Pet owners often claim their dogs cry. Darwin thought monkeys and elephants wept. But modern scientists believe the only animal to really break down in tears is us. So why do we do it, and why do we change the way we cry as we grow older? Amanda Smith investigates.

Charles Darwin thought that tears had no modern adaptive function.

A similar view was expressed in poetry by Tennyson when he wrote, 'Tears, idle tears, I know not what they mean'.

Professor Ad Vingerhoets from Tilburg University in the Netherlands has written extensively about our capacity to burst into tears, including his book Why Only Humans Weep: Unravelling the Mysteries of Tears.

'In the sense of producing emotional tears, we are the only species,’ he says. All mammals make distress calls, like when an offspring is separated from its mother, but only humans cry, he says.

I think that [crying in front of others] shows us how tears are a social phenomenon, it's something we do in public, it's showing other people what's going through our head. Dr Thomas Dixon, historian, Queen Mary, University of London

Anecdotally though, plenty of people today seem to believe that animals, especially their pets, can and do weep.

Certainly all land animals have the physiological ability to produce tears to lubricate their eyes.

Darwin’s own theory of tears claimed that ‘in our evolutionary past, babies would close their eyes very tightly to protect their eyes when they were screaming for their mothers,’ says Dr Thomas Dixon, director of the Centre for the History of Emotions at Queen Mary, University of London.

‘The closing of the eyes very tightly would squeeze the lachrymal glands and bring out the tears,’ he says. By a process of association thereafter, any kind of pain or suffering became connected with tears.

Darwin also thought that ‘animals could weep emotionally', says Dixon.

'He cited a macaque monkey in London Zoo that could do it, and he also firmly believed that Indian elephants when they were captured or distressed or dying would weep in grief.'

This is, however, out of line with current scientific thinking. ‘Darwin is a bit of an embarrassment for modern psychologists in that he was more willing to ascribe human emotions to animals than scientists tend to be today,' says Dixon.

Professor Vingerhoets says that weeping is not only confined to humans, it’s also a capacity that develops with age.

As you grow older the acoustic aspect, the howling, becomes less important and the visible signalling of tears becomes more essential.

The reasons why people cry also change.

'Powerlessness and being separated from loved ones stay important all over the lifespan', Vingerhoets says. However, while children from infancy through to adolescence cry as a result of physical pain, adults are less likely to. 'What is important for adults is what we call sentimental tearing—crying because we see others doing good to other people, self-sacrifice.’

With increasing age also comes increasing empathy.

'You also start crying more not only because of your own distress but also because of the distress and suffering of others,' Vingerhoets says.

The distress and suffering of others doesn't have to be real either. How often have you shed a tear over a movie, or a book?

Dr Dixon says that stage dramas going back to classical antiquity have always been a vehicle for a focused expression of emotion. 'In essence one of the main things that makes us cry is a story with moments of perhaps unexpected tragedy or perhaps longed-for resolution'.

There's also a purifying and ritual aspect to tears.

In one of the Psalms of the Old Testament, David says to God, 'You have stored my tears in your bottle and counted each of them'.

There is, indeed, a tradition of tear bottles in early Western cultures, both pagan and Christian. Professor Vingerhoets says that tear bottles have been found in ancient Egyptian, Greek and Roman societies. Rather than containing actual tears though, he says it's more likely that they contained ritual oils and perfumes.

'But what we know for sure is that tear bottles have been used by sailors and soldiers when they were separated from their loved ones. Also more recently, in the 19th century, they have been used when those mourning the loss of their loved ones collected their tears'. The mourning period was ended when the tears had evaporated from the bottle.

Tracks of my tears: the mystery of crying Sunday 21 July 2013 Listen to The Body Sphere to discover more about why we cry and how different cultures weep for different reasons. More This [series episode segment] has image, and transcript

Crying also has a social or performative nature, according to Dr Dixon. For example, saying something out loud in the presence of others is more likely to make you cry than if you are reading quietly to yourself.

'I think that shows us how tears are a social phenomenon. It's something we do in public, it's showing other people what's going through our head,’ Dixon says.

Yet surely there are also moments when we've all cried precisely because we're alone and no-one else is watching? Dixon admits that this is another of the complexities of crying.

'Nevertheless I would stand by the idea that if we can say anything about a general theory of tears, I think you'd have to involve their social nature, the fact it's something that we do in relation to other people,’ he says.

For those who are religious, 'there is always an audience potentially which is God’.

‘Even for those who think they really are just weeping for nobody apart from themselves, it's still a sort of performance. You're showing yourself things have really got bad, or whatever it might be'.

Find out more at The Body Sphere.

