Human male reproductive fluid, separate from spermatozoa (sperm)
Only 5 % of an ejaculation consists of sperm. The remaining 95 % is called “seminal plasma” which consists of a bewildering array of different ingredients, including various hormones, neurotransmitters, endogenous opiates, and immunosuppressants. Burch and Gallup (2006) have theorized that many of the different components in seminal plasma may have evolved to commandeer the female reproductive system in such way as to promote the reproductive best interests of males (Burch and Gallup 2006; Gallup and Burch 2006; Gallup et al. 2012).
Compounds in Seminal Plasma
Just as there appear to be context-specific mechanisms that promote sperm competition by adjusting the number of sperm in the ejaculate to take into account the probability of rival male sperm in the female reproductive tract, Burch and Gallup have championed the view that similar mechanisms operating at the level of the testicles may modify or adjust the composition of seminal plasma to fit the particular situation or setting in which ejaculation occurs. Gallup et al. (2012) refer to this as the “Topping off Hypothesis.”
For example, although it may appear counterintuitive, the chance of conception as a consequence of nonconsensual sex or rape is 2–6 times higher than is true for consensual sex (see Gallup et al. 2012 for a review). Contained in human seminal plasma are several compounds necessary for female reproduction, including follicle stimulating hormone (FSH) and luteinizing hormone (LH). During the ovulatory phase of the menstrual cycle, changes in FSH and LH serve to prompt the maturation and release of an egg. Researchers have theorized that during sexual assault, evolved mechanisms may operate to adjust the levels of these hormones in seminal plasma to increase the likelihood of induced ovulation and conception as a consequence. At least in principle, this hypothesis could be easily tested by assaying semen from victims of sexual assault or by collecting and assaying semen from sperm donors who watch either consensual or nonconsensual pornography while masturbating.
It is entirely possible that evolved mechanisms may exist that also function to adjust different aspects of the chemistry of seminal plasma to fit other reproductive contexts as well (Gallup et al. 2012).
- Burch, R. L., & Gallup, G. G., Jr. (2006). The psychobiology of semen. In S. M. Platek & T. Shackelford (Eds.), Female infidelity and paternal uncertainty. Cambridge: Cambridge University Press.Google Scholar
- Gallup, G. G., Jr., & Burch, R. L. (2006). The semen displacement hypothesis: Semen hydraulics, double mating, adaptations to self-semen displacement, and the IPC proclivity model. In S. M. Platek & T. Shackelford (Eds.), Female infidelity and paternal uncertainty. Cambridge: Cambridge University Press.Google Scholar
- Gallup, G. G., Jr., Burch, R. L., & Petricone, L. (2012). Sexual conflict, infidelity, and semen chemistry. In A. Goetz & T. Shackelford (Eds.), The Oxford handbook of sexual conflict in humans. New York: Oxford University Press.Google Scholar