Do crocodiles cry crocodile tearsThe idea that the crocodile cries whilst eating its victims is first mentioned by the Greek essayist Plutarch (AD46-119). Shakespeare called it the "mournful crocodile" that tricks its prey "with sorrow", in his play Henry VI, Part 2.? For centuries, people thought these cold-blooded monsters lured prey into their jaws with fake tears. And, in many modern languages, to "cry crocodile tears" refers to a false display of emotion.

Although scientists can't know what a crocodile is feeling behind that dead-eyed stare, thanks to recent research they now know a lot more about those tears. A Brazilian team has studied the chemical composition of samples taken from a range of animals. And, under the microscope, they look very similar to our own.

Tears are a mix of mucus, oil, and water. The thick mucus sticks the tear to the surface of the eye and stops it being washed away too quickly. This is very important for animals that live underwaterThe Loggerhead sea turtle tears are so thick with mucus that researchers described them as "like the worst snot you've ever seen" that needed very strong syringes to extract them.. The oil lubricates the eye to stop it drying out; the water contains proteins and minerals to keep it healthy.

Scientists hope their study of reptilian tears will help find new treatments for human eye problems. For example, we blink every four seconds to keep our eyes moist. But the caiman can hold a stare for up to two hours without batting an eyelidUnderwater, crocodiles use three eyelids to keep their eyes securely shut and protected from the murky swamp water.. How it does this without drying out could help doctors treat eye drynessAlso known as keratoconjunctivitis sicca, this is a common eye disease caused by a lack of oil in the basal tears., a major cause of blindness.

For millennia, humans have wondered why we cry. The Old TestamentThe first part of the Christian Bible, and is mainly based on the 24 books of the Hebrew Bible (Tanakh). explains tears as water formed from a weak heart, and medieval doctors assumed they were vapours produced by the heat of emotions. Even Darwin despaired and concluded that crying was essentially "purposeless".

But he was wrong. There are in fact three types of tear: basal, reflex, and emotional, each one with a different role. Humans are continually producing basal tears, a protective lubricant covering our eyes. Each year, we secrete 30 gallons from our tear ducts, although we make less as we grow older.

Cut into an onion and you'll soon be sobbing reflex tearsChopping onions creates a gas called syn-propanethial-S-oxide, which irritates the eyes. Reflex tears are also triggered by bright lights, wind, dust, and smoke. as your body tries to wash away the irritant fumes. These tears are rich in antibodies to attack any toxins that might get into your eyes.

But emotional tears are the real mystery. Only humans cry as an emotional response. This is despite anecdotal reports from dog owners and viral videos of crying elephantsRaju the rescued elephant and newborn baby Zhuangzhuang were two elephants that captivated social media with their weeping. However, biologists say there is no connection between their emotional state and their streaming eyes.. At some stage in our evolution, the emotional centre of our brains took control of our lacrimal systemMade up of various glands that produce and drain tears. and excitement, grief, and fear triggered the same reflex response as cutting an onion.

Crying may be designed to make us feel better, with some evidence that emotional tears contain oxytocin and endorphins, hormones to relieve stress and pain. But psychologist Ad Vingerhoets says the main reason we cry is social: "We cry because we need other people." It is a form of communicationMost animals disguise their weakness, but humans display their vulnerability with highly visible tears. This may be related to our unusually long childhood and dependence on others, reminding other members of our group that we need to be looked after., as human as language or laughter.

But humans are also skilled at deceiving and manipulating each other, behaviour biologists call Machiavellian intelligenceNamed after the 16th-Century political philosopher Niccolo Machiavelli, this theory argues that human intelligence evolved to help our ancestors compete for power in social groups by lying and manipulating one another.. "We learn early on that crying has this really powerful effect on other people," says psychologist Jonathan Rottenberg. And, so, crocodile tears quickly become another means of getting our own way.

Some say, no, this is a human-centric way of looking at the world. This research shows that many animals produce tears and each species has adapted them to its own environment. A sea turtle's viscous tears are as special as our own. And because we don't know what they are feeling, we assume our tears are more meaningful and unique.

Others say, yes, humans are the only animal that uses tears to show emotion. This is as unique as our ability to construct languages or create art. We feel empathy and compassion when we see people crying and we feel better after we have cried. We share with crocodiles the same mechanical response to external stimuli, but that is where the similarity ends.