Most of us think of tears as a human phenomenon, part of the complex fabric of human emotion. But they're not just for crying: All vertebrates, even reptiles and birds, have tears, which are critical for maintaining healthy eyesight. Now, 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.