Sperm competition in humans
Summary (2 min read)
Introduction
- Lutionary science, that female infidelity was a recurrent feature of modern humans’ evolutionary history has come the development of a unique area in the study of human mating: sperm competition.
- Male flour beetles sometimes fertilize females with a rival male’s sperm.
- This phenomenon was predicted and observed by researchers who study sperm competition.
- Humans also may experience fertilization by proxy (Gallup & Burch, 2004).
SPERM COMPETITION AS AN ADAPTIVE PROBLEM IN HUMANS
- For species that practice social monogamy—the mating system in which males and females form long-term pair bonds but also pursue extra-pair copulations (i.e., ‘‘affairs’’)—it is the extrapair copulations by females that creates the primary context for sperm competition.
- In species with paternal investment in offspring, selection often favors the evolution of adaptations that decrease.
ADAPTATIONS TO SPERM COMPETITION IN HUMANS
- The authors discuss adaptations men may have evolved in response to an evolutionary history of sperm competition.
- The authors limit their discussion of these adaptations to testis size, ejaculate adjustment, semen displacement, sexual arousal, and forced inpair copulation, as these adaptations have been investigated more rigorously than others.
Testis Size
- Across a range of animal species, males have relatively larger testes in species with more intense sperm competition.
- Because larger testes produce more sperm, a male with larger testes can better compete by inseminating a female with more sperm.
- Among the great apes, testes size varies predictably with the risk of sperm competition.
- Chimpanzees, in contrast, are highly promiscuous and, accordingly, males have relatively large testes, making up 0.30% of their body weight.
- The size of human testes falls between these two extremes at 0.08% of body weight, suggesting intermediate levels of female promiscuity and sperm competition in their evolutionary past (Shackelford & Goetz, 2006).
Ejaculate Adjustment
- The number of sperm recruited into a given ejaculate is not constant.
- A key hypothesis derived from sperm competition theory is that males will adjust the number of sperm they inseminate into a female as a function of the risk that their sperm will encounter competition from the sperm of other males.
- As the proportion of time a couple spends together since their last copulation decreases, there is a predictable increase in the probability that the man’s partner has been inseminated by another male.
- Additional analyses documented that the proportion of time a couple spent together since their last copulation negatively predicts sperm number ejaculated at the couple’s next copulation, but not at the man’s next masturbation (Baker & Bellis, 1993).
- Inspired by Baker and Bellis’s (1993) demonstration of male physiological adaptations to sperm competition, Shackelford et al. (2002) documented that human male psychology may include psychological adaptations to decrease the likelihood that a rival man’s sperm will fertilize a partner’s ovum.
Semen Displacement
- Features of the penis may have evolved in response to the selective pressures of sperm competition.
- Spines, ridges, and knobs on the penis of some waterfowl are positioned in a way to displace 48 Volume 16—Number 1 rival sperm, and these protuberances are larger in species for which the intensity of sperm competition is greater.
- The human penis does not have barbs and spines for removing rival sperm, but recent evidence suggests that the human penis may have evolved to function, in part, as a semen-displacement device.
- That the penis must reach an adequate depth before semen is displaced suggests that displacing rival semen may require specific copulatory behaviors.
- Men mated to women who placed them at high risk of sperm competition were more likely to use specific copulatory behaviors arguably designed to displace rival semen (e.g., more frequent thrusts, deeper thrusts) than were men mated to women who did not place them at high risk of sperm competition.
CONCLUSION AND FUTURE DIRECTIONS
- Sperm competition was first identified as a form of postcopulatory competition between males by Geoff Parker in the 1970s.
- Since then, evolutionary biologists and behavioral ecologists have described many anatomical, physiological, and behavioral adaptations to sperm competition in many nonhuman species.
- The question as to whether sperm competition has been an Volume 16—Number 1 49 important selection pressure during human evolution remains somewhat controversial, and further research is needed to establish the extent to which this might be the case.
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Frequently Asked Questions (15)
Q2. What are the future works in "Adaptation to sperm competition in humans" ?
The question as to whether sperm competition has been an Volume 16—Number 1 49 important selection pressure during human evolution remains somewhat controversial, and further research is needed to establish the extent to which this might be the case. An important avenue for future research is to identify adaptations not only in men but also in women. Thus, men ’ s adaptations to sperm competition are likely to be met by counteradaptations in women ( e. g., mechanisms that increase retention of sperm inseminated by men with ‘ ‘ good genes ’ ’ ; see Shackelford et al., 2005 ), and the study of such mechanisms is an important direction for future research.
Q3. What are the three types of tactics used to prevent female infidelity?
Anti-cuckoldry tactics fall into three categories: preventative tactics, designed to prevent female infidelity; sperm competition tactics, designed to minimize conception risk in the event of female infidelity; and differential paternal investment, designed to allocate paternal investment prudently in the event that female infidelity may have resulted in conception.
Q4. What is the effect of the penis on the sperm?
When the penis is inserted into the vagina, the frenulum of the coronal ridge makes semen displacement possible by allowing semen to flow back under the penis alongside the frenulum and collect on the anterior of the shaft behind the coronal ridge.
Q5. What is the key hypothesis derived from sperm competition theory?
A key hypothesis derived from sperm competition theory is that males will adjust the number of sperm they inseminate into a female as a function of the risk that their sperm will encounter competition from the sperm of other males.
Q6. What is the extent to which sperm competition occurred in ancestral human populations?
The extent to which sperm competition occurred in ancestral human populations would have depended largely on rates of female sexual infidelity and cuckoldry.
Q7. What is the purpose of the human penis?
The human penis does not have barbs and spines for removing rival sperm, but recent evidence suggests that the human penis may have evolved to function, in part, as a semen-displacement device.
Q8. What is the parsimonious explanation for the results of Pound (2002)?
When faced with cues of sperm competition, sexual arousal would have resulted in an increase in sperm transport upon ejaculation, thus enabling men to compete more effectively in such contexts.
Q9. What is the purpose of the penis?
That the penis must reach an adequate depth before semen is displaced suggests that displacing rival semen may require specific copulatory behaviors.
Q10. What is the parsimonious explanation for these results?
Pound argued that the most parsimonious explanation for these results is that male sexual arousal in response to visual cues of sperm competition reflects the functioning of psychological mechanisms that would have motivated adaptive patterns of copulatory behavior in ancestral males exposed to evidence of female promiscuity.
Q11. What is the common reason for the evolution of adaptations?
in species with paternal investment in offspring, selection often favors the evolution of adaptations that decreaseFL 33314; e-mail: tshackel@fau.edu.
Q12. What is the parsimonious explanation for the results of Pound’s hypothesis?
According to men’s self-reports and women’s partner-reports, men who use more sexual coercion in their relationships are mated to women who have been or are likely to be unfaithful.
Q13. What is the significance of the study?
Inspired by Baker and Bellis’s (1993) demonstration of male physiological adaptations to sperm competition, Shackelford et al. (2002) documented that human male psychology may include psychological adaptations to decrease the likelihood that a rival man’s sperm will fertilize a partner’s ovum.
Q14. What is the probability that a couple has been inseminated by another male?
As the proportion of time a couple spends together since their last copulation decreases, there is a predictable increase in the probability that the man’s partner has been inseminated by another male.
Q15. What is the main topic of the article?
As outlined in this article, however, there is mounting evidence that aspects of men’s sexual psychology and behavior, such as their attraction to and sexual interest in their partners, their copulatory behaviors, and sources of sexual arousal, may reflect adaptations to sperm competition.