As was noted in the previous section, although a dominant chimpanzee may try to monopolize time with a female in estrous, most of the competition in this species is carried out at the level of the sperm. The reproductive tract inside a female chimpanzee becomes the battleground for a sperm war that takes place over the course of several days. Only one sperm among all of the billions within her vaginal tract and uterus can win the prize of fertilizing the egg. In fact, only a very small percentage of sperm, possibly as little as 1%, are even capable of fertilization; the rest of the sperm have an entirely different function. This other 99% of sperm consists of what are called blockers, or kamikaze sperm. Their function is to prevent the sperm of other males from reaching the egg. There appear to be at least two types of kamikaze sperm (Baker, 1996). Type A sperm block the passage of sperm that enter the female after they do. Type B sperm actually attack sperm that have been delivered prior to themselves. This sperm competition in chimpanzees has necessitated the evolution of testicles large enough to produce adequate amounts of sperm. This is why the testicles of chimpanzees are much larger than those of men.
The testicles of gorillas are relatively tiny compared to those of chimpanzees. This is because of the sexual monopoly that is obtained by a single silverback male gorilla over his harem of females. In this type of mating system sperm competition is almost non-existent. Human testicle size is somewhat proportionally larger than that of the gorilla. Nevertheless, the general similarity suggests that early hominid mating systems were of a polygynous nature similar to that of the gorilla, where you have a single sexually active male (Hrdy, 1988). Conversely, humans display a greatly reduced sexual dimorphism for body size as compared to the gorilla, suggesting that humans trended toward a greater degree of monogamy in the latter stages of our evolution. Because this shift toward monogamy occurred within the context of a larger social group, evolved behavior and physiology aimed at combating infidelity is part of our legacy as modern humans.
Baker (1996) discovered evidence that human male sexual psychology has evolved to respond to the prospect that his mate has been inseminated by another man during his absence. He found that the volume of sperm a man ejaculates while having sex with his partner is unrelated to how long it has been since he last had an ejaculation; the important variable is the length of time that has passed since he last had sex with his wife. The volume of sperm may be as much as three times that of normal if the man has been separated from his wife for a long period of time. If the men were in proximity of their wives during a similar period and were sexually abstinent, their subsequent ejaculate did not show the same rise in volume. Baker & Bellis (1993, 1995) also found that female orgasm plays a role in sperm competition. When a woman has an orgasm the uterus starts to contract rhythmically, causing sperm to be drawn into the cervix; a kind of vacuuming effect. If a woman has had intercourse with several men within a short period of time, the sperm of the man associated with her orgasm has a much higher probability of fertilizing her ovum than that of men whose copulation did not result in orgasm. When we couple this finding with Thornhill’s data, showing that women have more orgasms with symmetrical men than with less symmetrical men, and with the findings of Gangestad and Thornhill (1998), showing that women during the time of their ovulation show a preference for the scent from symmetrical men, we can see a connection between sperm competition and female mate choice.
Both sperm competition and female mate choice have been evoked to explain the unique anatomical distinction of the human species. Although among our closest living primate relatives, chimpanzees have the largest testicles, humans have the largest penis in terms of length and thickness (see Figure 5.4). Diamond (1992) provides the following anatomical data: In the gorilla the length of the erect penis measures 1 ¼ inches, in the orangutan it averages 1 ½ inches, in the chimpanzee it averages 3 inches, and in man it averages 5 inches in length. Advocates of the sperm competition theory argue that a longer penis provides sperm delivery closer to the cervix and gives sperm a head start in their competition with rivals. Advocates of the female choice theory would argue that ancestral hominid females, at least in part, selected males who could provide more vaginal and clitoral stimulation. In all probability, there is some interaction between both of these processes such that a thicker penis provides more clitoral stimulation and, hence, increases the likelihood of an orgasm that would facilitate delivery of sperm into the cervix. Post coital uterine examinations of women show that sperm retention is positively correlated with the women’s self-reported sexual satisfaction (Graham-Rowe, 1998). Phylogenetic comparisons also suggest that the breasts of human females have evolved as a result of sexual selection pressures