Most of us think of tears as a human phenomenon, part of the complex structure of human emotion. But they are not just for crying: All vertebrates, even reptiles and birds, have tears, which are essential for maintaining healthy eyesight.
Now, a new study, published this week in the journal Limits in Veterinary Science, discovers that the tears of non-human animals are not so different from ours. The chemical similarities are so great, indeed, that the composition of the tears of other species – and how they adapt to their environments ̵1; can provide insights into better treatments for human eye disease.
Previously, scientists had studied closely the tears of only a handful of mammals, including humans, dogs, horses, camels, and monkeys. In the new study, Brazilian veterinarians first analyzed the tears of reptiles and birds, focusing on seven types: barn owls; blue-and-yellow macaw; street hawks; wide cayman; and sea curbs, hawks and green sea turtles. (Take our quiz: Which animal belongs to each eye?)
Tears, which are released from the tear ducts (in humans and some other mammals) or other similar glands, form an over-the-eye film that consists of three components: mucus, water and oil. The mucosa coats the surface of the eye and helps to connect the film to the eye, water is a natural saline solution containing important proteins and minerals, and the oil prevents the eye from drying out.
Humans are the only known species that produce exciting tears; the expression “crocodile tears,” which refers to a person’s emotional display, comes from the crocodiles ’mysterious tendency to shed tears while eating.
But tears play a major role beyond crying, 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 nourishment to the cornea, the clear outer layer of the eye, which has no blood vessels, he says.
“It’s an attractive look at such a diverse species,” Sebbag says of the new study.
How to analyze tears
Study leader Arianne Pontes Oriá, a veterinarian at the Brazilian Federal University of Bahia, already knew that the broad-leaved Kachaks – a “beautiful-eyed” alligator relative, could keep their eyes open without staring for up to two hours, she says. In contrast, people inflate every 10 to 12 seconds. The pulse distributes the tears all over the surface of the eye, keeping them moist and vision stable.
To analyze the tears of caymans and 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 line with various government agencies that regulate animal welfare, the team collectively collected tear samples in test strips or with a syringe from animal eyes, as well as tears from 10 healthy human volunteers. The scientists used special kits designed to measure the amounts of specific chemicals and compounds, such as electrolytes (a mixture of sodium and chloride) and proteins.
Strangely enough, given that birds, reptiles and mammals have different structures for producing tears, the tears of all species – including humans – had a similar chemical composition, with similar amounts of electrolytes, although the tears of birds and reptiles had slightly higher concentrations. The change may be because they live in water and air, which can be destructive to the surface of the eye – higher levels of electrolytes in their tears may be needed to protect against inflammation, Oric says. (Learn that 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 other species. Such proteins are important for maintaining ocular surface stability. Caimans and owls can 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, seeking very stable tears.
The researchers also analyzed patterns of crystallization of tears formed when they dried – a technique often used to diagnose eye diseases. That was the biggest surprise, Oric explains: “There were far more changes in their tear crystals than in the composition of the tears.” The teardrop crystals of sea turtles and caimans were extremely unique, she says, “again, probably because of their adaptation to aquatic environments.”
Sea turtles also had by far the thickest tears of all animals, which is why researchers had to collect them with a syringe. “They live in salt water, and so they need tears adapted to that environment,” Oric says. Having extra thick mucus in the tear film can protect the turtle’s eyesight; without the thick film, their tears would thin, making them useless.
Protecting the vision of sea turtles, humans, dogs and cats
By providing information on how to protect the vision of, for example, sea turtles, which are endangered, the study can inform conservation efforts. “If we understand what creates a healthy tear film, then we can understand how pollutants or other environmental effects can damage an animal’s eyes,” Oric says.
Learn how reptiles and bird use tears can also inspire new medications for conditions such as dry eye, which occurs when the tear ducts do not produce enough oil. The disease, common in cats, dogs and humans, can sometimes lead to blindness.
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 area, but massively poorly understood,” he says, “so this study is interesting at multiple levels.”