Is it sentimentality or morbid affectation? Why do animals shed tears?

||||

Written by reporter Liao Mailun Edited by Ding Lin

New Media Editor/Li Yunfeng

Interview experts

Li Xiangtao (Researcher at Beijing Museum of Natural History)

"Late at night, I suddenly dreamed of my youth, and cried in my dream, my makeup was stained with tears." "Things have changed, people have changed, everything has come to an end, tears flow before I can speak."... Since ancient times, literati and poets have often used tears to express sad emotions. But in daily life, tears are not exclusive to emotional expression. Even if there is "no ripple" in your heart, you may still "shed tears" because of dust or onions.

So in the animal world, do tears also express emotions similar to humans? What are the purposes of tears in various animals?

▼ ▼ ▼

Basic tears: The first priority of tears is moisturizing

The significance of tears can be seen from the creation of the Chinese character. The simplified Chinese character “泪” is a pictograph, composed of 氵 (water) and 目 (eye), which means “water flowing from the eyes” and does not carry sadness. In the animal world, the “first priority” of tears is not to express emotions, but they are a “must-have” to ensure normal physiological functions.

Under normal circumstances, the secretions of various glands such as the main and accessory lacrimal glands of the human eye together form tears. As a weakly acidic, transparent, colorless liquid, tears contain up to 98.2% water and a small amount of protein, inorganic salts, lysozyme, β-lysin, secretory immunoglobulin A and other substances.

Through the "blinking movement" of the eyelids (i.e. blinking), tears are evenly applied to the ocular surface to form a smooth tear film. In this process, the eyelids are like hardworking gardeners, constantly "watering" the ocular surface, shouldering the important physiological function of protecting and maintaining the ocular surface, ocular tissues and the survival of the eyeball.

The cornea is one of the most important structures in the eye's refractive system, but its normal refractive function is also inseparable from tears. The surface of the cornea, which is composed of cells, is not very smooth, and the presence of the tear film can fill in the unevenness of the cornea. When light passes through the cornea to the retina, the image will be clearer. It can be said that the tear film provides a strong refractive interface for clear vision. In addition, tears can also provide some nutrients and oxygen to the cornea that lacks blood supply.

▲The surface of the eye has three layers: the innermost is the mucus layer secreted by goblet cells (light brown in the picture), the middle is the tear layer secreted by the lacrimal glands and accessory lacrimal glands (turquoise in the picture), and the outermost is the lipid layer secreted by the meibomian glands (yellow in the picture). If any of the three components is problematic, it will lead to dry eye syndrome and other conditions (Source: The New England Journal of Medicine)

Although very important, these tears that ensure the normal physiological function of the eyeball are difficult to be perceived autonomously. Scientists call them "basal tears". Humans produce about 280 ml of "basic tears" every day. If there is a problem with its secretion, it will affect eye health. Many people work or play games in front of the computer for a long time, with high concentration, and the number of blinks is greatly reduced. In addition, the light, distance, and angle are incorrect, which leads to vascular nerve regulation disorders. The eyeball is not protected by the tear film and is likely to induce dry eyes. Patients will feel dry eyes, pain, itching, foreign body sensation, and even vision fluctuations. At this time, they often need the help of treatments such as artificial tears.

Tears, which are essential for eye health, are not unique to humans. In the animal world, many animals also secrete tears to keep their eyes moist, and they are not limited to the mammals we are familiar with. A 2020 study published in Frontiers in Veterinary Science showed that although the composition of the tears of various vertebrate subjects did not completely match that of human tears, they were closer than expected. Scientists have also found that the electrolyte composition of the tears of some birds and reptiles is comparable to that of human tears.

▲The tear crystal patterns of red-footed tortoises, caimans, buzzards, blue-capped parrots, barn owls and loggerhead sea turtles (Source: Frontiers in Veterinary Science)

▼ ▼ ▼

Reflective tears: dedicated "dust-repelling guards"

Many vertebrates, including mammals, can secrete tears, but some other animals "want to cry but have no tears." "Some animals are born without tear glands, such as invertebrates and fish," said Li Xiangtao, a researcher at the Beijing Museum of Natural History.

For animals that can secrete tears, lubricating the ocular surface is not the only function of tears. For example, when the spring breeze blows and the dust gradually dazzles the eyes, the second function of tears, namely flushing and diluting, is also triggered more frequently. With the help of a large amount of tears, foreign matter such as dust is washed out, thus protecting the cornea and conjunctiva from being injured by foreign matter.

In addition to dust, when sand, smoke, chemical irritants and small insects accidentally enter the eyes, our "emergency floodgates" will be quickly activated. Different from basal tears, this type of tearing is mainly caused by the stimulation of the ocular surface tissue by adverse physical, chemical and natural stimuli in the living environment. It is mostly caused by the secretion of tears after the trigeminal nerve endings such as the conjunctiva and cornea are stimulated. It is a response to invasive stimulation and belongs to the peripheral sensory type of "reflex tears" (depending on the different stimulation sites, reflex tears also include retinal type, central stimulation type, direct stimulation of lacrimal gland cells, etc.).

Unlike basal tears, reflex tears have a higher water content and contain anti-inflammatory ingredients. Reflex tears are not exclusive to humans, and some animals also rely on them to protect their eyes.

▲The camel in tears (Photo: Sean Scarff)

It is worth mentioning that some animals not only rely on tears to protect their eyes, but also rely on the "third eyelid" - the nictitating membrane. The nictitating membrane is a transparent or translucent eyelid that extends from the mucous membrane folds on the inner surface of the upper and lower eyelids at the inner corner of the eye. It can move upward or diagonally to shield the cornea of ​​the eyeball. However, not all animals have a nictitating membrane. This structure is unique to tailless amphibians, reptiles and birds among vertebrates, cartilaginous fish, sharks and a few mammals.

Among the many animals with nictitating membranes, birds have the most developed nictitating membranes. The nictitating membrane of birds is located in the front corner of the eye socket and is translucent. Its inner edge has a feather epithelium that can wash away dust on the cornea. When birds fly, the nictitating membrane can cover the eyeball to prevent dry airflow and dust in the air from causing damage to it.

At this point, many people who "cry in the wind" may express envy. If humans also have nictitating membranes, do we not need goggles when skiing? Unfortunately, the human nictitating membrane has degenerated and only remains in the inner corner of the eye, presenting as an immobile half-moon fold, and has lost the protective function of the bird's nictitating membrane.

▲The moment when the nictitating membrane of an American eagle owl flutters (Photo: Gary Cascio)

▼ ▼ ▼

Are “crocodile tears” hypocritical?

In addition to moisturizing and flushing, tears are also one of the ways we are familiar with to express emotions. When intense emotional changes such as grief, grievance, and sadness occur, humans may express their inner emotions by crying, sobbing, and sobbing. Compared with basal tears and reflex tears, this type of "emotional tears" is regulated by the nature and degree of human subjective emotions, and the outflow volume will be larger, part of which will enter the tear duct and flow out of the nasal cavity, and the other part will overflow.

William Fry, a psychologist at the University of Minnesota, once collected the tears of subjects in test tubes for analysis and testing. He found that compared with reflex tears, emotional tears contain more proteins, one of which is a chemical substance similar to analgesics. From this, he speculated that crying may help eliminate the biochemical toxins produced and accumulated by the human body due to emotional stress. However, the detailed composition of these toxins and their production and metabolic mechanisms need further research.

Humans are the only animals known to shed "emotional tears". Although there is a well-known proverb in the West called "crocodile tears", cold-blooded animals like crocodiles will not easily "break down".

In the 14th century, in the book "The Travels of Peter Mandeville", the author described in detail the scene of crocodiles "crying while eating people" in the form of personal experience. As the book became popular, related sayings also spread widely. In 1563, Edman Grindel, the Bishop of York and Canterbury in England, was the first to use "crocodile tears" to describe hypocrisy. Since then, people have gradually begun to use "crocodile tears" to describe those who hurt others while pretending to be kind, making it a synonym for "hypocrite".

But are crocodiles shedding tears really "false compassion"? The answer is no. Li Xiangtao pointed out that crocodiles' tears while eating are not an expression of emotion, but are related to their own physiological structure. Crocodiles have imperfect kidneys and can only use special glands to help excrete excess salt in the body, and their salt glands are located near the eyes.

Studies have shown that crocodiles have undifferentiated teeth and can only tear food but not chew it. Therefore, when crocodiles swallow their prey, the glands near their eyes are often stimulated and cause them to "cry."

Therefore, no matter what the reason, crocodile tears are not an expression of emotion, but a process for them to excrete salt and ensure their own health, or they must go through it during "dining". In addition to crocodiles, scientists have also found that sea turtles, sea snakes, sea lizards and some seabirds also have similar salt glands.

▼ ▼ ▼

Why do dogs have tears in their eyes?

So what are the emoticons of pets with tears in their eyes that we often see on the Internet? In fact, dogs with tears in their eyes are not sad or sentimental. Their tears are often an indicator of nutritional or health problems.

▲A dog with tears in his eyes

The tear glands of dogs are located above the eye sockets. Under normal circumstances, excess tears enter the tear sac and nasolacrimal duct through the tear duct opening (tear point) at the inner corner of the eye, and evaporate with the air when reaching the nasal cavity, so there will be no tears. If tears accumulate, the main reason is that the eyes are stimulated by adverse factors or the tear duct is blocked and cannot discharge tears normally, or it is in the early stage of infection of certain infectious diseases.

If a pet dog has local inflammation in the eyes, in addition to tears, it is generally accompanied by symptoms such as red eyes and swelling around the eyes; blocked tear ducts are accompanied by moist dermatitis below the inner corner of the eye and sticky purulent secretions squeezed out of the tear points; infectious diseases may be accompanied by fever, vomiting, diarrhea, runny nose, coughing, loss of appetite, etc. Once the above symptoms other than tears appear, be sure to send your dog to the doctor for treatment in time to avoid delaying the condition of your dog.

Li Xiangtao added that pet dogs' tears are not always caused by illness, they may also be "tears of happiness" caused by overnutrition: pet dogs eating too much meat or offal can also cause tears. If the dog owner finds that the dog has a lot of tears and too much eye mucus (eye boogers), and no other symptoms, it may be related to overnutrition, which can be adjusted by reducing the amount of meat fed.

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

Welcome to share to your circle of friends

Reproduction without authorization is prohibited