Migrating birds and butterflies fly thousands of miles in the autumn to escape cold winter temperatures, then return home in the spring.
Salmon swim hundreds of miles from the ocean through rivers until they reach the spawning ground where they were born.
During World Wars I and II, pigeons were trained to deliver messages between distant military units or to communicate with spies behind enemy lines.
How do these amazing creatures travel hundreds or thousands of miles without getting lost?
Compared to many animals, we humans are pathetic navigators. Some of us can barely navigate a parking lot.
It turns out that many animals have a secret sense that we don’t have: magnetosensation. They can sense the Earth’s magnetic field, so they always know which way is north. Basically, they have a compass inside their heads!
A crazy idea
Fifty years ago, people thought the idea that animals could sense magnetic fields was crazy. A secret sense that birds and fish and insects possess, but we humans can’t even imagine? Add that to the fact that Earth’s magnetic field is very weak, and scientists had no clue how even a strong magnetic field could be sensed by animals—it would need to somehow affect chemical or biological processes in the body.
Back then, people assumed that migrating animals used the sun or stars to navigate long distances. This may indeed be true for some animals. But evidence that animals can also sense magnetic fields slowly began to mount in the mid-20th century, led by students in Friedrich Wilhelm Merkel’s laboratory in Frankfurt, Germany.
In the autumn of 1957, graduate student Hans Fromme noticed that his caged European robins were starting to hang out in the southwest part of their cage, as if they were trying to begin their usual southwestward migration from Germany to Spain. But these robins lived in a closed room where they couldn’t see the sun or stars. How did they know which direction was southwest? Fromme proposed two possibilities: either the robins were either sensing the Earth’s magnetic field or they were receiving radio signals from the stars.
Merkel asked another graduate student, Wolfgang Wiltschko, to follow up on Fromme’s observation. Wiltschko didn’t believe Fromme’s magnetosensation hypothesis. He set out to instead prove that the birds were using radio signals. But as often happens in science, his experiments led him to exactly the opposite conclusion.
Wiltschko wanted to change the direction of the magnetic field surrounding the birds and see whether they moved to a different side of the cage. Since altering the magnetic field of the Earth isn’t really an option, he used electric coils to generate a new magnetic field around the birds’ enclosure. (As you may remember from high school physics, electric current flowing in a circular loop generates a magnetic field.)
Lo and behold, Wiltschko found that when he changed the direction of the magnetic field surrounding the birds, they reoriented to whichever direction was “southwest” according to his field. They really were using magnetosensation.
Wiltschko and his colleagues continued to investigate magnetosensation in birds, but it took a decade for their work to be accepted by other scientists. That’s the thing about scientists—we tend to remain skeptical about seemingly crazy discoveries for years and years, but if those experiments are repeated and the evidence continues to mount, we always come around.
A not-so-special sense?
Over the following decades, scientists discovered that magnetosensation is used not only by birds, but also by organisms such as butterflies, salmon, sea turtles, newts, lobsters, bees, dolphins, and even bacteria!
It’s easy to imagine that animals like birds and insects use magnetosensation to navigate as they travel long distances… but what about bacteria? Aside from the ones who’ve made a home in airplane seat cushions, they don’t usually travel thousands of miles in their lifetime. So why do they need a compass?
Scientists have proposed that magnetosensitive bacteria don’t actually care about crawling north or south, but that they instead use magnetosensation to tell which way is up or down. The Earth’s magnetic field has a downward component everywhere except the equator, since it emanates from the Earth itself (see picture above). In the northern hemisphere, following magnetic north will lead the bacteria downward, while the reverse is true in the southern hemisphere. Knowing which way is up or down can help bacteria navigate to layers of mud or marine sediment that have the ideal levels of oxygen and other nutrients.
What about humans?
So lots of animals possess this strange magnetic sense. What about us? Has evolution left us out of this exclusive club, or could magnetosensation be a latent power that we just don’t know we have?
In the 1980s, a scientist at the University of Manchester, Robin Baker, decided to test whether humans could sense magnetic fields. He blindfolded a bunch of undergrads, threw them in a van, drove them miles and miles along winding roads to various unknown destinations, then asked them to point which way was home.
Amazingly, they were able to point in the right direction more often than you’d expect by random chance. When Baker made them wear magnets on their heads to theoretically disrupt their sense of the Earth’s magnetic field, their accuracy dropped.
Unfortunately, these exciting results couldn’t be replicated by other groups. The current consensus is that we humans can’t sense magnetic fields. But some scientists haven’t given up.
A scientist at Caltech, Joe Kirschvink, has been conducting far more sophisticated experiments on humans than Baker did. Similar to Wiltschko’s experiments on birds, Kirschvink places human subjects in a cage lined with electric coils to generate a controlled magnetic field. As he varies the current to rotate the magnetic field, he records EEG activity from the subject to detect corresponding changes in brain activity. He says that there are EEG variations based on magnetic field changes, suggesting that our brains can indeed sense magnetic fields—a tantalizing but preliminary finding that must await further replication.
Magnets inside the body
Now that it’s become clear that many animals can sense magnetic fields, the obvious question is… how? Scientists have proposed three different types of magnetosensation mechanisms.
One idea is that magnetosensitive animals literally have a compass in their bodies. Compasses work because their needles are basically magnets that align with the Earth’s magnetic field. Could animals also have tiny magnets inside their bodies?
Yes! Tiny magnetic particles composed of a mineral called magnetite have been shown to mediate magnetosensation in bacteria. These particles are contained within structures called magnetosomes and are arranged in a linear chain. When they align with the Earth’s magnetic field, they literally rotate the bacterium’s body to aim it toward the bottom layers of the sediment.
Magnetite chains have also been identified in salmon, pigeons, and other animals, suggesting they may represent a general mechanism of magnetosensation. In these animals they are found within tiny spaces in the head associated with the trigeminal nerve, which conveys information about touch, temperature, and pain from the face to the brain. So if this nerve is sensitive to movements of magnetite particles in the head, it might also convey a sense of the magnetic field.
Sensing magnetism through electricity
A second mechanism that’s been proposed to mediate magnetosensation is based on “electromagnetic induction”. This sounds fancy, but it’s just the flip side of the same principle from high school physics that I mentioned above: just as electric current can generate a magnetic field, moving through a magnetic field generates an electric current.
So as an animal moves through the Earth’s magnetic field, it generates electric current. If it could sense that current then it could sense the magnetic field.
Now maybe sensing electricity sounds harder than sensing magnetism, but we already know of a few animals that can do so: electrosensitive fish like sharks and rays. Scientists are still testing whether their ability to sense electricity also enables them to navigate using the Earth’s magnetic field. (As you might imagine, sharks are not the ideal model organism for experiments.)
Sensing magnetism through light
The third and final mechanism that’s been proposed to mediate magnetosensation can be generally summarized as: chemistry. A magnetic field can affect certain kinds of chemical reactions. Specifically, it can affect reactions involving a “radical pair”, which refers to molecules with unpaired electrons.
I know, this is a biology blog and I’ve already asked you to dig up your knowledge from high school physics, and now I’m asking you to recall high school chemistry. But don’t you remember how electrons usually come in pairs and when one electron loses its partner, weird stuff can happen?
Electrons spin in a certain direction, and paired electrons normally spin in opposite directions. But unpaired electrons can spin in either the same or opposite directions. This matters because certain reactions only take place when the spins are the same, while other reactions take place only when the spins are opposite. Magnetic fields can flip which way the electrons are spinning and therefore influence the efficiency of those chemical reactions.
Now, there aren’t that many chemical reactions taking place in the body that meet all these criteria. But one molecule seems like a good candidate: cryptochrome, a protein in the eye that senses blue light. The chemical process by which blue light activates cryptochrome involves the formation of multiple radical pairs. So the idea is that cryptochrome would be more strongly or weakly activated by light depending on how the animal is oriented in a magnetic field.
An exciting study in 2008 found that without cryptochrome, fruit flies can no longer sense magnetic fields. So cryptochrome may indeed be the magnetic sensor in some animals. The role of cryptochrome in birds hasn’t been proven, but scientists have shown that the magnetic compass of birds is located in the eye and requires the presence of light, suggesting that birds may also rely on cryptochrome for magnetosensation.
What’s interesting is that we humans also have cryptochrome, and human cryptochrome can substitute for fruit fly cryptochrome in enabling flies to sense a magnetic field. So even if we don’t seem able to sense magnetism, we have molecules in our eyes that can!
One lesson we can take from the story of magnetosensation is that there are whole worlds out there, right in front of our eyes, that are completely invisible to us. We’re able to sense certain features of the world, like sights and sounds, but not others, such as magnetic fields.
Other animals have evolved different lifestyles that require different senses, and thus they experience different worlds that may be entirely foreign to us.
So our experience of the world isn’t “real”. It’s a construct of our brains, a biased representation of our surroundings—a meager impression of the richness that is truly out there.
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