By James Morgan
Science reporter, BBC News, Chicago
Sometimes the first kiss dooms a relationship
When you share a kiss with your lover on Valentine's Day, you may be revealing a lot more than you realise.
Locking lips not only stimulates our senses, it also gives us subtle clues about our suitability as mates, US scientists have found.
A man's saliva has a "cocktail of chemicals" hinting at his fertility and evolutionary fitness, they said at a conference in Chicago.
That may be why the first kiss is often the last - "the kiss of death".
"Kissing is a powerful adaptive mechanism - otherwise we wouldn't see it all over the world. Over 90% of human societies practice kissing," said anthropologist Helen Fisher, of Rutgers University in New Jersey, at the annual conference of the American Association for the Advancement of Science (AAAS) in Chicago.
"Chimpanzees and bonobos kiss. Foxes lick each other's faces. Birds tap their bills together and elephants put their trunks in one another's mouths.
"So why do we do it? I think it is a tool for mate assessment. When you kiss, you can touch, see, feel, taste somebody. A huge part of our brain lights up.
"This is a real assessment tool - and can be highly positive or highly negative. In one study, 66% of women and 59% of men had experienced a first kiss which killed the relationship. It was the kiss of death."
As well as acting as a "screening" mechanism for potential mates, Dr Fisher believes kissing evolved to stimulate what she has described as the three key brain systems for mating and reproduction.
The first of these is sex drive.
"Male saliva has testosterone in it. And men as a group seem to like wet kisses, with an open mouth and more tongue action.
"So it may be that, unconsciously, they are attempting to transfer testosterone - to trigger the sex drive in women and push them into being more sexually receptive."
Men also have a poor sense of smell, she said, so by open mouth kissing "they might be trying to pick up traces of a woman's oestrogen cycle, to figure out the degree of her fertility."
Elephants share our habit of kissing
The second mechanism is romantic love.
"Kissing is novel, at least at the beginning of a relationship, and novelty stimulates dopamine - which is associated with romantic love," said Dr Fisher.
Finally, kissing promotes what she referred to as "attachment" or "pair bonding".
It helps us to stay together "at least long enough to have children," she said.
To study the chemistry which underlies kissing and pair bonding, neuroscientist Dr Wendy Hill, of Lafayette College, recruited a group of college students.
The young lovers - 15 couples in all - were then split into two groups. Some were asked to smooch for 15 minutes, to the soundtrack of relaxing music. The others sat holding hands and talking.
"Afterwards, we measured the changes in their levels of cortisol - a stress hormone - in their saliva.
"Levels had declined for everyone in the kissing group. And the longer the relationship, the lower the cortisol."
Dr Hill also took blood samples from the couples to measure levels of oxytocin - a messenger molecule associated with trust and sexual intimacy.
After 15 minutes of kissing, the males saw a significant increase in the "pair bonding" chemical.
But in the females, a decrease in oxytocin was observed.
"This was very surprising," Dr Hill admitted. "We are exploring the possibility that the setting - a college health centre - was just not very romantic.
"Get your coat, love - you've pulled!"
"After all, this is a place where students go when they are ill. That may have had an effect on the females."
Dr Fisher is now running the study again "in a more romantic setting.
"We have a secluded room with a couch, flowers, candles, and a light jazz CD playing."
Interestingly, the females on birth control pills had significantly higher oxytocin levels, even before kissing began.
But with so few couples taking part in the study, which has yet to be published, it was not clear if there was any direct link between the two.