Snogging, smooching, pashing, macking, frenching, K-I-S-S-I-N-G. It’s ubiquitous in almost all human cultures, but what evolutionary information do we get when we play tonsil-hockey? And why, why, do guys like more tongue than girls?

Swapping spit can provide a goldmine of pheremonal and hormonal information about whether our lip-locking should lead to hip-knocking. This can help us assess if our potential children – woah there body, hold your horses – would be genetically compatible. Kissing also lets us get up close and personal with our partner’s sebaceous glands, and because these are regulated by sex hormones they can provide further hormonal advice. The taste and breath from someone’s mouth can also flag underlying health problems that we wouldn’t want getting into the fruit of our loins.

But what does this have to do with men and women’s different application of tongue? Just like they’re better at multi-tasking and taking selfies, women are better at reading the hormonal and pheremonal information in saliva than men are. As females are the more ‘investing sex’ (i.e. they have to use energy and resources to bake that bun in their oven), they have a greater need to suss out the health and quality of a potential partner before they have ‘the sex’. This means that it’s more important for them to pick up on the chemosensory cues in kisses to sort their friends into ‘more than’ and ‘just’. A woman’s ability to do this becomes even more effective when she’s ovulating, as it’s during this period that quality assurance is most necessary.

Hughes et al (2007) suggest that men have a bit more trouble reading the signs: “given that males are less sensitive to chemosensory cues, making assessments of a female’s fertility by means of kissing may be more difficult”. Because of this they “require greater salivary exchange… and for that reason prefer wetter, open mouth, tongue kisses”. What this all boils down to is that men need correspondingly more saliva to get their dose of bodily information from a kiss. Which could explain Johnny Depp’s tongue-wiggling antics in Cry Baby (1990).

A woman’s saliva has another important evolutionary function: “breath odor and saliva can provide cues to a woman’s fertility” (Hughes et al, 2007), meaning a woman’s spit can give a ‘reproductive-status-update’ to the person she’s kissing. When a woman is ovulating, she produces distinctive molecules in her saliva that act as green lights for reproduction. But at other times of the month it’s a different story. Just before a woman’s period starts, a rise in estrogen levels triggers the cells in her body to start shedding. It’s not just the cells in her uterus that begin to jump ship; this hormonal spike also sheds cells in her mouth, making it “ideal for bacterial growth” which can cause a woman to have “unpleasant breath near her menstrual period” (Hughes et al., 2007). So thanks body; back-pain, bloating, cravings and moodiness weren’t enough, you had to throw in bad breath to boot.

So can your first kiss with a person you thought was a spunk really be so bad that you’d never go there again? A test group was asked: “have you ever found yourself attracted to someone, only to discover after kissing then for the first time that you were no longer interested?”. 59% of males and 66% of females said yes, which suggests that those mechanisms our bodies have evolved to discourage sex between people who are genetically mis-matched could just be working. It definitely accounts for my first tongue kiss, which I wiped off using my sleeve like a windscreen wiper when I heard my mum come to pick me up, and have tried to wipe from my memory ever since.