Kissing, if interpreted as mouth-to-mouth contact between members of the same species with no food being transferred, is widespread in the Animal Kingdom, as well as in most (but not all) Human societies. Variants on this have been seen in Mammals, Birds, Fish and even Insects. If this process is narrowed to oral-oral contact with some movement of the lips, then it is still widespread in Old World Monkeys and Apes.
The advantages of kissing, viewed in a strictly evolutionary sense, are hard to define. Kissing does not appear to aid survival, or enhance reproductive success, and presents an opportunity for the transfer of infections. It has been suggested that kissing may enhance mate selection, enabling one kisser to evaluate the odour, health and/or social skills of the other, although there is no real way to prove this. Another suggestion is that kissing enhances arousal, thereby increasing the likelihood of mating success, though again, this is unprovable. Neither of these explanations can account for kissing between individuals who do not seem to be intent on mating, which is just as widespread as mating-related kissing. One theory which could account for this is that kissing is a social gesture which displays trust, coming intentionally close to the (potentially dangerous) mouth and teeth of a social peer or potential mate, while another is that the practice might facilitate the transfer of potentially beneficial microbes, though again, these hypotheses are impossible to prove one way or the other.
The meaning of kissing is subject to cultural interpretation in Human societies. Some societies do not kiss at all. In others kissing is an everyday, yet meaningful, event. It can also be a highly symbolic activity. Given this cultural variation in Humans, it is quite possible that kissing is also cultural in non-Human Primates, and may be present in some populations and not others.
While a number of researchers have come up with different possible explanations for the purpose of kissing, the origin of the practice does not appear to have been examined to date, despite the insight that understanding this could provide on the purpose of the practice. The presence of kissing in different, but closely related, species of Primates suggests that it has not appeared separately in each of these species, but that it first appeared in a common ancestral species, and was then adapted by each evolving lineage to meet their needs.
In a paper published in the journal Evolution and Human Behaviour on 19 November 2025, Matilda Brindle of the Department of Biology at the University of Oxford and the Department of Genetics, Evolution and Environment at University College London, Catherine Talbot of the School of Psychology at the Florida Institute of Technology, and Stuart West, also of the Department of Biology at the University of Oxford, present the results of a study in which they developed a comparative framework for the practice of kissing across different species of Primate, which was then used to examine the evolutionary history of the behaviour within the group.
A full systematic review of kissing in Primates in academic literature proved to be impractical. The lack of clear scientific terminology around kissing, combined with a general reticence to discuss such matters in scientific studies, and a very large body of work on kissing in fields such as literature, rendered specialist search engines such as Web of Science unable to produce useful results. Consequently, Brindle et al. settled on a non-systematic review of the literature, combined with searching platforms such as YouTube for evidence of kissing behaviour, with the intent of establishing evidence of kissing in a species, not any data about this. They note that absence of such data does not mean the behaviour does not exist.
In doing this, Brindle et al. set out to answer seven questions, namely: (i) Which primates have been observed kissing? (ii) Does kissing show a phylogenetic signal? (iii) When did kissing first evolve in this group? (iv) How many times did kissing evolve? (v) Has kissing been lost across the course of evolution in any lineages? (vi) Are Neanderthals likely to have kissed? And (vii) how well do different life history variables correlate with the occurrence of kissing?
For the purpose of their study, Brindle et al. defined kissing as 'non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer'. This definition still included behaviours seen in Animals such as Ants, Birds, and Polar Bears, but was much more common and widespread in Primates. Brindle et al. further restricted their study to Old World Monkeys and Apes, groups where there was sufficient data for useful comparisons to be made.
Brindle et al. make it clear that they were looking for the ultimate cause of kissing rather than the proximal one, which is to say, the reason why Primates first started kissing rather than the reasons for this occurring in any group of Primates who already had the behaviour.
They further note that kissing is likely to have begun as a modification of another behaviour. They note that a recent suggestion has been made that kissing began as a part of a oral grooming behaviour, but doubt the veracity of this, noting that this is not usually how Primates groom, and that no evidence was offered to support the hypothesis. They suggest instead that kissing might be a modification of oral-to-oral food transfer, a common phenomenon in Primates.
Because kissing has been speculatively linked to mating success, Brindle et al. compared the presence of kissing in a Primate species to the mating system used by that species. They also compared the presence of kissing to the diet, food sharing and premastication (chewing food before giving it to another individual, typically an infant) habits of each species, since these behaviours might give a species a preadaptation towards kissing. They note that species with frugivorous or omnivorous diets are more likely to share foods with infants, due to the patchy distribution of foods within the environment. Fruits and meats are also the foodstuffs most commonly premasticated before giving them to infants. Reliable data on premastication was only available for the Great Apes. Data on diets from Neanderthals and Modern Humans was included in the dataset.
Brindle et al. used a phylogenetic tree constructed using the 10kTrees Project V3.0 resource, which was cut to include only Old World Monkeys and Apes, including Humans and Neanderthals. Neanderthals were included not just because they nested within the available data, but because it has been suggested that there is evidence from the oral microbiome that Modern Humans and Neanderthals may have exchanged microbial species via kissing some time after the two species split.
The phylogenetic tree was constructed using a Bayesian Markov chain Monte Carlo framework with the practice of kissing identified as either present or unknown (since it is impossible to assume that a species does not kiss simply because we have not observed it). Neanderthals were also classified as unknown.
Kissing was found in all species of Great Ape except the Eastern Gorilla, Gorilla beringei. It was also observed in a variety of Old World Monkeys, although chiefly within the Papionini (Macaques and Baboons). Brindle et al. note that much of the data on the Papionini relates to same-sex kissing, at least in part because that was the subject of the studies which had looked at the behaviour in this group.
Brindle et al. conclude that kissing appeared in the common ancestor of all extant Great Apes some time after the split with the common ancestor of all extant Lesser Apes (Gibbons), i.e. between 21.5 and 16.9 million years ago. The trait has subsequently been retained, with the one possible loss in Eastern Gorillas. Kissing was found in eight species of Papionini, but it was impossible to determine that the trait was present in the common ancestor of the group; it has either evolved or been lost multiple times (possibly both). Brindle et al. also conclude that kissing was almost certainly present in Neanderthals.
Brindle et al. also note that there seems to be a strong association between kissing and multi-male mating systems (i.e., systems in which multiple males may compete for the right to mate with a female), with the kissing existing alongside long bonding of females to a single male only in Western Gorillas and some Human societies. Premastication was found in every species in which kissing has been observed, but there was generally an absence of data for species where kissing had not been observed, leading Brindle et al. to refrain from making a judgement on the relevance of this trait.
Brindle et al. conclude that kissing is present in all Great Apes except Eastern Gorillas, and that there is strong evidence that the trait appeared once in the group, between 21.5 and 16.9 million years ago. The data is less clear for Old World Monkeys, where the trait may have appeared once and subsequently been lost in many lineages, or appeared several times in different lineages.
The retention of kissing as a behaviour in Great Apes strongly suggests that it provides an evolutionary advantage, which Brindle strongly suspect is associated with sexual selection, although they do not believe this is sufficient to assert that this was the reason it originally evolved. They note that the limitations of the data they used leave plenty of opportunity for further research on the subject.
See also...
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