Granted it was a highly emotional moment in the “Life of Pi” movie. When Pi finally got ashore after his long ordeal at sea, he was so tired that he collapsed on the beach, only to see his travel companion, the Bengal Tiger Richard Parker, disappear into the jungle…..without looking back just once at Pi, his teacher and savior.

Nevertheless I was surprised to see tears welling up in the eyes of my young son with whom I watched this movie. Why do humans shed tears of sorrow, joy, sadness or happiness? What is the ultimate explanation for the tears we shed when we experience something emotional? Are other animals capable of crying too? If not, why did we become the tearful animal?

According to experts, crying is something humans share with other animals, notably the nonhuman primates. Particularly infants of these species produce crying sounds when they experience something unpleasant. There are two main differences between us and them though. In humans, not just children cry but adults cry too. Perhaps most significantly, when humans weep they actually produce tears. There is no other known animal capable of producing tears in response to something unpleasant. So why are we tearful?

The scientific debate about the origins of our weeping goes back some time. The great ethologist Charles Darwin himself did not think much of it. He considered human crying to be something quite insignificant: “"We must look at weeping as an incidental result, as purposeless as the secretion of tears from a blow outside the eye," he wrote in 1872.

Yet in the past century or so a dozen theories have been proposed for why we cry, some of them are quite imaginative. Let me mention a few theories, from the ridiculous to the sublime.

One theory for our tears has to do with our evolutionary past as animals living near coastlines. The “aquatic ape” theory proposes that many of our unique adaptations, including why we are keen swimmers -- unlike any other primate -- is because we evolved near the sea and needed to forage in the ocean. So why do we cry? The tears we produce are basically responses to living in a saline environment. Think of the tears that you get when seawater gets into your eyes.

The second, fairly ridiculous explanation is that our tears arrived when our ancestors started to build campfires and the smoke got in our ancestors’ eyes so they started to produce tears. Because fires were used in death rituals we came to associate – in some peculiar way -- our tears with negative life-events and that is why we still weep when we watch a sad movie.

A third set of theories for why humans cry -- which are somewhat more plausible -- is that crying has a signaling function. Weeping signals that we are defenseless and will not put up a fight when someone wants to harm us. As an extension of this, crying is perhaps a signal to others that we need their help in fighting off an aggressor. Other social animals use cry vocalizations for this purpose too. When we cry we convey the impression that we are innocent and weak – like children – and need the protection of others.

So what function do our tears have? We could cry for help without shedding any tears, and yet our tears are so unique to our species. There is an interesting twist here. The Dutch psychologist and crying specialist from Tilburg University, Ad Vingerhoets, proposes that alarm cries can be dangerous in an environment teemed with predators, because it indicates your location. Better to sit quietly. Yet how do you let others know that you are in distress? Precisely through showing them the tears in your eyes.

So is there any evidence for this cry-to-get-social-support hypothesis? First, children shed many more tears than adults do which is what you expect if crying is about wanting protection. Women also cry a lot more – about four times more – than men do, and this again is what you expect because women tend to be physically weaker and more defenseless. Finally, there is evidence that when we watch a sad movie in the presence of others, we produce many more tears than when we watch it by ourselves.

So, whenever we see someone being tearful in our presence, our automatic response is to offer support. That’s exactly what I did when I saw the tears in the eyes of my son after Richard Parker had left Pi. And that is precisely why tigers do not weep.

Vingerhoets, A. (2013). Why Only Humans Weep: Unravelling the mysteries of the tears. Oxford University Press.

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