Why do crocodiles cry?

Crocodiles do shed tears, but that's not because they are sad. Crocodiles have long been thought to be excess salt in their excreta. Sea turtles, sea snakes, sea lizards and other marine reptiles and some seabirds have salt glands growing near the eye sockets. Salt glands enable these animals to remove excess salt from seawater to get fresh water, so salt glands are their natural "desalinizer", so crocodiles are thought to be the same reason for tears. The excretion function of the crocodile kidney is very imperfect, and the excess salt in the body is excreted by a special salt gland, which is located right near the eye, but this is just speculation.

It was not until 1970 that biologists tested the composition of crocodile tears and found that the salinity of crocodile tears increased after living in the sea for a period of time, which seems to prove that crocodile eyes have the same salt glands as sea turtles. So it was written into zoology monographs and textbooks. But on the other hand, this experiment shows that the salt content of crocodile tears is significantly lower than that of sea turtles, sea snakes and other marine reptiles, indicating that the metabolic function of crocodiles is much better than that of other animals. therefore, some biologists (including the people who did this experiment) believe that the hypothesis that crocodiles have salt glands in their sockets cannot be denied.

The debate ended in 1981, when Taplin and Greg of the University of Sydney in Australia noticed that a clear liquid flowed from the surface of the crocodile's tongue, which they suspected was the secretion of the crocodile's salt glands. but the liquid secretion is too slow to collect and analyze, and they injected the crocodile with salt water to stimulate the secretion of salt glands. Finally, they injected the crocodile with methacholine-previous experiments have shown that injecting methylcholine into other marine reptiles stimulates the secretion of salt glands.

Sure enough, the crocodile tongue secretes liquid constantly, and it can be collected with a syringe to analyze the content of sodium, chlorine and potassium ions and measure the osmotic pressure. at the same time, crocodile tears are also collected for comparison. It was found that the salt concentration of these secretions was much higher than that of blood salt, about 3-6 times higher than that of blood salt, and the osmotic pressure was 3.5 times that of blood osmotic pressure, which was equal to that of sea water. Although the salt content of tears has also increased, it is only about twice the concentration of blood salt. They then dissected the crocodile's tongue and found salt glands on the mucous membrane of the tongue, which were similar in structure to those of other marine reptiles, especially those under the tongue of sea snakes. Since then, other studies have confirmed the findings.

So it seems that crocodiles excrete salt through tongue secretions rather than tears, so what is the role of crocodile tears? Crocodiles usually stay on land for a long time before they begin to secrete tears from behind the nictitating membrane, which is a transparent eyelid. When the crocodile dives into the water, close the nictitating membrane, which can not only see the situation underwater, but also protect the eyes. Another function of nictitating film is to moisturize the eyes, which needs to be lubricated with tears.

Kent Fleet, a zoologist at the University of Florida, observed and photographed four caimans and three alligators eating on land at a crocodile farm in 2007 and found that five of them did shed tears while eating, and some even had foamy eyes. There is no need to cry over processed food like dog food, which Flette speculates is because crocodiles blow together, oppressing the air and tears in the sinuses. The reason why crocodiles shed tears in textbooks, popular science books and knowledge competitions is still very scientific and rigorous.