Microscopically, Crocodile Tears Look Sort of Like Our Own

To the best of our scientific knowledge, only humans cry, which is to say the tears of other animals are not inspired by their emotional states, reports Katherine J. Wu for the New York Times. The reason we shed a tear seems to be the only thing that sets our sobs apart from other watery-eyed creatures—at the molecular level, tears are tears. Specifically, new research published last week in the journal Frontiers in Veterinary Science, found that the tears of birds and reptiles structurally aren’t so different from our own.

Learning about how and why other animals produce tears could help researchers develop better treatments for chronically dry eyes, the study's lead author Arianne Pontes Oriá, a veterinarian at Brazil’s Federal University of Bahia, tells the New York Times.

Previous studies examined the tears of mammals including dogs, horses, camels and monkeys, but the new study looked into the tears of slightly less relatable fauna, reports Virginia Morell for National Geographic. Researchers collected tears from barn owls, blue-and-yellow macaws, roadside hawks, broad-snouted caimans, as well as loggerhead, hawksbill, and green sea turtles. For comparison, Oriá and her team also collected tears from ten human subjects.

They’re not going to start blubbering when Bambi's mom dies in Bambi, but the new study finds, even reptilian crocodile tears follow the same basic formula that trickles down human cheeks: mucus, water and oil. Suspended in that watery solution are also salty electrolytes, urea (which is also found in urine) and proteins.

"Although birds and reptiles have different structures that are responsible for tear production, some components of this fluid (electrolytes) are present at similar concentrations as what is found in humans," says Oriá in a statement. "But the crystal structures are organized in different ways so that they guarantee the eyes' health and an equilibrium with the various environments."

With the help of 65 captive animals and 10 people, the researchers used small absorptive strips of paper (or in the case of the loggerhead sea turtles, a syringe) to humanely collect tear samples.

Some of the differences were subtle: reptiles and bird tears have slightly higher concentrations of electrolytes like sodium. Oriá tells National Geographic that this variance may be to help protect their eyes from inflammation caused by their environment, which for aquatic reptiles and birds in the study is often swift moving air or water.

Caiman tears were remarkably long-lasting, and though researchers still aren’t exactly sure why, Oriá says it may be due to added proteins present in the tears, per National Geographic. Caiman’s tears allow the scaly crocodilians to go without blinking for up to two hours at a time, according to the Times.

These proteins may be responsible for unique crystallization patterns the team observed when they dried out caiman tears. The sea turtles’ tears also exhibited striking, snowflake-like crystallized patterns when dried.

The sea turtles had the thickest tears in the group, forming a gooey layer on top of the turtles’ eyes. Oriá tells National Geographic that turtles’ mucus-laden tears help keep them from washing away underwater as well as protecting the turtle’s eyes from the salty ocean.

"This knowledge helps in the understanding of the evolution and adaptation of these species, as well as in their conservation," says Oriá in the statement.

Understanding the adaptations that various creatures use to keep their peepers lubricated and healthy may also inspire new treatments for people and animals with eye issues.

Alex Fox | | READ MORE

Alex Fox is a freelance science journalist based in California. He has written for the New York Times, National Geographic, Science, Nature and other outlets. You can find him at Alexfoxscience.com.