Kissing Across Species: An Evolutionary Look at Why We Lock Lips

In this NPR Shortwave episode, host Emily Kwong speaks with Matilda Brindle, an evolutionary biologist at the University of Oxford, about why kissing appears across many species. They discuss how kissing can be hygienically risky yet socially useful, and they define kissing in broad biological terms that cover humans, polar bears, ants, and birds. The conversation traces possible evolutionary routes from early ape ancestors to humans, distinguishing platonic from sexual kissing, and explores the roles of mate assessment, pre-copulatory arousal, oxytocin, and microbiome sharing. The episode also considers how culture and learning shape kissing and what future research might reveal about this universal behavior.

Overview and framing

Estimated transcript length: approximately 2,300 words. The episode frames kissing as a behavior that, despite seeming trivial or awkward, has deep evolutionary and social significance. Host Emily Kwong interviews Matilda Brindle, an evolutionary biologist at the University of Oxford, about why kissing persists across diverse species and what it might reveal about biology, culture, and communication. The conversation foregrounds a paradox: kissing transfers bacteria and poses hygienic risks, yet it also promotes bonding and social cohesion. Brindle emphasizes a broad operational definition of kissing to encompass a range of non-aggressive oral contacts across species, setting the stage to investigate its origins and functions beyond human romance.

"Kissing seems like a very unhygienic and vulnerable thing to do" - Matilda Brindle, Evolutionary Biologist, University of Oxford

Kissing across species and what counts as kissing

The discussion broadens to species beyond humans, highlighting examples such as ants, albatrosses, prairie dogs, and even polar bears. Brindle explains that the definition must accommodate organisms without lips or mouths, hence a wide umbrella that still fits an evolutionary narrative. The program also notes the microbiome transfer that accompanies kissing, citing that tens of millions of bacteria can be exchanged in a single encounter, illustrating the ecological dimensions of this behavior alongside its social implications.

Evolutionary hypotheses for kissing in primates

The conversation outlines two leading hypotheses for how sexual kissing evolved in primates: mate assessment, where kissing could reveal genetic compatibility or social compatibility through cues such as olfaction, and pre-copulatory arousal, where kissing facilitates copulation and fertilization. Brindle discusses how olfactory cues, particularly differences in major histocompatibility complex (MHC) similarity, might influence mate choice, potentially guiding immune-system diversity in offspring. The dialogue clarifies that both mechanisms could operate at different times or in different species, contributing to a complex evolutionary landscape for kissing.

"There are two leading hypotheses" - Matilda Brindle, Evolutionary Biologist, University of Oxford

Platonic versus sexual kissing and social function

The interview emphasizes a distinction between platonic kissing, often serving bonding and conflict resolution, and sexual kissing, which appears in sexual or romantic contexts. Brindle notes oxytocin release during affiliative interactions, which underpins bonding and social cohesion, and discusses the potential role of microbiome sharing in early life and maternal interactions. The conversation also acknowledges that platonic kissing varies across species and cultures, implying a mosaic of functions including tension reduction, parental and offspring bonding, and social integration in groups.

"There is a release of oxytocin associated with this, which helps us to bond" - Matilda Brindle, Evolutionary Biologist, University of Oxford

Cultural transmission and learning

A key point in the discussion is that kissing is not purely innate or fixed; humans learn how to kiss by observing others and through social practice. Brindle highlights that practices vary across cultures, such as different cheek-kissing patterns in Europe, illustrating the adaptability of this behavior as a social tool. The host and guest reflect on how sexuality and kissing are fluid across primates, arguing that culture interplays with biology to shape this behavior, rather than it being a static trait rooted solely in either nature or nurture.

"We do teach each other to kiss. We kind of see other people do it" - Matilda Brindle, Evolutionary Biologist, University of Oxford

What comes next for kissing research

Brindle closes with a call for systematic data collection on non-reproductive sexual behaviors and broader affiliative interactions across species. She argues for brave, open research into behaviors that may be uncomfortable to discuss but hold important insights for understanding social evolution, animal behavior, and human culture. The episode positions kissing as a fruitful domain for integrating observational data, phylogenetic analyses, and cross-species comparisons to reveal general principles about social behavior and evolution.

Overall, the show frames kissing as a multifaceted behavior with biological, ecological, and cultural dimensions, inviting readers to consider how universal actions emerge from a blend of evolutionary constraints and social learning.

"There is so much to be discovered... perhaps this is societally a little bit uncomfortable to look at, but actually it doesn't matter" - Matilda Brindle, Evolutionary Biologist, University of Oxford