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The human penis as a semen displacement device

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Inanimate models were used to assess the possibility that certain features of the human penis evolved to displace semen left by other males in the female reproductive tract. Displacement of artificial semen in simulated vaginas varied as a function of glans/coronal ridge morphology, semen viscosity, and depth of thrusting. Results obtained by modifying an artificial penis suggest that the coronal ridge is an important morphological feature mediating semen displacement. Consistent with the view of the human penis as a semen displacement device, two surveys of college students showed that sexual intercourse often involved deeper and more vigorous penile thrusting following periods of separation or in response to allegations of female infidelity.
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The human penis as a semen displacement device
Gordon G. Gallup Jr.*, Rebecca L. Burch
1
, Mary L. Zappieri,
Rizwan A. Parvez, Malinda L. Stockwell, Jennifer A. Davis
Department of Psychology, State University of New York at Albany,
1400 Washington Avenue Albany, Albany, NY 12222, USA
Received 30 August 2002; received in revised form 25 March 2003
Abstract
Inanimate models were used to assess the possibility that certain features of the human penis
evolved to displace semen left by other males in the female reproductive tract. Displacement of
artificial semen in simulated vaginas varied as a function of glans/coronal ridge morphology, semen
viscosity, and depth of thrusting. Results obtained by modifying an artificial penis suggest that the
coronal ridge is an important morphological feature mediating semen displacement. Consistent with
the view of the human penis as a semen displacement device, two surveys of college students
showed that sexual intercourse often involved deeper and more vigorous penile thrusting following
periods of separation or in response to allegations of female infidelity.
D2003 Elsevier Inc. All rights reserved.
Keywords: Penis morphology; Paternity; Semen displacement; Sperm competition; Sexual behavior
1. Introduction
To maximize the likelihood of paternity, human males have evolved several strategies.
Daly and Wilson (1998) list mate guarding, as a means of minimizing the chances of the
female becoming involved in an extra pair copulation, and using the degree to which
children show paternal resemblance to assess paternity. There is a third class of putative
paternal assurance tactics. In the event of extra pair copulation, strategies on the part of
1090-5138/03/$ – see front matter D2003 Elsevier Inc. All rights reserved.
doi:10.1016/S1090-5138(03)00016-3
* Corresponding author. Tel.: +1-518-442-4852; fax: +1-518-442-4867.
E-mail address: gallup@csc.albany.edu (G.G. Gallup Jr.).
1
Now at Colby College, Waterville, ME, USA.
Evolution and Human Behavior 24 (2003) 277 289
the resident male for preventing conception by an interloper may involve different
parameters of sperm competition. Baker and Bellis (1989) have shown that the less often
a man sees his partner over a 3-day period, the higher his sperm count upon resuming
sexual intercourse, implicating mechanisms that may function to compete with sperm of
rival males.
While the first few drops of ejaculate facilitate sperm transport, the last few have
spermicidal properties (Baker & Bellis, 1995). Therefore, in the event of a double mating,
the second male’s sperm would be met with a spermicidal layer left by the first. Men with
prostate or seminal vesicle dysfunction sometimes have a reversed seminal fluid sequence
and are often rendered infertile (Kvist, 1991). The first few drops of ejaculate also function to
protect sperm from spermicide left by previous males, and therefore improve the second
male’s chances of achieving conception (Lindholmer, 1973).
Species differences in genital morphology can arise by sexual selection (Dixson, 1987;
Eberhard, 1996; Verrell, 1992). Thus, the morphology of the human penis itself may have
been influenced by sperm competition. Relative to other primates, the human penis is larger
in both width and length, roughly twice as long and wide as that of our closest living relative,
the chimpanzee (Short, 1980). The shape of the human penis also differs from many other
primates because the glans is more exaggerated and uniquely configured (Izor, Walchuk, &
Wilkins, 1981). The diameter of the posterior glans is larger than the penis shaft itself, and the
coronal ridge, which rises at the interface between the glans and the shaft, is positioned
perpendicular to the shaft. The human penis may displace seminal fluid from other males in
the vagina by forcing it back around the glans. The effect of thrusting, according to this
analysis, would be to draw foreign semen back away from the cervix. Because of its elasticity,
the vagina expands around the penis during intromission creating suction that may further
enhance this effect. If a female copulated with more than one male within a short period of
time, this would allow subsequent males to ‘‘scoop out’’ semen left by others before
ejaculating (Baker & Bellis, 1995).
Although there are no empirical data that bear on the semen displacement hypothesis,
some behavioral implications of sperm competition in humans have been examined.
Shackelford et al. (2002) found that after separation, males in committed relationships rate
their partners as more attractive, show greater interest in copulating with them, and think that
their partners are more sexually interested in them.
To test the displacement hypothesis, we measured the magnitude of semen displacement
affected by various artificial genitals, different simulated semen recipes, different semen
viscosities, and different depths of thrusting. Two surveys were also conducted to corroborate
and extend the results obtained using laboratory analogs.
2. Experiment 1
In the first experiment, sexual encounters were simulated using artificial genitals and the
magnitude of artificial semen displacement was measured as a function of penis size, shape,
and depth of thrusting.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289278
2.1. Methods
The genital models are depicted in Fig. 1. The latex phallus B was 155 mm long and 33 mm
in diameter (Hollywood Exotic Novelties) with a coronal ridge extending approximately
5 mm from the shaft. The latex phallus D was the same length, but was 27 mm in diameter
with a coronal ridge extending 3 mm from the shaft (Hollywood Erotique Novelties). The
plastic shaft C lacked a coronal ridge, measured 155 mm in length with a diameter of 29 mm,
and was used as a control phallus. These dimensions are well within human parameters. The
typical erect human penis ranges from 127 mm to 178 mm in length, with a diameter of
24.5 mm (Masters & Johnson, 1966; Wessells, Lue, & McAninch, 1996).
The artificial vagina (California Exotic Novelties), marketed as a male masturbation
device, measured 113 mm in length, with a 51-mm vaginal barrel, and was 26 mm in
diameter. The vagina (labeled A) consisted of a hollow tube of highly flexible elastic material,
which when stretched measured approximately 102 mm in length. As shown in Fig. 1, the
‘cervical’’ end of this tube was closed with a rubber band to prevent semen leakage during
displacement trials. Simulated semen was created by mixing 7 ml of water at room
temperature with 7.16 g of cornstarch and stirring for 5 min. After trying different mixtures
of cornstarch and water, this recipe was judged by three sexually experienced males to best
approximate the viscosity and texture of human seminal fluid. To avoid thickening of the
semen over time, new mixtures of artificial semen were created after every two to three trials.
Five displacement trials were conducted with each phallus. For each trial, the vagina was
weighed after applying several drops of water-based lubricant (California Exotic Novelties) to
Fig. 1. Genital models: vagina A, phallus B, control phallus C, phallus D.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289 279
the vaginal opening and inserting 2.6 ml of simulated semen into the vagina with a syringe.
The volume of human ejaculate ranges from 2 to 6 ml with an average of 3 ml (Belsey,
Eliasson, & Gallegos, 1999). After being loaded with semen, the vagina was stretched and
held against a fiberglass base positioned at an angle of 45j(see Fig. 2), while a phallus was
fully inserted into the vagina and withdrawn. Upon withdrawing a phallus to within 25 mm
from the opening, the vaginal wall below the glans was pinched closed to prevent semen from
flowing back into the vagina. The amount of semen pulled to the outer portion of the vagina
was then removed and the vagina was weighed. The percentage of semen displaced was
calculated as follows:
ðweight of vagina with semen weight of vagina following insertion and removal of phallusÞ
ðweight of vagina with semen weight of empty vaginaÞ100
In the second phase of this experiment, the B phallus was inserted into the vagina at
different depths (25%, 50%, 75%, and 100%) to assess the effect of thrusting depth on semen
displacement. Displacement was measured in the same way and the percentage of semen
displaced was calculated over five trials for each depth.
2.2. Results
As shown in Fig. 3, the magnitude of displacement varied as a function of penis
morphology. Phalluses B and D each displaced approximately 91% of the simulated semen,
but phallus C, with no coronal ridge, displaced only 35.3% [ F(12) = 41.56, P< .0001].
Fig. 2. A typical semen displacement trial with phallus B. Note the collection of semen behind the coronal ridge.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289280
Videotapes of the trials revealed that when either penis B or D was fully inserted into the
vagina, rather than forcing the semen over the coronal ridge, the semen flowed back under
the penis through the frenulum and then collected over the top of the anterior shaft behind the
coronal ridge (see Fig. 2).
As shown in Fig. 4, variation in depth of thrusting was associated with significant variation
in semen displacement [ F(16) = 330.74, P< .0001]. When phallus B was inserted all the way
into the vagina, 90% of the semen (M= 90.44%, S.D. = 9.89) was displaced, whereas
insertion three fourths of the way only displaced about one third (38.95%) of the semen
(P< .001). There was no semen displacement under conditions of more shallow thrusting (see
Fig. 4). The correlation between depth of thrusting and semen displacement was r= .927
(P< .0001).
Fig. 3. Mean percent semen displacement as a function of phallus type.
Fig. 4. Mean percent semen displacement as a function of depth of thrusting.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289 281
3. Experiment 2
Using a different artificial vagina and a different semen recipe, a second experiment was
conducted to replicate and extend these effects. Displacement was measured across two levels
of semen viscosity, and the coronal ridge was removed from one phallus to assess the effect
on semen displacement.
3.1. Methods
As shown in Fig. 5, a different artificial vagina (New Concepts) measuring 138 mm in
length and 43 mm in diameter was used. Because it was longer than vagina A, the cervical
end of vagina E was tied off with a rubber band so that when stretched it was the same
length (i.e., 102 mm). The same three phalluses were deployed in this experiment. A new
recipe for simulating semen (see Acknowledgements) consisted of 18.8 g of sifted, white,
unbleached flour mixed with 250 ml of water. This mixture was brought to a boil, simmered
for 15 min while being stirred, and allowed to cool. The procedures for loading the vagina
with semen, inserting the phallus, and measuring displacement were the same as in the
previous experiment.
In the first phase, five displacement trials were conducted with each of the three
phalluses. In the next phase, semen viscosity was manipulated by diluting the flour and
water mixture with additional water to produce two levels of viscosity. The thick semen
consisted of approximately 0.147 g flour/ml water, and the thin semen consisted of 0.126 g
flour/ml water. To compensate for evaporation, small amounts of water were added
Fig. 5. Additional genital models: phallus D and vagina E.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289282
periodically to each semen mixture during testing to maintain the original viscosities. Three
sexually experienced males rated the dilute (thin) semen as the closest approximation to
human semen. Five trials were conducted with each level of semen viscosity using vagina E
and each of the three phallus types.
In the final phase of Experiment 2, we assessed the effect of the coronal ridge on semen
displacement. Using a knife, the coronal ridge was removed from phallus D taking care not to
destroy the glans. Fine sandpaper and an emory board were used to smooth the area that had
been cut away, and the resulting phallus (designated D ) is depicted in Fig. 5. Using the
low-viscosity flour and water semen mixture, five displacement trials were conducted with
both the intact and modified phallus.
3.2. Results
As was true for cornstarch, the dilute semen lead to comparable displacement effects as a
function of phallus morphology. Phallus B displaced an average of 97.8%, phallus D 86.7%,
and phallus C 28.9% [( F(12) = 61.66, P< .0001]. The effects of semen viscosity and phallus
type were examined using a 3 2 factorial design. As shown in Fig. 6, there was a main
effect of Phallus Type [ F(2,29) = 108.15, P< .0001], a main effect of Semen Viscosity
[F(1,29) = 13.49, P< .001], but no Phallus Type Semen Viscosity interaction.
As shown in Figs. 3 and 6, the displacement effect held up well across the two different
vaginas. Although there appeared to be slightly greater displacement effects for vagina A, the
differences between A and E were not statistically significant.
The two latex phalluses (B and D) differed from the plastic control phallus (C) not only in
terms of morphological features (e.g., the glans/coronal ridge), but also in terms of texture
(see Fig. 1). In an attempt to resolve this problem and isolate the critical semen displacement
feature, the coronal ridge was removed from phallus D. The magnitude of displacement using
Fig. 6. Mean percent semen displacement as a function of semen viscosity and phallus type.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289 283
thin viscosity flour semen was substantially affected. Phallus D, with an intact coronal ridge,
displaced over twice as much semen (86.69%) than did the same phallus (D ) without the
coronal ridge (40.03%) [t(8) = 7.49, P< .0001].
4. Survey 1
The results obtained with prosthetic genitals suggest that the magnitude of in vivo semen
displacement would also be expected to vary as a function of the depth of thrusting. An
anonymous survey was used to determine if allegations of female infidelity affect
parameters of intercourse in ways that are consistent with the results obtained using
inanimate analogs.
4.1. Methods
Three hundred and thirty-six undergraduate college students (131 males, 205 females) at
the State University of New York at Albany responded to an anonymous written question-
naire (approved by the Institutional Review Board). Forty-one respondents never had sexual
intercourse and were excluded from the analysis, leaving 122 males and 173 females.
The survey posed questions to participants about their sexual experiences, including
contexts in which they or their partner had ever been accused of cheating or being
unfaithful. They were asked if they had intercourse following allegations of infidelity, and
if so, how long intercourse lasted, how quickly thrusting occurred, and how deep thrusting
Fig. 7. Percent of males and females reporting changes in depth of thrusting following allegations of female
infidelity.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289284
was. Subjects were asked to rate changes in their sexual behavior compared to typical or
normal intercourse using three-point scales ( 1=less;0=no change;1=greater).
4.2. Results
Not all participants answered all questions. Of those who experienced intercourse
following allegations of female infidelity, just over half (54 males and 68 females)
reported noticing a difference in sexual behavior. As shown in Figs. 7 and 8, a greater
proportion of males and females who noticed a difference reported deeper thrusting after
allegations of infidelity [one-sample ttest, M= .685, t(110) = 11.968, P< .0001], and the
same was true for speed of thrusting [one-sample ttest, M= .686, t(85) = 9.983,
P< .0001]. Duration of intercourse and length of orgasm, however, did not differ from
typical encounters.
Broken down by sex, males reported that after allegations of infidelity, they thrust deeper
[one-sample ttest, M= .685, t(53) = 9.266, P< .0001] and faster [one-sample ttest, M= .579,
t(37) = 5.557, P< .0001]. Females also reported that males thrust deeper [one-sample ttest,
M= 98.685, t(56) = 7.839, P< .0001] and quicker [one-sample ttest, M= .771, t(47) = 8.517,
P< .0001].
5. Survey 2
Another survey was used to determine if periods of separation between couples change
sexual behavior in ways consistent with the displacement hypothesis.
Fig. 8. Percent of males and females reporting changes in speed of thrusting following allegations of female
infidelity.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289 285
5.1. Methods
Two hundred and eighty-six undergraduate college students (91 males, 195 females) at the
State University of New York at Albany responded to an anonymous questionnaire approved
by the University Institutional Review Board.
Participants were asked whether they were sexually active, if they were currently in a
sexual relationship, and how long it had lasted. They were also asked about instances in
which they and their partner had been separated, and if they had intercourse following
reunion, how long intercourse lasted, how quickly thrusting occurred, how deep and vigorous
thrusting was, the approximate number of thrusts, and the intensity of orgasm. Subjects rated
changes in their sexual behavior compared to normal encounters using five-point scales
(2=much less;1=less;0=no change;1=greater; 2=much greater). Forty-four
respondents never had sexual intercourse and two who declined to answer the question were
excluded from the analysis, leaving 81 males and 159 females.
5.2. Results
Not all participants answered all questions. Almost three out of four (73.3%, N= 176)
reported having intercourse with their partner following a period of separation. Length of
separation ranged from 1 to 438 days, with an average (mode) of 14 days. Of those who
experienced a sexual encounter following separation, 7 out of 10 (69.3%, 34 males, 88
females) noticed a difference in their own or their partner’s sexual behavior.
Using one-sample ttests (see Figs. 9 and 10), both sexes who noted a difference reported
that thrusting was deeper [M= .856, t(166) = 13.684, P< .0001], quicker [M= .566,
t(167) = 8.396, P< .0001], more frequent [M= .633, t(165) = 9.613, P< .0001], and more
Fig. 9. Percent of males and females reporting changes in depth of thrusting following periods of separation.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289286
vigorous [M= .524, t(163) = 7.437, P< .0001] following separation. When broken down by
sex, males reported deeper [M= .845, t(57) = 7.193, P< .0001], faster [M= .500,
t(57) = 4.216, P< .0001], and more vigorous [M= .418, t(54) = 3.543, P< .0001] thrusting,
as well as more thrusts [M= .597, t(56) = 4.984, P< .0001] after a separation. Females also
reported deeper [M= .863, t(108) = 11.794, P< .0001], faster [M= .600, t(109) = 7.327,
P< .0001], and more vigorous [M= .578, t(108) = 6.589, P< .0001] thrusting by their
partners, as well as more thrusts [M= .651, t(108) = 8.286, P< .0001].
6. Discussion
Simulating a sexual encounter in vitro, we found that phalluses with a glans/coronal ridge
configuration that approximated a human penis resulted in appreciable displacement of
simulated semen. Depth of thrusting was also an important parameter, with significant
displacement occurring only when the penis was inserted 75% or more of the way into the
vagina, forcing the semen under the frenulum and causing it to flow back around the shaft and
collect behind the coronal ridge (see Fig. 2).
A limitation of our attempts to model semen displacement was the greater rigidity of the
prosthetic as compared to real genitals. The artificial vaginas did not expand as readily as real
vaginal tissue nor did the phalluses compress, and, as a result, semen displacement was
assessed on the basis of a single insertion. The effects, however, were robust and generalized
across different artificial phalluses, different artificial vaginas, different types of simulated
semen, and different semen viscosities. In spite of variation in diameter and coronal ridge
configuration, phalluses B and D (see Fig. 1) produced roughly equivalent displacement,
suggesting that comparable effects would be obtained over a variety of prosthetic genital
parameters. Severing the glans from phallus D diminished displacement to levels commen-
Fig. 10. Percent of males and females reporting changes in speed of thrusting following periods of separation.
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289 287
surate with those obtained for control phallus C, thus implicating the coronal ridge as an
important semen displacement feature.
Whereas the different semen recipes led to comparable displacement effects, magnitude of
displacement varied with semen viscosity (see Fig. 6). Individual differences in semen
viscosity among males are associated with differences in fertility. Semen hyperviscosity
impairs sperm motility (Mendeluk, Munuce, Carizza, Sardi, & Bregni, 1997). Our data,
relating the magnitude of displacement to viscosity, suggest that the penis is more effective at
displacing low-viscosity, more mobile semen.
Masters and Johnson (1966) reported ballooning of the cervical end of the vagina during
intercourse, which, if true, might diminish a displacement effect. However, Weijmar-Schultz,
van Andel, Sabelis, and Mooyart (1999) recently conducted a magnetic resonance imaging
study of male and female genitals during coitus. Their results show that during complete
penetration, the penis can stretch the anterior wall of the vagina and even raise the uterus, but
they found no evidence of a ballooning effect.
While only suggestive, the data from both behavioral surveys are consistent with the
semen displacement hypothesis. Following separation from their partners, or when males
were sexually jealous, thrusting was rated as being deeper and more vigorous. Although
speed and duration of thrusting were not manipulated using prosthetic genitals, these have
been implicated in displacing vaginal secretions (O’Hara & O’Hara, 1999). Thus, many males
appear to alter their sexual behavior in ways that may displace a rival male’s semen in the
event of infidelity on the part of their partners. Baker and Bellis (1995) speculated that the
longer the duration of intercourse, the more material would be removed from the vagina. But
this assumes that number of thrusts vary with duration of intercourse. We found no
differences in duration of intercourse, but males and females both reported quicker, more
vigorous thrusting that could increase the number of thrusts per encounter.
The surveys have limitations. First, periods of separation may be confounded with time
since the last ejaculation. This assumes, of course, that males do not pursue alternative means
of achieving ejaculation during the interim. Because males often use masturbation as a
copulation substitute, this assumption may be unfounded. Moreover, time since the last
copulatory ejaculation poses the same adaptive problem even if the couple has not been
formally separated. Just as the likelihood of infidelity on the part of the female could vary as a
function of extended separation, the same would apply during briefer periods of informal
separation throughout the day (e.g., going to the store, work, or school). Unless the female is
sequestered and guarded on a continuous basis, as the time since last ejaculation increases so
would the risk of sperm competition. Thus, deeper, more vigorous thrusting as a function of
time since the last copulatory ejaculation could function as a more generalized sperm
competition mechanism.
Another potential problem with the survey results are demand characteristics; i.e., males
may think that deeper, more vigorous thrusting is a more appropriate response following
separation or female infidelity. If true, however, the mere existence of such a cognitive bias
would be consistent with our hypothesis. In addition, the accounts given by females of
different features of sexual behavior following separation or allegations of female infidelity
were the same as the males’. Also, contrary to demand characteristics, changes in sexual
G.G. Gallup Jr. et al. / Evolution and Human Behavior 24 (2003) 277–289288
behavior were specific to features related to semen displacement; i.e., there were no reported
differences in the duration of intercourse or incidence of orgasm.
Taken together, the results of the surveys along with the data derived from the artificial
genitals suggest that the genital anatomy and copulatory behavior of human males has been
shaped by a history of sperm competition.
Acknowledgments
We thank Steven Platek, Jack Maser, Aaron Goetz, Nicholas Pound, and Todd Shackelford
for helpful comments on an earlier version of this paper, and Brian Quintus, Todd
Shackelford, and Aaron Goetz for the flour and water semen recipe used in Experiment 2.
References
Baker, R. R., & Bellis, M. A. (1989). Number of sperm in human ejaculates varies in accordance with sperm
competition theory. Animal Behavior,37, 867 869.
Baker, R. R., & Bellis, M. A. (1995). Human sperm competition: copulation, masturbation, and infidelity.
London: Chapman & Hall.
Belsey, M. A., Eliasson, R., & Gallegos, A. J. (1999). World Health Organization laboratory manual for
examination of human semen and semen – cervical interaction. Cambridge, UK: Cambridge Univ. Press.
Daly, M., & Wilson, M. (1998). The truth about Cinderella: a Darwinian view of parental love. New Haven, CT:
Yale Univ. Press.
Dixson, A. F. (1987). Observations on the evolution of the genitalia and copulatory behavior in male primates.
Journal of Mammology,57, 1– 22.
Eberhard, W. G. (1996). Female control: sexual selection by cryptic female choice. Princeton, NJ: Princeton Univ.
Press.
Izor, R., Walchuk, S., & Wilkins, L. (1981). Anatomy and systematic significance of the penis of the pygmy
chimpanzee, Pan paniscus.Folia Primatologica,35, 218 224.
Kvist, U. (1991). Can disturbances of the ejaculatory sequence contribute to male infertility? International
Journal of Andrology,14, 389 393.
Lindholmer, C. (1973). Survival of human sperm in different fractions of split ejaculates. Fertility and Sterility,
24, 521 526.
Masters, W. H., & Johnson, V. E. (1966). Human sexual response. Boston: Little, Brown and Company.
Mendeluk, G. R., Munuce, M. J., Carizza, C., Sardi, M., & Bregni, C. (1997). Sperm motility and ATP content in
seminal hyperviscosity. Archives of Andrology,39, 223– 227.
O’Hara, K., & O’Hara, J. (1999). The effect of male circumcision on the sexual enjoyment of the female partner.
British Journal of Urology International,83, 79 84.
Shackelford, T. K., LeBlanc, G. J., Weekes-Shackelford, V. A., Bleske-Rechek, A. L., Euler, H. A., & Hoier, S.
(2002). Psychological adaptation to human sperm competition. Evolution and Human Behavior,23, 123 138.
Short, R. V. (1980). The origins of human sexuality. In: C. R. Austin, & R. V. Short (Eds.), Reproduction in
animals: Book 8. Human sexuality ( pp. 1 33). Cambridge: Cambridge Univ. Press.
Verrell, P. (1992). Primate penile morphologies and social systems: further evidence for an association. Folia
Primatologica,59, 114– 120.
Weijmar Schultz, W., van Andel, P., Sabelis, I., & Mooyart, E. (1999). Magnetic resonance imaging of male and
female genitals during coitus and female sexual arousal. British Medical Journal,319, 18 25.
Wessells, H., Lue, T. F., & McAninch, J. W. (1996). Penile length in the flaccid and erect states: guidelines for
penile augmentation. Journal of Urology,156, 995 997.
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... Semen displacement as a counterinsemination strategy is a mechanical means of competing with sperm from rival men (Baker & Bellis, 1995;Gallup et al., 2003). If a woman copulates with more than one man in a short period of time, the man can increase his chances of paternity by scooping out or displacing the semen deposited by rival males away from the cervix by capturing their sperm behind the penile coronal ridge (Gallup et al., 2003). ...
... Semen displacement as a counterinsemination strategy is a mechanical means of competing with sperm from rival men (Baker & Bellis, 1995;Gallup et al., 2003). If a woman copulates with more than one man in a short period of time, the man can increase his chances of paternity by scooping out or displacing the semen deposited by rival males away from the cervix by capturing their sperm behind the penile coronal ridge (Gallup et al., 2003). When compared with other primates, the human penis is appreciably longer than our closest ancestors, the chimpanzees (Short, 1980). ...
... In addition, the human penis has an enlarged glans and a pro-truding coronal ridge (Gallup & Burch, 2004;Izor, Walchuk, & Wilkins, 1981). Indeed, there is evidence based on laboratory simulations of intercourse that the uniquely shaped glans and coronal ridge of the human penis function to displace and remove rival male semen from the vagina (Gallup et al., 2003). ...
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In a sample of college couples, we examined the frequency of extrapair copulations, how these differ from intercourse with committed partners, and how knowledge of the other person's relationship status affects sexual behavior. More than 25% of both men and women reported engaging in 1 or more extrapair copulations. Those who cheated reported greater arousal, but the duration of intercourse was not affected. Both sexes achieved greater sexual satisfaction from extrapair copulations. When men engaged in intercourse with women they knew were in a committed relationship, thrusting was quicker, deeper, and more vigorous. Men also reported more intense orgasms and attempted to prolong intercourse for as long as possible when having sex with someone who was in another relationship. Women did not. Differences in various parameters of extrapair orgasmic experiences (latency to orgasm, frequency of orgasm, intensity of orgasm, and orgasm duration) were consistent with a priori predictions based on sex differences in fitness maximization (Gallup, Burch, & Petricone, 2012; Gallup, Towne, & Stolz, 2018).
... Among the traits indicating that the human penis did evolve under relatively strong selection are its comparatively large length and (especially) girth among the primates (Dixson 2018). In chimpanzees, which also exhibit a comparatively large penis, this is believed to be an evolutionary adaptation to sperm competition (Dixson & Anderson 2001, Gallup et al. 2003, Eberhard 2009, Dixson 2012, and it has been proposed that human penile morphology exhibits traits indicative of the evolution The missing human baculum I. Jakovlić of the penis as a semen displacement device (Gallup et al. 2003). This was contested on the basis of the fact that human penis is not unique in this aspect in comparison to other primates (Dixson 2009(Dixson , 2018. ...
... Among the traits indicating that the human penis did evolve under relatively strong selection are its comparatively large length and (especially) girth among the primates (Dixson 2018). In chimpanzees, which also exhibit a comparatively large penis, this is believed to be an evolutionary adaptation to sperm competition (Dixson & Anderson 2001, Gallup et al. 2003, Eberhard 2009, Dixson 2012, and it has been proposed that human penile morphology exhibits traits indicative of the evolution The missing human baculum I. Jakovlić of the penis as a semen displacement device (Gallup et al. 2003). This was contested on the basis of the fact that human penis is not unique in this aspect in comparison to other primates (Dixson 2009(Dixson , 2018. ...
Article
Unlike most primates and all other great apes, humans Homo sapiens do not possess a baculum (penile bone). I discuss the shortcomings of previous hypotheses to explain the loss of the baculum, argue that they fail to explain it fully, and show that the evolutionary history of this loss remains enigmatic. I also propose a new hypothesis: that conspecific aggression, in combination with the development of self‐awareness, may have played a role in the loss. If the presence of a baculum exacerbated the prevalence and severity of penile injuries resulting from blunt trauma to a flaccid penis, increasing ability to foresee the consequences of their actions would also enable hominins to realise that these injuries are a useful tool in male–male competition. This behavioural innovation, planned conspecific aggression with the goal of temporary exclusion of competitors from the breeding pool, would create an environment in which a genetic mutation for a penis without a baculum (or with an unossified baculum) would strongly increase the fitness of the mutant phenotype. Along with the hominin propensity for social learning and cultural transmission, this hypothetical scenario may explain why this phenotype became fixed in all human populations.
... In one of them, males stimulate female genitalia, which results in sperm ejection by females, and in the other one, males physically remove sperm stored in females (e.g. Waage 1979Waage , 1986aOno et al. 1989;Yokoi 1990;von Helversen and von Helversen 1991;Haubruge et al. 1999;Kamimura 2000;Gallup et al. 2003;Matsumura et al. 2014). Many examples of SRB have been reported across several insect orders, including dragonflies and damselflies (e.g. ...
... Given the widespread potential adaptive significance of such sperm removal in securing paternity success, it is not surprising that equivalent adaptations are also seen in other taxa (e.g. Naud et al. 2004;Wada et al. 2005;Galeotti et al. 2007;Calbacho-Rosa et al. 2013), and a similar function has been proposed for the human penis (Gallup et al. 2003). ...
Article
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Sperm removal behaviour (SRB) is known in many animals, and male genital structures are often involved in the SRB, e.g. rubbing female genitalia vigorously. However, it remains unclear how those male genital structures function properly without severe genital damage during SRB. In the present study, we focused on the bushcricket Metaplastes ornatus and examined the biomechanics of male and female genital structures, involved in their SRB as a model case. During an initial phase of mating, males of this species thrust their subgenital plate with hook-like spurs and many microscopic spines into the female genital chamber. By moving the subgenital plate back-and-forth, males stimulate females, and this stimulation induces the ejection of sperm previously stored in females. We aimed to uncover the mechanics of the interaction between the subgenital plate and genital chamber during SRB. The genital morphology and its material composition were investigated using modern imaging and microscopy techniques. The obtained results showed a pronounced material heterogeneity in the subgenital plate and the genital chamber. The material heterogeneity was completely absent in that of a second bushcricket species, Poecilimon veluchianus, which does not exhibit SRB. Finite element simulations showed that the specific material heterogeneity can redistribute the stress in the subgenital plate of M. ornatus and, thereby, reduces stress concentration during SRB. This may explain why only a few examined males had a broken spur. We suggest that the observed structural features and material heterogeneity in M. ornatus are adaptations to their SRB.
... The scraper shape of the penis is not intended only to pull sperm out (Gallup, 2003 or to reduce the acidity of the vagina but also to extract the bacteria it contains. ...
Preprint
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The effectiveness of sex to make children is remarkably low and totally random: there is no doubt that as soon as possible other methods to achieve it will be preferred. Yet humans have an enormous amount of sexual relations, devote a great deal of their resources to it, and most of the time seek to ensure that they are not fertile. Why? The usual explanations are insufficient to explain these behaviors. This article proposes that the most important purpose of sexual intercourse is to transmit not our genes but the genes of our microbiome, that is of other species’ genes. This hypothesis explains not only the frequency of sexual intercourse but also the frequency of non-fertilizing and risky sexual behaviors.
... For example, adaptations to the male intromittent organ in damselflies (Calopteryx maculata) remove sperm from prior copulations (Waage, 1979). Although sperm removal by males during copulation has not been reported in vertebrates, it has been suggested that the human (Homo sapiens) glans (distal end of the penis) might be shaped to move previously deposited sperm away from the cervix during thrusting (Gallup et al., 2003). In species with copulatory plugs (masses of coagulated semen deposited by the male), such as rodents and some primates, multiple intromissions and thrusting may be mechanisms to remove the plugs deposited by rival males Wallach & Hart, 1983). ...
Chapter
In internal fertilizers, copulatory behavior and genital morphology are intricately connected because there are many functional and morphological challenges that genitalia must overcome for successful transfer of male gametes into the female reproductive tract. In addition, selective forces can act either on copulatory behavior and genitalia independently or concurrently. However, collecting data on copulatory behavior can be challenging, and there are relatively few studies of genital morphology in Amniotes, which include reptiles, birds, and mammals. As a result, we know little about the relationship between copulation and genitalia in this group. Here we examine copulatory behavior in Amniotes using particularly well-known examples, and add information about the morphology and function of their genitalia to lay a framework for further research on the integration of these traits.
... It is also known that in the case of the women, adapting to the conditions of the multi-male mating is also indicated by: the ability of the female reproductive system to assess immune compatibility of sperm on the basis of its chemical structure (Birkhead and Pizzari 2002;Barratt et al. 2009), preference of the reproductive system for the sperm delivered during the orgasm (Baker and Bellis 1993b). In Homo sapiens, additional factors are male evolutionary adaptations to sperm competition, e.g., testis size (Møller 1988), ejaculate adjustment (Baker and Bellis 1993a;Shackelford et al. 2002), sexual arousal (men's sexual fantasies often involve multiple partners) (Pound 2002), ejaculate gametes' role (Baker and Bellis 1988;1989) and penis shape (Gallup et al. 2003;Mautz, et al. 2013). ...
Article
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The aim of the paper is to identify psychosomatic evolutionary adaptations of hominids, which direct them at maximizing their reproductive success, and on the basis of which their various social structures are built. Selected features of the hominid last common ancestor were extracted; by reducing the influence of the social structure, they were defined as the hominid “sexual nature”; these considerations were supported by the analysis of sexual jealousy as a function of socio-environmental conditions. The “sexuality core” of a hominid female was defined as “selective polyandry”—the female selects the best males among those available; and of a hominid male as “tolerant promiscuity”—the male strives for multi-male and multi-female copulations with sexually attractive females. The extracted “sexuality cores” condemn hominids to a patriarchal social structure and thus to sexual coercion and jealousy. The source of male sexual jealousy is limited access to females. Hominid female jealousy of the male results mainly from the need for protection and support. Hominids’ social structures are determined by females’ sexual selectivity or opportunism and by their continuous or periodic proceptivity and estrus signaling. Evolutionary functions developed by women: out-estrus sexuality, copulation calls, multiple orgasms, allow them to obtain the best possible spermatozoid. The institution of marriage blocks the influence of sexual selection in the species Homo sapiens.
... Evidence points to a long history of non-monogamous, extra-pair mating behaviour in our species (Baker and Bellis 1995;Buss and Schmitt 1993;Gallup et al. 2003;Kinsey et al. 1948;Symons 1979). Jealousy, often defined as distrust or resentfulness towards significant others due to suspected or known romantic contact with a rival or interloper, is inextricably linked with infidelity (Wurmser and Jarass 2011). ...
Article
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Research highlighting sex-differentiated jealousy resulting from imagined scenarios has now been reaffirmed when the infidelity-revealing message is discovered on a social media platform. Participants in the current study were presented with both sexually and emotionally charged infidelity-revealing scenarios featuring a same-sex sibling, a friend and a stranger in the format of a ‘Snapchat’ message. Men indicated significantly higher jealousy to sexual as opposed to emotional messages with the reverse pattern evident in women. Sex differences were also evident in the extent of jealousy elicited by ‘third-party’ identity. Women were significantly more jealous when the imagined infidelity occurred between their sister compared to both a best friend and a stranger with males showing significantly lower jealousy directed towards their brother compared to a stranger. These findings are supportive not only of a parental investment (PI) interpretation of sex differences in jealousy but also an interpretation consistent with aspects of inclusive fitness theory.
... Subscriber: SUNY Binghamton University; date: 10 January 2020 ejaculating (Baker & Bellis, 1995). Gallup et al. (2003) simulated sexual encounters using artificial models and found that the displacement of simulated semen was robust across different prosthetic humanlike phalluses (but only those with a pronounced glans), different artificial vaginas, different semen recipes, and different semen viscosities. ...
Chapter
Using a variety of different and often provocative examples, this chapter illustrates how evolutionary theory can be used to think about things in new and sometimes even counterintuitive ways. Examples include how semen sampling may be an evolved mate-choice mechanism, why pubic hair removal may promote pedophilia, why we owe our existence to the moon, why the risk of conception is higher as a result of being raped, why bottle-feeding your previous child may put your next child at risk of becoming autistic, and why smart people are attracted to evolutionary studies. The chapter argues that evolutionary theory enables people to think about human behavior and human existence in ways that are outside the box.
Article
Sperm competition occurs when the sperm of two or more males simultaneously occupy the reproductive tract of a female and compete to fertilize an egg. We used a questionnaire to investigate psychological responses to the risk of sperm competition for 194 men in committed, sexual relationships in the United States and in Germany. As predicted, a man who spends a greater (relative to a man who spends a lesser) proportion of time apart from his partner since the couple's last copulation reported (a) that his partner is more attractive, (b) that other men find his partner more attractive, (c) greater interest in copulating with his partner, and (d) that his partner is more sexually interested in him. All effects were independent of total time since the couple's last copulation and the man's relationship satisfaction. Discussion addresses two failed predictions and directions for future work.
Article
Comparative studies of genital anatomy and sexual behaviour in male primates show that penile morphology and copulatory patterns tend to be more specialized in species which have a multimale or dispersed (non-gregarious) mating system. The penis may be longer and more complex morphologically in such species. Copulatory patterns involving a series of intromissions or prolonged single intromissions are more common than in species with a monogamous or polygynous mating system. Elongation of the baculum occurs in species which prolong intromission into the post-ejaculatory period. Sexual selection may have favoured the evolution of such features in species where females mate with a number of males rather than with a single partner. Relative testis size is also greatest under such conditions and larger testes occur in some non-gregarious or seasonally breeding prosimians, as well as in anthropoids with multimale mating systems.