Each year, in late spring and early summer, female sea turtles will crawl out of the ocean under moonlight to lay their eggs in the sand, often returning to the same beach on which they were born many years earlier.

Sometimes when the turtles emerge to nest, researchers like Julianna Martin are watching patiently from the shadows.

For her doctoral research, Martin, a PhD student at the University of Central Florida, has been analyzing sea turtle tears. Yes, the tears of sea turtles. So on several summer nights in 2023 and 2024, she’d stake out beaches and wait for the turtles to start laying eggs. At that point, the reptiles enter a sort of “trance,” she said, allowing scientists like her to collect samples, including tears.

Martin told me she would army crawl up to the turtles on the sand and dab around their eyes with a foam swab, soaking up the goopy tears they exude. Sea turtles regularly shed tears as a way to expel excess salt from their bodies. (As far as we know, they are not sad.)

Martin would then take those tears back to her lab for analysis.

This odd work serves a purpose. Martin is examining sea turtle tears to see if they contain a specific kind of bacteria. Such a discovery, she said, could help unlock one of biology’s biggest and most awe-inspiring mysteries: how animals navigate using Earth’s invisible magnetic field.

The “holy grail” of sensory biology

After baby turtles hatch, they dig their way out of the sand and crawl into the ocean, where they embark on an epic journey that can take them thousands of miles across the open sea. Loggerheads that hatch in Florida, for example, swim across the Atlantic and reach islands off the coast of Portugal, before eventually returning to Florida’s beaches as adults to nest.

Remarkably, the turtles typically return to the same region of Florida or even to the same beach.

“These young turtles can guide themselves along that 10,000-mile migratory path despite never having been in the ocean before and despite traveling on their own,” said Kenneth Lohmann, a biologist at University of North Carolina at Chapel Hill who studies sea turtle navigation.

Researchers like Lohmann have learned that sea turtles, like many other species, seem to navigate using Earth’s magnetic field. That’s the subtle magnetic force — generated by the planet’s molten metal core — that surrounds Earth, not unlike the force around a bar magnet. The intensity and direction of the field vary across Earth’s surface, making it useful for navigation. Plus, the magnetic field is present even when other spatial cues, like light, are not.

What remains a mystery, however, is how animals sense these magnetic forces. Decades of research have failed to turn up a mechanism for so-called magnetoreception or any kind of specialized organ that can sense magnetic force. As Martin’s adviser Robert Fitak has written, it’s like knowing an animal can respond to something visual but not finding any eyes.

“It’s the last sense we effectively know nothing about,” sensory biologist Eric Warrant has said about magnetoreception. “The solution of this problem I would say is the greatest holy grail in sensory biology.”

Scientists have proposed a number of theories for how this might work. And all of them are totally bonkers.

The prevailing theory is rooted in quantum mechanics, and it is extremely complicated. The theory posits that when certain light-sensitive molecules known as cryptochromes absorb light, they produce something called radical pairs — two separate molecules each with one unpaired electron. Those two unpaired electrons are quantumly entangled, which essentially means that their spin states are interdependent: They either point in the same direction or opposite directions, and they ping-pong between the two.

This theory suggests that Earth’s magnetic field influences the spin states of those radical pairs, and that, in turn, affects the outcome of chemical reactions in the body of animals. Those chemical reactions — which animals can theoretically interpret, as they might, for example, smells or visuals — encode information about Earth’s magnetic field. (If you want to dive deeper, I suggest watching this lecture or reading this paper.)

Another theory suggests that animals have bits of magnetic material in their bodies, such as the mineral magnetite. According to this theory, those magnetic bits are influenced by Earth’s magnetic field — just like a compass — and animals can sense those influences to figure out where they’re going.

Martin and Fitak’s research is exploring this latter theory, but with an important twist. They suspect that sea turtles and other animals might rely on magnetite to sense Earth’s magnetic field but may not produce the magnetite themselves. Instead, they suggest, sea turtles may have a symbiotic relationship with magnetite-producing bacteria — literally living compasses — that sense the magnetic field and somehow communicate information back to the turtle.

This isn’t an outrageous idea. Magnetic bacteria — more technically, magnetotactic bacteria — is real, and quite common in aquatic environments around the world. Plus, there’s evidence that magnetotactic bacteria help another microscopic organism, known as a protist, navigate. The question is, could they help turtles navigate, too?

Magnetic bacteria is a thing

Magnetotactic bacteria are extremely cool. These microscopic organisms have what are essentially built-in compass needles, said Caroline Monteil, a microbial ecologist at the French research institute CEA. The needles comprise chains of magnetic particles produced by the microbes, which you can see under a microscope (shown in images below). Remarkably, those needles align the bacteria with Earth’s magnetic field lines, just like a real compass needle does. As the bacteria roam about, they move in line with the direction of the planet’s magnetic force.

Magnetic sensing is useful for the bacteria, said Fitak, an assistant professor at UCF. Magnetotactic bacteria need specific levels of oxygen to survive, and those levels tend to vary with depth. Deeper levels of sediment in a stream, for example, might have less oxygen. In most of the world, the direction of the magnetic field is at least somewhat perpendicular to Earth’s surface — meaning, up and down — allowing the bacteria to move vertically through their environment to find the optimal habitat, as if they’re on a fixed track.

In at least one case, magnetic bacteria team up with other organisms to help them find their way. A remarkable study published in 2019 found that microscopic organisms in the Mediterranean Sea called protists were able to sense magnetic forces because their bodies were covered in magnetic bacteria. When the authors put the north pole of a bar magnet next to a water droplet full of protists, they swam toward it. When they flipped the magnet, the protists swam away. (Different magnetic microbes are attracted to either north or south poles, often depending on where on Earth they live.)

You can actually see this in the video below.

It’s not clear how the magnetic bacteria are actually guiding the protist, said Monteil, the study’s lead author.

Now, returning to the turtles: The theory that Fitak and Martin are exploring is that sea turtles, like protists, might also have magnetotactic bacteria — those living compasses — in their bodies, and somehow be able to read them. Some microbes in the microbiome aid in digestion. Others provide directions. Maybe.

One idea, Martin says, is that the bacteria could aggregate near nerves in the turtles that provide information about their position in space. Some of those nerves are near the tear ducts, she said — which is ultimately why she was army crawling on the beach to collect turtle tears. The goal, she said, is to figure out if those tears contain magnetotactic bacteria. That would be one indication that these animals might be using bacteria for navigation.

“We’re not entirely sure how magnetotactic bacteria could be facilitating a magnetic sense, but that seemed like a good place to start,” Martin said.

While her research is still underway, Martin has yet to find evidence of magnetotactic bacteria in the tears of the 30 or so turtles she’s analyzed so far. That’s disappointing, she said, but it doesn’t rule out the possibility that these bacteria exist somewhere in the body of a turtle and help them navigate.

“There are so many other ideas about ways that magnetotactic bacteria could provide information to an organism about Earth’s magnetic field,” she said. “There’s a variety of other locations and other taxa that might be better for studying this theory.”

Other scientists who study animal navigation are skeptical.

It’s unlikely that symbiosis with magnetotactic bacteria is what enables sea turtle navigation, said Monteil. Part of the problem is that there’s no known mechanism through which the bacteria would communicate with the turtle. It’s also not clear what magnetotactic bacteria would get out of this relationship, if it is indeed symbiotic — could sea turtles provide the conditions bacteria need to survive? Maybe. Maybe not.

What’s more, Monteil said, is that magnetotactic bacteria are widespread in the environment, so even if Martin did find them in sea turtle tears, it would do little to prove the theory. Just because magnetic bacteria are present doesn’t mean they’re helping the animal navigate.

But then again, other theories are still entirely unproven, too — and some of them are a lot weirder.

“I don’t think it is impossible,” Monteil said of sea turtles and other organisms using magnetic bacteria to navigate. “Nothing is impossible. Life is amazing and has found ways to do things that we couldn’t imagine centuries before.”

“We don’t know until we know.”