Original cartoon from Alex Martin
Source: Original cartoon from Alex Martin

In the Western world, Valentine’s Day expressions of affection are closely linked to mouth-to-mouth kisses. I remember feeling envious years ago when a colleague found a card in his pigeon-hole “signed” with the lipstick imprint from a pair of (presumably female) lips. But does kissing have biological functions? And, if so, are they results of evolution? Many different functions have been proposed for kissing, ranging from assessing mate quality through strengthening pair-bonds to triggering sexual arousal. But there is also the key issue of hygiene. Amorous kissess—particularly the wet, open-mouth kind—can transfer microbes between partners. This could have adverse effects, but it might also beneficially match bacterial profiles between partners. Last but not least, kisses occasionally provoke severe allergic reactions in a partner.

//www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or FAL], via Wikimedia Commons.
Auguste Rodin’s famous sculpture Le Baiser (The Kiss), displayed in bronze in the Orangerie in Paris. The original sculpture, completed in about 1882, was based on Dante’s Divine Comedy, representing Francesca and Paolo, who were slain by Francesca’s husband when he caught them exchanging their first kiss.
Source: Yair Haklai [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or FAL], via Wikimedia Commons.

Origins of kissing

Intimate kisses are linked to romantic or explicitly sexual contexts, but the superficial kind can also figure in social interactions between acquaintances. Think of pretentious female friends exchanging vacuous “air kisses”. So which rôle came first? In his 1971 book Love and Hate, my one-time PhD mentor Irenäeus Eilbl-Eibesfeldt suggested one promising possibility. He proposed that mouth-to-mouth kissing began with direct transfer of pre-chewed food from mother to infant. Such an origin would explain a general social background to kissing without restriction to a romantic context.

Animal parallels to human kissing exist, not only in other primates and various non-primate mammals but even in other vertebrates such as birds and fish. In some cases there may be a link to oral contact during parenting. Certain rodents show mouth-to-mouth interaction between adults, as with the well-known “greet-kissing” of prairie dogs, preceded by a mother-infant equivalent.

Photograph taken by Dietrich von Holst in 1967.
Tree-shrew mother, Tupaia belangeri, immediately following the birth of 3 infants. One infant has just emerged (right), another has a bulging stomach from being suckled (middle), and the third is licking the mother’s mouth (left).
Source: Photograph taken by Dietrich von Holst in 1967.

My PhD research on tree-shrews revealed an intriguing pattern of oral contact. I discovered that mothers give birth to their 2-4 babies in a separate nest and visit them only once every two days for suckling. Each time, the standard pattern is that each infant rapidly fills its stomach with milk and then moves forward to lick the mother’s mouth. Only maternal saliva was delivered to the infant, perhaps providing the appropriate bacteria for its digestive tract. Whatever, the function, mouth-licking also occurs between mothers and juveniles and between adult pair partners.

Kisses and microbes

In 2014, Remco Kort and colleagues reported on a study of the relationship between intimate kissing and oral microbe communities. 21 couples provided information on previous kissing behavior using questionnaires, and an experiment was conducted to monitor microbes on the tongue and in saliva. Comparisons revealed that partners were generally distinctly more similar than unrelated individuals. Microbial populations on the tongue surface showed by far the greatest similarity. A single intimate kiss did not significantly increase similarity in microbe communities between partners. Nevertheless the degree of similarity in salivary microbes within couples was clearly associated both with kiss frequencies and with time elapsed since the most recent previous kiss.

Prior to a second intimate kiss, one partner was given a probiotic yoghurt to drink to determine numbers of marker bacteria (Lactobacillus, Bifidobacterium) transferred. Some 80 million bacteria were transferred to the recipient during an average intimate kiss lasting 10 seconds.

Overall, it emerged that frequent and continuous exchange of bacteria is needed to maintain sharing of salivary microbes, which is hence most evident with relatively frequent intimate kissing. Microbes on the upper surface of the tongue are also more similar between partners than among unrelated individuals, but no clear association with kissing was found. Moreover, it remains to be seen whether sharing bacteria between regular kissing partners benefits or negatively impacts health.

An intriguing, though entirely theoretical, suggestion made by Colin Hendrie and Gayle Brewer in 2010 is that kissing might be an evolutionary adaptation protecting against microbial disease. This was their starting premise: “As mouth-to-mouth sexual kissing exposes each participant to the diseases of the other, it must confer significant benefit.” They considered the specific example of human cytomegalovirus (HCMV), a pervasive infection that may threaten critical stages of embryonic/fetal development. Sexual kissing with exchange of saliva would efficiently ensure inoculation of a woman with HCMV from a particular man before pregnancy. But Hendrie and Brewer pushed their argument a little too far by proposing that the need to avoid of infection by other men would also have promoted the evolution of human monogamy. Why only in humans?

Severe and sometimes fatal allergic reactions to kissing have occasionally been reported. In a 2002 paper, Rosemary Hallett and colleagues noted that, when compiling an allergy database, 21 people out of around 400 with food allergies to nuts or seeds spontaneously reported adverse effects of kissing. The questionnaire used did not directly address kissing, so 5% is surely an underestimate. When Hallett and colleagues interviewed 17 of the patients concerned, peanuts, walnuts and other tree nuts were the foods implicated. In all cases, pronounced reactions began less than a minute after the kiss. The real frequency of allergic responses to kissing remains unknown, but it seems to be high enough to mandate caution. Yet it is probably too rare to evoke an evolutionary response.

Redrawn after figures in Hughes et al. (2007).
Survey results showing male/female differences in attitudes towards kissing. Above: Readiness to have sex without kissing (men distinctly more than women). Below: Preference for kissing before, during and after sex (women distinctly more than men at all stages).
Source: Redrawn after figures in Hughes et al. (2007).

Possible functions of kissing

In recent years, several investigators have attempted to identify potential functions of kissing. One general account published in 2007 by Susan Hughes and colleagues used anonymous surveys of around 1,000 American university students to explore sex differences. A key aim was to examine kissing in both short-term and long-term contexts. Survey questions were designed to explore possible functions of romantic kissing for mate assessment, strengthening pair bonds and promoting sexual arousal and receptivity. Results indicated that kissing was more important for women for mate assessment and for establishing/monitoring relationships with long-term partners. Men regarded kissing as less important, especially with short-term partners, and seemed to employ it more to increase the likelihood of coitus. Hughes and colleagues concluded that “kissing may play an important role as an adaptive courtship/mating ritual”.

Subsequently, in two companion papers published in 2013, Rafael Wlodarski and Robin Dunbar set out to examine possible functions of romantic kissing in more detail. Like Hughes and colleagues, they focussed on potential functions in mate assessment, pair bonding and sexual arousal. They also employed questionnaires, completed by just over 900 subjects. Results supported the notion that romantic kissing serves a useful function in mate assessment. Once again, women attributed greater importance to kissing overall, notably regarding the importance of kissing outside a sexual context. Furthermore, their responses indicated that an initial kiss was more likely to influence their attraction to a potential mate. Results also supported the hypothesis that kissing plays a part in pair bonding. By contrast, little evidence emerged to support the hypothesis that the main function of kissing is to increase sexual arousal.

Figure redrawn from an original in Wlodarski & Dunbar (2013b)
Importance of kissing during initial relationship stages (red) and established relationship phases (blue) across the menstrual cycle. Day 0 represents estimated day of ovulation.
Source: Figure redrawn from an original in Wlodarski & Dunbar (2013b)

In the companion paper, Wlodarski & Dunbar examined women’s attitudes to romantic kissing relative to inferred menstrual cycle stage at the time of completing the questionnaire. Findings indicated that, compared with women in the postovulatory phase of the cycle, women who completed the questionnaire during the preovulatory phase felt that kissing was more important during initial stages of a relationship. However, the observed effect was very weak: Overall, there was little variation across the cycle in women’s assessments of the importance of kissing.

Is kissing universal to humans?

Quite apart from relying on questionnaires, such studies of romantic kissing generally have a major flaw. Although they may identify attitudes towards kissing in British and American subjects, it is simply taken for granted that any sex differences identified are the outcome of evolutionary processes. But a major requirement here is that romantic kissing should be virtually universal across the human species. Yet there has been very little mention of possible cross-cultural differences. In fact, Hughes and colleagues, Wlodarski & Dunbar and various other authors have slavishly repeated the claim, apparently derived from Eibl-Eibesfeldt’s 1971 book, that “mouth-to-mouth sexual kissing is common in more than 90% of human cultures”.

In 2015, William Jankowiak and colleagues at last provided systematic cross-cultural information on the presence or absence of romantic-sexual kissing in 168 cultures. They compiled information from an extensive worldwide database in combination with a selective survey of ethnographic evidence, Romantic-sexual kissing is not even close to being a cultural universal, as it was lacking in 54% of societies. No evidence was found for regular occurrence of kissing in foragers or horticulturalists of Central America, Amazonia, Sub-Saharan Africa or New Guinea. Jankowiak and colleagues did, however, report a strong positive correlation between the frequency of romantic-sexual kissing and social complexity. But is this just another way of saying that kissing is most prevalent in the Western world? In a nutshell, this is a classic example of inferring evolutionary processes without a proper cross-cultural approach.

References

Eibl-Eibesfeldt, I. (1971) Love and Hate: The Natural History of Behavior Patterns (First Edition). London: Methuen.

Fisher, H.E. (1992) Anatomy of Love: The Natural History of Monogamy, Adultery and Divorce. New York/London: W.W. Norton Co./Simon & Schuster.

Hallett, R., Haapanen, L.A. & Teuber, S.S. (2002) Food allergies and kissing. New England Journal of Medicine 346:1833-1834.

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