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Why do humans kiss each other when most animals don't?

2015-07-15 02:33:18

A new study finds that half of human cultures don't practice romantic

lip-on-lip kissing. Animals don't tend to bother either. So how did it evolve?

Presented by

Melissa Hogenboom

When you think about it, kissing is strange and a bit icky. You share saliva

with someone, sometimes for a prolonged period of time. One kiss could pass on

80 million bacteria, not all of them good.

Yet everyone surely remembers their first kiss, in all its embarrassing or

delightful detail, and kissing continues to play a big role in new romances.

At least, it does in some societies. People in western societies may assume

that romantic kissing is a universal human behaviour, but a new analysis

suggests that less than half of all cultures actually do it. Kissing is also

extremely rare in the animal kingdom.

So what's really behind this odd behaviour? If it is useful, why don't all

animals do it and all humans too? It turns out that the very fact that most

animals don't kiss helps explain why some do.

Kissing could be quite a recent invention (Credit: Cultura Creative/Alamy)

According to a new study of kissing preferences, which looked at 168 cultures

from around the world, only 46% of cultures kiss in the romantic sense.

The study overturns the belief that romantic kissing is a near-universal human

behaviour

Previous estimates had put the figure at 90%. The new study excluded parents

kissing their children, and focused solely on romantic lip-on-lip action

between couples.

Many hunter-gatherer groups showed no evidence of kissing or desire to do so.

Some even considered it revolting. The Mehinaku tribe in Brazil reportedly said

it was "gross". Given that hunter-gatherer groups are the closest modern humans

get to living our ancestral lifestyle, our ancestors may not have been kissing

either.

The study overturns the belief that romantic kissing is a near-universal human

behaviour, says lead author William Jankowiak of the University of Nevada in

Las Vegas. Instead it seems to be a product of western societies, passed on

from one generation to the next, he says.

There is some historical evidence to back that up.

Not all cultures kiss (Credit: Christophe Courteau/NPL)

Kissing as we do it today seems to be a fairly recent invention, says Rafael

Wlodarski of the University of Oxford in the UK. He has trawled through records

to find evidence of how kissing has changed.

Is kissing something we do naturally?

The oldest evidence of a kissing-type behaviour comes from Hindu Vedic Sanskrit

texts from over 3,500 years ago. Kissing was described as inhaling each other's

soul.

In contrast, Egyptian hieroglyphics picture people close to each other rather

than pressing their lips together.

So what is going on? Is kissing something we do naturally, but that some

cultures have suppressed? Or is it something modern humans have invented?

We can find some insight by looking at animals.

Chimpanzees kiss and embrace after a fight (Credit: C.O. Mercial/Alamy)

Our closest relatives, chimpanzees and bonobos, do kiss. Primatologist Frans de

Waal of Emory University in Atlanta, Georgia, has seen many instances of chimps

kissing and hugging after conflict.

As far as we know, other animals do not kiss at all

For chimpanzees, kissing is a form of reconciliation. It is more common among

males than females. In other words, it is not a romantic behaviour.

Their cousins the bonobos kiss more often, and they often use tongues while

doing so. That's perhaps not surprising, because bonobos are highly sexual

beings.

When two humans meet, we might shake hands. Bonobos have sex: the so-called

bonobo handshake. They also use sex for many other kinds of bonding. So their

kisses are not particularly romantic, either.

These two apes are exceptions. As far as we know, other animals do not kiss at

all. They may nuzzle or touch their faces together, but even those that have

lips don't share saliva or purse and smack their lips together. They don't need

to.

A female can sniff out a good mate (Credit: Blickwinkel/alamy)

Take wild boars. Males produce a pungent smell that females find extremely

attractive. The key chemical is a pheromone called androstenone that triggers

the females' desire to mate.

Animals often release these pheromones in their urine

From a female's point of view this is a good thing, because males with the most

androstonene are also the most fertile. Her sense of smell is so acute, she

doesn't need to get close enough to kiss the male.

The same is true of many other mammals. For example, female hamsters emit a

pheromone that gets males very excited. Mice follow similar chemical traces to

help them find partners that are genetically different, minimising the risk of

accidental incest.

Animals often release these pheromones in their urine. "Their urine is much

more pungent," says Wlodarski. "If there's urine present in the environment

they can assess compatibility through that."

It's not just mammals that have a great sense of smell. A male black widow

spider can smell pheromones produced by a female that tell him if she has

recently eaten. To minimise the risk of being eaten, he will only mate with her

if she is not hungry.

Males can smell when it's best to mate a female (Credit: Visuals Unlimited/NPL)

Male black widow spiders can smell when it's best to mate a female (Credit:

Visuals Unlimited/NPL)

The point is, animals do not need to get close to each other to smell out a

good potential mate.

On the other hand, humans have an atrocious sense of smell, so we benefit from

getting close. Smell isn't the only cue we use to assess each other's fitness,

but studies have shown that it plays an important role in mate choice.

Men also make a version of the pheromone that female boars find attractive

A study published in 1995 showed that women, just like mice, prefer the smell

of men who are genetically different from them. This makes sense, as mating

with someone with different genes is likely to produce healthy offspring.

Kissing is a great way to get close enough to sniff out your partner's genes.

In 2013, Wlodarski examined kissing preferences in detail. He asked several

hundred people what was most important when kissing someone. How they smelled

featured highly, and the importance of smell increased when women were most

fertile.

It turns out that men also make a version of the pheromone that female boars

find attractive. It is present in male sweat, and when women are exposed to it

their arousal levels increase slightly.

Elephants show affection using their trunks (Credit: Laura Romin/Larry Dalton/

Alamy)

Elephants show affection using their trunks (Credit: Laura Romin/Larry Dalton/

Alamy)

Pheromones are a big part of how mammals chose a mate, says Wlodarski, and we

share some of them. "We've inherited all of our biology from mammals, we've

just added extra things through evolutionary time."

You could forego kissing and start smelling people instead

On that view, kissing is just a culturally acceptable way to get close enough

to another person to detect their pheromones.

In some cultures, this sniffing behaviour turned into physical lip contact.

It's hard to pinpoint when this happened, but both serve the same purpose, says

Wlodarski.

So if you want to find a perfect match, you could forego kissing and start

smelling people instead. You'll find just as good a partner, and you won't get

half as many germs. Be prepared for some funny looks, though.