Summary
Overview
Dr. Chris and Dr. Zand explore the science of kissing with evolutionary biologist Dr. Matilda Brindle from Oxford University. They investigate why humans kiss, how it evolved, what happens in our bodies during a kiss, and whether kissing is necessary for health and wellbeing. The conversation covers everything from mate assessment and immune system benefits to the exchange of 80 million bacteria during intimate kisses, while emphasizing that kissing, like other sexual behaviors, exists on a flexible spectrum.
Defining the Kiss: What Counts as Kissing?
Dr. Matilda Brindle explains her team's research definition of kissing as 'directed, intra-specific, mouth-to-mouth contact that couldn't be agonistic and didn't involve food transfer.' This deliberately broad definition was necessary to study kissing across multiple animal species, from humans to polar bears to ants. The definition excludes aggressive 'kissing' in French grunt fish and food transfer between mothers and offspring, focusing instead on affiliative contact.
- Scientific definition: directed, intra-specific, mouth-to-mouth contact that's non-aggressive and doesn't involve food transfer
- French grunt fish engage in aggressive mouth-locking to assert dominance - this doesn't count as kissing
- Pre-mastication (mothers pre-chewing food) looks like kissing but is excluded from the definition
- The broad definition needed to encompass different anatomies - flat faces, muzzles, beaks, and mandibles
" We needed this really broad definition that encompassed lots of different animals. So for example humans we have quite flat faces, we have movable lips, that's quite important in how humans kiss. But polar bears have muzzles, They've got really big teeth. They have extremely long tongues. "
Why Do Humans Kiss? Mate Assessment and Arousal
Kissing serves two primary evolutionary functions in romantic contexts. First, it acts as mate assessment - a 'try before you buy' health check where you can detect illness through breath and assess immune system compatibility through the major histocompatibility complex. Second, kissing functions as pre-copulatory arousal or foreplay, with female arousal potentially increasing fertilization chances during sex.
- Two leading hypotheses: mate assessment and pre-copulatory arousal (foreplay)
- Kissing allows health checks - you can smell illness on someone's breath
- Testing for immune system compatibility through major histocompatibility complex - you want slightly different immune systems for healthier offspring
- Female arousal during foreplay may increase chances of fertilization
" That could be basically a way of checking the quality of your mate. So try before you buy. And that could be things like a health check. So if someone is a bit unwell, you can often actually smell that on their breath. "
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