A comparative approach to the evolution of kissing

Abstract

Kissing can be observed across the animal kingdom. This presents an evolutionary puzzle, since the fitness benefits of kissing are unclear. We use a non-anthropocentric approach to define kissing as a non-agonistic interaction involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer. Using this definition we collate basic observational data across the Afro-Eurasian primates and employ Bayesian phylogenetic methods to reconstruct the evolutionary history of kissing. We find that kissing occurs in most extant large apes, and likely also occurred in Neanderthals (Homo neanderthalensis), first evolving in the ancestor to this group ~21.5–16.9 mya. Additionally, we highlight various life history variables that correlate reasonably, but not perfectly, with kissing across the apes (multi-male mating systems, non-folivorous diets, and premastication). With a major caveat about the quantity of available data at present, we hope that our results provide a useful starting point for further research into the adaptive function of kissing that highlights hypothesis generation and testing within a phylogenetic framework.

1. Introduction

Mouth-to-mouth contact occurs across the animal kingdom, including in many, but not all, human societies (Arbøll & Rasmussen, 2023; Jankowiak, Volsche, & Garcia, 2015; Watkins et al., 2019). Such contact is commonly referred to as “kissing”, which we more formally define as a non-agonistic interaction involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer. In nonhuman primates, kissing has been observed in most large apes (Table 1) and some Afro-Eurasian monkeys. Similar behaviours that are sometimes termed kissing, but which do not meet all the criteria in our definition, have been observed in a diversity of other organisms including mammals, birds, fish and even insects (e.g. (Cannon, 2024; Heinze, 2004; Hurd, 1997; Mercier et al., 2007; SloggerVlogger, 2020; Fig. 1; Section 2).

Table 1. Examples of kissing across the large apes (Hominidae).

Genus	Species	Kissing type	Details	Vignettes	Reference
Gorilla
	Gorilla beringei	No data
	Gorilla gorilla gorilla	Platonic	Appears to be rare	Affiliative behaviour in which “one animal puts lips on head or face of the other”.	Margulis, Atsalis, Bellem, & Wielebnowski, 2007, p.125
		Sexual	Appears to be rare	During a pause in male-male intromission “Jabir sits up and is face-to-face with Sampson followed by a few seconds of open-mouth play (similar to kissing)”.	Mead-Moehring & Moore-Jansen, 2006, p. 79
Homo
	Homo sapiens	Platonic	Common in some societies, absent in others	Platonic kisses take many different forms across human cultures, and may include other body parts such as the cheek, head, hands, or feet. “Cheek kisses in greeting are customary between genders in … many parts of the world to express warmth and respect”.	Kirshenbaum, 2011, p. 63
		Sexual	Common in some societies, absent in others	“Many couples supplement simple lip contact with the ‘deep-kiss’ which involves thrusting the tongue of one partner into the mouth of the other”.	Ford & Beach, 1951, p. 49
	Homo neanderthalensis	Unknown	Unknown	Shared oral microbes leaves open the possibility that modern humans and Neanderthals kissed.	Weyrich et al., 2017
Pan
	Pan paniscus	Sexual	Appears to be relatively common	“The mouth-to-mouth kiss. has a strikingly sensual character because of prolonged tongue-tongue interaction”.	de Waal, 1990, p. 383
	Pan troglodytes	Platonic	Appears to be relatively common	“Mouth-to-mouth kissing also occurs in chimpanzees, but because it is brief and tense it lacks sensuality”.	Wrangham, 1993, p. 65
				Predominantly a reconciliatory behaviour. For example “Hennie. approaches Nikki after he has slapped her. Hennie first offers her hand to the aggressor for a hand kiss, after which the two engage in a mouth-to-mouth kiss”.	de Waal, 2009, p. 40
		Sexual	Appears to be rare	“They put their lips together (kissing?). He then examined visually and with his fingers her external genitalia (2 min)”.	Bingham, 1928, p. 67
				“Wendy’s genitalia revealed exceptional openness and wetness. Billy took Wendy’s lower lip, while he lay on her, in his own lips and sucked, actually smacked, during the activities. When they got up, Billy had a complete erection”.	Bingham, 1928, p. 102
Pongo
	Pongo abelii	Platonic	Appears to be rare	Can be an affectionate gesture, often between family members.	Ape Tracker (Director) (2024)
	Pongo pygmaeus	Platonic	Appears to be rare	“Males may ‘kiss’ each other. While in some cases this mouth-to-mouth contact may be for the exchange of food or drink, in other cases it appears to be more of an affectionate gesture”.	Bagemihl, 1999, p285-286
		Sexual	Appears to be rare	“8-year-old Darril and 4-year old Syahrini appear to have hit it off, and begin to engage in what looks like kissing. Darril has an erection.”	Smithsonian Channel, 2020

Fig. 1.

Top panel: kissing across the animal kingdom (clockwise): Rhesus macaques (Macaca mulatta; credit: Paula Bronstein, Getty Images); Galapagos albatross (Phoebastria irrorata; credit: Vladimir Jurek, Shutterstock); Polar bears (Ursus maritimus; credit: Smiler99, Shutterstock); Wolves (Canis lupus, credit: Soren Wolf, Flickr); Prairie dogs (Cynomys ludovicianus, credit: Brocken Inaglory, Wikimedia Commons). Bottom panel: non-kissing mouth-to-mouth behaviours (left to right): premastication in orangutans (Pongo sp., credit: Sunsetman, Shutterstock); trophallaxis in ants (Camponotus compressus, credit: Rakeshkdogra, Wikimedia Commons); and kiss-fighting in French grunts (Haemulon flavolineatum, credit: Luiz A. Rocha, Shutterstock).

Kissing poses an evolutionary problem, since it does not appear to aid survival or reproduction in an obvious way, while the potential costs of disease transfer are high. What is its benefit or adaptive function? Several hypotheses have been developed that can vary according to kiss type (sexual versus platonic). Sexual kissing is hypothesised to affect reproductive success via pre- and postcopulatory sexual selection. For instance, kissing a potential reproductive partner may allow evaluation of mate quality and suitability via olfactory signals, for example by assessing their general health, genetic fitness, major histocompatibility complex (MHC), oral microbiome, or menstrual cycle phase (Durham, Malloy, & Hodges, 1993; Hendrie & Brewer, 2010; Kort et al., 2014; Poniewierka, Pleskacz, Łuc-Pleskacz, & Kłaniecka-Broniek, 2022; Wlodarski & Dunbar, 2013). Kissing may also serve to increase precopulatory arousal, increasing the chance of fertilisation (Brindle, Ferguson-Gow, Williamson, Thomsen, & Sommer, 2023). Close physical proximity to another individual’s mouth makes kissing a vulnerable act requiring some degree of trust between actors. Both sexual and platonic kissing have therefore been argued to serve an affiliative purpose in social species, that could help navigate relationships, mitigate tension, or increase bonding (Brooker et al., 2025; de Waal, 2009; de Waal & van Roosmalen, 1979; Wrangham, 1993). Finally, both platonic and sexual kissing may allow the transmission of microbes that benefit immunity (Kort et al., 2014; Lombardo, 2008; Raulo et al., 2018; Troyer, 1984).

Kissing can be both a meaningful everyday act and an important and symbolic behaviour in many human societies. Given that kissing is not universal across humans and is likely to be strongly culturally influenced (Jankowiak et al., 2015), it is also possible that it is a culturally learned behaviour in nonhuman primates. If this were the case, kissing might occur in some wild populations, but not others, equivalent to behaviours such as ‘eye-poking’ in capuchin monkeys, or ‘grass-in-ear’ behaviour in chimpanzees (Perry, 2011; Van Leeuwen, Cronin, & Haun, 2014; Whiten et al., 1999).

While these theories might explain the adaptive function of kissing, to date, no study has yet explored its evolutionary history or potential correlates. Across-species comparative studies provide a method to examine the possible function of behaviours (Cornwallis & Griffin, 2024). We can think of different species as the outcome of different experiments by natural selection and ask what conditions have been associated with the evolution and maintenance of a behaviour. Closely related species tend to be similar because they share traits by common descent rather than through independent evolution. Species therefore do not represent independent data points and it is necessary to examine how behaviours, or other traits, have evolved from a phylogenetic perspective (Felsenstein, 1985; Harvey & Pagel, 1991).

We used a comparative approach to examine kissing across primates and develop a framework for future research. Our first aim was to provide a definition of kissing that avoids anthropomorphism and works across taxa (Section 2). We then collated basic data on kissing across the primates – a clade in which this behaviour is more well-documented than others – as a starting point for analyses examining its evolutionary history within this group. In addition, we collected data on other factors hypothesised to either have played a role in favouring kissing, or predisposed a species to kiss: mating system, diet, food sharing (unresisted transfer of food from one food motivated individual to another; Jaeggi & Gurven, 2013) and premastication (pre-chewing food before sharing it with another) (Sections 3 & 4). Since records were limited, we collated broad, species-level data, without exploring intraspecific variation.

We conducted a targeted scoping review of kissing across the primates, and not a full systematic review, as a starting point to help establish a conceptual framework. While we generally advocate for a full systematic review when collating large comparative datasets on wellestablished areas of research (O’Dea et al., 2021), this method can be less useful when researching lesser-discussed or ‘taboo’ topics. These are often described euphemistically within the literature, as is the case for observations of masturbation in nonhuman animals, which renders even the most carefully constructed search strings ineffective (Brindle, 2022, or Brindle et al., 2023). Additionally, gathering data on subjects that are poorly explored in one discipline, but well-discussed in others, can be like finding a needle in a haystack. For example, kissing is a socio-cultural phenomenon extensively discussed in academic disciplines ranging from literature to history, the arts and psychology, but it is little addressed from the perspective of evolutionary biology. In instances like these, a targeted approach can be vastly more effective, both in terms of flexibility and efficiency. This is not to say that systematic review is never appropriate for exploring and understanding topics such as kissing, rather that there is a threshold (reached when a topic becomes more established) at which systematic review becomes a helpful and incisive tool, but before which a scoping review is likely to be more constructive. For example, the search string “kiss” OR “kissing” AND “primate*” OR “monkey*” OR “ape*” AND “reassur*” OR “affiliative touch” OR “social behaviour ethogram” returns 181,257 results from Web of Science, but of the first 200 papers (scanned by title and abstract), none was relevant to the present research. Crucially, therefore, it is important to view our analyses as a ‘proof of concept’ for the future work that could be done with greater data availability.

Nevertheless, the available data allow us to begin to explore a number of questions:

• (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?
• (vii)How well do different life history variables correlate with the occurrence of kissing?

2. What is kissing?

What constitutes a kiss? It has been argued that the distinctive features of kissing are “protruded lips and suction movement” (Lameira, 2024, p2). This is not a useful definition from an evolutionary perspective, since it is constructed to describe the form of a kiss (i.e. the physical movements that constitute this type of kissing) that occurs exclusively in certain populations of humans (Homo sapiens) and may be highly derived. A less anthropocentric approach might be to broadly define kissing as ‘oral-oral contact with some movement of the lips/mouthparts’. However, this would still encompass behaviours such as premastication, trophallaxis (oral-oral or anal-oral food and fluid transfer) and ‘kiss-fighting’ (agonistic oral fighting behaviour; Fig. 1), as well as non-targeted, or inter-species behaviours, such as dogs (Canis familiaris) licking the mouths of their owners. We exclude these behaviours from our definition as they have likely been favoured by evolution for different reasons.

We therefore define kissing as non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer. According to this definition, kissing occurs in taxa as diverse as ants, birds, and polar bears, but the majority of reports seem to be in the primates (Cannon, 2024; Heinze, 2004; Mercier et al., 2007; SloggerVlogger, 2020) (Table 1; Table 2; Supplementary Data). From here onwards, we focus on Afro-Eurasian monkeys and apes (Catarrhini), because there are sufficient comparable data to make the first steps in a comparative analysis. Any conclusions will of course depend upon the definition used, and a key future step will be to examine the data from different perspectives, such as different types of kissing (sexual versus platonic), as well as taking into account intraspecific variation.

Table 2. Primate taxa in which kissing has been observed, and sources.

Family	Species	Common name	Source
Hominidae
	Gorilla gorilla gorilla	Western lowland gorilla	Margulis et al., 2007; Mead-Moehring & Moore-Jansen, 2006
	Homo sapiens	Human	Jankowiak et al., 2015
	Homo	Neanderthal	Interpolated (see section 4.5)
	neanderthalensis
	Pan paniscus	Bonobo	de Waal, 1989; de Waal, 1990; Manson, Perry, & Parish, 1997; Wrangham, 1993
	Pan troglodytes	Chimpanzee	de Waal & van Roosmalen, 1979; de Waal, 1989; Goodall, 1968; Nishida, 1970; Shimada, 2014; Wrangham, 1993
	Pongo abelii	Sumatran orangutan	Ape Tracker, 2024; Pulfer-Focht, 2018
	Pongo pygmaeus	Bornean orangutan	Bagemihl, 1999; Edwards & Snowdon, 1980; Prudente, 2017; Smithsonian Channel, 2020
Cercopithecidae
	Macaca arctoides	Stump-tailed macaque	Bagemihl, 1999; de Waal, 1989
	Macaca fascicularis	Long-tailed macaque	Bagemihl, 1999
	Macaca mulatta	Rhesus macaque	Bagemihl, 1999
	Macaca nemestrina	Pig-tailed macaque	Bagemihl, 1999
	Macaca nigra	Crested macaque	Bagemihl, 1999
	Papio anubis	Olive baboon	Bolwig, 1963
	Papio cynocephalus	Yellow baboon	Bagemihl, 1999
	Semnopithecus entellus	Hanuman langur	Bagemihl, 1999

Note: all data gathered from Bagemihl (1999) are observations of same-sex kissing.

3. Different kinds of questions and explanations

To comprehensively understand a behaviour, it is important to distinguish between different questions and explanations (Davies, Krebs, & West, 2012; Scott-Phillips, Dickins, & West, 2011; Tinbergen, 1963). First, we can divide between mechanistic (proximate) and functional (ultimate) questions (Tinbergen, 1963). There are various reasons why an individual would be motivated to kiss another in the moment (i.e. its proximate mechanism), such as pleasure, excitement, comfort, or a desire for closeness. None of these, however, explain why kissing initially evolved and was retained in certain lineages (i.e. its ultimate, adaptive function). Proximate and ultimate questions are complementary and not competing, in that a full understanding requires answers to both, and a mechanistic answer cannot be given to an evolutionary question, or vice versa (Brindle et al., 2023; Scott-Phillips et al., 2011; West, El Mouden, & Gardner, 2011). Here, we focus on adaptive questions, exploring the function of kissing.

Second, we can ask whether kissing has been exapted from another behaviour. It has recently been argued that the physical act of kissing (protruded lips and suction movement) arose from oral allogrooming in primates, which humans subsequently evolved to conclude with a kiss (Lameira, 2024). However, this theory is not supported by the behaviour of extant nonhuman apes and no references or data are provided. More promising candidate behaviours are oral food transfer (such as premastication) or nursing in mammals, which may have been exapted into mother-offspring kissing in primates, since parental care is largely female-only in this group (Eibl-Eibesfeldt, 1971). Importantly, however, while exaptation may help explain how kissing evolved, this should not be conflated with why animals kiss. In other words, this tells us nothing about the mechanism or function of kissing. Again, the distinction here is between different types of question.

4. Methods

4.1. Kissing data

We compiled data on kissing across the Afro-Eurasian monkeys and apes via a non-systematic, targeted scoping review of the literature, with additional data from the video-sharing platform YouTube (Table 2, Supplementary Data). These data are likely not exhaustive, given that kissing appears to be rare and has not previously been formally defined - factors that often lead to a paucity of published reports and can make systematic review particularly challenging (Brindle et al., 2023). Instead, the data presented here are designed to be a starting point for future analyses.

We recorded kissing as ‘present’ if a species has a single observation, and ‘not observed’ if we did not encounter any reports. Importantly, a lack of observations does not indicate that kissing does not occur in a given species. Similarly, if kissing is recorded as ‘present’, it is not necessarily a common or universal behaviour within that species. We excluded observations of between-species kissing, such as when nonhuman animals have tried to kiss humans, as these may be nonrepresentative (I. Behncke and M. Brindle, pers. comm.). There are not sufficient data to begin to look at variation within a species, except in humans (Arbøll & Rasmussen, 2023; Jankowiak et al., 2015; Watkins et al., 2019).

4.2. Candidate life history variables

The comparative method tests adaptive hypotheses by looking for correlations with environmental or life history variables that reflect different functions (Cornwallis & Griffin). For example, the role of genetic relatedness (kin selection) in favouring cooperation has been tested by examining whether cooperation is more likely to occur, and occur at higher levels, in species where relatedness is higher within social groups (Cornwallis et al., 2017; Cornwallis, West, Davis, & Griffin, 2010; Downing, Griffin, & Cornwallis, 2020; Duffy & Macdonald, 2009; Fisher, Cornwallis, & West, 2013; Green & Hatchwell, 2018; Hughes, Oldroyd, Beekman, & Ratnieks, 2008; West, Cooper, Ghoul, & Griffin, 2021).

We present data on mating system as a proxy for the degree of postcopulatory selection pressure, since sexual kissing has been hypothesised to affect reproductive success (data from Brindle et al., 2023). We also collated data on diet, food sharing and premastication, as these traits may preadapt a species towards kissing (data from DeCasien, Williams, & Higham, 2017; Jaeggi & Van Schaik, 2011). For example, species with frugivorous or omnivorous diets may be more likely to share food with offspring when they find it, since these foods are spatially and temporally ‘lumped’, while leaves are more abundant and evenly distributed. Fruits and meat may also be processed for offspring via premastication, a behaviour requiring mouth-to-mouth contact and protrusion of the lips. These data were supplemented with additional literature for humans and Neanderthals (Homo neanderthalensis), which were not included in the original comparative datasets (see Supplementary Data for references). We collated data on premastication on a species-by-species basis where available for the large apes (Hominidae) only, since both kissing and premastication are poorly documented outside of this clade.

4.3. Phylogeny

We ran our analyses over a posterior distribution of 10,000 molecular phylogenies from the 10kTrees Project V3.0, a classic resource for conducting phylogenetic comparative analyses in primates (Arnold, Matthews, & Nunn, 2010). The benefit of using a block of 10,000 phylogenetic trees is that analyses are sampled from across this distribution, rather than a single tree, and therefore account for phylogenetic uncertainty. We cut the trees to only include Afro-Eurasian monkeys and apes, including Neanderthals.

We included Neanderthals in our analyses for two reasons. First, they are already represented within the phylogenies we employ. Second, microbiome data has suggested the possibility for kissing between modern humans and Neanderthals. While modern humans and Neanderthals diverged sometime between 450 and 750 ka, the oral microbe we share (Methanobrevibacter oralis) only separated into distinct strains between 112 and 143 ka (Weyrich et al., 2017). This suggests that commensal microbial species were transferred between modern humans and Neanderthals for some time after the two species split. Possible routes include food sharing, parental care (such as premastication), or kissing. Our data allow us to test whether kissing occurred in Neanderthals via a different analytical framework.

4.4. Ancestral state reconstructions

We used a Bayesian Markov chain Monte Carlo (MCMC) framework to reconstruct the evolution of kissing within the Afro-Eurasian monkeys and apes using the ‘multistate’ function within the programme Bayes-Traits V4 (Meade & Pagel, 2024). Analyses were carried out at the species level, with traits of subspecies marked as ‘missing’. We also classed Neanderthals as ‘missing data’ when running our ancestral state reconstructions. A reversible-jump (rj) hyperprior approach was taken, seeding from an exponential distribution with a range of 0–1. Each model was run for 10,000,000 iterations, with a burn-in of 1,000,000 iterations. Each model was run three times to check for equivalence, and the one with the median log marginal likelihood was chosen. Ancestral states of interest were reconstructed with the AddMRCA function, which generates the probability of a given state at a particular node. We interpret probabilities of >0.75 as providing high confidence of a given state, >0.65 as low confidence, and < 0.65 as equivocal.

4.5. Tip inference

We employed the same analytical framework to estimate whether kissing occurred in Neanderthals. We interpolated this using the ‘multistate’ function in BayesTraits. We estimated a distribution of models from our available data (with Neanderthals classified as ‘missing’) then used these models to estimate the unknown tip value.

5. Results

5.1. Phylogenetic distribution

Within the primates, kissing has been observed across the Afro-Eurasian monkeys and apes (Table 2; Supplementary Data). The majority of reports come from the large apes (Hominidae), with the exception of Eastern gorillas (Gorilla beringei; Fig. 2). Kissing also seems to occur outside of the apes, with reports across the Afro-Eurasian monkeys (Cercopithecoidea), chiefly within the Papionini, and predominantly in the context of same-sex sexual behaviour (although it is important to note that the source for these observations was focused on same-sex sexual behaviours and is unlikely to be representative of the full spectrum of kissing occurrences). Kissing showed a strong phylogenetic signal within the Afro-Eurasian monkeys and apes (D = 0.1; P (D < 1) < 0.01).

Fig. 2. Phylogeny illustrating the reconstructed evolutionary history of kissing within the apes (Hominoidea).

The occurrence of kissing and other life history variables is displayed at the tips of the tree (Supplementary Data). Left to right: kissing (observed/not observed); mating system (single/multi-male); diet (folivorous, frugivorous, omnivorous); food sharing (present/absent); and premastication (present/absent). At the tips and nodes of the tree, black circles indicate a trait has been reported or reconstructed as ‘present’; white circles indicate kissing has not been reported or was reconstructed as ‘absent’; grey circles indicate equivocal reconstructions (mean probability <0.65); circles are not present where data are missing. The Neanderthal tip represents the mean probability that kissing was ‘present’, based on Bayesian estimation. Maximum Clade Credibility tree created from a sample of 10,000 molecular phylogenies from the 10kTrees project (Arnold et al., 2010).

5.2. Ancestral state reconstructions

Ancestral state reconstructions (Bayesian rjMCMC ancestral state reconstructions; Fig. 2; Supplementary Information Table S1) across the Afro-Eurasian monkeys and apes suggest that kissing first evolved in the ancestor to the large apes, after the split with the small apes (Hylobatidae), somewhere between 21.5 and 16.9 mya (Chatterjee, Ho, Barnes, & Groves, 2009). Kissing appears to have been retained in subsequent hominid nodes, with the exception of ancestral gorillas, in which the reconstruction was equivocal. Within the large apes, therefore, kissing appears to have evolved once and has potentially been lost once. Ancestral state reconstructions suggest that kissing was absent in ancestral Macacina and Papionina, despite this behaviour being present in eight species within these tribes. It is possible, therefore, that kissing has evolved up to seven times independently in this group. Importantly, however, given the lack of data outside of the large apes our power to make inferences in this group is extremely limited, and this is therefore likely to be an overestimation. Consequently, we focus our discussion on kissing within the apes.

5.3. Neanderthals

Our analyses suggest that kissing was present in Neanderthals (Bayesian rjMCMC interpolation; P_present = 0.843, SE = 0.002; Fig. 2). This result, together with previous research indicating that humans and Neanderthals transferred commensal oral microbes, provides further evidence in support of the hypothesis that humans and Neanderthals kissed one another (Weyrich et al., 2017).

5.4. Candidate life history variables

Considering the hypothesis that sexual kissing may increase reproductive success, the presence of kissing seems to be associated with multi-male mating systems in the apes, except for Western gorillas (Gorilla gorilla) and (most) human societies. Diet shows a less-clear pattern, although Western gorillas are the only folivorous species in which kissing has been observed. Food sharing does not seem to show a relationship with the presence or absence of kissing. Finally, premastication occurs in every species in which kissing is present, but no data were available for species in which kissing has not been observed, so we cannot appropriately assess whether the two traits have a relationship. We stress though, that any discussion of this data is speculative, because we lack phylogenetic power to carry out formal analyses (Section 6).

6. Discussion

Our preliminary analyses suggest that: (i) kissing occurs in most large apes, excluding Eastern gorillas, and at least eight Afro-Eurasian monkeys. Within this group: (ii) kissing shows a strong phylogenetic signal; (iii) kissing evolved in the ancestor to the large apes ~21.5–16.9 mya; (iv) more data are needed to establish how many times kissing evolved in the Afro-Eurasian monkeys and apes; (v) kissing may have been lost at least once in Eastern gorillas; (vi) kissing most likely occurred in Neanderthals; and (vii) various life history variables correlate reasonably but not perfectly with kissing across the primates (multi-male mating systems, non-folivorous diets, and premastication; Fig. 2).

We suggest that kissing is an ancient and phylogenetically conserved trait within the large apes, including extinct human species. This conclusion is consistent with kissing having been observed in a variety of animals. We highlight promising future avenues of enquiry for understanding why kissing evolved, with our preliminary data highlighting a potential association with a high degree of postcopulatory sexual selection (via the proxy of mating system). While more data are needed, premastication appears to be a strong candidate trait for how kissing evolved, such that it may have been exapted into the behaviour we now describe as ‘kissing’.

As more data become available, future research could focus on exploring these life history variables in more detail, as well as other traits, such as the suckling associated with nursing in mammals and olfactory health checking behaviours. In addition, it would be useful to establish whether there is indeed evidence that extant nonhuman apes kiss the recipients of oral allo-grooming as a final stage, in order to appropriately test the hypothesis that kissing is exapted from this behaviour (Lameira, 2024).

6.1. Limitations to data

The results we present highlight the merits of taking a cross-species, comparative approach for uncovering the evolutionary history of nonfossilising traits, such as kissing. However, our analyses were limited in several ways that may have affected our preliminary conclusions.

First, a paucity of data outside of the large apes meant that we could not adequately test adaptive and exaptive hypotheses because (i) we lack power to confidently reconstruct ancestral states outside of the apes; and (ii) the small number of evolutionary transitions to kissing mean we do not have the power to formally test hypotheses regarding its adaptive function. Instead, correlations with life history variables are better viewed as the basis for hypothesis generation, pending more comprehensive data.

Second, our data on kissing were categorised as ‘present’ or ‘not observed’. Clearly, this absence of evidence does not indicate that kissing does not occur. Future studies could address this systematically, by asking experts whether they have observed kissing in species for which there are gaps in the database (following Brindle et al., 2023). These ‘confirmations’ of absence could be weighted by how well-studied a given species is, to quantify the level of uncertainty.

Third, classifying kissing as ‘present’ is a coarse measure. It would be more useful to gather data on how common kissing is in those species in which it has been observed. Categorising kissing in this way also ignores intraspecific variation. For instance, while kissing is commonplace in many human societies, it does not seem to occur at all in others (Jankowiak et al., 2015). It is important to establish whether this is also the case in nonhuman animals: is kissing widespread, or does it only occur in certain populations? If it is patchily distributed, this could indicate that kissing is a socially learnt, cultural phenomenon. It is also possible that demographic features correlate with kissing behaviour, such as age or sex, and the relationships between individuals, such as whether they are related or unrelated. Nonetheless, even when there is variation within species, this does not prevent insights being gained from broad across species studies (Cornwallis & Griffin, 2024).

6.2. What is a kiss?

Whilst our definition of kissing is deliberately broad, to be applicable across taxa, it could be defined more narrowly. One behaviour that is close to our definition of kissing is mouth-play – which may sometimes take the form of oral-oral contact – as can be observed between motherinfant bonobo pairs (Pan paniscus; Z. Clay, pers. comm.). This behaviour is clearly affectionate, and reinforces bonds, but does not necessarily constitute kissing as mouth-to-mouth contact does not appear to be the primary aim and there is little movement of the lips. Our definition also does not differentiate between sexual, platonic, or other forms of kissing. Given that these types of kiss may serve different adaptive functions, it would be useful to explore where they differ. Examining the data from different perspectives is a key step for future research.

Another limitation is that currently, among nonhuman primates, most reports of kissing are from captive or sanctuary living individuals, or do not state the living conditions (but see Nishida, 1970 and Shimada, 2014 for examples of kissing in free-living chimpanzees). For instance, to our knowledge, bonobos have not yet been observed kissing in the wild despite this behaviour occurring regularly in captivity (Z. Clay and B. Fruth, pers. comm.). This is surprising given that the occurrence of other behaviours, such as masturbation or the construction and use of sex-toys, do not seem to vary according to whether individuals are freeliving or captive (Brindle et al., 2023).

6.3. Conclusions

This paper is a starting point for future comparative research on the evolution of kissing. We highlight the limited available comparative data on kissing and make predictions about its adaptive function by exploring potential life history correlates. Where do we go from here? A key first step is for behavioural researchers interested in sexual behaviour to include mouth-to-mouth kissing in existing ethograms, documenting demographic (e.g. age; sex; rank), environmental (e.g. freeliving or captive) and contextual data (e.g. relationship between actors; whether the kiss occurred in a sexual or affiliative context). This information will determine whether there is sufficient power to adequately test adaptive hypotheses within a comparative framework. An alternative approach would be to test these hypotheses experimentally in species that exhibit kissing behaviour, by exploring its associated costs and benefits at the species level. Taken together, these approaches will help us to establish whether a kiss is more than simply “a lovely trick designed by nature to stop speech when words become superfluous” (Bergman, n.d.).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.evolhumbehav.2025.106788.