Asked by: Irina Bylchenko, London
It’s not just humans that enjoy a smooch; lots of animals have courtship behaviours involving the mouth. Pigeons touch beaks, cats and dogs nuzzle each other, male fruit flies lick the females. At the most basic level, kissing is just a way of tasting and touching a potential mate, as part of the process of assessing suitability.
In primates though, kissing might also be a behaviour that has transferred from maternal feeding. We depend for our first meals on our ability to suckle, and the positive feedback mechanisms that evolved to encourage infants to do this last into adulthood. Kissing triggers lots of hormone changes, including raising oxytocin levels – the hormone that creates a sense of attachment.
Kissing is virtually universal in all human cultures, so it’s possible that it is instinctive. But on the other hand, we generally have sex face to face with our mouths already close together, so it’s an easy behaviour to stumble upon.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.