همسر حفظ، جابجایی اسپرم و رقابت اسپرم انسان : یک تحقیق مقدماتی از تاکتیک برای جلوگیری از خیانت زن

Sperm competition is the competition between the sperm of two or more males to fertilize a female’s egg(s). We examined how men under a high recurrent risk of sperm competition might attempt to prevent and correct their partners’ sexual infidelity. Three hundred and five males drawn from universities and surrounding communities rated their partner’s physical attractiveness and personality characteristics (to assess their recurrent risk of sperm competition), and reported their use of tactics designed to prevent partner infidelity (mate retention tactics) and their use of specific copulatory behaviors arguably designed to displace the semen of rival men (semen-displacing behaviors). As hypothesized, men at a high recurrent risk of sperm competition were more likely to use mate retention tactics and to perform semen-displacing behaviors.

Competition between males to fertilize a female’s egg(s) can occur before, during, and after copulation (Parker, 1970; Birkhead & Møller, 1998). When the sperm of two or more males simultaneously occupy the reproductive tract of a female and compete to fertilize her egg(s), sperm competition occurs (Parker, 1970). Sperm competition has been documented or inferred to exist in many species, ranging from molluscs (Baur, 1998) and insects (Simmons, 2001) to birds (Birkhead & Møller, 1992) and humans (Baker and Bellis, 1993a and Baker and Bellis, 1993b; Gallup et al., 2003; Shackelford, 2003; Shackelford, Goetz, LaMunyon, Quintus, & Weekes-Shackelford, in press; Shackelford et al., 2002; Smith, 1984; Wyckoff, Wang, & Wu, 2000). For species that practice social monogamy, the mating system in which males and females form long-term pair bonds but also pursue extra-pair copulations (e.g., most birds and humans), female sexual infidelity creates the primary context for sperm competition (Birkhead & Møller, 1992; Smith, 1984). Males of such species may have adaptations that decrease the likelihood that a rival male’s sperm will fertilize his partner’s egg(s)––adaptations that decrease the likelihood of being cuckolded, unwittingly investing resources in genetically unrelated offspring. Male sexual jealousy, for example, is one of the most widely researched human anti-cuckoldry adaptations. Male sexual jealousy is hypothesized to motivate men to deter a mate from a sexual infidelity or a permanent defection from the mateship, and to deter rivals from mate poaching (e.g., Buss, Larsen, Westen, & Semmelroth, 1992; Daly, Wilson, & Weghorst, 1982; Harris, 2003; Symons, 1979; White & Mullen, 1989). Others have described more specific adaptations that may combat sperm competition. Baker and Bellis (1993a), for example, demonstrated that men may have physiological adaptations that function to increase the likelihood that their sperm will out-compete rival sperm to fertilize their partner’s egg(s). In a study of couples in committed, sexual relationships, Baker and Bellis (1993a) documented that, at the couple’s next copulation, men inseminated more sperm when the couple had spent a lesser proportion of their time together since their last copulation. As the proportion of time together decreases, the likelihood of female infidelity increases, creating a higher risk of sperm competition (Baker and Bellis, 1993a and Baker and Bellis, 1995). Inseminating more sperm following a separation may function to outnumber or “flush out” rival sperm that may be present in the reproductive tract of the female (Baker & Bellis, 1993a; Parker, 1970). This temporally variable risk of sperm competition produces specific physiological responses apparently designed to “correct” any female sexual infidelity that might have occurred while the couple was separated. Some men, however, may be mated to women who recurrently place them at a high risk of sperm competition. Female physical attractiveness and certain personality characteristics that attract rival men, for example, may increase the likelihood of female sexual infidelity and, therefore, place her partner at a high recurrent risk of sperm competition. Because a woman’s physical attractiveness indexes her reproductive value and fertility ( Singh, 1993; Symons, 1979), physically attractive women are desired partners for long-term, short-term, and extra-pair mateships ( Buss, 1989; Buss & Schmitt, 1993; Li, Bailey, Kenrick, & Linsenmeier, 2002; Regan, Levin, Sprecher, Christopher, & Cate, 2000). Accordingly, physically attractive women are more likely to have had men try to poach them away from their current partners ( Schmitt & Buss, 2001), and men married to more physically attractive women devote more effort to retaining their mates ( Buss & Shackelford, 1997). Direct evidence that physically attractive women are more likely to commit infidelity comes from research examining women’s waist-to-hip ratios (WHR) and their sexual behavior. WHR is a key component of female physical attractiveness (Dijkstra & Buunk, 2001; Singh, 1993; Streeter & McBurney, 2003). Low WHR is judged cross-culturally to be physically attractive, perhaps because it is a reliable indicator of reproductive age, sex hormone profile, and disease resistance––features associated with health and fertility (Singh, 1993). Hughes and Gallup (2003) documented that women with low WHR reported committing more infidelities and having more sexual partners than women with high WHR. Thus, because physically attractive women attract more mate poachers and commit more infidelities, they may put their partners at a high recurrent risk of sperm competition. Another set of factors that may place a man at a high recurrent risk of sperm competition is his partner’s personality traits. The five-factor model of personality describes five dimensions of stable individual differences in personality (Surgency, Agreeableness, Conscientiousness, Emotional Stability, and Openness to Experience; Goldberg, 1982; Norman, 1963). Schmitt and Buss (2001) found positive and significant relationships between a woman’s Surgency and Openness to Experience and the likelihood of having had someone try to poach her away from an existing mateship. Similarly, Gangestad and Simpson (1990) found that women who are more socially dominant and extraverted (high in Surgency) are significantly more willing to have sex without indicators of commitment and emotional closeness. Sexual promiscuity, in turn, is a good predictor of infidelity (Hughes & Gallup, 2003). Women who are higher in Surgency and Openness to Experience, therefore, are more often given the opportunity to engage in extra-pair copulations. No data exist on the relationship between Surgency and Openness to Experience and the actual occurrence of infidelity, but because infidelity can only occur if the opportunity for infidelity exists, a greater opportunity for infidelity translates, on average, to a greater probability of infidelity. Although not all infidelity generates sperm competition, the occurrence of sperm competition depends, in large part, on female infidelity (Baker & Bellis, 1995; Smith, 1984). In summary, men mated to women who are physically attractive, high in Surgency, and high in Openness to Experience may face a high recurrent risk of sperm competition. Ancestral men mated to such women would have reaped reproductive benefits if they were able to prevent or correct their partner’s sexual infidelity.