Most of us think of tears as a human phenomenon, part of the complex fabric of human emotion. But they’re not just for crying: All vertebrates, even reptiles and birds, have tears, which are critical for maintaining healthy eyesight.
Now, a new study, published this week in the journal Frontiers in Veterinary Science, reveals that non-human animals’ tears are not so different from our own. The chemical similarities are so great, in fact, that the composition of other species’ tears—and how they’re adapted to their environments—may provide insights into better treatments for human eye disease.
Previously, scientists had studied closely only the tears of a handful of mammals, including humans, dogs, horses, camels, and monkeys. In the new study, Brazilian veterinarians analyzed the tears of reptiles and birds for the first time, focusing on seven species: barn owls, blue-and-yellow macaws, roadside hawks, turquoise-fronted Amazon parrots, broad-snouted caimans, loggerhead sea turtles, and red-footed tortoises. (Take our quiz: Which animal does each eye belong to?)
Tears, which are emitted from tear ducts (in humans and some other mammals) or other similar glands, form a film over the eye that’s composed of three ingredients: mucus, water, and oil. The mucus coats the eye’s surface and helps to bind the film to the eye, the water is a natural saline solution containing crucial proteins and minerals, and the oil prevents the eye from drying out.
Humans are the only known species to produce emotional tears; the expression “crocodile tears,” which refers to a person’s phony display of emotion, comes from the mysterious tendency of crocodiles to release tears as they eat.
But tears play key roles beyond weeping, notes Lionel Sebbag, a veterinary ophthalmologist at Iowa State University, in Ames, who was not involved in the new research. They help with vision by lubricating the eye and clearing it of debris. They also protect the eye against infection and provide nutrition to the cornea, the eye’s clear outer layer, which lacks blood vessels, he says.
“It’s a fascinating look at such a diverse range of species,” Sebbag says of the new study.
How to analyze tears
Study leader Arianne Pontes Oriá, a veterinarian at Brazil’s Federal University of Bahia, already knew that broad-snouted caimans—an alligator relative with “beautiful eyes,” can keep their eyes open without blinking for up to two hours, she says. People, by contrast, blink every 10 to 12 seconds. Blinking distributes tears across the surface of the eye, keeping them moist and vision stable.
To analyze the tears of caimans and the six other species, Oriá and her colleagues worked with 65 captive animals at a conservation center, an animal care facility, and a commercial breeder in Brazil. In compliance with various government agencies that regulate animal welfare, the team humanely collected tear samples on test strips or with a syringe from the animals’ eyes, as well as tears from 10 healthy human volunteers. The scientists used special kits designed to measure the amounts of particular chemicals and compounds, such as electrolytes (a mixture of sodium and chloride) and proteins.
Surprisingly, given that birds, reptiles and mammals have different structures for producing tears, all the species’ tears—including those of humans—had a similar chemical makeup, with similar amounts of electrolytes, although bird and reptile tears had slightly higher concentrations. This difference may be because they live in water and air, which may be disruptive to the surface of the eye—higher levels of electrolytes in their tears may be needed to protect against inflammation, Oriá says. (Learn how mice spy on their predators by sniffing their tears.)
Human tears, as well as those of caimans and barn owls, had higher levels of protein compared to the other species. Such proteins are important for maintaining the stability of the ocular surface. Caimans and owls may have high protein concentrations because both species have large eyes and long intervals between blinks; caimans also live with their eyes submerged in fresh water for long periods of time, requiring highly stable tears.
The researchers also analyzed the crystallization patterns the tears formed when they dried—a technique often used to diagnose eye diseases. Here was the biggest surprise, Oriá explains: “There was much more variation in their tear crystals than in the tear composition.” Sea turtles’ and caimans’ tear crystals were strikingly unique, she says, “again, probably due to their adaptation to aquatic environments.”
The sea turtles also had by far the thickest tears of all the animals, which was why the researchers had to collect them with a syringe. “They live in salt water, and so they need tears adapted to that environment,” Oriá says. Having extra thick mucus in the tear film likely protects the turtles’ vision; without the thick film, their tears would be diluted, rendering them useless.
Protecting the vision of sea turtles, people, dogs and cats
By providing information about how to protect the vision of, for example, sea turtles, which are endangered, the study could inform conservation efforts. “If we understand what makes up a healthy tear film, then we can understand how pollutants or other environmental effects can harm an animals’ eyes, Oriá says.
Learning how reptiles and birds’ use tears may also inspire new medications for conditions such as dry eye, which occurs when tear ducts don’t produce enough oil. The disease, common in cats, dogs, and people, can sometimes lead to blindness.
The research illustrates how little we know about tears and how they work in humans and other animals, notes Brian Leonard, a veterinary ophthalmologist at the University of California, Davis.
“It’s an important but massively poorly understood field,” he says, “so this study is interesting on multiple levels.”
Correction: On August 21, 2020, the list of species the veterinarians studied has been corrected to include turquoise-fronted Amazon parrots and red-footed tortoises, and to exclude two other turtle species.