Female orgasm is a strange phenomenon as the female can in be entirely passive, yet still reproduce, so there is no obvious explanation for them to spasm during copulation. The male, on the other hand, needs to perform a thrusting motion to inject sperm as deeply as possible, especially to outcompete contending sperm, and this behavior needs to be incentivized with positive emotion such that the male reproduces at all given a passive female. Some suggested the female orgasm is merely a vestige of the male orgasm or that it mechanically somehow increases chances of conception.
Bonding to get male investment[edit | edit source]
Female orgasms and copulatory vocalizations may have the function of bonding with the male in order to get their investment. The male, in turn, is also interested in bonding for paternity assurance. With sexual affection, the female can assure sexual exclusivity, so the man is more inclined to invest his resources in her and the offspring. Bonding presumably also increases reproductive success given how much especially children depend on the presence of a family and resources (Morris, 1967).
This suggests women desire bonding more with a good looking and physically strong partner, which makes sense as a both is predictive of high status, which in turn predicts resource availability and other goodies.
This would also agree with female orgasms as a means of sperm retention. If female orgasm does retain sperm, it is advantageous for women to orgasm with men with complementary or 'desirable' genes. For example, MHC (Major Histocompatibility Complex) genes may affect mate choice and attraction.
Suppose MHC compatibility has any influence on mate choice. In that case, one could speculate that MHC compatibility would significantly affect women's likelihood to achieve orgasm with partners of a similar immune system surface protein polymorphisms. Evidence does suggest that women rate men with dissimilar MHC complex gene's body odour as more pleasant, though only when they do not use oral birth control, which likely would play a role in enhancing women's likelihood of achieving orgasm with such men as a woman's level of physical attraction to her partner is the major determinate of when and whether she will achieve orgasm during sexual intercourse.  MHC complexes differ across race as well, which may/may not explain differences in race attraction. The importance of MHC compatibility/heterogeneity may also vary by race or ethnicity. Some studies that examine the effects of MHC similarity on attraction only finding significant effects among East Asians.
Therefore, selective orgasms helps female to retain sperm from the fittest males while expelling sperm from the unfit males. Accordingly, there may even be mechanisms for "sperm dumping", though evidence for these mechanisms in humans is lacking. It is somewhat uncertain as to how female orgasm in humans affects fertility. If female orgasm increase the chances of a female becoming pregnant, then it follows that women whom are more orgasmic (that have more orgasms) are more fertile and thus produce more children. However it seems that female orgasm and reproductive success, by proxy fertility, are not related. However one paper in 2016 has found that female orgasm does affect sperm backflow, however the sampled used was tiny (n=6), a fact prominently noted by the study authors.
Others also claim that oxytocin released by human female orgasm may aid in the transportation of sperm transported to the ovum and thus increase fertility, however these claims are controversial. The evidence suggests that female orgasm has, at best, a mild effect on sperm retention, but has little to no effect on fertility or on reproductive success.
Decline of female orgasm[edit | edit source]
One study found gender equality and sexual education since the 1970s have not helped women to become more orgasmic, rather women report less often orgasms, despite of their greater openness about sex, an increase in number of sex partners, and more frequent masturbation. Explanations for this range from lower availability of high status, highly confident men, lowered level of male educational and economic status (Safarinejad, 2006), increased incidence of female depression. Perhaps even a behavioral sink could explain it, which could be due to increased mutational load due to ecological mildness of modern civilization and/or due to pollutants disrupting biological development.
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Evidence suggests that women's moaning, which is much louder than men's, does not even require an orgasm to occur with 80% of women reporting having produced such vocalizations without orgasm in one study. In an informal survey of 5,000 British women, 46% admitted they had faked orgasms. This also points to moaning to have a largely separate function from successful copulation act.
Similarly, it has been suggested, the loud vocalizations of the hominid female raises the social status of the copulating males, as well as raising the vigilance of the males the female has chosen to defend her from unwanted males, which is thought to occur in a somewhat promiscuous mating context that involves sperm competition.
Evolutionary hypotheses regarding the development of the female orgasm and female copulatory vocalizations[edit | edit source]
There is a lot of variation in female copulatory cocalization in even closely related primates. For example, a 2005 paper listed at least 15 different hypotheses for the existence of female copulatory vocalization. The 15 hypothesis are listed as follows:
- Nonadaptive by-product of sexual intercourse
- Nonadaptive phenomenon maintained by phylogenetic inertia
- Self-stimulates the occurrence of ovulation
- Promote synchronization of male and female orgasm
- Strengthen the pair bond
- Honest signals with which females advertise their reproductive status
- Honest signals with which low-ranking females advertise their sexual motivation
- Advertise mating to other females and inhibit breeding synchrony
- Advertise mating to other females and promote breeding synchrony
- Advertise the presence of a male partner and reduce female harassment
- Incite male competition and increase the probability of mating with dominant males
- Incite male sperm competition to ensure that sons will inherit the best sperm (sexy son)
- Incite multiple male matings to reduce paternity certainty and the risk of infanticide
- Announce paternity certainty to promote paternal investment
- Encourage mate guarding by the consort male
Thus, as female copulatory vocalizations' functionality is hypothesized to vary greatly, it would be a mistake to extrapolate the function of human female copulatory vocalizations from the function of other female primate vocalization. A rudimentary reading of the 15 hypotheses, in addition to some prior understanding of basic biology in humans, will cast doubt on a few of the hypotheses as likely explanations for human female vocalization.
For example, hypothesis no. 13, which posits that female vocalization is ultimately about reducing paternity certainty cannot be true for Human Female Vocalization as Humans have altricial offspring (requiring investment from both parents). Fathers who are uncertain in their children's paternity do not invest in them (and may even kill them). Cross-culturally, suspicions of infidelity on behalf of the female partner is one of the main motivators of female-directed violence and homicides by males. If paternal confusion made males less likely to kill their partners and offspring due to the possibility that the offspring of promiscuous women are theirs, one would not suspect mate-guarding related homicidal jealousy to be such a violent impulse among men. Paternal confusion would be expected to be less in such instances, as there are generally fewer rivals regarding their female partner's affections than the hypothesized contexts of historical widespread female promiscuity. The fact that men who have heavily invested in a woman seem to have such a strong, innate, impulse to react with violent rage when confronted with their partner's betrayal suggests that men may be adapted to 'make good' on their misdirected investments in such women by killing them and potentially their offspring if they suspect that they are not the real fathers of said offspring.
It also suggests that despite the potential retaliation men could suffer from their murdered/otherwise victimized partner's relatives or society as a whole, female-directed violence serves as a method of controlling women's sexuality in general, inducing in them a fear of the potential consequences of infidelity, which is suggested by the leniency with which such crimes were treated with throughout history when cheating on behalf of the wife was certain. It also likely serves as a way of attempting to diminish the reputation loss that a cuckolded man generally suffers.
Female vocalization for the sake of paternity uncertainty only works as a hypothesis in 'polygynandrous' mating systems where males do not invest in their children, and the females are highly promiscuous (like we see in bonobos and chimpanzees). If a chimpanzee, for example, knew that the offspring birthed by a particular female was not his (as she did not engage in coitus with him) then he would, of course, attempt to kill the child (or at least act aggressively towards it). If the female chimp has sex with him, and thus introduces paternity uncertainty, then the chimp has no reason to act aggressively towards the child as there is at least a chance that the child is his; as he invests nothing more than his sperm, he doesn't lose much by leaving the child alone. The same cannot be generally said for human males as paternity investment is much higher for human males; thus, even the tiniest amounts of paternity uncertainty in human males will result in aggressive behaviour.
However, as human males differ in terms of the investment they put into their relationships and offspring, with the largest source of this variance being argued to represent inter-individual differences in life history speed, this hypothesis may have some validity among men adapted to ecological niches that require much less paternal investment on behalf of the males, and among individual fast life history strategists, who may only react with jealousy in response to instances of infidelity perpetrated by women in partnerships they have heavily invested in (which they are less likely to enter, in any case, unless enforced by religious etc., norms). In such strongly fast-life history niches, the polygynandrous mating system that is required by this hypothesis may be possible, as compared to harsher, but still predictable ecological niches that require stronger pair bonds, high male investment, and some level of long term pair bonding for most offspring to have a strong chance of survival.
Sperm competition and female copulatory vocalizations[edit | edit source]
One issue with the idea that female copulatory vocalization serves to encourage sperm competition is the concealed ovulation of human females. Unlike our cousins, human females do not advertise their fertility and are constantly sexually receptive (excluding when menstruating). Human's low levels of fertility compared to other mammals, human male's high investment in their partners and offspring, and human female's concealed ovulation compared to other primates incentivizes human males to mate-guard and continuously have sex with females to help ensure pregnancy; this results in a more monogamous mating system where sperm competition is reduced. These two factors indicate that a certain amount of pair-bonding (at least in the short term) is necessary to produce offspring in most cases instead of wild, promiscuous mating.
As males do not have any obvious clues that a human female is ovulating like with most other mammals, sperm competition is less frequent as females will be infertile most of the time they have sex and males are less likely to have sex with the same female at the same time (as the sexual activity does not converge and peak at ovulation, due to the males being unaware of it, and is thus flattened). Therefore traits that do well under sperm competition will not be selected for in human males as heavily as they are in other primates. It is unlikely that human female copulatory vocalizations would be selected to encourage sperm competition when she would not be fertile the majority of the time she has sex. Thus, most attempts at encouraging sperm would be futile.
It is more likely that the function of female copulatory vocalizations would be to increase paternity certainty. Hence, males invest in their children and do not kill them, to encourage protection from other females and to strengthen the pair bond so that the male doesn't stray and invest his resources elsewhere. One might argue that human copulatory vocalization is a vestigial behaviour, though this does not rule out this behavior existing because it was increasingly selected for a diametrically different reason (promoting monogamy vs. promoting promiscuity) as humans increasingly transitioned from a promiscuous species to a pair-bonding species, as has been argued occurred throughout humanities evolutionary history as low-rank rank males increasingly focused on a resource provision strategy in order to increase their mating success, selecting against female promiscuity in the process.
Interestingly, a 2008 paper reveals that copulatory calls in chimpanzees aren't as simple as to simply serve to encourage sperm competition either. It found that females didn't produce calls mating with low-status males; thus the reason for copulatory calls here is not to call other males to engage in mating; neither did calls result in increased fertility and likelihood of conception. Females were more likely to produce calls with high-status males. They suppressed calls if high-status females were nearby, indicating that the reason behind calls in chimpanzees is expected to avoid aggression from other females and gain favour with high-status males. As a general rule of thumb, female primates prefer to have more control over what male's genes want to pass on and develop behaviours to subvert sperm competition; the fact that female chimpanzees and bonobos are choosier during ovulation seems to support this.
Grooling vs orgasm[edit | edit source]
Similar to vocalizations, also grooling may not necessarily be related to positive emotion in women. Instead, they may grool upon any sexual visual stimulus or stimulus of a powerful man, presumably as a protection of rape to reduce chances of skin tears and infections.
References[edit | edit source]
- Mogielnicki C, Pearl K. 2020. Hominid sexual nature. [Article]
- Suschinsky, K. D., Lalumière, M. L., & Chivers, M. L. (2008). Sex Differences in Patterns of Genital Sexual Arousal: Measurement Artifacts or True Phenomena? Archives of Sexual Behavior, 38(4), 559–573. [Abstract]