The radio was on but I wasn't paying much attention. A woman was being interviewed about a book. Suddenly my attention was grabbed. She claimed that promiscuous male sexual behaviour characteristic of some animal species correlated with the relative size of the species' male genitalia. By this criterion, she reckoned that human males are biologically predisposed to be mildly promiscuous.
This was news to me and, reckoning that my readers would like to be kept informed on such matters, I decided to investigate. What I found was interesting, but unconvincing.
First, let me briefly review the primary theory in biology, the theory of evolution by natural selection. Every species contains natural variety among its members.
Those variations help their owners to cope better with the natural environment, allow them to survive and reproduce more successfully than their fellows.
This naturally selects these particular characteristics and distributes them over time on a widespread basis throughout the species at the expense of other characteristics that are less well suited to the environment.
Biologists must therefore interpret current biological form in the light of its evolutionary history. Apes are our evolutionary primate cousins, and if you wished to pick an animal to symbolise strength and virility, the male gorilla automatically suggests himself.
Although not a matter that I ever pondered, I would have assumed the male gorilla to be handsomely endowed in genitalia. But the truth is that, relative to his mass, the gorilla has a very small penis and small testicles compared to the male human.
All of which prompts the question, posed in one publication in stark and vulgar form: "If the male gorilla is so big and tough, why are his balls so small?"
To move on to some specific measurements, the average length of an erect penis in a gorilla is 1.25 in. It measures 1.5 in an orangutan, three inches in a chimpanzee, and five inches in a man. Testicle size, and the conspicuousness of the penis varies correspondingly.
The gorilla's flaccid penis is so demure that it is not readily visible. So how does evolutionary biology explain such marked differences in penis and testicle size? Let us look more closely at some arguments commonly used.
The male gorilla is much larger than the female, which is common across the animal kingdom in species that practice polygamy, i.e., where dominant males mate with multiple females.
The massive size of the male gorilla is interpreted to have resulted from competition among males for access to limited females, with natural selection favouring the evolutionary success of the largest, most powerful males. This evolutionary "arms race" escalated over time, producing massive males equipped with huge canines and prominent secondary sexual characteristics to attract mates.
But when you reach the top of the gorilla community-power ladder you have a relatively quiet life and pretty much uncontested access to all the females. The dominant ape, so the argument goes, doesn't need large genitalia producing lots of sperm in order to compete with the sperm coming from other apes. In his case a little penis and small testicles go a long way - surely the ultimate demonstration that size doesn't matter.
The male and female chimpanzee are of similar size. Chimpanzee societies also have power pyramids, but they are more complex and "political" than with gorillas and orangutans. Dominant males have more frequent, but not exclusive, access to females.
Male chimps therefore rely largely on their sperm rather than on physical strength and ferocity in order to compete with other chimps in the race to father offspring. The male who inserts the most sperm in the female has the best chance of fertilising the egg in a situation where several males often copulate in quick succession with the same female. Under these circumstances natural selection would produce males with large penises and testicles.
The human male is slightly larger than the female. Relative to his body size, his penis and testicles are much larger than the gorilla's but not as large as those of the chimpanzee. Therefore, according to evolutionary theory, humans should be intermediate between gorillas and chimpanzees in respect both of promiscuity and polygamy.
Another aspect of human anatomy in need of explanation is the size of the female breast. Relative to body size, it is much bigger than the breast of the female gorilla and significantly bigger than the breast of the female chimp. Much of the human breast size is accounted for by fat, not milk glands, and human breast size is not explained by the need to nurse infants.
Evolutionary psychologists explain the large female breast as a secondary sexual characteristic that evolved to attract mates. According to Desmond Morris in The Naked Ape (McGraw Hill, 1987) this development coincided with the switch over from front-to-rear to front-to-front mating. In other words, the arousal previously elicited in the male by the sight of the female's pendulous buttocks bisected by a central cleavage was now elicited by the mimicking breast arrangement on the front.
So there you have it. Biologists must interpret biological form in the light of evolution, but sometimes this leads to flimsy and uncomprehensive speculation.
For example, in the case of chimpanzees, where male and female are of similar size, it is possible that female preferences might play a part in the natural selection of the size of the male genitalia.
A somewhat similar situation developed with Freudian interpretation of adult behaviour relying so heavily on history of psychosexual development. As the saying goes, "sometimes a cigar is just a cigar".
Readers who wish to look further into the area of evolutionary psychology could consult The Third Chimpanzee by Jared Diamond (New York: Harper, 1992) and The Moral Animal: The new Science of Evolutionary Psychology by Robert Wright (Vintage, 1994). A useful summary article by Frank Miele appears on the Internet at www.skeptic.com
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.