Semen and Vaginal Chemistry
Seminal fluid possesses more than just a transport medium for spermatozoa. Seminal fluid also contains dozens of compounds, including ovulatory- and pregnancy-related hormones. Many of these compounds can also be absorbed through the vaginal wall.
Semen is a reproductive fluid. Seminal fluid has been selected over generations to be composed in such a way that it not only houses sperm but also increases probability of conception and pregnancy maintenance (coagulates to retain sperm in the vagina, decreases acidity for sperm survival, etc.). This has a direct effect on the fitness of the human male.
Ney (1986) suggested that the high concentrations of estrone and estradiol, testosterone, dihydrotestosterone, follicle stimulating hormone and luteinizing hormone, prolactin, and 13 different prostaglandins in semen could be biologically active if absorbed by the vagina. Researchers have administered many of these hormonal compounds through the vagina and determined high levels of absorption that elevated blood levels for hours after administration (Schiff et al. 1977; Villanueva et al. 1981).
What is in seminal fluid? Glucocorticoids that aid in the stress response and reaction to pain, androgens that increase sexual desire, endogenous opiates that increase pain relief and pleasure, immunomodulators which increase sperm longevity and appear to have significant effects on the launching of pregnancy, and myotonia enhancing compounds that aid in orgasm. Seminal fluid also contains a host of ovulatory compounds (including estrogens, luteinizing hormone, luteinizing hormone releasing hormone, and follicle stimulating hormone) and over a dozen compounds that aid in the initiation and maintenance of pregnancy. Beyond these, there are also several affiliative and psychotropic hormones and neurotransmitters (see Burch and Gallup 2006 for review).
The major question when faced with these findings is why: why do humans have such elaborate and unique seminal fluid? The answer lies in the evolutionary history of human females. Women are concealed ovulators. Males cannot synchronize insemination with ovulation if they cannot detect ovulatory signals. The chief male strategy then becomes surveillance and support of the female along with consistent coitus, ensuring paternity. In fact, Sillén-Tullberg and Moller (1993) found that this shift from ovulatory signals to concealed ovulation emerged and then triggered a shift in the human mating system, pushing humans to become monogamous. This placed a significant pressure on males, forcing them to choose between monogamy (with its paternal certainty) and promiscuity (with its high reproductive pay off). Males that could, through semen chemistry, better synchronize insemination with ovulation would have a reproductive advantage. Research has shown that other parameters of ejaculate (volume and sperm counts, for example) vary with context of ejaculation (masturbatory/coital) and are greater during coitus. It is possible the same is true for seminal compounds.
Strategies such as this have been found in other species, with “ovulation induction factors” found in several species, and in many, this factor is a potent stimulator of luteinizing hormone (Adams and Ratto 2013). Human males have five times more luteinizing hormone in their semen than circulating blood levels (and also Luteinizing hormone releasing hormone) and researchers have long argued that coitus-induced ovulation occurs in humans as well (Jöchle 1973). If concealed ovulation led to ovulation induction strategies in humans, how does seminal fluid compare in other primates who do not possess concealed ovulation? Chimpanzees, our closest living primate relatives, are conspicuous ovulators. If the composition of human semen has evolved to compensate for the loss of ovulatory signals, then levels of luteinizing hormone and follicle stimulating hormone should differ between human and chimpanzee semen. Levels of luteinizing hormone were lower (and more variable) in chimp than human semen and follicle stimulating hormone was completely absent (Burch and Gallup 2006).
The effect of seminal compounds on female physiology does not end there. The triggering of ovulation does not guarantee conception, and conception does not guarantee pregnancy. Human seminal fluid also contains a host of pregnancy inducing and maintaining compounds that actually assist the zygote in implantation and the formation of the pregnancy (Relaxin; Human Chorionic Gonadotropin; Human Placental Lactogen; Pregnancy-Specific β 1-Glycoprotein; Placental proteins 5, 12, and 14; and Pregnancy-associated plasma protein). Several fertility studies have shown that seminal fluid exposure effects positive outcomes. Live birth rates in couples undergoing In Vitro Fertilization are significantly improved when women engage in intercourse circa embryo transfer (Bellinge et al. 1986; Tremellen et al. 2000). Treating women who suffer recurrent miscarriages with seminal plasma pessaries improves pregnancy success (Coulam and Stern 1993). Preeclampsia studies show a cumulative benefit of semen exposure over time: limited sex or condom use is linked with increased risk (Klonoff-Cohen et al. 1989; Robillard et al. 1995) and the effect is partner-specific (Dekker et al. 1998).
Some of these effects might also be result of a variety of cytokines and growth factors that culminate in improved endometrial receptivity to the implanting embryo. Cytokines have embryotrophic properties and also contribute directly to the optimal development of the early embryo (Robertson 2005). Cytokines and male alloantigens in seminal fluid are sufficient to induce a state of “active immune tolerance” to paternal alloantigen and this priming positively influences fetal survival in later gestation (Robertson et al. 2009).
Perhaps the most fascinating (or at least the most publicized) effect of seminal fluid is that on female psychological functioning. Seminal fluid contains a variety of neurotransmitters and mood regulators; Tyrosine, DOPA, Norepinephrine, Serotonin, Melatonin, and Thyrotropin-Releasing-Hormone. Very little research has been done on the effects of these compounds on the female psyche.
Gallup et al. (2002) reported that females who engaged in intercourse but never used condoms showed significantly lower scores on the Beck Depression Inventory than those who used condoms more frequently and those who never engaged in intercourse. Depression scores between females who used condoms and those who did not engage in intercourse were not significantly different. In then “no condom” and “little condom use” groups, time since last sexual intercourse and depressive symptoms were positively correlated. Being in a relationship, length of that relationship, use of oral contraceptives, and frequency of sex did not affect depressive symptoms.
In short, human semen biochemistry appears to have been selected to suppress the female immune system and increase female sexual interest, bonding, the potential for orgasm, the probability of conception and pregnancy maintenance, partially in response to concealed ovulation. This process also appears to have the byproduct of affecting female physiological and psychological functioning.
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