Females are the dominant sex in most primate communities and may determine social evolution. Communal living is particularly helpful to females because a group of females is better at finding and defending the food which enables them and their young to survive than individual animals. Having a group also helps in keeping watch for predators. But chimpanzees, our closest relatives, have a patriarchal society in which males are dominant. Usually there is an ‘alpha’ male who controls the group and keeps order during any disputes. Males seek domination because it brings them better mates, and they try to enlist the support of other males. But the female influence is also important in choosing the alpha male. An alpha male must be accepted not only by other males but by the females. Female chimpanzees will show their acceptance of the alpha male by presenting their hindquarters. Sometimes a group of dominant females will oust an alpha male whom they do not like, and back another male whom they would prefer to see as leader. Female chimpanzees also have a hierarchy, and sometimes young females may inherit their status from their high-ranking mothers. The more dominant females will get together to boss the weaker females. But all this female behaviour is aimed at gaining better access to resources such as food, and not, as in the case of males, to outright domination of the group.
Clear differences exist between the mating systems of different primates. Polygyny, in which one male mates with more than one female while each female mates with only one male, is thought to be the fundamental mating system of animals and is quite common, even in humans. But chimpanzees have a promiscuous mating system: each female copulates with many males and vice versa. Sexual selection is related to male competition and female choice, as females invest far more in their offspring, while males may contribute nothing more than their sperm, though they can provide protection and food. Female primates use a variety of body movements, facial expressions and vocalisations to indicate a desire for sex. Female sexual skin swellings can be attractive to males, particularly in chimpanzees and baboons. Males and females may use similar facial expressions to initiate sex. In female primates, hormones related to ovulation can affect both sexual behaviour and the genitalia, unlike humans where there is little effect. Copulation occurs with the male mounting from the rear. Females respond with orgasm less frequently than do males. Primates manipulate and stimulate their own sexual organs in an activity related to human masturbation. Male–male mounts are common but copulation is rare, and there is no real evidence for lesbianism.
There is great variability in primate sexual behaviour. Males are more interested in mating and try to impregnate as many females as possible, while females are more selective. The basis of female choice of a male for sex is not understood, but males tend to prefer older, not younger, females as mates. There is thus a big difference between chimpanzee behaviour and that of their human relatives. The reason for this difference may be that whereas chimpanzees have a promiscuous mating system, humans typically form longer-term relationships, which means young females are more desirable as mates than older ones. But there may also be a genetic determinant for human males to prefer young females, since, as we will see, this has an evolutionary advantage with respect to health and child care.
Food is an especially important issue for primates, especially for mothers looking after their young. A nucleus chimpanzee group is composed of males who hunt for meat, while females gather insects and may thus have greater manual dexterity. Primate females seem to be keener tool users; for example, they extract ants from anthills using stems from plants. Female chimpanzees exchange meat for sex with males, and males who shared meat with females can be observed to attract twice as many females. For females, who have difficulty obtaining meat on their own, this provides food without the exertions and risks of injury when they hunt for themselves. Females often co-operate to gather food.
The two human sexes have evolved different characteristics to fit their evolutionary requirements. Both sexes must get great pleasure from sex and must want to have children, though both factors are more significant for the female as it is she that bears the child. Care of young children must thus be genetically programmed into both females and males, especially into the former. Both males and females must also be genetically programmed not to mate with close relatives, as this, known as incest, can have deleterious genetic consequences for their offspring. Sexual differences in the brain must control these behaviours.
Breeding with close relatives brings severe genetic defects. The closer the parents are related, the greater are the chances that the offspring will be damaged. When two parents are brother and sister, or parent and child, the likelihood becomes very high. The reason for this is that a child of unrelated parents who inherits one abnormal gene may be unaffected, since there is likely to be a normal gene on the other chromosome, but if close relatives mate there is a high likelihood of both genes being similarly abnormal.
Even animals avoid incest. Chimpanzee females usually leave their community to join another before mating and males may also emigrate. Chimp mothers do not generally mate with mature sons or brothers, and there is evidence to suggest that young females avoid the sexual advances of older males that could have sired them. All of these behaviours serve to avoid incest with its negative genetic consequences. While chimps are promiscuous, several studies have shown that they can sense those to whom they are related, and thus avoid having sex with them. In humans there is a similar mechanism designed to cause the avoidance of sibling incest.
In the past human males seeking desirable mates found that they had to compete with other males, and they still do. Aggressive behaviour was one way of seeing off rivals, used either to drive them away or even to kill them. Since aggression was profitable in terms of finding mates and passing on genes, it could be that this is why men have a genetically inherited trait which makes them act aggressively towards other men. But showing such aggression towards females would be likely to be counter-productive, because it could cause them to be rejected as potential mates and fathers to offspring.
Females did best if they chose good mates and nurtured their children. Since the reproductive success of males is not easy to assess, men were likely to be selected if they looked like being willing to take greater risks than females, claims Daniel Nettle. Women, by contrast, are more anxious than men, which relates to their evolution of harm-avoidance behaviour. Such behaviour might be adaptive for women. A female with dependent young or who is pregnant is more vulnerable to unexpected attack than a male. Protecting her offspring, and her own ability to breed are evolutionary priorities. Any conflict within her group can also be a major threat, as it is difficult for her to leave the group and join another.
A basic difference between the sexes in the importance of social skills and behaviour may be part of our evolutionary heritage, particularly for women. Evolution may have selected men to be more aggressive and more risk-taking.
Scheib and her team say it seems likely that men today find younger women attractive because during the course of our evolution males who chose younger women were more successful in breeding than those who were drawn to females that were too young or too old to conceive. These early successes by men have echoes today, because carefully selected partnerships gave rise to better genes, and it was these that were passed down through evolution. So women today may look for successful men who can best support their children. They use their sex appeal to achieve this end. The anthropologist Donald Symons has observed that people everywhere understand sex as ‘something females have that males want’.
Sex incurs a heavy cost compared to asexual reproduction, as two parents are involved in making one child. But the cost of sex in terms of time and energy is almost trivial for the male compared to the female who will bear a child. It is therefore in the male’s best interest to spread his genes by mating with as many females as he can. The female, by contrast, has to be very selective as to which man she chooses. She has in mind the children who might be produced, and wants them t
o be fathered by a male who will be supportive in bringing them up as well as contributing good genes to the union. Thus her criteria are aimed at selecting the best possible male, not just the first who comes along.
So the mating behaviour of our ancestors probably involved the most powerful and dominant males obtaining access to multiple female sexual partners. Men, more than women, can thus be seen to have evolved a greater desire for casual sex. But for women casual sex can have the consequences of an unwanted pregnancy with all its long-term implications. Females in those ancient times were significantly smaller than males and so were likely to submit sexually to the dominant male’s advances and may not have generally understood that sex leads to pregnancy. With time the size difference between males and females diminished and relationships between the sexes started to involve staying with a single partner, and perhaps the beginnings of romantic love. A possible fundamental negotiation, already mentioned, was the female exchanging sex for meat and protection provided by the male, and this could have been the origin of marriage among early humans. We have seen that the exchange of meat for sex occurs in chimpanzees. Early human societies are known to have engaged in meat-for-sex behaviour, with the best male hunters probably having the greatest number of sexual partners. Is today’s money perhaps the equivalent of meat?
In ancient societies the most reliable suppliers of food were women who collected it rather than the men who hunted for it. This resulted in women making major technical contributions. Women’s food-gathering activities gave rise to the domestication of animals as well as agriculture. It was women working with digging-sticks–one of the earliest tools–to procure food from the ground that led to the planting of crops. According to Veer Gordon Childe, an Australian archaeologist who wrote Man Makes Himself (1936) and What Happened in History (1942), every single food plant of any importance, as well as other plants such as flax and cotton, was discovered by the women in the pre-civilised epoch. Primitive women were also the first potters, using clay to create cooking and storage vessels. It has been suggested that skilled tool-making and tool use in humans thus began as a largely female domain. All the basic cooking techniques–boiling, roasting, baking, steaming–were developed by women. The social anthropologist Robert Stephen noted in The Mothers: A Study of the Origins of Sentiments and Institutions in 1927:
The weaving of bark and grass fibres by primitive woman is often so marvellous that it could not be imitated by man at the present day, even with the resources of machinery. The so-called Panama hats, the best of which can be crushed and passed through a finger ring, are a familiar example.
Perhaps the least-known activity of primitive women was their work in the construction of homes, whether huts, skin lodges, wigwams or camel-hair tents. So long as hunting was an indispensable full-time occupation for men, it required women to work cleverly at home. The late Oxford evolutionary biologist William D. Hamilton made a clear comment on the two sexes:
People divide roughly, it seems to me, into two kinds, or rather a continuum is stretched between two extremes. There are people people, and things people.
Men hunting and fighting, while women attend to agriculture and the home, is a typical pattern in primitive societies. (The hunting experience of ancient men, which involved rapid action, and the foraging activity of women, which required long periods of searching, could explain why traditionally women like to take their time browsing in shops while men would rather go in for what they want and rush out with it.) When ancient man had killed his game, he sometimes sent his woman to fetch it, and even if he brought it home himself, he left it to her to deal with it. The discovery of agriculture by women, and their domestication of cattle and other large animals, enabled men to devote much less time to hunting. Spending more time at home, they began to teach themselves the crafts that the women had already mastered, and to make improvements in their tools. The invention of the plough and the use of domesticated animals to pull it was crucial. Men’s freedom from hunting, together with their greater physical strength, made it easy for them to dominate women, who were committed to caring for children.
However, behaviour that seems to go beyond the single aim of improving or encouraging reproductive fitness, such as altruism, is harder to explain. It is difficult to see a genetic link. But research has suggested that any behaviour that is not directly beneficial to an individual but which potentially benefits that individual’s relatives can leave its mark on the legacy of the group. This can be clearly seen in the behaviour of today’s parents towards their children. The process of natural selection would have favoured this kind of altruistic behaviour in humans because children carry their parents’ genes into the next generation. So there is a sound genetic basis for this sort of parental altruism. Similarly there is also a common advantage to living in an altruistic society.
The evolution of human sex required the development of specific body structures like the penis and breasts. How could the evolution of breasts, which gave milk to the newborn, have come about? Even Darwin found it a very difficult problem. He wrote, ‘If it could be demonstrated that any complex organ existed, which could not have been formed by numerous, successive, slight, modifications, my theory would absolutely break down.’ Small modifications in the lineage of mammalian ancestors gave rise to lactation and the breasts. It is far from easy to understand. An important piece of evidence was provided by the platypus, a monotreme mammal, which has a patch on its breast that produces milk secretions which the infants suck. Mammary glands are thought to have evolved from some mosaic of the different milk-secreting glands in our ancestors. Among our primate cousins, the monkeys and the great apes, the breasts of females enlarge only slightly during pregnancy and lactation, and in the non-pregnant state remain almost completely flat, with merely the nipple projecting.
So why are breasts so important in the sexual life of humans? Why is their size so crucial for human attractiveness and female sexual self-esteem? It must be related to the change in sexual intercourse, from the male mating from the rear to front-to-front contact. Some anthropologists have proposed that the swell of female breasts is equivalent to the swollen fertile buttocks of our female primate ancestors which attracted males. Human female breasts may have become permanently enlarged due to sexual selection, as they potentially resembled the region of the distended buttocks which are employed to signal appeasement as well as sexuality in primates. Over the course of evolution female breasts became permanently enlarged, and thus a permanent sex symbol to signal that the human female was continuously sexually receptive.
But why then do men have breasts and nipples? This is almost certainly due to males being essentially modified females, as we have seen. Females have everything required for reproduction except some means for getting the egg to start developing. The advantage of having two sexes resulted in the evolution of males to provide sperm. The evolution of males involved modification of female development, but vestigial breasts in the male were no disadvantage, and so they have persisted.
Richard Dawkins has speculated that the absence of the penis bone in humans, although present in our nearest related species, the chimpanzee, and other primates, may be due to sexual selection by females looking for an honest advertisement of good health in prospective mates. Human erection relies on a blood-pumping mechanism, and failure to achieve an erection would be a warning to females of possible physical or mental ill health. It has also been speculated that the loss of the penile bone occurred because humans evolved a courtship pattern in which the male tended to accompany a chosen female all the time to ensure paternity of her children, and thus could enjoy frequent matings of short duration. Primates with a penis bone only infrequently encounter females, but engage in longer periods of copulation which the bone makes possible, thereby maximising their chances of fathering the female’s offspring.
It seems clear that evolution led to a number of genetically determined sexually different characteristics in humans, both physical and beha
vioural. These are likely to include greater empathy and less physical risk-taking in females, and more aggression by males. Both sexes developed sexual desire for the opposite sex and the habit of face-to-face copulation. But what evidence for these effects can be found in the male and female brain?
5
Brain
Here’s all you have to know about men and women: women are crazy and men are stupid. And the main reason women are crazy is because men are stupid.
George Carlin
It is claimed by a leading American neuroscientist, Larry Cahill, that clear sex differences exist in every brain lobe, including cognitive brain regions. In this chapter I focus on structural differences between male and female brains, and will examine some functional differences in later chapters. As we shall see, brain-imaging techniques report sex-related differences in visual skills, emotional responses, sexual arousal, facial processing, memory and language abilities.
The brain is composed of billions of nerve cells–neurons–and supporting cells, and the working of the nervous system depends on the formation of neuronal circuits in which the neurons make even more billions of connections with each other. Neurons consist of a cell body containing the nucleus with its chromosomes and genes, and extensions, which can be several feet in length, that transmit or receive electrical signals. From the cell body a long extension, the axon, may contact distant nerve cells to which it transmits signals. The axon may be insulated by a fatty coat of myelin along its length to improve its ability to conduct the nerve impulse. Damage to this coating is a feature of multiple sclerosis–a disease more common in women than men. The neuron’s insulated extensions form the brain’s white matter, while the cell bodies form the grey matter. On the cell body are other, highly branched short extensions, dendrites, with which other nerve fibres make contact, so that a single neuron can receive as many as 10,000 separate inputs. Astonishing!
Why Can't a Man Be More Like a Woman? Page 4