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