A new study, published this week in the journal Frontiers in Veterinary Science, reveals that non-human animals' tears are not so different from our own. The chemical similarities are so great, in fact, that the composition of other species' tears—and how they're adapted to their environments-may provide insights into better treatments for human eye disease.
Previously, scientists had studied closely only 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.
Tears, which are released from tear tubes, form a film over the eye that's composed of three ingredients: mucus, waler, and oil. The mucus coats the eyes surface and helps to attach the film to the eye, the water is a natural salty solution containing crucial proteins and minerals, and the oil prevents the eye from drying out.
Humans are the only known species to produce emotional tears; the expression "crocodile tears," which refers to a person's phony display of emotion, comes from the mysterious tendency of crocodiles to release tears as they eat.
But tears play key roles beyond weeping, notes Lionel Sebbag at Iowa State University, who was not involved in the new research. They help with vision by lubricating and clearing the eye. They also protect the eye against infection and provide nutrition to the cornea, the eye's clear outer layer, which lacks blood tubes, he says.
Learning how reptiles and birds' use tears may also inspire new medications for conditions such as dry eye, which occurs when tear tubes don't produce enough oil. The disease, common in cats, dogs, and people, can sometimes lead to blindness.
