History of Sperm Competition in Humans
KeywordsSperm Competition Regular Partner Female Orgasm Mating Plug Sperm Competition Risk
Many species have faced sperm competition (when sperm from two or more males compete to fertilize an egg in a female’s reproductive tract) in their ancestral past to varying degrees. In the history of human evolution, the common mating system of social monogamy led to decreased levels of sperm competition relative to some other primates; however, it was not completely absent. Evidence of this history of sperm competition is seen in both men and women in terms of their genetic makeup, anatomy, and evolved mating psychology.
In humans, there is evidence that they repeatedly faced sperm competition in their ancestral past. That is, women mated with two or more men in a sufficiently brief window of time to allow sperm from more than one man into their reproductive tract that subsequently would compete to fertilize ova (Parker 1970). Because the most common mating system in humans is social monogamy (Brown et al. 2009), sperm competition can have a large impact on men’s reproductive success through their regular partner’s engagement in extra-pair mating and subsequently producing offspring sired by another male. This scenario leads to men unwittingly investing in genetically unrelated children (Trivers 1972).
This situation does occur in contemporary human societies, with an average rate of 3.1% of children having a different biological father than the one socially regarded as their father (Voracek et al. 2008). However, this rate sharply increases with degree of paternity uncertainty as men who reported having low confidence in paternity of their children experience a nonpaternity rate of 29.8%, while those with high paternity confidence experience only a 1.7% nonpaternity rate (Anderson 2006). Additionally, women report “double mating” or mating with more than one male within a 5-day span at least once in their lifetimes at a rate of 17.5% (Baker and Bellis 1993a). As such, men currently exhibit both physiological and psychological adaptations to sperm competition in order to avoid cuckoldry (and the subsequent reproductive cost of raising unrelated children) and women exhibit adaptations to manipulate the outcome of sperm competition in their favor (Shackelford et al. 2015). These adaptations would not have arisen without the presence of sperm competition in humans’ evolutionary past. The following presents evidence of this history of sperm competition in humans.
Evidence in Men
In some primate species, females commonly mate with multiple males (polyandry) and in other species they mainly mate with one male (monandry; Harcourt et al. 1995). In species practicing polyandry, the risk of sperm competition is greater, and the male genitalia of these species evolved to adapt to this greater risk. Specifically, more sperm is needed to be produced to outcompete that of rivals, and polyandrous species have larger testes capable of producing enough sperm to meet the demands of sperm competition (Harcourt et al. 1981; Short 1979). Relative testis size and the number of sperm per ejaculate correlate positively with the degree of polyandry; for example, monandrous gorillas have relatively small testes, while highly polyandrous chimpanzees have rather large testes in proportion to body size (Harcourt et al. 1995). Humans have testes that are intermediate in size (in proportion to body size) compared to both the gorilla and the chimpanzee, which signals an intermediate level of sperm competition risk in humans (Smith 1984).
Similarly, the length and shape of the human penis has been noted as evidence of a history of sperm competition. The length is longer than that of any other ape (Short 1979) which is argued to give men an advantage in sperm competition. Specifically, the long penis can deposit sperm close to the cervix which gives the sperm an advantage in the race toward the egg as they would have a shorter distance to travel (Baker and Bellis 1995; Smith 1984). In terms of shape, it has been proposed that the coronal ridge of the penis is designed to displace rival semen that was previously deposited in a woman’s reproductive tract (Baker and Bellis 1995). Empirical evidence to support this hypothesis was provided by Gallup et al. 2003, who used artificial genitals to conclude that phalluses with a coronal ridge displaced up to 90% of simulated semen while phalluses lacking a coronal ridge only displaced 35.3%. In further detail, this displacement seemed to function by the coronal ridge allowing semen to flow back and collect under it; however, this only was able to occur when at least 75% of the length of the penis was inserted into the simulated vagina (suggesting again that length is important for sperm competition). Finally, the refractory period that men experience after an ejaculation (period in which an erection cannot be achieved) can also be argued as evidence of the semen displacing ability of the human penis, as it would be reproductively costly for a man to displace his own sperm (Gallup and Burch 2004).
At the genetic level, there also exists evidence of some history of sperm competition in humans. Upon examination of protamine genes (genes involved in the production of functional spermatozoa), they seem to be somewhat similar to those found in polygamous rodents as well as being thought of as comparable to the more polygamous chimpanzee in primate terms (Martin-Coello et al. 2009; Wyckoff et al. 2000). Other research looking at genes that encode seminal fluid proteins suggest a more moderate history of sperm competition. Specifically, in many nonhuman species (including other primates such as the gorilla and chimpanzee), proteins in seminal fluid function to form a “mating plug” within the female reproductive tract as a means of blocking sperm from rival males in subsequent matings (Lievers and Simmons 2014), and the rate of evolution of a certain gene involved in the production of the proteins necessary to form the mating plug is positively correlated with the level of sperm competition within a species (Clark and Swanson 2005). Among primates, the rate of evolution of this gene is only moderate in humans compared to the highly promiscuous chimpanzee (Dorus et al. 2004).
Beyond anatomical evidence, men try to boost their paternity confidence of offspring that may result from sexual relations with a given woman and therefore have evolved preferences for mates that will provide them with the least amount of sperm competition, even in short-term contexts (Shackelford et al. 2004). Consistent with these preferences, men are most likely to report a desire to pursue a short-term relationship with a woman who is neither married nor involved in casual sexual relationships as this situation presents no risk of sperm competition; men were least interested in a short-term relationship with a married woman, as frequent inseminations from a husband present a high sperm competition risk (Shackelford et al. 2004).
Another part of men’s mating psychology related to the threat of sperm competition is the strength of male sexual jealousy. Not only do men attempt to choose mates that pose a low sperm competition risk, once finding a low-risk partner they try to further minimize the opportunities for her to engage in extra-pair copulations with other men, and this is often motivated by jealousy (Shackelford et al. 2015). Men react with stronger jealousy to just imagining their regular partner engaging in sexual intercourse with someone else compared to women (Buss et al. 1992). This jealousy and its use as a mate retention tactic tend to keep men’s regular partners away from potential rivals in order to avoid the consequences that can be attached to that jealousy. In the most extreme of cases, male sexual jealousy can lead to men murdering their partners; this is also the leading motive for men to kill their partners (Buss 2006). Such strong reactions to potential extra-pair copulation are evidence of sperm competition as a recurring theme among humans’ evolutionary past.
Evidence in Women
While men’s adaptations to sperm competition are designed to avoid cuckoldry, women’s adaptations to sperm competition are designed to ensure its outcome serves their best reproductive interests. These adaptations concern both precopulatory and postcopulatory female mate choice (Shackelford et al. 2015). In precopulatory mate choice, women have the most reproductive success from forming a long-term relationship with a man who is willing to invest in her and her children to increase the chances of offspring survival (Trivers 1972). However, the type of mate that a woman can secure as a long-term investing mate and the type of mate that can offer the most in terms of good genes that could be passed to her offspring may be different from one another (Gangestad et al. 2015). Specifically, men with traits signaling “good genes” such as muscularity, masculinity (in the body, face and voice), symmetry, and social dominance typically have high testosterone levels and subsequently are less likely to display a willingness to be faithful and invest in one woman (Pollet et al. 2011). Therefore, it is adaptive for women to strive to obtain both paternal investment and good genes to maximize reproductive success by having a long-term mate and engaging in extra-pair copulations with men of superior genetic quality when conception risk is the highest (Gangestad et al. 2015).
This speculation has been supported by research finding that while women’s sexual attraction to their regular partner remains unchanged across the ovulatory cycle, their sexual attraction to and likelihood of sexual fantasizing about men other than their partner is more likely to occur during the fertile window of the ovulatory cycle (Pillsworth and Haselton 2006). This also translates into behavior as women are more likely to engage in extra-pair sex when conception risk is high, especially when their regular partner is not of relatively high genetic value (Garver-Apgar et al. 2006). Not only do women engage in extra-pair mating during high fertility, but they also strategically schedule copulations with their regular partner to manipulate sperm competition outcomes as well. When women have recently engaged in an extra-pair mating, they tend to delay copulation with their regular partner in order to give the rival male an advantage in sperm competition (Gallup et al. 2006).
In postcopulatory mate choice, female orgasm seems to be the driving force in sperm competition outcomes. It has been speculated that the female orgasm is an adaptation for selective sperm retention, as its occurrence causes uterine contractions that facilitate the retention and transfer of a greater number of sperm into the reproductive tract (Baker and Bellis 1993b, 1995). Because women want the best quality genes for their offspring, the finding that women mated to more symmetrical men (a sign of genetic quality) have more copulatory orgasms than women mated to less symmetrical men (Thornhill et al. 1995) might be support for the argument of female orgasm being an adaptation. In addition to the presence of female orgasm during copulation for sperm retention, the timing of such orgasms may be just as (if not more) important for this purpose (Smith 1984). According to research by Baker and Bellis (1993b), the optimal timing for female coital orgasms for greater sperm retention is between 1 min before and 45 mins after male ejaculation. This was concluded after examining seminal and vaginal fluid that was ejected from the vagina after copulation (known as flowback), and less sperm was ejected when female orgasm occurred during this time frame. Furthermore, for women with extra-pair partners, they report having more of these high-retention orgasms within this time frame with the extra-pair partners compared to their regular partners (Baker and Bellis 1993b). This is also indicative that women may promote sperm competition and bias the outcome in favor of an extra-pair partner that likely is of greater genetic quality.
Related to female orgasm as an adaptation concerning sperm competition outcomes is men’s interest in ensuring their partner achieves orgasm. One study found that men who spent more than 50% of time away from their partner since the couple’s last copulation reported a greater interest in their partner’s copulatory orgasm than men who spent less than 50% of time away from their partner (McKibbin et al. 2010). More specifically, sperm competition risk (as measured by percentage of time spent away from partner) moderated the effect of relationship satisfaction and investment on interest in partner orgasm, such that men who reported a greater sperm competition risk and greater relationship satisfaction and investment were particularly likely to report an interest in and attentiveness to their partner’s copulatory orgasm. The association between relationship satisfaction and investment and interest in partner orgasm was not significant for men who reported lesser sperm competition risk, suggesting this interest is motivated by wanting to get an advantage in potential sperm competition.
Humans have faced a significant degree of sperm competition in their evolutionary history. Between the highly promiscuous chimpanzee and the monandrous gorilla, the level of sperm competition in human mating systems is moderate, but this moderate amount is enough to warrant the development of adaptations in response to sperm competition in both men and women. For men, they have evolved adaptations to both avoid sperm competition and to also have an advantage in winning such competition on the occasions in which it does occur. Their genitalia evolved to generate enough sperm to be competitive in a lottery or scramble scenario and also to displace rival sperm that may already be present in a woman’s reproductive tract. Genetic indicators of moderate sperm competition exist in men as well. Psychologically speaking, men have evolved preferences to avoid choosing a long-term mate that would be of high sperm competition risk and, once in an exclusive sexual relationship, men have evolved strong sexual jealousy to deter their partners from engaging in extra-pair copulations.
In women, sperm competition adaptations function to manipulate its outcome in their favor. In terms of reproductive success, it would be optimal for women to have a regular partner from which they can secure paternal investment and also strategically engage in extra-pair copulations during periods of high conception risk to secure genes superior to those that can be obtained from the long-term partner. To aid in utilizing this strategy, women’s sexual preferences change throughout the ovulatory cycle such that they are more attracted to men other than their regular partner during times of high fertility. Women also can manipulate the outcome of sperm competition in the incidence of “double mating” by strategically having orgasms during copulation with the partner that is deemed genetically superior to retain more of his sperm. For being a moderate selection pressure on human mating dynamics, sperm competition has had considerable influence on the evolution of human sexuality.
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