Whether you're pecking granny's cheek or tangling tongues with that special someone, there's a lot of biology in a kiss. From lip action to brain power, kissing is a coordinated affair of chemicals, electricity, and fireworks.
But, let's start at the lips, one of the most touch-sensitive parts of your body; even the softest kiss can be an unforgettable experience. So, what makes our lips so sensitive? It just so happens that each lip is jam-packed with millions upon millions of touch receptors. Because the receptors are smushed together in such a small space, even the smoothest caress can activate thousands of them, which partly explains why kisses feel so good...and why lip piercings feel so painful!
Did You Know?
Other body parts richly endowed with touch receptors include the tongue, fingertips and genitals.
With kisses come sparks — literally. Each of those millions of touch receptors in the lips is actually the start of an electrical wire called a neuron, a specialized cell that can generate and conduct bioelectricity. When the pressure from a kiss excites your lips' touch receptors it sends electric sparks shooting along the neurons to different parts of your body, most notably to the head, where these neurons hook your lip receptors right into your brain.
It's in the brain that a kiss with granny differs from a passionate embrace. Neurons can branch many times, plugging receptors into different regions of the brain. A group of these regions is called the reward pathway. The reward pathway serves as a pleasure centre for your body that makes you feel good when you kiss, do well on a test, or score the winning goal.
If the sparks from a kiss reach the reward pathway they set off a chemical explosion of neurotransmitters in the brain. Neurotransmitters, like receptors, can trigger electrical charges that racing along neurons. So, release of these neurotransmitters in the reward pathway sends sparks shooting all over your body that, among other things, pump up your heartbeat, put butterflies in your stomach, and send tingles all the way to your toes. These physical and chemical effects of a kiss are what many refer to as "fireworks". Of course whether or not a kiss will cause fireworks depends on what we anticipate from a kiss — and most people do not anticipate fireworks from granny.
Did You Know?
Addictive drugs like alcohol, cigarettes, and marijuana and even addictive behaviours like gambling can artificially ramp up activity in the reward pathway. This makes these addictive habits difficult to ignore and hard to resist.
The pleasurable highs of kissing aside, it seems a bit odd that we would engage in such an unhygienic activity. After all, those who exchange an embrace also swap spit, bacteria, and bits of breakfast...all of which seems like more than enough reason to avoid a smooch. So how did humans come to kiss at all? Well, it turns out that our closest evolutionary cousins, the chimpanzees, regularly kiss and make up after a fight, indicating that kissing behaviour may have developed at a time before humans and chimps diverged into separate species. Some scientists think that kissing may allow partners to sniff out the quality of a potential mate.
In research conducted at the University of Chicago in 2002, women were asked to smell the t-shirts of men who each carried different genes for fighting diseases and infections. More often than not, women preferred the scent of a man whose genes would work well with her own to combat a wider range of germs, which could confer greater resistance to children they potentially decide to have by passing on the combination of their genes. So, kissing may provide the perfect opportunity to catch a whiff of your potential significant other to see if their genes are up to snuff with your own.
Then again biology isn't the only thing behind a kiss. Kisses are usually cultural decisions. After all, some societies see kissing as insulting, and a tribe in New Guinea hadn't even heard of kissing until Europeans arrived. In other words how you feel about kissing largely depends on the family, society, and culture you grow up in. In the end, when and whom you kiss is your choice — except maybe when it comes to granny.
For more on the Science of Kissing:
Diamond, J. (2002) The Third Chimpanzee: The Evolution and Future of the Human Animal. Harper Perennial, New York, NY
Martini, FH. (2004) Fundamentals of Anatomy and Physiology, 6th edition. Prentice Hall, Upper Saddle River, NJ
Jacob, S, McClintock, MK, Zelano, B, Ober, C. (2002) Paternally inherited HLA alleles are associated with women's choice of male odor. Nature Genetics, 30, 175-179.
Maldonado R, Valverde O, Berrendero, F. (2006) Involvement of the endocannabinoid system in drug addiction. Trends in Neurosciences, 29(4), 225-232.
Chris Damdar graduated with high distinction from the University of Toronto holding degrees in evolutionary biology and professional writing. Although he managed to co-author a scientific study and win several science scholarships, Chris has caved into his love of writing and has set his sights on a career in science journalism. He is a cellist, amateur composer, administrative assistant at the Institute of Genetics, and a freelance writer living in Toronto. His writings have been published in The Medical Post, YESmag, The Varsity, and now, CRAM Science.