Men sometimes engage in noncopulatory sexual behaviors, such as cunnilingus and other kinds of sexual foreplay. Men involved in long-term romantic relationships, in particular, tend to provision their partners with oral sex. Potential adaptive functions of cunnilingus in humans are discussed with a focus on the hypothesis that men use oral sex provisioning as part of a general benefit-provisioning, long-term mating strategy. Other potential adaptive functions are also considered, including the infidelity detection hypothesis and several hypotheses concerning sperm competition adaptations. Some research has proposed the possibility that men may use oral sex as a form of infidelity detection, wherein they might be able to smell or taste the semen of rival males in their partner’s vagina. Other research has posited that men might perform cunnilingus in order to induce orgasm in their partners, thereby increasing the amount of sperm retained in her reproductive tract after ejaculation. Still others have suggested that men might perform cunnilingus to increase their own arousal, thus increasing their subsequent ejaculate volume. These adaptive perspectives are couched within the wider literature on oral sex, which includes data regarding the frequency of oral sex in adolescent, preindustrial and non-Western samples, as well as women’s desire for receiving oral sex. Regarding the relative infrequency of cunnilingus in preindustrial and non-Western samples, in particular, men’s provisioning of oral sex is considered as potentially being a part of an evolved cognition for benefit-provisioning mate retention in general, rather than oral sex itself serving a specific adaptive function. Specifically, oral sex may be one type of sexual favor that men, especially those in Western cultures, sometimes provide to their long-term partners. Additional data regarding the increased sexual and relationship satisfaction in women who engage in a wider variety of sexual practices and who more frequently experience orgasm further supports the mate retention hypothesis of men’s provisioning of oral sex. Nevertheless, the available literature investigating these potential adaptive functions is currently insufficient to draw any decisive conclusions. Finally, gaps in the current literature and suggestions for future research that may help determine the evolved nature of men’s oral sex provisioning are discussed.

Female engagement in oral sex, both receiving and performing, is a sexual behavior found throughout the animal kingdom, yet it remains an underdeveloped research area. Regardless, proximate and ultimate causes can explain engagement in oral sex. For instance, there have been various sociocultural impacts that have led to fluctuations in the performance frequency of oral sex over human history. In this chapter, we highlight the results of research on oral sex for women. First, cross-cultural and historical practices regarding oral sex are presented to explain various fluctuations in the behavior. We also examine cross-species examples of oral sex. Second, we present contemporary research on oral sex and aspects that impact its frequency, such as peer pressure, age, and myths regarding the behavior. Third, proximate mechanisms (sexual pleasure and partner satisfaction) for engagement in oral sex are discussed. Fourth, we describe six evolutionary perspectives on oral sex and introduce one additional theory—a sexual communal perspective that integrates other models. Lastly, we note limitations and future directions for studying women's engagement in oral sex.