Most of us think of tears as a human phenomenon, part of the complex structure of human emotion. But they’re not just for crying: All vertebrates, even reptiles and birds, have tears, which is crucial for maintaining healthy eyesight.
Now, a new study, published this week in the journal Borders in Veterinary Science, revealed that the tears of non-human animals are not too different from our tears. In fact, the chemical similarity is so great that the composition of tears in other species – and how they adapt to the environment ̵1; could provide a deeper understanding of better treatments. for eye disease in humans.
Previously, scientists only studied the tears of a few mammals, including humans, dogs, horses, camels and monkeys. In the new study, Brazilian veterinarians for the first time analyzed the tears of reptiles and birds, focusing on seven species: barn owl; macaws green and yellow; roadside street vendors; wide muzzle caimans; and sea turtles, tortoises, and green sea turtles. (Do our quiz: What animal each eye belongs to?)
Tears, which are secreted from tear ducts (in humans and some other mammals) or other similar glands, form a membrane on the eye that includes three components: mucus, water, and oil. Mucus covers the surface of the eye and helps bind membranes to the eyes, water is a natural salt solution that contains important proteins and minerals, and oils that keep the eyes from drying out.
Humans are the only species known to produce emotional tears; the phrase “crocodile tears”, referring to a person’s artificial expressions of emotion, comes from crocodile’s mysterious tendency to release tears when they eat.
But tears play a role beyond crying, notes Lionel Sebbag, veterinarian ophthalmologist at Iowa State University, in Ames, noting that tears are not involved in the new thing. research. They help strengthen eyesight by lubricating the eyes and clearing debris. They also protect the eye against infection and nourish the cornea, the transparent outer layer of the eye, where blood vessels are lacking, he said.
“It’s a fascinating look at such a diverse range of species,” says Sebbag of the new study.
How to analyze tears
Study leader Arianne Pontes Oriá, a veterinarian at the Bahia Federal University of Brazil, knew that the broad-snouted caimans – a cousin of the crocodile with “beautiful eyes”, can open their eyes without blinking. for two hours, she said. In contrast, people blink every 10 to 12 seconds. Blink distributes tears across the surface of the eyes, keeping them moist and stable vision.
To analyze the tears of the caimans and six other species, Oriá and her colleagues worked with 65 animals kept in a conservation center, an animal care facility and a commercial breeder. trade in Brazil. In order to comply with various governmental animal welfare agencies, the research team humanely collected tear samples on test strips or with syringes from the animal’s eyes, as well as tears from 10 subjects. healthy volunteers. Scientists have used special kits designed to measure the amount of specific chemicals and compounds, such as electrolytes (a mixture of sodium and chloride) and proteins.
Surprisingly, since birds, reptiles and mammals have different structures to produce tears, the tears of all species – including humans – have the same chemical makeup, with similar amounts of electrolytes, although the tears of birds and reptiles are slightly higher. This difference may be because they live in water and air, which can disrupt the surface of the eye – a higher amount of electrolytes in their tears may be needed to protect against inflammation, Oriá to speak. (Learn how mice track predators by sniffing their tears.)
Human tears, as well as the tears of caimans and barn owls, have a higher protein content than other species. Such proteins are important for maintaining the stability of the eye surface. Owls and Caimans birds can have a high protein concentration because both species have large eyes and long interval between blinks; Caimans also live with eyes submerged in fresh water for long periods of time, requiring highly stable tears.
Researchers also analyze the forms of crystals that tears form when they dry – a technique commonly used to diagnose eye diseases. This was the biggest surprise, Oriá explained: “There are more variations in their tear crystals than in the tear composition.” “Again, maybe because they are adapted to the water environment,” she said.
Sea turtles also have the thickest tears among any animal species, which is why researchers had to collect them with syringes. “They live in salt water, so they need tears to adapt to that environment,” Oriá said. There is extra thick mucus in the tear film that protects the turtle’s vision; Without the thick film, their tears would dilute, rendering them useless.
Protect the sight of sea turtles, humans, dogs and cats
By providing information on how to protect visibility, for example sea turtles, endangered species, research can inform conservation efforts. “If we understand what makes a healthy tear film, then we can understand how pollutants or other environmental influences can harm an animal’s eyes,” Oriá said.
Finding out how reptile and bird’s tear uses can also inspire new drugs for conditions like dry eyes, which occur when tear ducts don’t produce enough oil. This disease is common in cats, dogs and humans, and can sometimes lead to blindness.
Research shows little is known about tears and how they function in humans and other animals, says Brian Leonard, an ophthalmologist at the University of California, Davis.
“It’s an important area but very poor understanding, so this research is very interesting on many levels,” he said.