The Earth, with its constantly moving molten iron core, is surrounded by a vast and glorious magnetic field. It’s as much a part of the planet as the oxygen or water that it protects, shielding the planet from solar wind being blasted at us from the Sun.
And, like water and oxygen, lifeforms on Earth have evolved to make use of this resource. We may need to take a compass with us on low-tech hikes – but plenty of birds, fish, mammals and more have gone for a much more direct route.
The big daddies of magnetoreception: birds
If ever there were an avian idiom that was wholly misleading, it’s this: “bird-brained”. Sure, our wingéd brethren occasionally fly straight into glass windows or kill up to 100 million humans in less than two years – but they’re not actually stupid. Some of them can do math; others can craft tools to solve practical problems.
And most of them, it turns out, can do quantum physics.
“I had never thought that I would get to a place where we would start understanding the quantum mechanical mechanism that goes on inside a bird,” said Henrik Mouritsen, a biologist from the University of Oldenburg in Germany and co-author of a 2021 paper on how European robins perceive the Earth’s magnetic field.
“What exactly the bird is seeing we do not know,” he told the June 2021 Nature podcast, “because we cannot ask the bird.”
Nevertheless, researchers think it’s likely that birds not only use the Earth’s magnetic field to navigate – they also literally see the field in some way. And the key to this ability is, indeed, quantum physics: “We found cryptochromes in the eyes of birds and so did others,” Mouritsen explained, “and it looks like the cryptochrome 4 from the migratory birds are significantly more magnetically sensitive than the same molecule from a chicken.”
Cryptochromes, despite their mysterious name, are not some molecular boogeyman. They’re proteins, common in both plants and animals – and before their role in magnetoreception was discovered, they were mostly known for the role they played in the regulation of circadian rhythms. But in birds, they seem to be doing something else, too.
“The current working theory is this: When a particle of light, or photon, hits bird cryptochrome, its energy can perturb molecules within the protein,” explained Katherine J. Wu in a 2019 PBS Nova article on the subject. “The disturbance catapults a pair of molecules into an unstable state so fragile that it can be affected by even the subtle energetic pulse of Earth’s magnetic field.”
That, friends, is what’s called quantum entanglement: a precarious state in which two particles become intricately linked, even after being separated physically. “You can only describe the two together,” Erik Gauger, a quantum scientist at Heriot-Watt University who has studied magneto-sensing in birds, told Nova. “You can’t describe just one by itself.”
What happens then is – well, it’s sort of similar to how our brains figure out where a sound is coming from. Just as the microsecond difference in timing between our ears picking up a sound clues us into the direction of its source, so too is the birds’ understanding of the surrounding magnetic field dependent on timing – only, here, it’s the amount of time these entangled particles spend in one state before forced into another by the magnetic field.
The weird thing is, though – well, okay, all of this is pretty weird, but the even weirder thing about it is that, well, this shouldn’t really be possible. Birds can be discombobulated by artificial radio frequencies as weak as 1/3,000 the strength of the Earth’s magnetic field, which implies that the quantum pair are entangled for way longer than is usually possible – like, five orders of magnitude longer.
“It seems nature has found a way to make these quantum states live much longer than we’d expect, and much longer than we can do in the lab,” Gauger told Nova. “No one thought that was possible.”
An attempt was made: mammals
While birds harness the power of the Earth’s magnetic field to traverse vast migratory routes, other animals have a more, uh… prosaic uses for it.
“Expecting magnetoreception in dogs is reasonable given the extraordinary homing abilities of dogs and closely related species like red foxes, coyotes and grey wolves,” explains one paper, published in the journal Frontiers in Zoology back in 2013.
“Having been inspired by our hitherto observations in other animals, we monitored spontaneous alignment in dogs during diverse activities,” the authors write, “and eventually focused on excreting (defecation and urination incl. marking) as this activity appeared to be most promising.”
That’s right: we’re talking about dog poop. Dogs, given by their biology access to the entire planet’s geomagnetic forcefield, use it to… go to the bathroom.
Specifically, they use it to go to the bathroom facing north. Or south; either way, “during calm magnetic field conditions revealed a highly significant axial preference for North–South alignment during defecation,” the paper reports.
Why? Who knows. No, seriously: the dogs apparently definitely do poop along the major axis of the compass, but the researchers couldn’t tell whether they do it consciously – whether they, like birds, can “see” (or, perhaps more likely for a dog, “smell”) the Earth’s magnetic field, and line themselves up as appropriate – or if it’s more of a subconscious thing.
“The study was truly blind,” the researchers wrote. “[N]o one, not even the coordinators of the study, hypothesized that expression of alignment could have been affected by the geomagnetic situation, and particularly by such subtle changes of the magnetic declination.”
Now, dogs may have the funniest use for the Earth’s magnetic field, but they’re far from the only fuzzies out there exploiting it. Cows, it turns out, may also prefer a north-south alignment (they don’t really do much with it, but they’re cows, so that’s hardly surprising) as do deer, boar, and, bucking the pattern a little, carp.
Foxes, at least, have a more wily use for magnetoreception. “Red foxes hunting small animals show a specific behavior known as ‘mousing’,” explains one 2011 paper; “The fox jumps high, so that it surprises its prey from above.”
You’ve seen this before; it’s adorable. But what you might not have noticed – unless you had a compass to hand – was the direction they jumped in, which as it turns out was almost certainly north-east.
“Foxes on the prowl tend to direct their jumps in a roughly north-eastern compass direction,” the paper reports. “The direction of attacks was independent of time of day, season of the year, cloud cover and wind direction.”
Not only do foxes seem to prefer this axis, they’re also much more successful along it. The report found that foxes jumping to the north-east were successful nearly three times out of four; those who jumped south-west – i.e. the same direction, just facing the other way – won out three times out of five. Any other direction? Less than an 18 percent success rate. That’s like, one time out of five or six.
So, clearly, magnetoreception is something you don’t have to have wings to enjoy. But we know what you’re thinking: if humanity’s best friend can sense the Earth’s magnetic field – even if it’s just to poop – can we?
Lagging behind: humans
Out of all species, humans have one big advantage when it comes to studying their perception of the world – namely, we can actually ask them what they sense. So: can you see any magnetic fields right now?
Okay, maybe that’s a little simplistic. “We have not as a species lost the magnetic sensory system that our ancestors [millions of years ago] had,” said Joseph Kirschvink, the Nico and Marilyn Van Wingen Professor of Geobiology at Caltech and leader of a 2019 research project that claimed to find evidence for magnetoreception in humans.
“We are part of Earth’s magnetic biosphere,” Kirschvink told The Guardian at the time.
Now, unlike our four-legged and two-winged friends, this sense didn’t manifest itself as some orientation superpower or all our toilets facing one way. But sit a human in an aluminum cage and fire weird electromagnetic interference at their brains, Kirschvink and colleagues found, and our brainwaves start “freaking out,” he explained.
But does that mean humans have a magnetoreceptive “sense” of some kind? That’s more debatable.
“Given that a number of other animals can sense Earth’s magnetic field, it is certainly within the realm of possibility that humans can as well,” Kenneth Lohmann, an expert in magnetoreception from the University of North Carolina at Chapel Hill who was not involved in the research, told The Guardian.
Nevertheless, Lohmann said, “it is one thing to find a subtle change in brain activity in response to a weak magnetic field, and another thing to show that people really detect and use magnetic field information in a meaningful way.”
