Ejaculate Adjustment in Response to Sperm Competition Risk in Humans

Previous research suggests that human males, like males of many mammalian and avian species, adjust their ejaculate quality in accordance with sperm competition risk. Men who spend less time with their regular female partner since the couple's last copulation produce ejaculates with more sperm at the couple's next copulation (Baker & Bellis, 1993). We conducted a conceptual replication of this research to investigate whether sperm competition risk predicts ejaculate adjustment in human males using additional measures of sperm competition risk (e.g., perceptions of partner infidelity, presence of potential sexual rivals) and updated laser-optic semen analysis technology. We collected data from 34 heterosexual couples (age range 18-32 years) from a university population who completed self-report surveys on their relationship dynamics and provided six ejaculate samples (three copulatory and three masturbatory) across a 45-day period. Time spent together since the couple's last copulation was not significantly associated with ejaculate quality. However, sperm concentration for copulatory ejaculates was higher for men who perceived more potential sexual rivals. Discussion situates the current results within the literature on human sperm competition and suggests several directions for future research.