The size difference between men and women, the relative size of men’s penis and testes, and the presence of permanent breasts and buttocks in women, are excellent clues for guessing about early human behavior.
Size Difference
For mammals generally, the size difference between females and males tells us something about their mating habits. Where females and males are the same, or nearly the same in size, they tend to have monogamous mating systems. In this context, “monogamy” means that a single male is paired with a single female, both of whom defend territory and care for young, although either partner may occasionally mate with an outsider. While many monogamous animals pair for a season, some such as gibbons, pair for life.
Where males are much bigger than females, as is the case with gorillas, we see a system where the most powerful male beats back the competition and maintains a harem.
“This association of monogamy with equality in size and of polygamy with inequality of size was noted as early as the nineteenth century….These correlations have stood the test of statistical analysis by modern researchers with more and more reliable data”. ((The woman that never evolved. Blaffer Hrdy, Sarah. (pp.24, 134-135,25, 72) ))
Human males are not substantially bigger than females, indicating an evolutionary history of partial, or “quasi-monogamy”.
“In other words, body size suggests, on average, that early human males might have had two or three mates at a given time, while testis size suggests that women also found more than one simultaneous mate over the eons that our bodies were evolving.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Testes and Sperm Competition
For male primates, humans included, the size of the testes relative to body weight, indicates how many mates the female of the species typically has.
“Human testes weigh about the same as those of a 450-pound gorilla, which suggests that men compete with men at the sperm level more than gorillas compete with other gorillas. Because gorilla groups are widely separated, a gorilla female is not likely to go off among the giant nettles to mate with a neighboring male unless she opts to change groups completely. It would be taking a huge physical risk to sneak off for an adulterous liaison and sneak back, hoping to escape detection…. Gorillas compete by muscle power to establish a harem; they do not need to compete by quantities of sperm.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
“Males who produce more sperm flood out their opponents. Chimpanzees and bonobos, whose females mate with many males, have far larger testes for body size than humans, and much larger than gorilla harem masters. Chimp testes are about 4 ounces for a 100-pound gorilla; gorillas are about 2.5 ounces for a 450-pound animal. Gorillas keep rivals away by teeth, muscles, and reputation, and so do not have to share their mates. Human testes weigh about 2.5 ounces for a 175-pound man–suggesting that our females are more promiscuous than gorillas, but less so than chimps” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Penis Size
The enormous male gorilla, who fights competing males, and keeps a harem of smaller females to himself, has a tiny little penis. He doesn’t have to be a great lover, just a great fighter. Fortunately, most human males differ from this, themselves having relatively huge members.
“Another peculiarity of humans is that the penis is twice the size for body weight as that of any other primate. Indeed, gorillas and orangutans are strikingly under endowed in this respect. Among bonobos, the penis is longer than our own, but thinner… the two apes with most frequent sex, ourselves and bonobos, are the two with huge penises.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
And while male scientists like Jered Diamond, and Paul R. Ehrlich offer amusing and rather contrived theories about why this is so (see below), we see that female scientists like Allison Jolly, and Sarah Hrdy simply point out what would seem like common sense to almost any woman: the large human penis is probably the delightful result of generations of sexual selection by human women, and this indicates that early women had choices, and preferences as well..
“The possibility has been raised that substantial penis length is related to sperm competition and ejaculation as close as possible to the egg, an explanation that seems unlikely….” **5(Paul R. Ehrlich) ((Human Natures: Gene, Cultures, and the Human Prospect. Ehrlich, Paul R. (pp. 69,86, 207) ))
“…Jared Diamond concludes that the function of a large penis is to show off to other males. He may be influenced by his time in New Guinea, among people whose chief clothing is the exaggerated and erect penis sheath. It is true that men’s genitalia seem to interest other men, in urinals and locker rooms, more so than women. But the reproductive advantages of giving pleasure to females, promoting mate choice, and stimulating female orgasm by tactile means should not be underestimated” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
“Both female and male acquired permanent attractiveness–big breasts, big penis–which continually advertised both sexiness and reassurance to the mate.” **4 (Sarah Hrdy) ((Mother Nature: Maternal instincts and How They Shape the Human Species. Blaffer Hrdy, Sarah ))
Female Shape, Sexual Selection, and Quasi-Monogamy
For species whose males invest little or nothing in rearing young, females are very choosy in selecting the best mate. Males of these species compete to be chosen, and are not themselves choosy. This is not surprising since the female is biologically limited in the number of offspring she can produce, and invests heavily in that production. For her, mating indiscriminately with many males is not a good reproductive strategy; a better plan is to mate with only the best males. For a male of such species, however, contributing nothing but sperm, the act of mating is a low investment affair. For him, mating many times, with many females, does increase his chances of producing surviving offspring, his only real limitation being the number of females who are willing to mate with him. Therefore, the male’s best reproductive strategy is to invest his energy into being as attractive as possible to those choosy females. When a species develops characteristics that have no value other than to attract mates, we get the beauty and wonder that is produced by sexual selection. The most famous example of this is the magnificently tailed peacock, whose elaborate train does nothing to enhance survival, but is necessary for attracting mates. To the choosy peahen, herself undecorated, the tail advertises quality. In fact, researchers have found a truth in such advertising; peacocks having long, richly decorated tails with symmetrical patterns show better immune health than their small-tailed, less ornate, or lopsided brothers. So much for low investment males. For high investment males, however, the tables turn, and the gender that is usually chosen has earned the right to choose.
“Male choice does occur, however, in monogamous (or quasi-monogamous) species like sea-horses and humans, where both mates have a similar interest in their brood and are willing to spend time assessing, and being assessed by, potential mates…. It is less often pointed out that playing hard-to-get is also a male trait wherever that male has high parental investment in the offspring of his official mate.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Permanent Breasts and Buttocks
“Breast milk is secreted by milk glands, not the fat that surrounds them, which indicates nothing about milk production. Flat-chested and big-breasted women are equally able to feed babies, as are chimps, who have nothing visible but a nipple. Wide pelvic bones aid easy birth, but panier fat deposits alongside the pelvis do nothing of the sort… women’s fat is distributed in the same zones as other mammals but particular fat deposits are exaggerated, like any other sexually selected signal…. The hourglass shape of nubile young women signals the same primitive message to young men. The contrast between small waist and wide hips and breasts says that the woman is fertile (as evidenced by her breast and hip fat) but not pregnant (as evidenced by her small waist). It is the waist-to-hip ratio that attracts mates.(Of course, even waist-to-hip ratio, real and preferred, is affected by culture. Machinga Indians of the upper Amazon think thin-waisted women look starved, not attractive.)” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Permanent breasts, buttocks, and fat hips are products of sexual selection. Having no function other than to attract mates, these qualities indicate that early men where choosy about who they stayed to raise young with. If men had not been a valuable part of the early human family then women would not have evolved to be continuously attractive and sexually receptive to them.
Sex as a Social Lubricant
“Sex for passion, for intimacy, for relief of tension, even for material reward, for all the reasons people now make love–sex in early humans became a sexual lubricant…. Sexual eagerness evolved in women along with their other social needs.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Most animals mate only when it is time to reproduce. But humans, like bonobos, have evolved a highly social sexuality. In fact, you could say that our highly social sexuality has evolved us.
“…[I]n his 1967 book, the naked ape, Desmond Morris argued that the development of continuous receptivity, breasts, buttocks and orgasm in women was critical to the evolution of human beings because they cemented the pair-bond between mates by providing mutual rewards for both partners. The reduction of body hair and the unusually large size of the male penis compared with that of chimpanzees and gorillas are explained by similar arguments about mutual attraction.” ((The woman that never evolved. Blaffer Hrdy, Sarah. (pp.24, 134-135,25, 72) ))
Monogamy, what is it good for?
Monogamy has its good points. Males who help rear young increase the chances that their offspring survive by watching for predators, guarding against other males, feeding, and nurturing.
“Monogamous males also help with child care. They are fairly sure of paternity, so they are good fathers.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
“But then why tolerate a male? Perhaps for his help in watching for predators…probably the males stayed originally as paladins, and then gradually made themselves useful as nannies.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))
Primates are especially monogamous, monogamy being several times more common in primates than is typical of other mammals.
“Among mammals generally, primate males stand out for the unusual role they play in rearing offspring….primate males are not just fighting machines, built to conquer other males and reap their rewards in copulations. Were this the case the primate female would be a different creature, vastly more submissive, less assertive, and less complex …” ((The woman that never evolved. Blaffer Hrdy, Sarah. (pp.24, 134-135,25, 72) ))
Yet like so much in life, monogamy is a trade off. A male who stays to rear young greatly reduces his opportunity to roam great distances looking for new females. And with such a heavy investment in his mate, the monogamous male also has a vested interest in her behavior. If she has sex with other males, than he stands the chance of raising someone else’s offspring, himself potentially remaining childless.
“…preventing female primates (or birds for that matter) from pursuing extra-group partners is a problem common to pair-bonded males alike.” (( Tree of Origin: What primate behavior can tell us about social evolution. Frans deWaal, editor))
From her point of view, the female who bonds with a male must share her territory and its limited food supply.
“But along with clear-cut benefits of having males around…their presence also imposes a variety of costs…some are minor and easily discounted (for example males take up space). Other costs are chronic and more serious: males usually compete with young for finite resources…” ((The woman that never evolved. Blaffer Hrdy, Sarah. (pp.24, 134-135,25, 72) ))
The female may also find herself with reduced opportunities to join with other mates. This is the root of the battle of the sexes, for females have a tendency to roam as well. So why did females evolve such a strong and persistent wanderlust? The reasons are many: to obtain superior genes from a male who is other than her “official” mate, to gain support from a variety of men who know that they may be the father of her children. to set up potential “back-up” daddies should her mate abandon her or die, and to stave off infanticidal males who won’t risk killing a child that might be theirs.
Thus monogamy, or the appearance of it, is a compromise that curtails the mating opportunities of both partners.
“This in turn, could invoke severe conflict both between the mates and within the female’s own psyche over how much she was committed to stick with her official mate, and how much she was still free to roam.” (( Lucy’s Legacy: Sex and Intelligence in Human Evolution. Jolley, Allison))