SECTION: HUMAN FEMALES COMPARED TO OTHER PRIMATES
From Drmills
Human Female Morphological Adaptations
As we noted earlier, it is generally the case that females of most species are morphologically better adapted to their ecology than are their conspecific males. In general, females are the "correct" size, color, shape, etc., given their environment. Males, due to the added effects of sexual selection, are more often morphologically "deviant" (e.g., with fighting equipment such as horns, or "charming" equipment such as large, colorful tails). In general, across most species, males are morphologically strange.
Despite these generalizations across most animal species, human males are not particularly morphologically strange. Except for their slight size dimorphism and greater upper body strength, human males do not have specialized fighting equipment (e.g., antlers, etc.), nor do they have distinctive coloration or other characteristics to "charm" females. In this sense, human males are remarkably morphologically unremarkable.
In humans, the situation is actually reversed--it is the females that are morphologically "strange," at least when they are compared to other female primates. All female primates, with the one exception of human females, "advertise" their fertile periods, with what is called a period of "estrus." They advertise by changing colors, by emitting scents, and by "presenting" more frequently to males. This causes males to compete for sexual access to the female in estrus. Presumably, the female gets to mate with males who have proven themselves in competition with other males.
In contrast, human females give no clues - visual, olfactory or behavioral - that display their ovulation. Human females have "concealed" their ovulations. In a morphological sense, it is as if human females were trying to hide their fertile periods from human males (and possibly even from themselves). Why? Human females are morphologically strange in a second respect—they have permanently swollen breasts and buttocks. The breasts of other primates swell only when they are pregnant, and then only by a relatively small amount. It is as if human females, with their permanently swollen breasts, were trying to deceive males into thinking that they were already pregnant! If the original purpose of this was designed to inspire male sexual disinterest, ironically, it has exactly the opposite effect today. It may be possible, however, that swollen breasts indicate a woman's fertility status. Indeed, young women tend to have fuller, rounded breasts, and research has shown that breasts may swell during a woman's ovulatory phase. Finally, by primate standards, human females are strange in that they are willing to copulate even during their infertile times of the month. Other primate females are far more discriminating, and generally refuse (or are unable) to copulate except during their estrus periods.
In the following sections, we will examine a few of the more unusual and unexpected aspects of human female morphology.
Concealed ovulation
Any animal that can perceive can be deceived. And the deception need not be performed sneakily, at the level of the plotting mind, but can work unconsciously, at the level of the evolving body.
-- Lynn Margulis and Dorion Sagan, Mystery Dance, 1991, p. 103.
Both sexes may attempt to deceive the opposite sex. Many female mammalian species are only sexually accessible during periods of estrus, a period of time during which she is ovulating and capable of becoming pregnant (Moir and Jessel, 1989). In such species copulation is reserved for the sole purpose of reproduction. For example, the female baboon is only sexually receptive during one week of her cycle, the exact week when she is capable of being fertilized. During this time, she displays powerful visual and olfactory cues that serve as a sexual invitation to males. When she is no longer ovulating, these cues disappear, and the males who were once powerfully drawn to her displays are no longer interested.
In contrast, human females do not display such vivid clues when they ovulate. Her sexual receptivity and appeal to males generally remains constant, regardless of where she is in her monthly cycle. Unlike baboons, human females do not blatantly advertise ovulation through physiological changes in appearance. Without such vivid displays, the female’s reproductive status is largely undetectable. Concealing ovulation from males has served female reproductive interests very well.
However, recent evidence has suggested that there may be subtle cues in humans which may help males detect the timing of ovulation, suggesting that human ovulation may not be completely concealed.
One possible way that the timing of ovulation may be revealed is through women’s body odors that signal the reproductive status of women. A study conducted by Doty et al. (1975) showed that both men and women reported that vaginal odors were less intense and less unpleasant during preovulatory and ovulatory phases than during the menstrual, early luteal, and late luteal phases. However, using vaginal odors to determine the approximate time of ovulation may not be very practical.
Kuukasjaarvi et al. (2004) conducted a study to examine whether men use olfactory cues when distinguishing between potential partners and to identify fertile women. In addition, this study included female raters to see whether the ability to detect the ovulatory period is sex-specific. The results showed that male and female raters showed a preference for odors of women whose cycles were near the ovulatory phase, indicating that both sexes may have the ability to detect women’s reproductive statuses. Another study conducted by Singh and Bronstad (2001) revealed that men rated the odors of shirts during the follicular phase as more sexy and pleasant compared to those worn during the luteal phase.
Other studies, such as Roberts et al. (2004), have found that women are judged as being more physically attractive during the follicular phase.
Although body odors and facial attractiveness may serve as possible cues for detecting ovulation, there are some less obvious changes that signal this fertile period. Asymmetry of women’s paired soft tissue, such as fingers, ears, and breasts, is lowest during ovulation. Furthermore, a female's skin color tends to be the lightest, smoothest, and free of blemishes when they are nearly ovulating.
As mentioned earlier in this section the human female can be sexually receptive, and therefore active, throughout her entire menstrual cycle. In fact, women experience a peak in desire and sexual pleasure directly before menstruation begins, at a time when it is highly unlikely that conception would occur (Margulis and Sagan, 1991). Women can also be receptive to intercourse during pregnancy and nursing. The human male’s sexual interest is not determined by the female’s period of ovulation, nor can the human male tell when the female is ovulating. The male does not become sexually disinterested when conception is not physically possible.
Obviously then, concealed ovulation in humans is seemingly reproductively inefficient in comparison to the estrus cycles of other primates. Because the time of ovulation is unknown, successful conception usually requires many attempts throughout a monthly cycle and can even take years. Why did we evolve this way? There are several different hypotheses, some which are described below (Diamond, 1985):
Hypothesis 1: Concealed ovulation serves to reduce aggression among males, whose cooperative hunting is essential to the group as a whole.
Concealed ovulation helped preserve harmony within early social groups. If a woman showed visible signs of ovulation, it is likely that males would fight for access to her. Expending valuable energy fighting would then steal time and energy away from hunting, and virtually destroy chances of cooperation among the members of the group.
Hypothesis 2: Concealed ovulation evolved to keep "husbands" from straying to sexual partners other than their "wives."
By keeping their mates guessing as to when conception was likely to occur, females were more able to control the male desire for sexual variety. In a perfect reproductively successful male world, "wives" fertility would be easily observable so that he could impregnate her, then abandon her and move on to the next fertile female. Concealing ovulation therefore helped to keep the "husband" around and interested throughout her cycle.
Hypothesis 3: By being in a constant state of potential sexual receptivity, a female could potentially receive more resources from males.
By being sexually receptive all the time, females have a better chance of benefiting from the resources of males. For example, a male chimpanzee is more likely to share the meat from a small kill with an estrus female as opposed to a non-estrus female (as noted by Donald Symons of UCSB). It is also possible that women may have developed the ability to appear as if they are in a constant state of estrus in order to receive a regular food supply from the male hunters. He also suggests that females’ constant state of attractiveness could serve to seduce a genetically superior male and pass his genes to her offspring.
Hypothesis 4: Concealed ovulation evolved to sustain male commitment to a female – by requiring him to guard his paternity.
If a male could recognize signs of ovulation, he could use that knowledge to fertilize a female and then safely go off philandering the rest of the time. Not knowing the time of ovulation, a man must copulate with his wife often to have a chance of fertilizing her, leaving him less time to dally with other women. The male gains confidence in the paternity of his children, and he needn’t worry that his wife will suddenly attract many competing men by turning bright red on a particular day.
Hypothesis 5: Concealed ovulation evolved to confuse paternity, and thus reduce infanticide.
Sarah Hrdy was impressed by the frequency with which many male primates kill infants not their own. The mother is thereby induced to come into estrus again, and often mates with the murderer. As a counter-measure, women evolved concealed ovulation to manipulate men by confusing the issue of paternity. A woman who distributed her favors widely would thereby enlist many men to help feed (or at least not kill) her infant, since many men could suppose themselves to be the infant’s father.
A study by Heistermann, Ziegler, van Schaik, Launhardt, Winkler, and Hodges, (2001) monitored the hormone content of feces and showed that female langurs have greater periods of receptivity and less predictable periods of ovulation. This, combined with multiple matings, decreases the assurance of paternity of those male langurs that had reproductive access. The authors argue that this strategy was adopted by female langurs to confuse paternity, to garner increased resources, or to reduce the likelihood of male langurs harming their young.
Hypothesis 6: Concealed ovulation evolved to keep women ignorant of their own ovulation.
Concealed ovulation developed to keep women uncertain about the time when they could conceive. One motivating factor for this could be the intense pain and dire risks of childbirth. If a woman was certain about when she ovulating, she could avoid intercourse during that time to prevent pregnancy, thus leading to fewer offspring than women who cannot sense their ovulation.
Burley (1979) states that concealment of ovulation from the outside world, but more specifically from the female herself, evolved as a method to combat abstinence, or overall birth control. Burley goes on to explain that the explicit processing and conscious thought present in humans could potentially interfere with the reproductive success of the species. Specifically, females may choose to avoid copulation during times of fertility in order to prevent the discomfort of childbirth, the burden of raising children, or for other various cultural reasons. The avoidance of pregnancy could seriously threaten the viability of a species in general, or potentially threaten communal living groups who depend on reproduction to maintain population. Burley’s theory is possibly supported by the discrepancy present in the birth rates of 1st world countries vs. those in 3rd world countries. Those present in wealthier cultures have greater access to contraceptives and medical knowledge, thus their birth rates are lower as women are better able to control their own reproduction. This is not true of poor countries or cultures where modern medicine is not mainstay.
Other Theories
Other research indicates that there may not have been directly relevant selection pressures to facilitate the evolution of concealed ovulation. External factors, such as bipedal locomotion, and other physiological changes may have precipitated the lack of external cues of ovulation. Additionally, research indicates that smell is still a significant signal of fertility in women, and that smell may have been all that was required in the environments our ancestors lived in.
Pawlowski (1999) advances the argument that overt female ovulation may have diminished in human females, and consequently cyclical receptivity of males, due to various reasons other than sexual selection pressures. Pawlowski supports this claim in two ways; first, she establishes that constant sexual receptivity is not exclusive to humans, and second, she provides evidence that external signs of ovulation may have diminished as the physical needs of the species changed.
When looking at sexual receptivity in various primates, Pawloswski (1999) notes that constant receptivity is not human specific. Several species of primates, such as orangutans, have shown sexual receptiveness regardless of where in their reproductive cycle they are. Also, current research indicates a leveling off of the hormones associated with oestrus, a period of heightened reproductive drive in females. This downward trend could potentially be explained by “endocrinological, ecological, and cultural conditions.”
In regards to the physical cues associated with ovulation and how they are not present in human women, Pawlowski hypothesized that as our species evolved various physical aspects such bipedal movement or the increase in adipose tissue, that may have interfered the ability to display a physical sign of ovulation.
In order to promote reproduction through copulating at peak fertility, most mammalian species developed estrus, which is a female's physiological, and quite obvious, signal to males that she is fertile and able to copulate. Estrus is sometimes referred to as "heat," where females usually only copulate when they are fertile. However, unlike most mammals who have evolved ways to flaunt their fertility, human females developed concealed ovulation that is so effective, females themselves aren't aware when they are ovulating. The question of how these women were the most successful in producing and raising offspring has led some to believe that their ovulation is detectable, though just not as obvious. Statistically, others have found that no significant differences are apparent in reality. For example, research has been conducted on pheromone secretion, hypothesizing that a signal of ovulation is in fact given, which is intended to increased sexual activity. In response, Brewis (2005) conducted statistical research on the patterns of copulation of couples throughout the female's cycle, and found no greater prevalence of copulation in the four days surrounding ovulation. In fact, recorded peak times of copulation were found during every stage of the cycle, with the exception of menstruation. Thornhill (2007) also reported that in couples, no higher instances were found of proceptivity (initiation of mating) and receptivity (allowing mating) by females around her peak ovulatory phase. Another evolutionary theory suggests that concealed ovulation developed as sexual activity lost its exclusive reproductive meaning early in the evolution of humans (Benagiano 2009). Since copulation became more recreational, the ability for females to exhibit "extended sexuality" became the favored trait. Therefore, females who were capable of intercourse at any time, fertile or not, were more sought by males (Gangestad & Thornhill, 2007). The increase of recreational sex had a specific impact on women who displayed their fertile phase. Females with displayed estrus, who could also mate outside of their fertile phase, were able to use this as a sort of birth control. In knowing when they were most fertile, they could avoid copulation during that time to avoid pregnancy. Clearly, avoiding pregnancy is not favored in evolution. Therefore, the females who did not display estrus, whose ovulation was not as obvious, would continue to copulate throughout their peak fertility. Whether their intentions were to reproduce or not, those who were unable to perceive their ovulation eventually became pregnant since they were unable to know when to abstain. As evolution continues on, women have adapted ways to more successfully adjust to men, while men adapt ways to better read women, and all the while nature is adapting new ways to trick both of them into reproducing. In a study conducted in 2004 (Spencer et al.), researchers found that different pheromones produced by ovulating women, and breasfeeding women had significant influences on the sex drives of the people around them. The pheromones produced by breastfeeding women increased the sexual desires for the women around them in different ways. Those who were in relationships experienced an increase in sexual desire, while the women who were single experienced an increase in sexual fantasies. The pheromones produced by ovulating women more obviously targeted men who found the demeanor and walk of ovulating women more attractive than of women who were not ovulating, or who were on the pill. According to theory, the adaptations from estrus to concealed ovulation prove to have been favorable for reproduction, for those at the time. Though it seems confusing that being unaware of fertility would in fact be the most beneficial, researching the process step by step does reveal a sound theory overall.
Estrus Rediscovered? Women's colorful clothes and makeup
As we have learned, the human female body gives no obvious signs to indicate ovulation. However some theorists believe that women choose to wear bright clothing or especially noticeable make-up to mimic estrus (Margulis and Sagan, 1991). Bright clothing attracts attention to the female and her body, which could be similar to the way that some female primates change color to attract mates during ovulation. The clothing and make-up thus act as a signal to men. Margulis and Sagan admit that there is an "eerie similarity between the swelling and brightening of the lower parts of estrus monkeys and the clinging Day-Glo hot pink pants of a street corner prostitute" (1991, p. 121).
They also suggest that perhaps with the development of clothes, we have lost physical signs of estrus that were once so prominent in our ancestors, but we still have psychological intuitions of what such signs may represent. Women mimic these signs with makeup and clothes. The make-up industry is continually coming up with new eyeshadows and lip glosses to enhance women’s natural beauty. Victoria’s Secret has been quite successful marketing their push-up bras and control-top pantyhose to enhance a natural looking voluptuous female figure (in other words, to enhance a woman’s signs of estrus).
Because women’s sexual desires fluctuate across the cycle, as the time of ovulation approaches, women dress to impress potential mates. Haselton et al. (2007) found that women’s sexual motivation tends to increase near ovulation. During periods of high fertility, women report an increased occurrence of extrapair copulations, masturbation, and sexual fantasies. Women are also more likely to engage in “self-ornamentation” through grooming and wearing clothes that are more attractive during the high fertility phase of the ovulatory cycle. Photographs were taken of women in their high fertility phase and low fertility phase and judges rated those who were “trying to look more attractive.” The photographs revealed that during women’s high fertility phase, women engaged in active ornamentation (Haselton et al., 2007). Grammer et al. (2005) linked the clothes women chose to wear with their motivations. These researchers interviewed women at a discotheque and found that women who rated their clothing as sexy and bold also stated that their goal was to flirt or find a sex partner.
According to literature reviews of the 70's, 80's, and 90's, the general consensus of research of human ovulation concluded that females experienced concealed ovulation, as opposed to estrus. Unlike most other mammals who, in estrus, display their fertile phase physiologically and only mate during that time, human females are unaware of when they are most fertile and are both able and willing to copulate at any phase of their cycle. However, more recent research has found several examples of increased sexual behavior and perceived attractiveness during the time a female is ovulating, suggesting that the fertile phase is not completely undetectable.
Many studies touch on the concept of "co-evolution." Unlike most mammalian species whose females present their fertility to males, human females generally do not advertise when they are fertile. As women have evolved a way to conceal their ovulation, men have also evolved ways to detect the subtle differences that are apparent when females are near ovulation (Gangestad 2005). A study conducted in 2005 (Provost et al.) measured the perceived attractiveness of women by men, when women were shown walking. Men rated 35 women, who were shown once during their non-fertile phase, and once during their fertile phase. Overall, the men rated the ovulating women's gaits as significantly more attractive. Researchers therefore concluded that some information regarding female fertility is encoded in the way women walk, signaling a heightened sense of sexuality.
In addition, evolutionary theory states that if women do experience a heightened sense of sexuality similar to the effects of estrus when they are fertile, their preferences in a mate should also change. In reality, female preferences do significantly shift. When women are not ovulating, they prefer slightly more feminine facial features in men, who appear to display caring and kind qualities that one would look for in a long-term mate. However, near mid-cycle when females are most fertile, they abandon their preference for long-term mating qualities and instead look for men who display the masculine traits that the female would want her offspring to have (Gangestad 2007). Women who were not on hormone regulators, such as the pill, began to prefer men with traits associated with body symmetry, social dominance, facial and body masculinity, masculine vocal qualities, and tall height. The article also suggests that this adaptation encourages women, however slightly, to stray from their relationships during their fertile phase to seek out extra-pair copulations with males who more closely fit the masculine description (Gangestad 2007).
Similar to this hypothesis, Gangestad et al. (2005) found that even though the rates of extra-pair copulations increased for women mid-cycle, their in-pair copulations remained average. Also, women who were in relationships with more asymmetrical men experienced greater interest in extra-pair copulations during ovulation than women in relationships with more symmetrical men. These findings also support the hypothesis that women seek copulation with men whose genes they would want for their offspring.
In evolutionary response to women seeking extra-pair copulations, men have adapted to become more protective of their partners when they are fertile. Women report their partners increased vigilance on knowing their whereabouts when they are mid-cycle, and their partners report increased concern (Haselton & Gangestad, 2006). Men also react to other men differently during that time. They perceive dominant men as even more so, which indicates an increased awareness of possible competition in other men.
The female orgasm: Byproduct or exaptation?
There exists disagreement among researchers regarding the function of the female orgasm. In order for it to be adaptive, it must somehow promote fitness. In other words, a trait is considered an adaptation when "'there is some evidence that it has evolved (been modified during its evolutionary history) in specific ways to make it more effective in the performance of [a particular task]" (Puts, p. 104). In the human male, the orgasm is a sexually evolved adaptation with the purpose of creating muscle contractions that cause the penis to ejaculate. Basically it is a delivery system in which the male can move semen from the male to female in order to fertilize her eggs and pass on his genes. Several theories suggest that female orgasm is adaptive, by serving to make fertilization more likely or by contributing to female mate choice. Others disagree, and suggest that the female clitoral orgasm is simply a physiological by-product of the male capacity for orgasm.
From Buss 1999, p 175-176 :
"Once it was thought that a woman’s orgasm functioned to make her sleepy and keep her reclined, thereby decreasing the likelihood that sperm would flow out and increasing the likelihood that she would conceive. But if the function of orgasm were to keep the woman reclined so as to delay flowback, then more sperm would be retained. That is not the case. Rather, there is no link between the timing of the flowback and the number of sperm retained (Baker & Bellis, 1995).
Women discharge roughly 35 percent of sperm within thirty minutes of the time of insemination. If the woman has an orgasm, however, she retains 70 percent of the sperm, ejecting only 30 percent. This 5 percent difference is not large, but if it occurred repeatedly, in woman after woman, generation after generation, it could add up to large selection pressure over evolutionary time. Lack of orgasm leads to the ejection of more sperm. This evidence is consistent with the theory that a woman’s orgasm functions to draw the sperm from the vagina into the cervical canal and uterus, increasing the probability of conception."
Some researchers view the female orgasm as a means of positive reinforcement, promoting pair bonding and keeping the female from being unfaithful to her mate. Still others suggest that females respond to genetically superior males who demonstrate physical and intellectual fitness. A choosy female might only orgasm when she is with a male to whom she feels genuine attraction. He proves his fitness to her by being attentive and sensitive to her needs.
Miller suggests that “pleasurable orgasms promote future copulations with the favored male through reinforcement learning and emotional attachment” (Miller, 1998, p. 19). In other words the purpose of the female orgasm is to be pleasurable. This theory would lead one to predict that the male that provides the most stimulation would be most preferred. That pleasure, in turn, creates an emotional bond between the male and female and increases the chance that they will copulate again and possibly form a long-term relationship. In addition to being a motivator in the relationship, Miller also suggests that another “role for orgasms is to advertise happiness” (Miller, 1998, p. 19). In other words, the orgasm advertises a woman's pleasure with her partner and is an indicator of “female sexual satisfaction, commitment, and fidelity” (Miller, 1998, p. 19). So, not only does the female orgasm create an emotional bond, but in reality it indicates her exclusivity to him.
If a woman finds a sexual partner that has the ability and interest in satisfying her, she can then conclude that this attention paid to her would also translate into attention that this partner would pay to their potential offspring. The idea is that a good, attentive lover could make a caring, devoted father, and thus increase the resources and improve the children’s chances for survival. The emotional component of sexual orgasms for women is imperative, for women that are secure in relationships are more likely to experience orgasms often.
An alternative argument for the function of the female orgasm comes from Comer et al. (1995), who hypothesized that the female “orgasm is an adaptation for manipulating the outcome of sperm competition resulting from facultative polyandry” (Comer et al., 1995, p. 1601). They suggest that the human female is mildly polyandrous as a strategy for attaining the best possible genes and pathogen resistance for their offspring. The authors argue from the standpoint that women judge male fitness or physical attractiveness through fluctuating asymmetry; the lower a male's fluctuating asymmetry, the higher heritable fitness of the male. The importance of the orgasm in this study is that they believe “orgasms occurring near the time of or after male ejaculation particularly favor sperm retention” (Comer et al., 1995, p. 1603). Therefore, it increases the chances of pregnancy. In an empirical test of their hypothesis, Comer et al. (1995) surveyed heterosexual adult couples. They found that “as predicted, women with men who possessed lower fluctuating asymmetry were reported by both partners as having more orgasms during copulation” (Comer et al., 1995, 1601).
A subsequent follow up study by Bleske et al. (2000) was designed to replicate Comer et al.’s (1995) study on female orgasm and attractiveness. Instead of basing the study on fluctuating asymmetry, they focused on physical attractiveness and they controlled for variables such as the womens’ relationship satisfaction, relationship duration, and age. The results indicated that “women mated to more attractive men (relative to women mated to less attractive men) are more likely to report an orgasm at their most recent copulation” (Bleske et al., 2000, p. 299). However, Daly and Pound (2000) argue that “at present there is no evidence that the female orgasm, copulatory or otherwise, have any effect on conception rates” and “it is yet to be shown that a woman who copulates with two men during a single fertile period is more likely to conceive a child sired by the one whom she experiences a copulatory orgasm” (p. 620).
Other theories of the female orgasm suggest that it is not an evolved adaptation, but simply a by-product from structural similarities of male morphology. Both the clitoris and the penis grow from the same type of cells in the fetus. Left undifferentiated, the clitoris forms in the female fetus. When differentiated (masculinized), a penis forms in the male fetus. Therefore, the clitoral orgasm is merely a side effect of the male capacity for orgasm and serves no adaptive purpose.
The presence of an evolutionary byproduct should be seen as "[sharing] a common developmental origin with an adaptation" (Puts, p. 105). A commonly referenced byproduct is that of the male nipple. Differing from the female nipple (which is an adaptation), the male nipple most likely evolved due to the developmental link that males share with females. However, as Puts (2005) points out, merely explaining the developmental connection between males and females does not fully explain the male nipple as an example of a byproduct (p. 105). As it is concluded, not only should the connection with adaptations be defined within a byproduct, it should be recognized that byproducts do not "appear to have been modified over their evolution to provide function efficiently" (Puts, p. 105). Where adaptations show the function of the product or action, the byproduct views express an absence of proposed functions.
The byproduct approaches have appeared to revolve around the type of tissue creating the clitoral and inter-vaginal regions that result in a female orgasm with stimulation. As it was pointed out by Venis (2005) "Lloyd prefers the theory that since the penis and clitoris arise from the same undifferentiated embryological organ, women get the erectile and nervous tissue necessary for orgasm as a by-product of the selection pressure for the male-sperm delivery system" (p. 800).
It should also be understood that the female orgasm is not the most consistent and predictable sexual characteristic. As it has been stated, "human sexual tactics are condition-dependent" (Thronhill, p. 99). Each female may experience similar physiological responses during an orgasm (ie. deep breathing, and contraction of the uterus), however, the psychological responses and actions taken by each female will realistically differ. Particular women orgasm within particular situations, these climaxes should not be depicted as an absolute occurrence. For example, a female's ability to have an orgasm may be partly biologically-based. Dawood, Kirk, Bailey, Andrews, and Martin (2005) found that genetics can account for 31% of variance in the frequency of sex induced orgasm, 37% of the variance of the frequency in non-sex induced orgasms with a partner, and 51% of the variation in the frequency of orgasms during masturbation.
Topic: The Role of Oxytocin in Bonding and Attachment
Contributor: Stacey Hewitt
Class: Psyc 452, Spring 2009 
Recent research has linked the hormone oxytocin to attachment behavior in both mother-child relationships as well as male-female sexual relationships. This same hormone is released during lactation and childbirth as well as during orgasm and it fosters attachment in both scenarios. Oxytocin can help us to better understand the dynamic of sexual relationships by looking at the bond between a mother and child. Due to the release of oxytocin during orgasm, woman can become attached to their partners similarly to the way that they become attached to their children. This provides a possible explanation for why men can have casual sex much easier than women and tend to be less attached than women in sexual relationships. For women, attachment to sexual partners can be as strong as the attachment of a mother and child. Oxytocin can also provide an explanation for the female orgasm. Males may be able to ensure that their partners are faithful and reduce paternity threat by bringing their partner to orgasm and causing the release of oxytocin.
Attachment between a mother and child can be essential to the child’s survival. If the mother is not attached to her child she may refuse to feed or protect her baby and greatly threaten the child’s chances of survival. In a study conducted by Thomas Insel (2000) rats were injected with oxytocin and within 30 minutes they began to display maternal behaviors. Other neuropeptides did not have this same effect. Without oxytocin rats normally avoid their offspring all together. After the injection of oxytocin, however, they built a nest for their pups, exhibited grooming behavior and even defended their offspring from an intruder at their own risk. Oxytocin is strong enough to cause a non-maternal species to exhibit even the most extreme maternal behaviors.
Oxytocin also plays a very important role in sexual relationships between females and their partners. In a study done by Blaicher et. al. (1999) oxytocin levels were measured in females before and after sexual stimulation. The levels of oxytocin in the females were significantly higher after orgasm than they were before, supporting the fact that oxytocin is also released during orgasm. Due to this release of oxytocin, females will form a bond with their sexual partner. It is therefore difficult for women to have casual sexual counters because although they may want that type of interaction, they cannot control the release of oxytocin and a bond might be formed anyhow. From the male perspective, the release of oxytocin in females and the inability to have casual sex is beneficial. If the female becomes attached to the males she will be less likely to seek out other sexual partners and the male does not have to worry about paternity.
Research by Marazziti et. al. (2006) suggests a link between oxytocin and anxiety of romantic attachment. Their findings suggest that oxytocin may serve to counteract anxiety due to their findings that levels of oxytocin were positively correlated with anxiety level. When an individual has a severe anxiety of dating, their body may produce more oxytocin to mask this anxiety. Much of the anxiety regarding romantic relationships is the result of trust issues. If a female with trust issues also has an increased level of oxytocin, she will become attached to her partner in spite of this anxiety. She can put aside her fears regarding the relationship and actually try and form a bond instead of avoiding the situation all together. Companies have even developed products containing oxytocin for individuals with anxiety regarding relationships (see related websites). These products are based off of findings that raised oxytocin levels in one person can result in raised oxytocin level of those around them. If someone was to increase their oxytocin level then then their partner would be more willing to commit and/or trust them.
References
Blaicher, W., Gruber, D., Bieglmayer, C., Blaicher, A.M., Knogler, W., Huber, J.C.(1999). The Role of Oxytocin in Relation to Female Sexual Arousal. Gynecological and Obstetric Investigation, 47, 125-126.
Insel, T.R. (2000). Toward a neurobiology of attachment. Review of General Psychology, 4(2), 176-185.
Kendrick, K. (2000). Oxytocin, motherhood, and bonding. Experimental Psychology, 85(s1), 111-124.
Marazziti, D. (2006). A relationship between oxytocin and anxiety of romantic attachment. Clinical Practice and Epidemiology in Mental Health, 2(28).
Related Websites:
http://www.verolabs.com/
http://www.apa.org/monitor/feb08/oxytocin.html
Permanently swollen breasts
Only human females develop and maintain full breasts from the time of puberty. Female primates such as chimpanzees, orangutans, and gorillas develop full-looking breasts only during lactation. When they are not lactating, their breasts are relatively flat. Male chimpanzees ignore females who are producing milk, knowing that lactating females are not fertile. Male chimps pay great attention, however, to flat-chested females, who, presumingly during ovulation, are fertile. This stands to reason that, like chimps, ancestral human males should have found swollen breasts unattractive since during lactation the female is not fertile. This poses a problem a problem for evolutionary psychologists. Why do human females differ so greatly from other primates? How could swollen breasts, an obvious clue to lactation and infertility, possibly contribute to fitness? Why do human males generally find full-looking breasts to be so attractive, so much so that many women artificially enhance their appearance through cosmetic surgery?
Desmond Morris suggests that female breasts resemble "a pair of fleshy, hemispherical buttocks." He reasons that this resemblance serves to bring the male’s interest around to the front of the female body, which encourages ventral intercourse, increases the chances that the female will orgasm, helps bond the couple, and maintains the monogamous, parental unit. Morris thinks that the male attraction to smooth, rounded buttocks is inborn and could be the "key female gender signal for the human species," even if mimicked in the form of a pair of breasts.
Perhaps the evolution bloated breasts and buttocks can be simply explained by their indication of health of the female. An unhealthy, malnourished female may not be able to maintain adequate fat reserves, burning up the fat to stay alive. It would not be in her reproductive interest to become pregnant in a time of ill health, and many females actually stop menstruating and ovulating when fat reserves are spent. Male mate choice may have favored females who displayed health and fertility through adequate fat reserves.
Yet another explanation may be closely knit to concealed ovulation. Bloated breasts may have hidden fertility from jealous and philandering males. If a female can maintain a consistent appearance throughout her cycle, she can manipulate her mate into staying close to guard his paternity, forgoing other mating opportunities. This consistent ambiguous appearance may have also served to keep a male interested and attentive, so that he may gain sexual access often enough to impregnate her, and not abandon her as soon as she became pregnant.
The hypothesis of Furnham, Dias, and McClelland (1998) is based on the notion that the form and structure of an individual female can correlate with judgments passed on the female’s attractiveness, age, and the appeal for a relationship. The findings suggest that even though evolved permanently swollen breasts are more alluring to males there are restrictions to a female’s attributed attractiveness. For the human breasts to make an effect on males, in reference to attractiveness, breast size is compared to overall body weight and waist-to-hip ratio. Jasieńska, Ziomkiewicz, Ellison, Lipson, and Thune (2004) found that the reason males focus in on the breasts size and waist-to-hip ratio is because it serves as cues to fertility. The study found that women with large breasts and low waist-to-hip ratio had higher fertility assessed through the daily measurements taken of 17-β-oestradiol and progesterone, two hormones essential for the capacity to reproduce successfully. Females in these categories had 26% higher mean 17-β-oestradiol and 37% higher mean mid-cycle of 17-β-oestradiol levels than women in other categories of body shape, such as low waist-to-hip ratio with small breasts or large waist-to-hip ratio with large breasts (see Figure 1). Swollen breasts not only help males asses the fertility of a female but also benefit the female with attaining a healthy body for procreation.
Manning et al. (1999) argue that breast size is a strong indicator for body size and composition, which leads to a good set of genes. Females with superior genes produce large symmetrical breasts. Fluctuating asymmetry can also be connected to menstrual cycles. Manning, Scutt, Whitehouse, Leinster, and Walton (2003) found that breast size varies throughout a female’s menstrual cycle. Fluctuating asymmetry is highest at the beginning and end of an ovulatory cycle when the female is the most infertile and lowest in the middle of the cycle when the female is most fertile. Thus, these fluctuations in breast size indicate when a female is ovulating. For males, swollen, symmetrical breasts can be indicative of when a female is most fertile (the prime for copulation and chances of pregnancy).
The hymen: What function?
The hymen, a word meaning marriage or matrimony in French, is a thin fold of tissue that partly or fully covers the opening to the vagina in many virgins. Monkeys and apes are not known to have hymens, nor is any other animal other than the human. The purpose of the hymen or how it evolved is uncertain, and it may have come about as a genetic mutation.
In the beginning stages of the development of the fetus there is no opening into the vagina at all. A thin layer of tissue that covers the vagina at this time usually divides incompletely leaving excess skin prior to birth, forming the hymen. Some women have no hymen at birth at all, due to the tissue dividing completely while they were still in the womb. The size and shape of these openings varies greatly from person to person (Emans, 2000).
There are about twelve different structures of the hymen that have been noted when females are born. The first hymen is called annular. This hymen forms a mid-size ring around the opening of the vagina. When this hymen starts to erode from sexual activity or other activities such as sports, the hymen starts to be become less ring-like. This forms a crescentic, or lunar, hymen. A moon-shaped flap of skin forms above the vaginal opening. A female with some sexual or masturbatory (internal) experience most likely looks like this. When the hymen is examined by professionals for signs of sexual abuse, they are usually most interested in the posterior part of the hymen, from the 3 o'clock to 9 o'clock position, which is normally where the hymen tears when the vagina is first penetrated. When women give birth the complete structure of the hymen is gone. One in 2000 girls is born with an imperforate hymen, which causes great complications in the vaginal opening. Often at birth, doctors will cut holes in this type of hymen to promote better health. The cribriform hymen is rare hymen that has lots of tiny holes that let menstrual and other fluids go through with no problem but tampons may be very uncomfortable. An interesting type of hymen is a rare denticular hymen, so called because it looks like a set of teeth surrounding the vaginal opening.
The hymen may have been selected for by males who wanted to ensure that their partners were virgins. In many societies which value chastity, the significance of the hymen is a traditional or religious belief that an intact hymen indicates virginity. A lack of previous sexual activity then signifies a lack of infidelity, promiscuity and cuckoldry. Males may have selected for females with hymens, and provided preferentially for females who could prove their chastity and fidelity.
However, it is impossible to confirm that a woman is not a virgin by examining the hymen (Otis, 2008). A physician checks for the appearance of the hymen routinely of baby girls at birth and again during all future pelvic examinations to make sure it doesn’t have any bad effect on menstrual cycles or causes any physical ailments. In cases of suspected rape or sexual abuse a detailed examination of the hymen may be carried out, however the condition of the hymen alone is often open to misinterpretation due to the number of possible ways in which one can many ways to rupture the membrane, especially if the patient has reached puberty (Emans, 2000).
Menopause
Menopause is the point in a woman’s life, usually between the ages of forty-five and fifty, when menstruation stops, and she loses the ability to conceive. While the biological cause of reproductive senescence (menopause) has been known, i.e. due to an "exhaustion of ovarian oocyctes (Kirkwood & Shanley, 2001, p. 282), there is no clear reason why oocyte stock did not increase as the lifespan of Homo sapiens increased. Such reproductive senescence is a rare phenomenon in the natural world, especially among primates. After all, why would any organism develop traits that lead to a sacrifice of fecundity? For this reason, menopause has proven to be an evolutionary puzzle; one often shrouded in speculation. This is especially the case for human females where its occurrence is an inevitable part of the aging process. Several evolutionary explanations have been proposed to explain this phenomena. These explanations center around two themes. The first is that menopoause as a byproduct of increased quality of living, and thus lifespan among humans. The second is that menopause is an adaptation by which women switch over from direct reproductive effort (i.e. mating and producing new offspring), to parental investment and kin investment, which may take the form of grandmothering.
The Altriciality-Lifespan hypothesis suggests that if a female lives long-enough she will eventually become infertile (Pecci, 1995). This notion is supported by some aenecdotal evidence. For example, in the wild it is very rate to find a mammal that lives long enough to reach a stage of infertility, however one report by Jane Goodall in 1975 identified an approximately 50 year old female chimpanzee who did go through menopause. In captivity however, where the lifespan of animals tends to exceed that of their wild counterparts, menopause is common, such as with Rhesus Monkey's who live into their thirties and commonly go through this "change of life". Evidence from physical anthropology also supports the byproduct hypothesis. For example, skeletal records indicate that is wasn't until the Holocene that many women lived past their fourth decade. This suggests that expected menopause might be an incidental side-effect of increased life-expectancy and a newly emergent feature of human female sexuality.
The Altriciality-Lifespan hypothesis, however, is not the most widely accepted explanation for menopause. Rather, the evolutionary logic of the "Grandmother hypothesis" has garnered the most support (Kuhlem, 2007). While multifaceted in it's explanation, this hypothesis can be summarized based on what we have already learned about kin selection theory. By switching from a reproductive strategy of producing new offspring to a strategy that focuses on aiding current offspring and closely related family, a woman would produce several possible benefits for her kin, each of which translates to reproductive fitenss (Hawkes, O’Connel, Blurton Jones, Alvarez, & Chranov, 1998). For example, by producing fewer new offspring a woman would decrease the amount of resources necessary to gestate, lactate, and care for a new child. This would open-up more resources to the offspring that she currently has, reducing offspring competition. The same benefit might extend to other kin. Further, by ceasing direct reproduction she would increase the liklihood that she would not leave a young, helpless child behind due to of age-related mortality of either herself (e.g. death during childbirth) or the father (absent father hypothesis: Kirkwood & Shanley, 2001). Evidence for these benefits have been identified in several cultures. In two different studies, Kuhlem, and Hawkes and her collegues, showed that pre-and-post menopausal women have nearly identical foraging rates in hunter-gatherer tribes. Other findings by Sear and colleuges (2000) showed that in a population of rural Gambian children only the maternal grandmother's had the ability to imporve the nutritional status of their grandchildren, illustrating the importance of maternal over paternal investment.
Further, Shanley, Sear, Mace, & Kirkwood (2007) tested two of these evolutionary explanations for menopause: the likelihood of maternal mortality with age and the grandmother hypothesis, assessing whether menopause enhanced fitness in comparison to women whose fertility was continued past the age of 50. Their results yielded the fact that child survival depended on maternal survival, but only during the first two years of life. The maternal grandmother also had a significant influence on child survival after the first two or three years, thus supporting the grandmother hypothesis. Although the researchers did not find strong statistical support for the reason why menopause occurs around the age of 50, they hypothesized that a woman living beyond 50 and remaining fertile would require the maintenance and repair of a woman’s body and her ovaries, a process that may be too costly. Each of these studies identified a reproductive division of labor between mothers and grandmothers which was argued to have evolved in order to suppliment the nutritional needs of the mother for times of pregnancy and decreased foraging ability. By aiding one's kin, a woman is able to ensure a larger number of her genes survive to future generations than she could produce directly.
More recently, some scientists have argued that neither the Grandmother hypothesis nor the Altriciality-Lifespan hypothesis assess the complexity of the cost-benefit analysis inherent in evolution by natural selection. They aruge that only by synthesizing these two perspectives under the framework of life-history theory (LHT) it is possible to accurately assess whether or not the fitness benefits gained by menopause (decreased mortality, better ability to care for existing offspring, ability to assist in the care of grandchildren) outweigh the cost of producing offspring past the age 40. LHT has been employed by researchers to explain how organisms allocate resources toward different goals. Because there are a limited amount of resources, the way in which an organism allocates its resources (toward either mating or somatic effort, for example) is likely to reflect the environmental context. The strategy that works best, in other words, depends on the situation. Hall (2003) used an energetics-based approach to explain menopause. Energetics “refers to nutrient transformations of a physical, chemical, or biological system, and thus provides the metabolic cost of key adaptations” (Hall, 2003, p. 84). Hall identified that negative biological influences, such as tobacco smoking, can cause early menopause and increase the pace of egg cell loss. Older females would be expected to have fewer menstrual cycles due to the deterioration of oocytes and the increasing chance of early fetal loss. Menopause also decreases the chances of a woman to die during childbirth, especially if they are over the age of 50. In addition, due to the fact that menstruation consumes energy, menopause would help older women save their energy for other activities, such as investing in other kin. Such a combined model has been quantitatively assessed and has demonstrated that the enhanced fitness produced by menopause does in fact outweigh the costs (Shanley & Kirkwood, 2001). A cost/benefit analysis of each perspective separate from the other reveals that neither the benefits suggested by the Altriciallity-Lifespan hypothesis nor the Grandmother hypothesis can enhance fitness to where it would outweigh the costs of forgoing further reproduction. Furthermore, if true, grandmothering would strengthen selection against late-acting deleterious mutations by increasing fitness of longer-lived females through the reproductive success of their daughters (Johnson & Kapsalis, 2004). This is an intriguing line of research and one that is sure to insight to this evolutionary puzzle.
