The Unusual Reason Scientists Collect Sea Turtle Tears

Every year in late spring and early summer, female sea turtles crawl out of the ocean in the light of the Moon to lay eggs in the sand, often returning to the same beach where they were born years before. Sometimes, when the turtles choose a nest site, researchers like Julianne Martin patiently watch them from the shadows.

For her dissertation, Martin, a graduate student at the University of Central Florida, is analyzing sea turtle tears. So for several summer nights in 2023 and 2024, she watched the beaches and waited for turtles to start laying eggs. At that point, she said, the reptiles go into a kind of "trance" that allows scientists to collect samples, including tears.

Martin says that to collect samples, she crawls up to the turtles on the sand and wipes their eyes with a foam swab to absorb the tears they secrete. Sea turtles shed tears regularly to flush out excess salt from their bodies. Martin then takes these tears back to his laboratory for analysis.

This strange job serves a specific purpose. Martin is examining sea turtle tears to see if they contain a special kind of bacteria. Such a discovery, she says, could help solve one of biology's biggest and most awe-inspiring mysteries: how animals navigate using the Earth's invisible magnetic field.

After the 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 out to the open sea. Loggerhead turtles hatched in Florida, for example, swim across the Atlantic Ocean and reach islands off the coast of Portugal before returning to Florida beaches to nest as adults.

Remarkably, the turtles usually return to the same region of Florida or even the same beach. "These young turtles can navigate 15,000 kilometers on their own, despite never having been in the ocean before and traveling independently," says Kenneth Lohmann, a biologist at the University of North Carolina at Chapel Hill who studies sea turtle navigation.

Researchers like Lohmann have found that sea turtles, like many other species, seem to orient themselves using the Earth's magnetic field. It's a subtle magnetic force - generated by the planet's molten metallic core - that surrounds the Earth, not unlike the force around a magnet.

The intensity and direction of the field varies across the Earth's surface, making it useful for navigation. In addition, the magnetic field is present even when other spatial landmarks, such as light, are absent. However, it remains a mystery how animals sense these magnetic forces.

As Martin mentor Robert Fitak writes, "it's like knowing that an animal can respond to something visual, but not finding any eyes."

"It's the last sense we actually know nothing about," sensory biologist Eric Warrant says of magnetoreception. "Solving this problem, I would say, is the greatest holy grail in sensory biology." Scientists have proposed several theories for how it might work.

The prevailing theory is rooted in quantum mechanics, and it is extremely complex. According to this theory, when certain light-sensitive molecules known as cryptochromes absorb light, they create so-called radical pairs - two separate molecules, each with one unpaired electron. These two unpaired electrons are quantum entangled, which means that their spin states are interdependent: they point either one way or opposite, and they move between the two.

According to this theory, the Earth's magnetic field affects the spin states of these radical pairs, and this in turn affects the results of chemical reactions in animals. These chemical reactions, which animals can theoretically interpret, such as odors or visual images, encode information about the Earth's magnetic field.

Another theory suggests that animals have bits of magnetic material, such as the mineral magnetite, in their bodies. According to this theory, these magnetic pieces are influenced by the Earth's magnetic field - like a compass - and animals can feel this influence to figure out where they are going.

Martin and Fitak's study explores this theory, but with an important twist. They hypothesize that sea turtles and other animals can rely on magnetite to sense the Earth's magnetic field, but cannot produce magnetite themselves.

Instead, the researchers hypothesize that sea turtles may have a symbiotic relationship with magnetite-producing bacteria - literally living compasses - that sense magnetic fields and somehow relay the information back to the turtle.

Magnetotactic bacteria are pretty cool. According to Caroline Monteilh, a microbial ecologist at the French research institute CEA, these microscopic organisms essentially have built-in compass needles.

The needles are made up of chains of magnetic particles produced by microbes, which can be seen under a microscope. Remarkably, these needles align the bacteria along the lines of the Earth's magnetic field, just as a real compass needle does. The bacteria move according to the direction of the planet's magnetic field.

According to Fitak, an assistant professor at the University of California, magnetic sensitivity is useful for bacteria. Magnetotactic bacteria need a certain level of oxygen to survive, which tends to change with depth. For example, a stream with deeper sediments may have less oxygen.

Over most of the globe, the direction of the magnetic field is at least somewhat perpendicular to the Earth's surface - that is, up and down - allowing bacteria to move vertically in search of optimal habitat as if they were on a fixed path.

In at least one case, magnetic bacteria team up with other organisms to help them find their way. A study published in 2019 found that microscopic organisms in the Mediterranean Sea called protists are able to sense magnetic forces because their bodies are covered with magnetic bacteria.

When the authors placed the north pole of the magnet next to a drop of water filled with protists, the protists swam toward it. When they turned the magnet upside down, the protists swam away.

Different magnetic microbes are attracted to either the north or south pole, which often depends on where they live on Earth. According to Monteilh, lead author of the study, it is still unclear exactly how magnetic bacteria guide protists.

The theory that Fitak and Martin are exploring is that sea turtles, like protists, may also have magnetotactic bacteria - these living compasses - in their bodies and somehow be able to read them. Some microbes in the microbiome help with digestion. Others, perhaps, provide guidance.

One idea, Martin said, is that the bacteria may accumulate near the turtles' nerves, which relay information about their position in space. Some of those nerves are near tear ducts, she said - which is why she crawled on the beach to collect turtle tears.

The goal is to find out if there are magnetotactic bacteria in these tears. This would be one indication that these animals can use bacteria to navigate. "We don't quite understand how magnetotactic bacteria might contribute to magnetic sensing, but it seems like a good start," Martin says.

This is disappointing, she says, but doesn't rule out the possibility that these bacteria exist somewhere in the turtle's body and help it navigate. "There are many other ideas about how magnetotactic bacteria can provide the body with information about the Earth's magnetic field," she says. "There are many other places and other taxa that might be better suited to study this theory."

Other scientists who study animal navigation are skeptical. "It is unlikely that symbiosis with magnetotactic bacteria is what enables sea turtle navigation," says Monteilh. "Part of the problem is that the mechanism by which the bacteria could communicate with the turtle is unknown. It's also unclear what the magnetotactic bacteria would gain from this relationship if it is indeed symbiotic - could sea turtles provide the conditions the bacteria need to survive? Maybe. Maybe not."

Moreover, according to Monteilh, magnetotactic bacteria are widespread in the environment, so even if Martin finds them in a sea turtle's tears, it would do little to prove the theory. Just because magnetic bacteria are present doesn't mean they help the animal navigate. But then again, other theories have yet to be proven, too - and some of them are much weirder.

"I don't think it's impossible," Monteilh says of sea turtles and other organisms that use magnetic bacteria to navigate. "Nothing is impossible. Life is amazing and finds ways to do things we couldn't imagine centuries ago. We can't know until we find out."