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Sperm competition occurs when a female copulates with two or more males within a sufficiently brief time period, resulting in the sperm of the different males competing to fertilize ova. In this chapter, we present the evidence of sperm competition in humans. We review the literature indicating anatomical, psychological, and physiological adaptations to human sperm competition. These adaptations include traits that produce prudent sperm allocation, copulatory interest, forced in-pair copulation, sexual fantasy, female orgasm, and semen-displacing copulatory behavior. Given the evidence of apparently convergent adaptations to sperm competition in humans and nonhumans, we conclude that sperm competition has been a important selection pressure that shaped human nature.
Human sperm competition—p. 1
[in press, In D. M. Buss (Ed.), The handbook of evolutionary psychology (2nd ed.). New York: Wiley.
Human Sperm Competition
Todd K. Shackelford1, Aaron T. Goetz2, Craig W. LaMunyon3, Michael N. Pham1, & Nicholas Pound4
1. Department of Psychology, Oakland University, Rochester, Michigan, USA
2. Department of Psychology, California State University, Fullerton, Fullerton, California, USA
3. Department of Biological Sciences, California State Polytechnic University, Pomona, California,
4. Department of Psychology, Brunel University, Uxbridge, Middlesex, UK
Human sperm competition—p. 2
Sperm competition was first defined as “the competition within a single female between the sperm
from two or more males for the fertilization of the ova” (Parker, 1970, p. 527). As a form of sexual selection
that occurs after the initiation of copulation (termed post-copulatory sexual selection), sperm competition has
shaped the evolution of adaptations across a wide range of taxa (Birkhead & Møller, 1998; Birkhead,
Hosken, & Pitnick, 2009). These adaptations (a) increase a male’s probability of fertilization when sperm
competition occurs, and (b) allow females to bias paternity toward favorable males (Eberhard, 1996).
Sperm Competition in Non-Human Species
Sperm competition has been reported in many species, from molluscs (Baur, 1998) and insects
(Simmons, 2001) to birds (Birkhead & Møller, 1992) and mammals (Gomendio, Harcourt, & Roldán, 1998).
In species with internal fertilization, sperm competition can occur when a female mates with multiple males
within a sufficiently short time period so that sperm from two or more males simultaneously occupy her
reproductive tract. Although the outcome of such competition depends on many factors (e.g., mating order,
male accessory secretions, and the shape, number, and size of female sperm storage organs), the number of
sperm transferred is often the most important factor and, consequently, a male can increase the probability of
fertilizing ova by inseminating more sperm (Parker, 1970; 1990a). However, because the costs of ejaculate
production are non-trivial (e.g., Dewsbury 1982; Pitnick, Markow, & Spicer, 1995), males must trade-off
ejaculate production costs against the potential benefits of delivering more sperm in any particular ejaculate.
Thus, one of the first hypotheses generated by sperm competition theory was that males will deliver more
sperm when the risk of sperm competition is high (Parker, 1982, 1990a). Across species, therefore, sperm
competition risk should predict investment in sperm production, whereas within species males are predicted
to exhibit prudent sperm allocation, inseminating more sperm when the risk of sperm competition is higher
(Parker, Ball, Stockley, & Gage, 1997). Consistent with the first prediction, investment in sperm production
is greater in species with high levels of sperm competition (e.g., Gage, 1994; Harcourt, Harvey, Larson, &
Short, 1981; Møller, 1988). In nematodes, where sperm size correlates with sperm competitiveness
(LaMunyon & Ward, 1998), species with greater sperm competition risk produce larger, more costly, sperm
(LaMunyon & Ward, 1999). Moreover, experimental exposure to higher levels of sperm competition leads to
Human sperm competition—p. 3
rapid evolution of increased testis size in yellow dung flies (Scathophaga stercoraria) (Hosken & Ward,
2001) and experimental removal of sperm competition in fruit flies results in the evolution of lower
investment in sperm production (Pitnick, Miller, Reagan, & Holland, 2001).
Within-species, individual males in many species are capable of prudent sperm allocation (for
reviews, see delBarco-Trillo, 2011; Kelly & Jennions, 2011) and adjust the number of sperm they deliver
during each insemination in response to (often indirect) cues of sperm competition risk. These cues can be
any auditory, chemosensory, tactile, or visual stimuli that predict whether a female’s reproductive tract (in
internal fertilizers) or the spawning area (in external fertilizers) contains or will soon contain sperm from
rival males. Prudent sperm allocation has been demonstrated in many taxa, but perhaps rats (Rattus
norvegicus) are of most relevance to humans. Male rats adjust the number of sperm they inseminate
depending on the amount of time they have spent “guarding” a particular female prior to copulation (Bellis,
Baker, & Gage, 1990) and when mating in the presence of a rival male (Pound & Gage, 2004). In another
mammalian example, male voles (Microtus pennsylvanicus) deliver more sperm when exposed to the odor of
another male (delBarco-Trillo, 2004).
For males, lack of success in sperm competition means loss of fertilization opportunities. However,
in species with substantial male post-copulatory parental investment, males also risk cuckoldry — the
unwitting investment of resources into genetically-unrelated offspring — and the associated loss of the time,
effort, and resources spent attracting a partner (Trivers, 1972). Both males and females of socially
monogamous species pursue extra-pair copulations, and female sexual infidelity creates the primary context
for sperm competition (Birkhead & Møller, 1992; Smith, 1984). Consequently, selection pressures associated
with sperm competition can also generate adaptations that function not only to maximize sperm competition
success but also to reduce the risk of cuckoldry.
Has Sperm Competition Been an Adaptive Problem for Humans?
Smith (1984) argued that facultative polyandry (i.e., female infidelity) was the most common
ancestral context for sperm competition in humans. Other contexts in which sperm competition might have
occurred include consensual communal sex, courtship, rape, and prostitution, but Smith (1984) argued that
Human sperm competition—p. 4
these contexts may not have occurred with sufficient frequency over human evolution to provide selection
pressures for adaptations to sperm competition comparable to female infidelity.
Male anatomy and physiology provide evidence of an evolutionary history of sperm competition.
Across primate species, relative testis size and the number of sperm per ejaculate correlate positively with the
degree of polyandry, which determines sperm competition risk (Harcourt et al., 1981; Harcourt, Purvis, &
Liles, 1995; Short, 1979). As a proportion of male body mass, human testes are larger than those in
monandrous species such as the gorilla and orangutan but smaller than testes in the highly polyandrous
chimpanzee (Harcourt et al, 1995). Similarly, human ejaculates contain an intermediate number of sperm.
Smith (1984) argued that these traits indicate polyandry, and therefore that sperm competition was an
important selection pressure during human evolution.
Do Women Generate Sperm Competition?
Evolutionary analyses of human sexual psychology have emphasized the benefits to men of short-
term mating and sexual promiscuity (Buss & Schmitt, 1993; Symons, 1979). However, for men to pursue
short-term sexual strategies and engage in consensual sex with multiple females, there must be women who
mate non-monogamously (Greiling & Buss, 2000). Ancestral women may have benefited from facultative
polyandry in several ways (reviewed in Greiling & Buss, 2000). First, they may acquire resources, either in
direct exchange for sex (Symons, 1979) or by creating paternity confusion as a means to elicit investment
(Hrdy, 1981). Second, women may secure genetic benefits for their offspring by copulating opportunistically
outside their pair bond with men of superior genetic quality (Smith, 1984; Symons, 1979). Jennions and
Petrie (2000) provide a review of the genetic benefits of multiple mating to females in non-human species.
Multiple mating by women is a prerequisite for sperm competition to occur, but not all patterns of
polyandry will generate post-copulatory competition between men: Women must copulate with two or more
men within a sufficiently short time period such that there is overlap in the competitive lifespans of the rival
ejaculates. Several studies have shown that the length of this competitive window is five days (Wilcox,
Dunson, Weinberg, Trussell, & Baird, 2001; Wilcox, Weinberg, & Baird, 1998). Using an estimate of five
days, Baker and Bellis (1995) documented that 17.5% of British women self-reported “double-mating” in
Human sperm competition—p. 5
such a way as to generate sperm competition (in the absence of barrier contraception) at some point during
the first 50 copulations in their lifetimes.
Large-scale studies of sexual behavior have not collected data on the frequency with which women
double-mate specifically, but many have recorded how often they engage in concurrent sexual relationships,
more generally. Laumann, Gagnon, Michael, and Michaels (1994), for example, found that 83% of
respondents reporting five or more sexual partners in the past year also reported that at least two of these
relationships were concurrent. Moreover, a major study of sexual behavior in Britain—the National Survey
of Sexual Attitudes and Lifestyles conducted between 1999 and 2001 (Johnson et al., 2001)—revealed that
9% of women overall, and 15% of those aged 16-24 years, reported having had concurrent sexual
relationships with men during the preceding year. Not all concurrent sexual relationships involve double-
matings, but it is likely that many do.
Based on survey findings that women report more frequent double-matings when conception rate is
highest, Bellis and Baker (1990) argued that women “schedule” their copulations in a way that actively
promotes sperm competition and thus encourages fertilization by the most competitive sperm. Bellis and
Baker argued that this finding cannot be attributed to men’s preferences for copulation with women at peak
fertility, because it arose from increases in the frequency of extra-pair copulations, whereas a general male
preference for copulation at times of high conception risk would be expected to also increase the frequency
of in-pair copulations during this time. Bellis and Baker may have been too quick to dismiss the possibility
that men prefer to copulate with a woman during peak conception risk, however. If women pursue extra-pair
copulations to secure genetic benefits from extra-pair partners (e.g., Gangestad & Simpson, 2000), then while
pursuing extra-pair partners they should simultaneously avoid in-pair copulations (Gallup, Burch, &
Mitchell, 2006). Consequently, the absence of a spike of in-pair copulations at times of high conception risk
may not reflect the motivation of the in-pair male.
Polyandrous Sex in Women’s Fantasies
Sexual fantasy may provide insight into the evolved psychological mechanisms that motivate sexual
behavior (Ellis & Symons, 1990; Symons, 1979). A large empirical literature has addressed sex differences
Human sperm competition—p. 6
in sexual fantasy (reviewed in Leitenberg & Henning, 1995), and much of this work has been conducted
from an evolutionarily-informed perspective (e.g., Ellis & Symons, 1990; Wilson, 1987). Given the
asymmetric costs of sexual reproduction, female reproduction is limited by the ability to bear and rear
offspring, whereas male reproduction is limited by sexual access to females (Trivers, 1972). Consequently, it
has been hypothesized that men more than women will have sexual fantasies that involve multiple,
anonymous sexual partners who do not require an investment of time, energy, or resources prior to granting
sexual access (Ellis & Symons, 1990), and empirical investigations have confirmed this hypothesis. Indeed,
one of the largest sex differences occurs for fantasies about sex with two or more members of the opposite
sex concurrently: men report this fantasy much more than do women (Leitenberg & Henning, 1995).
Tests of the hypothesis that men more than women fantasize about concurrent sex with two or more
partners have, nevertheless, provided data on women’s polyandrous sexual fantasies. Although this work
indicates that men are more likely than women to report fantasies of concurrent sex with multiple partners,
polyandrous sex is something about which women fantasize. Large-scale surveys indicate that a significant
minority of women report fantasies of polyandrous sex, imagining themselves as a woman having sex with
two or more men concurrently: 18% in the US (Hunt, 1974) and 15% in the UK (Wilson, 1987). Similarly,
smaller scale studies found that 15% to 41% of women report sexual fantasies involving more than one man
(Arndt, Foehl, & Good, 1985; Davidson, 1985; Pelletier and Herold, 1988; Person, Terestman, Myers,
Goldberg, and Salvadori, 1989; Sue, 1979). Rokach (1990) reported that, although sex with more than one
partner accounted for 14% of the sexual fantasies reported by a sample of 44 men, it accounted for 10% of
the fantasies reported by a sample of 54 women. Price and Miller (1984) report that polyandrous sex was
among the 10 most frequently reported fantasies in a small sample of college women.
If sexual fantasy reflects sexual desires and preferences that might sometimes be acted upon, then
previous research indicates that polyandrous sex is not an unlikely occurrence, given the well-established
finding that women more than men are the “gatekeepers” of sexual access—including when, where, and the
conditions under which sex occurs (Symons, 1979). If, as Symons (1979) has argued, sexual fantasy provides
a window through which to view evolved human psychology, then human female sexual psychology may
Human sperm competition—p. 7
include design features dedicated to the pursuit of polyandrous sex, with the consequence of promoting
sperm competition.
Men’s Adaptations to Sperm Competition
Sperm competition can take one of two forms: contest competition, in which rival ejaculates actively
interfere with each other’s ability to fertilize an ovum or ova, and scramble competition, which is akin to a
race or lottery. In mammals, there are theoretical reasons to believe that most sperm competition takes the
form of a scramble, and modeling studies and experimental findings support this view (Gomendio et al.,
1998). Male adaptations to scramble competition are likely to take the form of physiological, anatomical, and
behavioral features that increase the male’s chances of fertilizing ova in a competitive environment in which
the ability to deliver large numbers of sperm is a crucial determinant of success.
Is There Evidence of Prudent Sperm Allocation by Men?
Sperm competition theory predicts that investment in sperm production will vary with sperm
competition risk across species (Parker, 1982, 1990a, 1990b), and adaptations to high levels of sperm
competition may include anatomical, physiological, and behavioral traits that deliver large numbers of
competitive sperm. Sperm competition theory also predicts that, when sperm competition risk varies between
matings, males will allocate resources prudently, adjusting the number of sperm inseminated at each
copulation. Prudent sperm allocation occurs even in species in which overall levels of sperm competition are
not especially high—but sufficiently variable to select for the evolution of such facultative mechanisms.
Ejaculates appear costly to produce for human males. Frequent ejaculation, especially occurring
more frequently than every other day, results in decreased sperm counts (Tyler, Crockett, & Driscoll, 1982),
suggesting limits to sperm production. Men hardly seem limited by sperm production, however, given the
apparent wastage of sperm that occurs in humans. Sperm are continuously lost in the urine, and entire
ejaculates are lost during nocturnal emissions and masturbation, although masturbatory ejaculates contain
fewer sperm than do copulatory ejaculates (Zavos & Goodpasture, 1989). Baker and Bellis (1993a) suggest,
however, that these lost sperm are older and less competitive, and that non-copulatory ejaculations increase
the number of younger, highly-competitive sperm ejaculated at the next copulation. Given the cost of
Human sperm competition—p. 8
ejaculates, human males may have evolved the ability to modulate ejaculated sperm numbers depending on
sperm competition risk at copulation. The number of sperm in a man’s ejaculate varies considerably between
ejaculates (e.g., Mallidis, Howard, & Baker, 1991). Although clinicians treat this intra-individual variability
as “noise” when determining the “true” values of a man’s semen parameters, sperm competition theory
predicts that some of this variability might reflect prudent sperm allocation in response to the temporal risk
of sperm competition.
Evidence indicating that men adjust ejaculate composition in response to adaptively-relevant aspects
of the sociosexual environment was first reported in a series of articles by Baker and Bellis. In the first report
for a sample of copulatory ejaculates (Baker & Bellis, 1989a), one from each of 10 couples, there was a
negative rank-order correlation (rs = -0.95) between “objective” sperm competition risk—the percentage of
time the couple had spent together since their last copulation—and the estimated number of sperm in the
ejaculate. No such relationship was identified for masturbatory ejaculates. Baker and Bellis (1989a) argued
that objective sperm competition risk indexes risk of female double-mating and, therefore, that these findings
are consistent with the hypothesis that there is a positive association between the number of sperm
inseminated and the risk of sperm competition.
What Baker and Bellis (1989a) reported, however, was not direct evidence of prudent sperm
allocation by men from one copulation to the next in response to variation in sperm competition risk. Their
study was based on a single ejaculate per couple, with the finding that men who had spent the most time
apart from their partners since their last copulation produced copulatory ejaculates containing the most
sperm. It could be that men who tend to produce larger ejaculates also tend to spend a greater proportion of
their time between copulations apart from their partners.
In a follow-up to this initial report, Baker and Bellis (1993a) addressed the aforementioned problems
by including in their analyses multiple ejaculates from each participating couple. For a sample of 40
specimens produced by five men, non-parametric analyses indicated a negative association between the
number of sperm inseminated and objective sperm competition risk. Although Baker and Bellis argued that
these results demonstrated prudent sperm allocation in response to a cue of increased sperm competition risk,
Human sperm competition—p. 9
several alternative interpretations are possible. For example, changes in ejaculate composition may depend
on changes in female sexual behavior induced by partner absence providing different stimuli prior to, and at
the time of, ejaculation. This may be significant because quality differences between ejaculates obtained via
uninterrupted coitus and those obtained via coitus interruptus (Zavos et al., 1994) indicate that sexual stimuli
present at the moment of ejaculation may be important determinants of sperm numbers.
Physiological Mechanisms Associated with Prudent Sperm Allocation
Although the findings of Baker and Bellis (1993a, 1995) suggest that men are capable of prudent
sperm allocation, the physiological mechanisms involved in the adaptive regulation of ejaculate composition
are poorly understood. However, factors known to affect semen parameters may provide some clues. In
longitudinal studies, individual men exhibit substantial variability in ejaculate parameters such as volume
and sperm concentration (e.g., Mallidis et al., 1991), in part because both parameters are affected by the
duration of ejaculatory abstinence (e.g., Blackwell & Zaneveld, 1992). There also is evidence that the context
in which an ejaculate is produced is important. For example, ejaculates produced during copulation are
superior to those produced via masturbation (Zavos, 1985), having greater volumes, greater sperm numbers,
and higher grades of sperm motility (Sofikitis & Miyagawa, 1993; Zavos & Goodpasture, 1989).
The mechanisms that cause copulatory ejaculates to contain more sperm than masturbatory
ejaculates are not fully understood, but the greater intensity and duration of pre-coital stimulation increases
the number of motile sperm with normal morphology in copulatory ejaculates (Zavos, 1988). There is
contradictory evidence on whether sexually stimulating visual material can improve semen parameters for
masturbatory ejaculates (Handelsman et al, 2013; Yamamoto, Sofikitis, Mio, & Miyagawa, 2000; van Roijen
et al., 1996), but there is a positive association between the duration of pre-ejaculatory sexual arousal and
sperm concentration for masturbatory ejaculates when multiple specimens are collected from individual men
(Pound et al., 2002) although this is not apparent in between-male studies (Elzanaty, 2008; Handelsman et al,
Relationships between semen quality and the duration of sexual arousal also have been documented
in domesticated farm animals when specimens are collected for artificial insemination (for review, see
Human sperm competition—p. 10
Pound, 2002). Given the relationship between duration of pre-ejaculatory sexual arousal and variation in
ejaculate sperm counts across species, males may achieve adaptive changes in ejaculate composition through
behavioral changes that prolong arousal prior to ejaculation (Pham, Shackelford, Welling et al., 2013; Pound,
Psychological Mechanisms Associated with Prudent Sperm Allocation
Males in many non-human species can adjust the number of sperm they inseminate in response to
sperm competition risk. Baker and Bellis (1993a) suggest that human males share this capacity. Shackelford
et al. (2002) investigated men’s psychological responses to sperm competition risk, hypothesizing that
psychological mechanisms evolved to motivate male behavior to increase the probability of success in sperm
competition. For men, the absence of a regular partner (e.g., objective sperm competition risk) may provide
the key information processed by male psychological mechanisms and which subsequently motivates a man
to inseminate his partner as soon as possible, to combat the increased risk of sperm competition (Shackelford
el al. 2002). Nevertheless, total time since last copulation might have important effects on a man’s sexual
behavior, perhaps increasing feelings of sexual frustration whether that time has been spent apart or together.
Shackelford et al. (2002) assessed the relationships between male sexual psychology and behaviors
predicted to be linked to objective sperm competition risk, while controlling for the total time since a
couple’s last copulation. Shackelford et al. suggested that men might respond differently to cues of sperm
competition risk depending on the nature of their relationship with a particular woman. Satisfaction with, and
investment in, a relationship are likely to be linked, with the result that a man who is more satisfied may have
more to lose in the event of cuckoldry. For this reason, when examining the responses of men to increases in
the proportion of time spent apart from their partner since their last copulation, Shackelford et al. controlled
for the extent to which the participants were satisfied with their relationships.
Shackelford et al. (2002, 2007) found that men who spend a greater proportion of time apart from
their partner since their last copulation (and, therefore, incurred greater sperm competition risk) rate her as
more attractive, report that other men find her more attractive, report greater interest in copulating with her,
and indicate that she is more interested in copulating with him. Pham and Shackelford (2013a) reported a
Human sperm competition—p. 11
similar association between men’s partner-directed copulatory interest and the proportion of time apart from
a partner since the couple’s last copulation, but only among men who perceive that their partner spends more
time with other men. Starratt, McKibbin, and Shackelford (2013) documented that men who are
experimentally primed with thoughts of partner infidelity report greater partner-directed copulatory interest.
Taken together, these findings suggest that men are sensitive to cues to sperm competition risk and adjust
accordingly their partner-directed copulatory interest.
The cuckoldry risk hypothesis predicts that men at greater sperm competition risk are more likely to
sexually coerce their partner (Goetz & Shackelford, 2006; Lalumiere et al., 2005; Thornhill & Thornhill,
1992; Wilson & Daly, 1992). In socially monogamous avian species, in-pair forced matings often follow
immediately a female’s extra-pair copulation (Bailey, Seymour, & Stewart, 1978; McKinney & Stolen,
1982). In humans, previous research documents a positive relationship between men’s partner-directed
sexual coercion and their partner’s infidelity risk. Men who rape their female partners often accuse their
partner of infidelity prior to the act (Finkelhor & Yllo, 1985; Russell, 1982). Female victims of intimate
partner violence rate their abusers as more sexually jealous when the abuse also includes rape (Frieze 1983;
Gage & Hutchinson 2006). Men who report sexually coercing their partner are more likely to report
perceiving their partners as being unfaithful, and women who report being sexually coerced are more likely
to report being unfaithful (Goetz & Shackelford, 2006). Even after controlling for men’s dominant
personalities and controlling behaviors, men’s sexual coercion tactics are positively correlated with their
perception or knowledge of partner infidelity (Goetz & Shackelford, 2009). Men’s sexual coercion is
positively correlated with the occurrence of partner directed insults involving accusations of their partner’s
infidelity (Starratt, Goetz, Shackelford, & Stewart-Williams, 2008). The proportion of time spent apart from
a partner since the couple’s last copulation predicts men’s partner-directed sexual coercion, but only among
men who perceive a greater risk of partner infidelity (McKibbin, Starratt, S
Men’s partner-directed copulatory interest in response to sperm competition risk may manifest as
frequent copulations. In socially monogamous avian species, males use frequent copulations to increase rates
of sperm transfer into the female reproductive tract, thereby increasing their chances of success in sperm
Human sperm competition—p. 12
competition (Birkhead, Atkin, & Moller, 1987; McKinney, Cheng, & Bruggers, 1984). Similarly in humans,
men at greater sperm competition risk (Kaighobadi & Shackelford, 2008), and who perform more frequent
behaviors to minimize sperm competition risk (Shackelford, Goetz, Guta, & Schmitt, 2006), perform more
frequent copulations with their partner.
The Influence of Sperm Competition on Men’s Reproductive Anatomy and Copulatory Behavior
Human testis size suggests an evolutionary history of intermediate levels of sperm competition
(Smith, 1984), and other aspects of male reproductive anatomy may provide insights as well. Human males
have a penis that is longer than in any other ape (Short, 1979), but in relation to body weight it is no longer
than the chimpanzee penis (Gomendio et al., 1998). Several arguments have been offered to explain how the
length and shape of the human penis might reflect adaptation to sperm competition. A long penis may be
advantageous in the context of scramble competition, which combines elements of a race and a lottery,
because being able to place an ejaculate closer to the cervix may increase the chance of fertilization (Baker &
Bellis, 1995; Short, 1979; Smith, 1984).
Gallup et al. (2003) empirically tested Baker and Bellis’s (1995) hypothesis that the human penis
may be designed to displace semen deposited by other men in the reproductive tract of a woman. Gallup et
al. found that artificial phalluses with a glans and a coronal ridge approximating a human penis displaced
more simulated semen than did a phallus lacking these features. They suggested that when the penis is
inserted into the vagina, space around the frenulum allows semen to flow back under the penis and collect
behind the coronal ridge, facilitating its extraction. Displacement of simulated semen only occurred,
however, when a phallus was inserted at least 75% of its length into the artificial vagina, suggesting that
successful displacement of rival semen may require specific copulatory behaviors. Following allegations of
female infidelity or separation from their partners (contexts in which the likelihood of rival semen being
present in the reproductive tract is relatively greater), both sexes report that men thrusted deeper and more
quickly at the couple’s next copulation (Gallup et al., 2003). Such copulatory behaviors may increase semen
Human sperm competition—p. 13
In an independent test of the hypothesis that successfully displacing rival semen may require specific
copulatory behaviors, Goetz et al. (2005) investigated whether and how men under a high risk of sperm
competition might attempt to “correct” a female partner’s sexual infidelity. Men in committed, sexual
relationships reported their performance of specific copulatory behaviors arguably designed to displace the
semen of rival men. As hypothesized, men mated to women who place them at a high recurrent risk of sperm
competition were more likely to perform semen-displacing behaviors such as an increase in number of
thrusts, deepest thrust, average depth of thrusts, and duration of sexual intercourse.
Sperm competition theory has informed research on other male sexual behaviors, such as oral sex.
Evidence suggests that oral sex was a recurrent feature across human evolution, occurring in most cultures
and in several other species (see Pham & Shackelford, 2013a; 2013b). It is frequently depicted in modern
pornography (Mehta & Plaza, 1997), and appears in Paleolithic cave paintings (Angulo & Garcia, 2005).
However, whether oral sex is an adaptation remains unclear. Previous researchers have suggested that men
perform oral sex to (i) assess a partner’s reproductive health (Baker, 1996), (ii) detect rival male semen near
or in the vagina (Baker, 1996; Kohl & Francoeur, 1995; Pham & Shackelford, 2013b; Thornhill, 2006), (iii)
manipulate female mechanisms that may bias the outcome of sperm competition (Pham, Shackelford, Sela, &
Welling, 2013), (iv) sexually satisfy the woman, thereby reducing the likelihood of her mating with another
man (Pham & Shackelford, 2013c), or (v) increase male sexual arousal and consequent semen quality
(Pham, Shackelford, Welling et al., 2013). Additionally, oral sex may facilitate “fertility-detection”: Previous
research suggests that men use olfactory cues to detect women’s fertility status (reviewed in Haselton &
Gildersleeve, 2012), and men report vaginal fluid as more pleasant smelling when the fluid is produced at
high fertility versus low fertility (Cerda-Molina, Hernández-López, Claudio, Chavira-Ramírez, &
Mondragón-Ceballos, 2013).
The Influence of Sperm Competition on Men’s Mate Selection
To minimize sperm competition risk, men may have evolved mate preferences that function to select
as short-term sexual partners women who present a low risk of sperm competition (Shackelford, Goetz,
LaMunyon, Quintus, & Weekes-Shackelford, 2004). Men’s risk of sperm competition increases with a
Human sperm competition—p. 14
prospective short-term partner’s involvement in one or more relationships. Women who are not in a long-
term relationship and who do not have casual sexual partners, for example, present a low risk of sperm
competition. Consequently, such women may be perceived as desirable short-term sexual partners. Women
who are not in a long-term relationship but who engage in short-term matings may present a moderate risk of
sperm competition, because women who engage in short-term matings probably do not experience difficulty
obtaining willing sexual partners. Women in a long-term relationship may present the highest risk of sperm
competition. The primary partner’s frequent inseminations might therefore make women in a long-term
relationship least attractive as short-term sexual partners.
As predicted, Shackelford et al. (2004) found that men’s sexual arousal and reported likelihood of
pursuing a short-term sexual relationship was lowest when imagining that the potential short-term partner is
married, next lowest when imagining that she is not married but involved in casual sexual relationships, and
highest when imagining that she is not married and not involved in any casual sexual relationships. These
results suggest that, when selecting short-term sexual partners, men do so in part to avoid sperm competition.
The Influence of Sperm Competition on Men’s Sexual Arousal and Sexual Fantasies
Men’s sexual fantasies often involve multiple, anonymous partners (Ellis & Symons, 1990; Symons,
1979). These fantasies tend to be embodied in pornography produced for men, incorporating multiple, low-
investment matings with highly fertile women (Malamuth, 1996). However, much pornography contains
visual cues of sperm competition risk. Pound (2002) analyzed pornographic images on internet sites and
showed that depictions of sexual activity involving a woman and multiple men are more prevalent than those
involving a man and multiple women. Similar results were found in both an online survey of self-reported
preferences and in a preference study that unobtrusively examined image selection behavior. McKibbin,
Pham, and Shackelford (2013) reported that the number of images on adult DVD covers depicting multi-
male interactions with one woman predicted DVD sales rank better than the number depicting multi-female
interactions with one man. Finally, anecdotal reports from the “swinging” or “partner-swapping” community
suggest that men experience increased arousal in response to the sight of their partner interacting sexually
with other men (Talese, 1981; Gould, 1999).
Human sperm competition—p. 15
Pound (2002) argued that males should find mate sharing to be aversive because of the potential loss
of paternity to a competitor’s ejaculate. However, sexual arousal in response to cues of sperm competition
risk may be an adaptive paternity assurance mechanism because it may motivate earlier or more frequent
copulation. Moreover, increased arousal in response to cues of sperm competition risk may play a proximate
role in ejaculate adjustment mechanisms (Pound, 2002). Consistent with this hypothesis, Kilgallon and
Simmons (2005) reported that men who view pornography depicting two men interacting with one woman
(cueing sperm competition), relative to men who view pornography depicting three women (cueing absence
of sperm competition) ejaculate a higher percentage of motile sperm. Thus, although men should avoid
sexual instances with risk of sperm competition, they may actually prefer the arousal associated with the
same instances when experiencing the fantasies involved in viewing pornography.
Women’s Adaptations to Sperm Competition
If sperm competition was a recurrent feature of human evolutionary history, we would expect
women to possess adaptations that allow them to influence its outcome. Specifically, women may have
evolved mechanisms to determine which men achieve paternity; i.e. adaptations for both pre-copulatory and
post-copulatory choice. In this context, “post-copulatory female choice” refers to female influence that
follows initiation of copulation (Eberhard, 1996).
Precopulatory Female Choice: Promoting and Avoiding Sperm Competition
Bellis and Baker (1990) documented that women in committed relationships are more likely to
double-mate when the probability of conception is highest. This observation suggests that women have
psychological adaptations that actively promote sperm competition, with the result that their ova will be
fertilized by the most competitive sperm. Under certain conditions, however, it may be advantageous for
women to avoid sperm competition. For example, Gangestad, Thornhill, and Garver (2002) documented that,
as women enter the high conception phase of their menstrual cycle, they are sexually attracted to, and
fantasize about, men other than their regular partner. Additionally, Gallup et al. (2006) documented that
women delay copulations with their regular partner following their extra-pair copulation. Favoring
copulation with an extra-pair partner to the exclusion of a primary partner reduces competition for the extra-
Human sperm competition—p. 16
pair sperm when conception is likely. Thus, women’s sexual attraction to and fantasy about men other than
their regular partner may qualify as a precopulatory female adaptation. But because men have been selected
to be sensitive to their partner’s increased interest in extra-pair copulation near ovulation (Gangestad et al.,
2002), women may have post-copulatory adaptations that favor sperm from one man over another.
Postcopulatory Female Choice: A Function for Female Coital Orgasm?
One such female post-copulatory adaptation may be orgasm. The human clitoris and penis develop
from the same embryonic tissue, prompting Symons (1979) and Gould (1987) to argue that female orgasm is
a by-product of male orgasm. Others have hypothesized, however, that female orgasm may be an adaptation
(e.g., Alexander, 1979; Baker & Bellis, 1993b; Hrdy, 1981; Smith, 1984). For example, female coital orgasm
may afford selective sperm retention (Baker & Bellis, 1993b; Smith, 1984). Female orgasm causes the cervix
to dip into the seminal pool deposited by the male at the upper end of the vagina and this may result in the
retention of a greater number of sperm (Baker & Bellis, 1993b; 1995). Baker and Bellis (1993b) and Smith
(1984) contend that by strategic timing of orgasm, women may select preferentially the sperm of extra-pair
partners, who are likely to be of higher genetic quality than in-pair partners.
Baker and Bellis (1993b) estimated the number of sperm in ejaculates collected by condoms during
copulation and by vaginal “flowbacks” (i.e., ejected seminal and vaginal fluids) when condoms were not
used, and documented that women influence the number of sperm retained in their reproductive tract through
the presence and timing of coital orgasm. Coital orgasms that occurred between one minute before and 45
minutes after their partner ejaculated were linked with greater sperm retention than coital orgasms that
occurred earlier than one minute before their partner ejaculated. Baker and Bellis also provided evidence that
women with a regular partner and one or more extra-pair partners had fewer high sperm retention orgasms
with their regular partner and more high sperm retention orgasms with their extra-pair partners.
Missing from Baker and Bellis’s (1993b) study, however, was the explicit demonstration of higher
sperm retention associated with partners of higher genetic quality. Thornhill, Gangestad, and Comer (1995)
established this link and documented that women mated to men with low fluctuating asymmetry (indicating
relatively high genetic quality) reported more copulatory orgasms than did women mated to men with high
Human sperm competition—p. 17
fluctuating asymmetry (indicating relatively low genetic quality). Women mated to men with low fluctuating
asymmetry did not simply have more orgasms, but specifically reported more copulatory orgasms likely to
result in greater sperm retention. Another indicator of high genetic quality and related to fluctuating
asymmetry is physical attractiveness. Shackelford et al. (2000) found that women mated to more physically
attractive men were more likely to report having a copulatory orgasm at their most recent copulation than
were women mated to less attractive men.
Although female orgasm as an adaptation for post-copulatory female choice between rival ejaculates
is plausible, the functional significance of the female orgasm is still hypothetical (Pound & Daly, 2000).
Baker and Bellis’s (1995) evidence that women retain more sperm if they experience orgasm between one
minute before and 45 minutes after their partner ejaculates than at other times (or not at all) assumes that the
number of sperm ejaculated is identical regardless of whether or when the woman has an orgasm. This
assumption may be false, however, because the duration of pre-ejaculatory sexual arousal is positively
associated with the number of sperm ejaculated (Pound, 2002; Zavos, 1988).
Men’s concerns with whether their partner achieves orgasm suggests that female orgasm may be an
adaptation (see Thornhill et al., 1995). Consistent with this, McKibbin, Bates, Shackelford, Hafen, and
LaMunyon (2010) found that sperm competition risk moderates the association between men’s relationship
investment and their interest in their partner’s copulatory orgasm.
Women may pretend orgasm to appease their partner, suggesting the existence of female counter-
adaptations to men’s interest in their orgasm (Thornhill et al., 1995). Women may pretend orgasm to signal
their relationship satisfaction to their partner, thereby minimizing the likelihood of their partner’s infidelity
or relationship defection (Muehlenhard & Shippee, 2010). Women who perceive a greater risk of partner
infidelity are more likely to pretend orgasm (Kaighobadi, Shackelford, & Weekes-Shackelford, 2012). A
tendency to pretend orgasm with a desired partner would seem to be inconsistent with the hypothesized
sperm retention function of genuine orgasm. If female orgasm functioned to retain preferentially sperm from
men of high genetic quality, we might predict that women would pretend orgasm more frequently with men
of low genetic quality to “avoid” retaining his sperm from genuine orgasm, while simultaneously satisfying
Human sperm competition—p. 18
him, possibly to continue securing non-genetic benefits. To reconcile these differences, future research
should investigate whether the frequency with which women pretend orgasm correlate with measures of their
partner’s “good genes” (e.g., masculinity, muscularity, fluctuating asymmetry; Frederick & Haselton, 2007).
Direct evidence of preferential use of sperm by females is absent in humans, particularly because it is
methodologically difficult to study female influence of sperm behavior within the female reproductive tract.
Even in non-human animals, evidence of female manipulation of sperm is scarce and circumstantial.
Although there have been observations of females discarding stored sperm when mating with a new partner
(Davies, 1985; Etman & Hooper, 1979), most studies infer female manipulation based on patterns of sperm
storage or offspring paternity (see Eberhard, 1996). Because much of post-copulatory competition occurs in
the reproductive tract, it is likely that human females have evolved adaptations in response to sperm
The bulk this chapter focuses on men’s adaptations, which reflects the historical and current state of
research and theory. Intersexual conflict between ancestral males and females, however, produces a co-
evolutionary arms race between the sexes, in which an advantage gained by one sex selects for counter-
adaptations in the other sex (Rice, 1996, Shackelford & Goetz, 2012). Thus, men’s numerous adaptations to
sperm competition are likely to be met by numerous adaptations in women.
Concluding Remarks
We describe the far-reaching consequences of female infidelity and consequent sperm competition.
First identified in non-human species in the 1970s, and not considered in humans until the late 1980s,
evolutionary-minded researchers are only beginning to uncover its possible role in shaping human anatomy,
physiology, and psychology. Sperm competition may have influenced men’s and women’s reproductive
anatomy and physiology, men’s attraction to and sexual interest in their partners, men’s copulatory
behaviors, men’s short-term mate selection, and men’s sexual arousal and sexual fantasies, so understanding
its role will be challenging but necessary if we are to achieve a comprehensive understanding of human
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... Still, the production of sperm by males is not unlimited. According to Shackelford et al. (2016), sperm are costly to produce, and the frequency of ejaculation as well as the variable circumstances of ejaculation (copulation, masturbation, nocturnal emission) influence both the number and quality of sperm (e.g., motility) contained in the ejaculate. Consequently, a phenomenon called "prudent sperm allocation" has been discovered in number of species, including humans (Shackelford et al. 2016). ...
... According to Shackelford et al. (2016), sperm are costly to produce, and the frequency of ejaculation as well as the variable circumstances of ejaculation (copulation, masturbation, nocturnal emission) influence both the number and quality of sperm (e.g., motility) contained in the ejaculate. Consequently, a phenomenon called "prudent sperm allocation" has been discovered in number of species, including humans (Shackelford et al. 2016). Prudent sperm allocation refers to the (unconscious) ability of males to adjust the number of sperm delivered by the male "during each insemination in response to auditory, chemosensory, tactile, or visual cues of sperm competition (Shackelford et al. 2016: 428). ...
... Sex, as a subject of study, has long been a complex matter. Biological, evolutionary, cultural, political, and psychological influences have consistently contributed to evolving societal views toward those engaging in different forms of sexual practices (Best and Williams, 1997;Shackelford et al., 2015;Lucas and Fox, 2020). For example, sex work has often been a target of stigmatization (Goffman, 1986): sex work was associated with pathologic conditions, drug addiction, and sexually transmitted diseases in the past (Vanwesenbeeck, 2001;Comte, 2010). ...
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Student sex work is a current phenomenon all over the world, increasingly reported by the media in recent years. However, student sex work remains under-researched in Germany and is lacking direct first-hand reports from the people involved. Further, sex work remains stigmatized, and therefore, students practicing it could be at risk of social isolation and emotional or physical danger. Therefore, this study examines students working in the sex industry focusing on their personal experiences and attitudes toward them. An online questionnaire was completed by 4386 students from Berlin universities. Students who identified themselves as sex workers (n = 227) were questioned with respect to their motivations to enter the sex industry, characteristics of their job, feelings after the intercourse, and perceived risks. Student non-sex workers (n = 2998) were questioned regarding knowledge of and attitudes toward student sex workers. Most student sex workers reported that they entered the sex industry due to financial reasons (35.7%). The majority reported offering services involving direct sexual intercourse. Disclosing their job to friends, family, or others was associated with less problems with social isolation and in romantic relationships. With a total of 22.9%, student non-sex workers reported never having heard about students working in the sex industry. The most frequent emotions mentioned by them with regard to student sex workers were compassion and dismay (48.9%). There was no difference in happiness between student sex workers and non-sex working students. Through this research, it becomes evident that there are similarities between the student’s motivations to enter the sex industry, their feelings, and the problems they have to face. Moreover, prejudices still prevail about the life of student sex workers. Increasing understanding of student sex work might help those sex workers to live a less stigmatized life and thereby to make use of support from others. The universities as institutions could form the basis for this, e.g., by openly supporting student sex workers. This could help to encourage the rights of student sex workers and to gain perspective with respect to the sex industry.
... This is of particular importance in species with internal insemination where females mate with multiple males, leading to an evolutionary cross-species association of greater sperm production capacity with increasing sperm competition. Among the great apes, chimpanzee males are well known as having exceptionally large testes for their body size on this account, while human males have moderately large testes and gorilla and orangutan males have relatively small ones since alphas typically monopolize mating (Ramm, et al., 2014;Shackelford, et al., 2016). Females in many species have evolved the capacity for post-copulation sexual selection via cryptic choice of which sets of sperm to allow access to their ova, based on assessments of the mate value of the sperm depositors (Brennan & Prum, 2015;Kekäläinen & Evans, 2018). ...
This essay describes Reproductive Channeling Theory, which proposes that an “ultimate” purpose of the stress response is to promote reproduction of those who are least distressed and hinder that of those who are most distressed. This hypothesis emerges from research showing that traumatic or chronic stress can cause erosion of telomeres and epigenetic changes that lead to ill health and speeded up life history which may mean early death. Status hierarchy positions are set through winner-loser effects plus social selection, with the lowest status individuals tending to be the most distressed and most liable for speeded up life history. The purpose of this speeding up of life history appears to be to (1) beat the environmental odds against reproduction (individual level selection), and also to (2) more rapidly exit the reproductive arena through accelerated aging and social withdrawal, removing apparently poor genes for the current environment and freeing up resources for kin and other group mates (inclusive fitness selection).
... One of the traits desired by men in a long-term mating partner is sexual fidelity, because selecting a faithful partner reduces the probability of extra-pair copulations (EPCs) and increases paternity certainty (Buss 1989;Mogilski et al. 2014). Although women also benefit from faithfulness in a longterm partner, EPCs can be more detrimental to male reproductive success because they could lead to cuckoldry (i.e., investing resources in genetically unrelated offspring; Shackelford et al. 2015). Therefore, the ability of men to accurately assess whether a potential long-term partner is likely to be sexually faithful-even with minimal information about the potential mate-may have been favored by selection. ...
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Previous research indicates that men can accurately assess women’s mating orientation from facial photographs (DeLecce et al. Archives of Sexual Behavior, 43, 319–327, 2014). The current study investigated whether this ability is moderated by men’s own mating orientation. To that end, 89 men completed the Sociosexual Orientation Inventory (SOI)—an assessment of mating orientation—and rated the perceived faithfulness of 55 women (who also completed the SOI) depicted in facial photographs. Although men were indeed accurate in their faithfulness perceptions of the female targets, men’s mating orientation did not moderate the negative association between their faithfulness ratings and the self-reported mating orientation of the female targets. Limitations of the current study and directions for future research are addressed in the discussion.
Compared to the male orgasm, the female orgasm is poorly understood in humans. There is continuing debate about whether female orgasm is an adaptation or if it is a byproduct of the male orgasm. Additionally, even among scholars who agree that the female orgasm does serve an adaptive function, there are multiple proposed hypotheses as to what that function could be and insufficient evidence to differentiate among them. There are also questions concerning whether these hypotheses are mutually exclusive. However, there is one feature related to the female orgasm that is much clearer from the research: male interest in inducing orgasm in their female partners. This chapter will address male interest in inducing female orgasm from the perspective of sperm competition theory. First, the importance of avoiding cuckoldry and the use of tactics to combat sperm competition (including inducing female orgasm) will be discussed. Next, there will be a brief review of hypotheses for the possible adaptive function of female orgasm, with special attention paid to hypotheses related to the quality of the male partner. The chapter will then discuss specific behaviors men use to induce orgasm in their female partners, as well as the relational contexts in which men are more versus less interested in inducing female orgasm. Then, men’s individual differences in terms of traits (e.g., warmth and sense of humor) will be discussed with regard to how they moderate both interest and success in inducing female orgasm. The chapter closes with suggestions for several future directions in this understudied area of research.
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Due to the characteristics of internal fertilization, human fertilization, to a certain extent, is concealed. Meanwhile, adultery existed in early human society. Therefore, males suffer from the uncertainty that their putative children may not be their biological offspring, which is paternal uncertainty. To cope with this uncertainty, males are unconsciously looking for cues that might indicate a genetic similarity between them and their offspring. Among these cues, the facial resemblance is one of the indirect ways that males could infer if there is a biological connection in terms of fatherhood. Three methods are there to measure facial resemblance, including morph software synthesis, third–party rating, and self-rating. Previous studies have shown that the more similar the facial characteristics between a father and his offspring are, the more paternal resources the father would be willing to invest during parenthood. The less anxious and healthier a father is during parenthood, the higher sexual aversion a father would experience in the relationship with their children. The present research suggested three directions for future studies. First, it is valuable to look at the cultural differences in the impacts of father-child facial resemblance. Second, in order to enhance its internal and external validity, future work can adopt new methods to assess the father-child facial resemblance. Last, it would be an exciting direction to explore the relationship between the subjective assessments and the objective assessments of father-child facial resemblance.
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The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple matin!: can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use precopulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings of the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.
We carried out a prospective study of 221 healthy women who were attempting pregnancy. During the study, women collected daily urine samples and kept daily records of intercourse. Ovulation and early pregnancy losses were later identified by immunoassays of urinary human chorionic gonadotrophin and steroid metabolites. We have used these data to examine whether the risk of early pregnancy loss was higher with post-ovulatory ageing of the oocyte. 192 pregnancies were ranked by the probability that the oocyte might have aged before fertilization. There was a statistically significant increase in the risk of early loss as the likelihood of oocyte ageing increased (P < 0. 05). No similar risk was observed for clinical miscarriages. Post-ovulatory ageing of the oocyte prior to fertilization may cause early pregnancy failure in humans as it does in several other mammalian species.
Sexual conflict-what happens when the reproductive interests of males and females diverge-occurs in all sexually reproducing species, including humans. This book is the first volume to assemble the latest theoretical and empirical work on sexual conflict in humans from the leading scholars in the fields of evolutionary psychology and anthropology. Following an introductory section that outlines theory and research on sexual conflict in humans and non-humans, ensuing sections discuss human sexual conflict and its manifestations before and during mating. Articles in these sections address a range of factors topics and factors, including: sexual coercion, jealousy, and partner violence and killing; the ovulatory cycle, female orgasm, and sperm competition; chemical warfare between ejaculates and female reproductive tracts. Articles in the next section address issues of sexual conflict after the birth of a child. These articles address sexual conflict as a function of the local sex ratio, men's functional (if unconscious) concern with paternal resemblance to a child, men's reluctance to pay child support, and mate expulsion as a tactic to end a relationship. The book's concluding section includes an article that considers the impact of sexual conflict on a grander scale, notably on cultural, political, and religious systems. Addressing sexual conflict at its molecular and macroscopic levels, it is a resource for the study of intersexual behavior.
It is suggested that sperm competition (competition between the sperm from two or more males over the fertilization of ova) may account for the fact that sperm are so small and so numerous. In the entire absence of sperm competition, selection may favour an increase in sperm size so that the sperm contributes nutriment to the subsequent viability and success of the zygote. However, an extremely low incidence of sperm competition is adequate to prevent sperm size increasing. Vertebrate sperm should remain at minimal size provided that double matings (one female mated by two males) occur more often than about 4 times the ratio of sperm size:ovum size. The classical theory that sperm are small simply because of the difficulties of ensuring that ova do get fertilized may also explain sperm size, and both effects (sperm competition and ensuring fertilization) are likely to contribute to the stability of anisogamy. Large numbers of sperm can be produced because sperm are tiny and the optimal allocation of reproductive reserves to ejaculates is not trivially small even when double matings are rather rare. It is suggested that of its total mating effort, a male vertebrate should spend a fraction on sperm that is roughly equivalent to a quarter of the probability of double mating. © 2006 Springer Science+Business Media, Inc. All rights reserved.