Short Wave

Why do we kiss? It's an evolutionary conundrum

February 13, 2026

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  • Kissing is considered an evolutionary conundrum because it is inherently risky, involving the transfer of up to 80 million bacteria in a 10-second kiss and physical vulnerability due to teeth. 
  • Evolutionary biologist Matilda Brindle defines kissing broadly as a non-agonistic, intra-specific, oral contact with movement but no food transfer, a definition necessary to encompass behaviors across species like polar bears, ants, albatrosses, and prairie dogs. 
  • The leading hypotheses for the evolution of sexual kissing in primates are mate assessment (checking genetic compatibility via MHC) and pre-copulatory arousal to increase the chance of fertilization, while platonic kissing is linked to bonding (oxytocin release) and sharing beneficial microbiomes. 

Segments

Introduction to Kissing Conundrum
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(00:00:25)
  • Key Takeaway: Kissing is a widespread behavior across species, yet its evolutionary purpose remains elusive despite being unhygienic and physically risky.
  • Summary: The episode of Short Wave, “Why do we kiss? It’s an evolutionary conundrum,” opens by highlighting the paradox of kissing: it transfers an estimated 80 million bacteria in 10 seconds and involves physical risk, yet it is practiced by humans and animals like polar bears and prairie dogs. Evolutionary biologist Matilda Brindle is introduced as the expert who will detail the evolutionary drivers of this behavior. The segment also notes that not all human cultures engage in kissing.
Defining Animal Kissing
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(00:05:25)
  • Key Takeaway: Kissing is scientifically defined as a non-aggressive, same-species oral contact involving mouth movement but no food transfer, which accommodates diverse animal behaviors.
  • Summary: The scientific definition of kissing must be broad to apply across species, including those without lips. This definition requires the interaction to be non-aggressive and between members of the same species, involving mouth-to-mouth contact with movement, excluding food exchange. Examples of surprising kissing animals include ants, albatrosses, and prairie dogs, with polar bear kissing being noted for its unusual frothing and foaming.
Evolutionary History and Primate Kissing
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(00:07:08)
  • Key Takeaway: Kissing likely existed in the common ancestor of apes 21 million years ago, and in primates, it manifests as distinct platonic and sexual behaviors.
  • Summary: Phylogenetic research suggests kissing was present in the ancestor of all apes living 21 million years ago, possibly resembling the Miocene ape Pliobates Cataloniae. Primate kissing is categorized into platonic forms, like chimpanzees making up after arguments, and sexual forms, such as the sensual, tongue-heavy kissing observed in bonobos. Distinguishing between these types relies on observing the surrounding environmental context, such as whether the interaction occurs within a broader sexual sequence.
Hypotheses for Sexual Kissing
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(00:09:43)
  • Key Takeaway: Sexual kissing in primates is hypothesized to function either for mate selection by assessing genetic compatibility (MHC dissimilarities) or for increasing pre-copulatory arousal to favor fertilization.
  • Summary: One leading hypothesis suggests sexual kissing aids mate selection by allowing individuals to detect olfactory cues related to Major Histocompatibility Complex (MHC) compatibility, favoring mates with dissimilar genes to boost offspring immunity. The second hypothesis posits that kissing serves as pre-copulatory arousal, tipping the scales toward successful fertilization once mating is already intended. Platonic kissing, conversely, is linked to releasing oxytocin for bonding and sharing microbiomes, which is particularly beneficial for infants.
Culture and Learned Behavior
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(00:12:54)
  • Key Takeaway: While kissing has an evolutionary origin, its specific expression, especially in humans, is heavily influenced by learned behavior and cultural variation, demonstrating behavioral flexibility.
  • Summary: The prevalence of platonic versus sexual kissing varies significantly across primate species, such as chimpanzees favoring platonic and bonobos favoring sexual kissing. In humans, the practice is highly flexible, differing between individuals and societies, suggesting that kissing is a learned behavior taught through observation. The interplay between innate evolutionary roots and cultural learning dictates how and why people kiss.