Most of us think of tears as a human phenomenon, part of the complex fabric of human emotion. But they are not just for crying: All vertebrates, even reptiles and birds, have tears, which are critical for maintaining a healthy face.
Now, a new study, published this week in the journal Frontiers in Veterinary Science, shows that the tears of non-human animals are not so different from our own. The chemical similarities are so great that the composition of the tears of other species – and how they are adapted to their environment – can provide insight into better treatments for human eye disease.
Previously, scientists had only studied 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 macaw; Hawksen by road; broad-nosed caimans; and woodcock, hawksbill, and green sea turtles. (Take our quiz: to which animal does each eye belong?)
Tears, emitted from tear ducts (in humans and some other mammals) or other similar glands, form a film over the eye that consists of three ingredients: mucus, water, and oil. The mucus covers the surface of the eye and helps to bind the film to the eye, the water is a natural salt solution that contains important proteins and minerals, and the oil prevents the eye from drying out.
Humans are the only known species that produce emotional tears; the term “crocodile tears”, which refers to the false representation of a person’s emotion, comes from the mysterious tendency of crocodiles to release tears when they eat.
But tears play important roles outside of 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 removing it from debris. They also protect the eye from infection and provide nutrition to the cornea, the clear outer layer of the eye that has no blood fats, he says.
“It’s a fascinating look at such a wide range of species,” says Sebbag of the new study.
How to analyze tears
Study leader Arianne Pontes Oriá, a veteran at the Federal University of Bahia in Brazil, already knew that broad-shouldered caimans – an alligator-relative with “beautiful eyes” – can keep their eyes open for up to a maximum of two hours, she says. People, on the other hand, blink every 10 to 12 seconds. Cutting scattered tears over 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 animals in captivity at a conservation center, an animal care facility, and a commercial breeder in Brazil. In accordance with various government agencies that regulate animal welfare, the team has collected human tears on test strips as with a syringe from the eyes of animals, such as tears from 10 healthy human volunteers. The scientists used special kits designed to measure the amounts of certain 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 tears of the species – including those of humans – had a similar chemical makeup, with similar amounts of electrolytes, although birds- and reptile tears had slightly higher concentrations. This difference may be because they live in water and air, which can be disturbing 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, such 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. Caymans and owls can have high protein concentrations because both species have large eyes and long intervals between flashes; caimans also live with their eyes submerged in fresh water for long periods of time, requiring very stable tears.
The researchers also analyzed the crystallization patterns that formed the tears when they dried – a technique often used to diagnose eye diseases. Here was the biggest surprise, Oriá explains: “There was much more variation in her tear crystals than in the trian collection.” Tear crystals of sea turtles and caimans were remarkably unique, she says, “again, probably due to their adaptation to aquatic environments.”
See: How animals and humans see the world differently
The sea turtles also had by far the thickest tears of all animals, and the researchers had to collect them with a syringe. “They live in salt water, and so they need tears that are adapted to that environment,” Oriá says. Having extra thick mucus in the tear film probably protects the vision of the turtles; without the thick film their tears would be diluted, making them useless.
Protect the vision from sea turtles, humans, dogs and cats
By providing information on protecting the vision of, for example, endangered sea turtles, the study was able to inform conservation efforts. “If we understand what makes a healthy tear film, we can understand how pollutants or other environmental effects can damage an animal’s eyes,” says Oriá.
Learning how reptiles and birds use tears can also inspire new medications for conditions such as dry eye, which occurs when tears do not produce enough oil. The disease, often found in cats, dogs, and humans, can sometimes lead to blindness.
The study illustrates how little we know about tears and how they work in humans and other animals, says 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.”
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