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By and large, though, the chemical recipe for tears, which include a slurry of water, fats, proteins and charged minerals such as sodium, seems to be pretty similar across various species. The few variations that exist seem to track with habitat, the researchers found. Animals that spent most of their time on land, for instance, had more proteins in their tears than their seafaring counterparts, but they also had less urea — a molecular waste product that’s also found in urine.

Dr. Oriá’s team also previously found chemical similarities among dog, horse and human tears, all of which seem to flow quite freely. That might be a mammal thing, Dr. Oriá said. But perhaps domestication, which prompted a big shift in scenery for these previously wild animals, tamed their tears, too.

That an animal’s surroundings heavily influence the composition of tears, which are constantly exposed to the outside world, “makes a lot of sense,” Dr. Echeverri said. “Most of our other liquids are waste that we get rid of, or internal. Tears have to deal with the environment from moment to moment.” (But tears aren’t universal, Dr. Echeverri noted. Invertebrates, which have very different body plans, have had to concoct tear-free methods of keeping their eyes clean. Some spiders, for instance, use bristlelike hairs on their legs to brush away dust and debris.)

Some of the weirdest tears out there, Dr. Oriá said, come from loggerhead sea turtles, whose eyes secrete fluids so viscous they are practically sap — and impossible to collect with the supplies she and her students typically use to sop up specimens.

“We tried paper strips, we tried micropipettes, nothing,” she said. “The mucus stuck in everything.”

They finally worked out a method of sucking up the sludge with a superstrong syringe.

Dr. Thomasy suspects the tears’ texture helps them stick to the eyeballs of loggerhead sea turtles, even when they’re underwater. On land, though, it makes for quite the spectacle. “I would guess it would look like the worst snot you’ve ever seen,” she said.



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