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Oral sex as infidelity-detection

https://doi.org/10.1016/j.paid.2012.11.034Get rights and content

Abstract

An evolutionary history of human female infidelity and consequent sperm competition may have caused the evolution of male counter-adaptations. The infidelity-detection hypothesis for oral sex proposes that men perform oral sex to gather information about their partner’s recent sexual history. We tested this hypothesis with data secured from 231 men in committed, sexual, heterosexual relationships. We found support for two derivative predictions: men at a greater recurrent risk of sperm competition expressed greater interest in, and spent more time performing, oral sex on their partner, even after controlling statistically for relationship length, relationship satisfaction, and sexual intercourse duration. The discussion addresses limitations of this research and highlights directions for future research, including distinguishing empirically the infidelity-detection hypothesis from alternative hypotheses for oral sex.

Highlights

► We test the idea that oral sex functions to detect partner’s sexual infidelity. ► Men with more attractive partners have more interest in performing oral sex. ► Men with more attractive partners spend more time performing oral sex.

Introduction

When a female copulates with two or more males within a sufficiently brief time period, the sperm from the different males simultaneously occupy the female’s reproductive tract and compete for fertilization of ova (Parker, 1970). Sperm competition has been documented or inferred to exist in many different species, including humans (Baker and Bellis, 1993a, Baker and Bellis, 1993b, Birkhead and Møller, 1998, Smith, 1984, Shackelford et al., 2002, Shackelford et al., 2007).

Female infidelity is an important context for human sperm competition (Smith, 1984). Humans typically form long term pair-bonds, but women occasionally pursue extra-pair copulations, placing their regular partner at risk for cuckoldry, or unwitting investment of time and resources into genetically unrelated offspring. The reproductive costs of cuckoldry may have caused the evolution of mechanisms that reduce the likelihood of cuckoldry (Shackelford, 2003). Some anti-cuckoldry mechanisms appear to be designed to prevent female infidelity by minimizing extra-pair copulation opportunities (Buss, 2002). Other anti-cuckoldry mechanisms appear to be designed to anticipate or to “correct” female infidelity, motivating the regular partner to enter his sperm into competition with rival male sperm that may already be present or that soon may be present in his partner’s reproductive tract (Shackelford, 2003).

Among men in committed relationships, those who spend a greater proportion of time apart from their partners since the couple’s last copulation (and thus experience greater risk of sperm competition) ejaculate more sperm at the couple’s next copulation (Baker & Bellis, 1993a). Inseminating more sperm increases a man’s chances in the “sperm raffle” to fertilize his partner’s ova (Parker, 1970). Men who spend a greater proportion of time apart from their partner since the couple’s last copulation also report greater desire to copulate with their partner, find their partner to be more attractive, and report that other men find their partner to be more attractive (Shackelford et al., 2002, Shackelford et al., 2007).

Men mated to more attractive women are at greater recurrent risk of sperm competition, because more attractive women are more likely to be pursued and successfully lured away by rival men (Goetz et al., 2005, Schmitt and Buss, 2001). Goetz et al. (2005) documented that men at greater recurrent risk of sperm competition perform deeper and more vigorous copulatory thrusts during sex with their partner, according to both men’s reports and women’s reports. These copulatory behaviors may function to displace from his partner’s reproductive tract another man’s semen, which complements the semen-displacing morphology of the human penis (Gallup et al., 2003).

Oral sex has been documented in many species, including bonobos (Palagi, Paoli, & Tarli, 2004), orangutans (Schurmann, 1982), lemurs (Koyama, 1988), short-nosed fruit bats (Tan et al., 2009), and humans (Leichliter, Chandra, Liddon, Fenton, & Aral, 2007), but the evolutionary function of oral sex is unclear in each of these species.

Informed by sperm competition theory, Thornhill (2006) hypothesized that oral sex performed by a man on his regular partner (cunnilingus) may function to detect the presence of rival semen following her sexual infidelity. Cunnilingus may allow men to taste and smell rival semen near or within the vagina, providing cues to a woman’s recent sexual history. This hypothesis was inspired by research on non-humans documenting the increased frequency of male genital licking and sniffing during female oestrus (Dugmore et al., 1984, Palagi et al., 2003, Pennington et al., 1986), as well one study in humans documenting that men rate vaginal fluid as more pleasant-smelling when the woman is at peak fertility (Doty, Ford, Preti, & Huggins, 1975).

In the present study, we test the infidelity-detection hypothesis of oral sex: if cunnilingus functions to detect rival semen, then men at greater recurrent risk of sperm competition will report greater interest in performing cunnilingus on their partner (Prediction 1) and will perform cunnilingus for a longer duration, to better detect rival semen (Prediction 2).

Men engage in lengthier durations of sexual intercourse with more attractive women (Goetz et al., 2005), so the predicted greater duration of cunnilingus for men at greater recurrent risk of sperm competition may be a byproduct of sexual intercourse duration. Men’s relationship satisfaction and the length of the relationship also are correlated with the performance and frequency of oral sex (Santtila et al., 2008). We assess and control statistically for these potential confounds in tests of the predictions.

Section snippets

Participants

Two hundred and thirty-one men in a committed, sexual relationship lasting at least one year participated, in exchange for extra credit in a course. All participants reported having had sex with their partner at least once in the past week. The mean participant age was 25.2 years (SD = 7.8) and the mean relationship length was 43.4 months (SD = 56.9).

Materials

Participants reported their age and current relationship length on a questionnaire. Following Shackelford et al., 2002, Shackelford et al., 2007,

Results

Following Goetz et al. (2005), we constructed a relationship satisfaction measure (α = .85) from the mean of four variables: sexual satisfaction, emotional satisfaction, overall satisfaction, and commitment to partner. Also following Goetz et al. (2005), we constructed a recurrent risk of sperm competition measure (α = .83) from the mean of four variables: how sexually and physically attractive the participant views his partner, and how sexually and physically attractive the participant believes

Discussion

This is the first study to test an evolutionary explanation for oral sex in humans. The results support the hypothesis that cunnilingus may function to detect female infidelity (Thornhill, 2006). Men mated to more attractive women—and, therefore, exposed to a greater recurrent risk of sperm competition—report greater interest in performing cunnilingus on their partner and report performing cunnilingus for a longer duration, supporting Predictions 1 and 2. These effects remained even after

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