Biologist Richard Short, while studying apes in the 1970s, noticed an interesting fact that cried out for explanation — namely, the huge size disparity seen in the testicles of different ape species. Gorillas, for example, despite their formidable size, have testicles four times smaller than those of chimpanzees, which weigh four times less than gorillas do.
After studying the structure of gorilla and chimp societies in light of this fact, Short came to the conclusion that male testicle size may be directly correlated with the adulterous proclivities of the females of the species, as well as the amount of competition for females among the males. This theory seems to hold up not only among chimps and gorillas, but also among monkeys, rodents, dolphins, whales, all studied bird species, and likely among human beings as well.
Sperm Competition
Once the pattern was noticed, it wasn't terribly difficult to theorize about the reasons for the observation. In chimpanzee societies, females may mate with several males, who compete vigorously for access. So it is to the males' advantage to have very large testicles and very high sperm production in order to try to outcompete all the other males attempting to fertilize the female in question.
In gorilla societies, by contrast, one dominant male controls access to a harem of females, and therefore has little need to produce excessive sperm or the large testicles needed to hold them, for the paternity of the offspring of members of his harem is practically assured.
Human Testicle Size and Female Polygamy
Human beings fall somewhere in the middle of the male competition/female promiscuity spectrum. The testicles of human males are considerably larger than those of gorillas, but smaller and less productive than those of chimpanzees. This would suggest, according to the theorized pattern, that there is some degree of competition among males for access to females, but not nearly as much as in chimpanzee societies. It also suggests that human females are far less prone to profligate mating than their chimpanzee counterparts.
And indeed, this does seem to be the broad finding in studies of human societies; while males generally attempt to maximize their reproductive impact by attempting to mate with as many females as possible, females generally prefer monogamy, largely committing adultery with only one extracurricular male whose genetic fitness is higher than that of her mate.
Monogamous Birds
It was long believed that many species of birds, including swans and sparrows, were completely monogamous. Monogamy would make a great deal of sense from an evolutionary perspective; male birds can usually do a large part of the work involved in raising offspring: Nest building, food collection, egg guarding, and so on, and so it would be to the offspring's advantage to have the male around.
But later studies showed that birds followed a similar pattern of male competition, female adultery, and testicle size. The bird species with the largest testicles are those where several males mate with one female, just as in chimps; the birds with the smallest testicles are the so-called "lekking" species, where the female chooses her mate from a pool of performing males; and medium-sized testicles tended to appear in colony-living birds like herons and sparrows, where mostly monogamous females commit adultery with one or two other superior males in an attempt to maximize the genetic fitness of their offspring. This is the same pattern found in human societies, an unsurprising finding considering the medium-sized testicles of the human male.
Source:
Ridley, Matt. "Monogamy and the Nature of Women." The Red Queen: Sex and the Evolution of Human Nature. New York: Macmillan Pub., 1994. Print.
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
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