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

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. 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. 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. 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. 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. 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 . 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.


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