magnetism to help find their way back to a nest.

It may be that these animals literally “see” magnetic fields, because cryptochrome is is found in nerve cells in the eye. But both birds and insects also contain magnetic iron compounds such as magnetite, so it remains to be proven whether just one or both systems work. For example, homing pigeons are famous for an impeccable sense of direction, as long as they have access to either the sun or the Earth’s magnetic field. They appear to keep a massive, detailed internal “map”, a memory of positions and landmarks, then follow it using those directional cues. It is thought that, similarly to trout, bird brains receive information about magnetic fields from magnetite-containing cells in their beaks.

However, recent research has thrown doubt upon those cells’ ability to provide nervous signals, and instead attention is turning to the bird inner ear. On the other hand, migrating songbirds have cryptochrome-containing cells in their eyes which activate when they use magnetoception. So the question of whether bird magnetoception is “smelled” or “seen”, so to speak, remains open.

Small mammals like mice and bats also use magnetism to find their way home when they can’t use sight or smell. There is relatively little evidence pointing to any particular mammal organ as a magnetoceptor. Strangely, cows and deer seem to align north-to-south when grazing. This could be to do with the sun, except that their orientation gets disturbed when they graze near high-voltage power lines. If the lines are west-to-east, the animals line up that way. If they run in any other direction, the animals arrange pretty much randomly. Power lines produce magnetic fields, so grazers seem to have magnetoception, too – though since the original findings, debate has continued as to whether they are really valid or not. No one knows why cows would prefer a north-south alignment, and there have been no clues as to how they find it.

You’ve probably never thought you could tell which way the magnetic field in your vicinity was pointing. However, there is evidence for human magnetoception, albeit controversial, including that brain activity reacts to the activation and deactivation of weak magnetic fields. Our eyes contain crytochrome, and our sinuses (in the nose) contain magnetic bones, so the tools for detecting magnetism seems to be present.

In fact, human cryptochrome works just as well as fruit fly cryptochrome for magnetoception. Yet there’s no obvious purpose for a human magnetic sense, being as we are creatures of the day with excellent vision who rarely migrate without a map. So while scientists work towards a full understanding of biological compasses in animals, perhaps you’ll look out (or smell out) for a sense of magnetism next time you navigate somewhere. You might have what it takes, after all.

Image from Fishandfeather.com

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