The ‘crocodile tears’ are surprisingly similar to our own –

The ‘crocodile tears’ are surprisingly similar to our own

Most of us consider tears to be a human phenomenon, part of the complex fabric of human emotions. But they are not just for crying: all vertebrates, even reptiles and birds have tears, which are important for maintaining abdominal light.

Now, a new study, published this week in the journal Frontiers in Veterinary Science, Shows that non-human animal tears are not very different from our own. The chemical similarities are indeed so great, that the composition of tears of other species – and how they adapt to their environments – may provide insight into better treatments for human eye disease.

Previously, scientists had closely studied only the tears of 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 species: barn owls; Blue-and-yellow macaw; Roadside hawkers; Broad-snout caimans; And loggerheads, hawkbills and green sea turtles. (Take our quiz: Which animal does each eye have?)

Tears, which are excreted from the tear ducts (in humans and some other mammals) or other similar glands, form a film on the eye that is made up of three ingredients: mucus, water, and oil. Mucus coats the surface of the eye and helps to bind the film to the eye, water is a saline solution containing important proteins and minerals, and oil prevents the eye from drying out.

Man is the only known species causing emotional tears; The expression “crocodile tears,” which refers to the performance of a person’s spirit, elicits tears from the mystical tendency of crocodiles, as they eat.

But tears play an important role beyond crying, notes ophthalmologist Lionel Sebbag, a veterinarian at Iowa State University in AIIMS who was not involved in the new Research. They help with vision by lubricating the eye and clearing out debris. They protect the eye against infection and provide nutrition to the cornea, the clear outer layer of the eye, which lacks blood vessels, they say.

“It’s a fascinating look at such a diverse species,” Sebabag says of the new study.

How to analyze tears

Arianne Pontes Oria, a veterinarian at the Federal University of Bahrain, Brazil, already knew that broad-snowed caimins – relatives of a crocodile with “pretty eyes”, could blink their eyes for two hours without blinking eyelids, she says . In contrast, people blink every 10 to 12 seconds. In the blink of an eye, tears are shed on the surface of the eyes, which keeps them moist and the vision stable.

To analyze the tears of Caimans and six other species, Oriá and his 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 regulating animal welfare, the team humanely collected samples from animal eyes on syringes as well as tears from 10 healthy human volunteers on test strips. The scientists used special kits designed to measure the amount of specific chemicals and compounds, such as electrolytes (a mixture of sodium and chloride) and protein.

Surprisingly, given that birds, reptiles and mammals have different structures to produce tears, tears from all species — including humans — had a similar chemical makeup, with the same amount of electrolytes, However bird and reptile bears had slightly higher concentrations. This difference can occur because they live in water and air, which can be disruptive to the surface of the eye – high levels of electrolytes in their tears may be necessary to protect them from inflammation, says Oria. (Find out how rats sniff their prey by sniffing their tears.)

Human tears, as well as those cammins and barn owls, had higher levels of protein than other species. Such proteins are important for maintaining the stability of the ocular surface. Simmons and owls may have high protein concentrations because both species have large gaps and long gaps between the eyelids; Cymons also live with their eyes immersed in fresh water for a long time, requiring extremely stable tears.

Researchers also analyzed the crystallization patterns of tears when they dried up – a technique often used to diagnose eye diseases. The biggest surprise here was, Oria explains: “There was a lot more variation in their tear crystals than in tear composition.” The tear crystals of sea turtles and caimins were strikingly unique, she says, “again, perhaps due to their adaptation to aquatic environments.”

Look: how animals and people see the world differently

Sea turtles were also by far the thickest tears of all animals, which is why researchers had to collect them with syringes. “They live in salty water, and so they need tears suited to that environment,” Oria says. The possibility of excess thick mucus in the tear film protects the eyes of turtles; Without a thick film, their tears will be diluted, rendering them useless.

Protect the sight of sea turtles, people, dogs and cats

For example, by providing information about how to protect the sight of sea turtles, which are endangered, the study can inform conservation efforts. “If we understand what makes a healthy tear film, we can understand how pollutants or other environmental effects can harm the eyes of animals,” Oria says.

Learning how the tears of reptiles and birds can induce new drugs for conditions such as dry eye, which occurs when the tear ducts do not produce enough oil. The disease, common in cats, dogs and people, can sometimes lead to blindness.

This research reveals how little we know about tears and how they work in humans and other animals, notes Brian Leonard, a veterinarian ophthalmologist at the University of California, Davis.

“This is an important but largely poorly understood area,” he says, “so this study is interesting on many levels.”


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