While kissing might feel like one of the most natural things in the world, this familiar behavior is quite mysterious—various animals also kiss, despite a lack of practical benefits and a real risk of disease transmission.
To shed light on the smooching enigma, researchers have attempted to reconstruct the evolutionary history of kissing in the primate family tree, which includes mammals such as monkeys, apes, and humans. The team’s findings suggest that kissing is an ancient trait, evolving in the ancestors of great apes (such as humans) 21.5 to 16.9 million years ago and sticking around to this day in most surviving great ape species.
“This is the first time anyone has taken a broad evolutionary lens to examine kissing,” Matilda Brindle, an evolutionary biologist at the University of Oxford, said in a university statement. “Our findings add to a growing body of work highlighting the remarkable diversity of sexual behaviours exhibited by our primate cousins.” Brindle is the lead author of a study published today in the journal Evolution and Human Behavior.
What’s a kiss?
First, the team had to scientifically define what kissing is. That’s harder than it sounds, given that many mouth-to-mouth behaviors might seem like kissing and the definition had to be consistent across different species. They ultimately decided on an incredibly romantic description: non-aggressive mouth-to-mouth contact without the transfer of food. Pucker up.
Brindle and her colleagues then gathered previously documented information on modern primate species kissing, focusing on the monkeys and apes that evolved in Europe, Africa, and Asia, including chimpanzees, bonobos, and orangutans (all three of which have been recorded kissing). Treating kissing as an evolutionary “trait,” the team ran a computer model 10 million times to simulate different primate evolution scenarios and estimate the chances of different ancestors kissing.
“By integrating evolutionary biology with behavioural data, we’re able to make informed inferences about traits that don’t fossilise – like kissing. This lets us study social behaviour in both modern and extinct species,” said Stuart West, co-author of the study and an evolutionary biologist at the University of Oxford.
Kissing partners
This method revealed that Neanderthals likely kissed, too. In addition to previous evidence demonstrating that humans and our now-extinct cousins transferred saliva and interbred with each other, the results strongly indicate that Homo sapiens and Neanderthals also smooched each other.
“While kissing may seem like an ordinary or universal behaviour, it is only documented in 46% of human cultures,” explained Catherine Talbot, co-author of the study and an assistant professor at Florida Institute of Technology’s school of psychology. “The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behaviour or cultural invention. This is the first step in addressing that question.”
Naturally, there are some important limitations to point out, given the methodology used. The paper is based on previously recorded behaviors and computer simulations, and not direct observations. This is particularly precarious when it comes to extinct species, including Neanderthals. What’s more, data beyond great apes are sparse, limiting how far the findings can be stretched. The results also depend on the assumptions built into the models, which means the outcomes could vary with different parameters.
At the very least, and as noted in the press release, the study offers a framework for future work and provides a way for primatologists to record kissing behaviors in nonhuman animals using a consistent—if not a complete buzzkill—definition.
