Are humans the only animals that聽cry? If so, why?
Mike Follows
Sutton Coldfield, West Midlands, UK
It depends what we mean by 鈥渃ry鈥�. If we mean making a sound when distressed, humans are certainly not alone. Distress calls are widespread in the animal kingdom and are examples of an ancient form of communication. A hungry, frightened or separated youngster may attract food or protection by calling, while adults may warn others of danger.
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Humans are unusual because we also cry tears. Lots of animals produce tears, but ordinarily these have nothing to do with being sad. Basal tears keep the eye moist, while reflex tears respond to irritants such as smoke or onions. Emotional tears, by contrast, can accompany sadness, grief, anger, relief, joy and even laughter.
The most plausible explanation for emotional tears is their use in communication. They provide a conspicuous signal of distress or vulnerability and may encourage sympathy or reduce aggression. But if tears are such an effective signal, why don鈥檛 newborn babies use them from the outset?
Emotional tears may not originally have evolved as a signal, but simply accompanied intense emotional arousal
Babies certainly cry, but initially sound is the important signal because a baby needs to attract a parent who may not be looking at them. A loud cry works at a distance and in darkness; tears only work if somebody can see your face.
Emotional tears may not originally have evolved as a signal, but simply accompanied intense emotional arousal. Once other humans began responding to a tearful face, however, natural selection could reinforce its signalling function. But any useful signal is open to exploitation. Someone able to cry convincingly without experiencing the corresponding emotion may manipulate the response of others, known as crocodile tears.
Perhaps this is why tears alone aren鈥檛 enough to convince us that someone is genuinely upset. We also look for confirmation in facial expression, voice, circumstances and previous behaviour. Actors complicate matters further because some can deliberately summon a genuine emotional response and cry on demand.
The expression crocodile tears comes from the old belief that crocodiles wept while eating their victims, apparently regretting an act while continuing to benefit from it. The idea is old enough for William Shakespeare to have used it in Othello. Curiously, there is some real biology behind the story. Crocodilians (which include alligators, caimans and gharials, as well as crocodiles) have lacrimal glands and can appear to shed tears while feeding. The secretions have nothing to do with emotion.
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