The results of the current research support the hypothesis that a man who spends more time performing cunnilingus produces an ejaculate with greater estimated volume (hypothesis 1), even after controlling for potential confounds, including the total time the performers spent in physical contact and the age and attractiveness of the actress. We did not find support for the hypothesis that a man who spends more time copulating will produce an ejaculate with greater estimated volume (hypothesis 2). The current research is the first to document a relationship between the time spent performing cunnilingus and ejaculate quality.

An alternative hypothesis for prolonged copulation is that men perform prolonged copulations to spend more time displacing rival semen that may be in a woman’s reproductive tract (Goetz et al. 2005). Gallup et al. (2003) used artificial genitals and artificial semen and provided evidence that the shape of the human penis facilitates semen displacement from the vagina during copulatory thrusting. Goetz et al. (2005) found that men who perceive their partner to be more attractive (i.e., presenting greater sperm competition risk) perform more semen displacement copulatory behaviors (e.g., deeper, more vigorous copulatory thrusts), including prolonged copulations. Although the results of the current research did not support the ejaculate adjustment hypothesis for prolonged copulation, future research can extend the results of Goetz et al. by conducting a content analysis of amateur pornography to assess whether men who copulate with a more attractive woman also spend more time copulating with her.

The current research is guided by a male perspective because male sexual arousal mediates the relationship between copulatory behaviors and ejaculate quality. However, women play significant roles in copulatory behavior. For example, men perform cunnilingus to satisfy a woman (Pham and Shackelford 2013b), often upon her request (Backstrom et al. 2012), and women can determine the duration of copulation depending on the sexual position (e.g., when a woman is mounted on top of a man). Women also have reproductive interests in their partner’s ejaculate quality. When women’s interests and men’s interests are aligned (i.e., sexual cooperation), women may benefit from their partner’s higher quality ejaculate (e.g., when a couple is attempting to conceive). Thus, the results of the current research should be interpreted not as the exclusive product of a male strategy, but instead as the product of male and female sexual strategies that are variably in conflict or cooperation (see Shackelford and Goetz 2012).

Professional pornography—in contrast with amateur pornography—uses superior camerawork and, therefore, produces higher quality video of ejaculations. However, producers and directors of professional pornography may dictate the time professional actors spend performing sexual behaviors. Thus, the duration of sexual behaviors in professional pornography may not accurately reflect the duration of these behaviors in normal, natural circumstances. Furthermore, having sex publicly in exchange for payment from viewers (i.e., professional pornography) likely did not occur in ancestral environments. People might perform different sexual behaviors in amateur pornography than in professional pornography. However, because ejaculate adjustment is an autonomic reflex—unconsciously controlled by the male sexual response cycle (Jones and Lopez 2013)—the relationships between sexual behaviors and estimated ejaculate volume documented in the current research might be generalizable to normal, natural circumstances. Nevertheless, future research might attempt to replicate results of the current research using content analyses of amateur pornography.

A limitation of the current research is reliance on visual estimates rather than actual assessments of ejaculate volume. However, our hypotheses (e.g., time spent performing cunnilingus is positively correlated with ejaculate volume) depend only on reliable estimates of volume—not on the accuracy of ejaculate volume estimates. In other words, it is irrelevant whether coders correctly identified ejaculate volumes in teaspoons, only that they correctly identified the relative differences in ejaculate volumes between scenes. The independent coders achieved reasonable inter-rater agreement in estimating ejaculate volume (α = 0.78), indicating that these estimates can be made with sufficient reliability to warrant inclusion in empirical analyses. Nevertheless, future research could secure motion capture video of participants during copulation to directly measure sexual behavior and could collect the ejaculate (e.g., via condom) produced from that copulation to directly measure ejaculate volume.

Additionally, we coded scenes in which the man ejaculated outside the vagina. Such ejaculations do not result in fertilizations and, therefore, represent non-adaptive encounters. Indeed, men produce lower quality ejaculates when they withdraw their penis from the vagina immediately prior to ejaculation (Zavos et al. 1994). Future research can employ fluorescent dyes in which researchers can measure ejaculate volume inside the female reproductive tract, a methodology that has been used in non-human research measuring the fertilizing success of ejaculates (e.g., Lymbery et al. 2016).

We could not determine when the actor last ejaculated before each scene. This may explain why the two regression models accounted for a small percentage of the variance: The duration of abstinence is consistently one of the strongest predictors of ejaculate volume (WHO 2010). Indeed, in studies that investigate the strategic adjustments in ejaculate quality, participants are instructed to abstain for at least 48 h prior to submitting their ejaculates for analysis (Joseph et al. 2015; Kilgallon and Simmons. 2005; Leivers et al. 2014) Future replications of the current research could secure information about the most recent ejaculation prior to each copulation to increase the explanatory power of the models.

Additionally, the small effect size may be attributable to the human olfactory system. Humans are less dependent on chemosensory systems than are other primates (Gilad et al. 2003). The relationship between cunnilingus and ejaculate quality in humans may be remnants of adaptations found in species closely related to humans (i.e., other primates). If this is the case, then we expect much stronger effect sizes when replicating the current research to non-human primates.

The quality of men’s masturbatory ejaculates are dependent on several features of the pornography they consume, including cues to sperm competition (Kilgallon and Simmons 2005) and the actress’s mate value (Leivers et al. 2014). Future research could extend our findings by investigating whether men produce different quality ejaculates depending on whether cunnilingus is depicted in the pornography they consume.

Variation in quality between ejaculates has been historically interpreted by andrologists as “noise,” with the WHO (2010) recommending that clinicians secure two or three ejaculates from a man to determine his “true” fertility. However, a growing body of research indicates that men strategically adjust their ejaculates (reviewed in Pham and Shackelford 2014)—including adjustments in non-sperm chemicals that affect fertilizing success (Burch and Gallup 2006; Davis and Gallup 2006). The current research contributes to this literature by documenting that men’s pre-ejaculatory sexual behavior may affect ejaculate quality. Further, the current research contributes data from humans to the substantial non-human literature documenting that copulatory behaviors that do not contribute directly to reproduction contribute indirectly to reproduction by affecting sexual arousal and consequent ejaculate quality. Finally, the current research is the first to document a relationship between the time spent performing cunnilingus and ejaculate quality.