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........

© Vox