There is a higher rate of left-handedness among men than among women, and this may be due to the higher levels of prenatal testosterone in males. A minor physical characteristic that shows sex differences is the ratio of the length of the right-hand second digit (index finger) to that of the fourth digit (ring finger). In men the second digit tends to be shorter than the fourth, while in women the second tends to be the same size or slightly longer than the fourth. Lower 2D:4D ratios imply a higher testosterone exposure in the embryo.
The effect of hormones such as testosterone on the development of male features of the embryo is fundamental to understanding biological differences between the sexes, including physical characteristics such as size and strength as well as particular skills and emotions. More than fifty years ago a paper was published about the effect of treating the embryos of female guinea pigs with male hormone, which masculinised their sexual behaviour. Since then there have been numerous studies showing that sex hormones present early in development affect sexual differentiation and sex-specific behaviour. It is not permissible to inject male hormones into the human womb where female embryos are developing to observe the effect, but it is possible, as we shall see, to study the development of individuals with disorders that cause an increase in testosterone in the womb, as well as measuring hormone concentration during pregnancy.
Hormones are not the sole contributor to the differences in typical male and female development. Genes on the sex chromosomes influence the development of both the body and the brain, and directly influence neural developmental and some identifiably sex-specific behaviour. These direct genetic actions have a wide influence and can include actions caused by locally produced hormones or other, non-hormonal, messenger molecules.
One informative disorder of sexual development occurred on the island of Hispaniola in the Caribbean, where children who had been identified as girls at birth began to develop male sexual characteristics at puberty, and grew up to become men. This was found to be due to a faulty gene for the enzyme 5-alpha reductase, which is required to make testosterone potent. Its absence in embryonic development was resulting in babies being born with female characteristics even though they had a Y chromosome, but at puberty the secretion of large amounts of testosterone caused them to grow a penis. The defect was traced back for seven generations. We will also examine examples of another genetic disease, congenital adrenal hyperplasia (CAH), which increases testosterone during development and so masculinises the development of the female embryo, and causes changes in the female body and later behaviour.
Further evidence for the key role of genes and hormones in human sexual development is illustrated by rare cases of abnormal development. Testosterone can enter cells and then bind to a receptor. If XY males have a mutation that prevents testosterone binding, they develop as females in external appearance, even though they have testes that secrete testosterone, and they do not develop male secondary sexual characteristics at puberty. Thus there are males with this insensitivity who despite their Y chromosome are usually indistinguishable from girls and women. Conversely, genetic females with a completely normal XX constitution can develop as male in external appearance if they are exposed to male hormones during their embryonic development. There are also rare cases of XY individuals who are female due to part of the Y chromosome with the SRY gene being lost, and XX individuals who are physically male due to that part of the Y chromosome being transferred to the X chromosome. The sex chromosomes can have negative effects when combined in an abnormal manner that results in there being three of them, instead of the usual two. Some individuals who have three sex chromosomes (either XXX, XXY or XYY) have clearly marked difficulties in speech and language, motor skills and intellectual achievement. Mental ability has been found to be poorest in females with XXX, while males with XYY have normal-range intelligence.
After birth, development continues mainly as growth but at the age of about ten to fourteen years old puberty occurs, bringing major physical changes through which a child’s body develops into that of an adult capable of reproduction. Puberty is begun by hormonal signals from the brain to the ovaries and testes, which respond by producing a variety of hormones that stimulate growth. The principal sex hormone for males is testosterone, while the hormone that dominates female development is an oestrogen, estradiol. Females also produce testosterone, which is made in their adrenal gland and ovaries. The concentration in their body is only one twentieth that of males. There is a growth spurt in the first half of puberty which only stops once puberty is complete. Puberty introduces much wider changes into bodies of boys and girls than merely genitalia, and results in radical developments. Females develop functional breasts and oestrogen widens the pelvis to help with childbearing. Males become sexually competent, develop facial hair and their voices break.
The average height of men in the United Kingdom is five feet nine inches, and for women just under five feet four inches. American men are about an inch taller, and American women half an inch. According to the Guinness Book of Records 2013 the shortest living man–at one foot nine and a half inches–is Chandra Bahadur Dangi of Nepal, and the tallest living man is Sultan Kösen of Turkey, at eight feet three inches. The tallest man ever was Robert Pershing Wadlow (1918–40) from the United States, who was over eight feet eleven inches tall. The tallest living woman is Yao Defen of China at seven feet seven and a half inches, and the shortest Jyoti Amge of India at two feet seven inches.
On average males are physically stronger than females and this has a significant effect on lifestyle, particularly in relation to defence, hunting and aggression. Men have more total muscle mass than women because of higher levels of testosterone. As a result gross measures of body strength suggest that women are between fifty-two and sixty-six per cent as strong as men in the upper body, and between seventy and eighty per cent as strong in the lower body. Males typically have about fifty-six per cent greater lung volume proportional to body mass, and larger tracheae and bronchi. Men also have larger hearts and a ten per cent higher red blood cell count: more haemoglobin, and hence greater oxygen-carrying capacity. An important consequence of these structural differences is that maximal exercise capacity in women may be more limited by lung capacity than in men.
It is worth noting some other physical differences that result from development. Both males and females have an Adam’s apple but it is larger in men because their larynx, which contains the vocal cords, is larger. The Adam’s apple is actually the thyroid cartilage which forms part of the larynx protruding in the front of the neck. In most women it is not easy to see. Women, who have smaller vocal cords, normally have higher-pitched voices than men. But this is not the only difference. Timbre, the tone quality differentiating one spoken sound from another, also matters and men tend to speak more monotonously while women tend to use a wider range of tones.
Differences are found also in the special senses. In sense of smell, for example, women are significantly better than men in odour detection and identification. It has been claimed that this superiority can even be found in newborn babies. Female sensitivity to male-specific odours is many times stronger during ovulation than during menstruation. Women are also significantly more likely than men to suffer from cacosmia, where a normally benign smell is registered as unpleasant. Symptoms include dizziness, headaches and anxiety, and while there can be more serious causes, links have been found to common household solvents such as paint. Women are better at hearing at high frequencies than men and have even more acute hearing around the time of ovulation. A boy may need a teacher to speak more loudly than girls if he is to follow a class, but he can also block out background noise in the classroom better than girls.
As we shall see from the behaviour of newborn children, there are also psychological differences specified in development as the brains of males and females are different at birth.
One may ask why specific differences should have been favoured and selected for by evolution. And how could th
ey confer advantages for reproduction?
4
Two Sexes
You don’t have to be naked to be sexy.
Nicole Kidman
What is the origin of men and women? And why are there two sexes? An opinion poll in 1999 found that one in two Americans believed the Bible’s account of the origin of human beings, namely that God made man, and women came from his rib, so women were clearly a second-class derivative. Questioning our origins has a long history, but it was only with Darwin’s theory of evolution that any proper scientific understanding became possible.
It was a common belief in most religions and cultures that gods had created men and women just as they created earth and nature. Those early times also fostered other concepts of human origins. The Greek philosopher Anaximander in the sixth century BC believed in a gradual development of life from moisture under the influence of warmth and suggested that humans originated from animals of another sort. Two centuries later Aristotle proposed that the world and the animals and plants living in it were eternal, uncreated and unchanging. The Chinese creation myth of Pan-ku is one of the best known of many such stories as to how the world began. The giant Pan-Ku burst out of a cosmic egg which had been floating in a formless, chaotic void. Far from taking just seven days, Pan-Ku spent over 18,000 years separating the earth and the sky and the opposites of nature, including men and women. On his death, different parts of his body became the different features of the earth with his left eye becoming the sun and his right the moon, his blood the rivers, and the fleas on his body scattered to become fish and animals and hence the human race.
Darwin’s theory of evolution, first published in 1859, was a fundamental and wonderful change. We humans, he argued, evolved from ape-like animals. The core of his brilliant discovery is that physical and behavioural characteristics can be inherited from one generation to the next, and that these can change so that organisms’ offspring can differ from their parents. If this potentially gives them an advantage they will be more successful in reproducing and will increase in numbers more quickly than organisms that lack this inherited advantage. Random change in inherited characteristics, and selection for reproductive advantage, sums it up. The inheritance of characteristics was only later discovered to be entirely due to genes, made of DNA, which are the only structures in the cell that replicate and so can be passed from generation to generation and undergo random change.
Evolution is based on changes in genes which control how the embryo develops. The genetic constitution of an individual is present in the fertilised egg and represents the combination of genes from male and female parents–sperm and egg. Natural selection determines which combinations of genes persist, and in animal evolution this is based fundamentally on successful reproduction. Dogs provide a nice counter-example, as their evolution was based on artificial selection. Dogs were first domesticated from wolves in the Middle East and Europe about 14,000 years ago but in Southeast Asia, the area from where many of our modern breeds were derived, only 7,000 years ago, and recent research from the UC Davis School of Veterinary Medicine says that the dogs of Southeast Asia were on a separate evolutionary track as they could no longer cross-breed with wolves. Breeds from the East may have been the ancestors of many of today’s game dogs and were bred to undertake specific tasks such as hunting game. Ancestors of the Afghan hound and the greyhound have been identified at sites in Mesopotamia dating to around about 5000 to 4000 BC, and scent hounds, that hunt by smell rather than by sight and whose descendants include the modern bloodhound, foxhound and dachshund, are recorded from around 3000 BC. Guard dogs for protecting animals as well as people also appeared at around this time. Their descendants were selected by humans for various features, so that they were selectively bred into hundreds of breeds each with its own size, shape and distinctive appearance and abilities. (I am grateful for the evolution of the wire-haired fox terrier, one of which was my childhood friend.) Since humans were intervening to make the selection of animals with specific characteristics, there was very rapid progress, compared with human evolution that had to take its own course: it is estimated that we branched off from the last common ancestor of human and chimpanzee about five to seven million years ago.
Differences in the genes and in their control regions are the raw material on which evolution depends. Evolutionary changes in genes must occur in the germ cells that give rise to the next generation, in our case either sperm or eggs. The evolutionary changes in the genes are expressed either as changes in the nature of proteins themselves, or where and when they are made during the development of the embryo. Those that give rise to the organisms best adapted to their environment with respect to reproduction survive and continue to reproduce–this is the essence of evolution. Thus if genetic changes cause the embryo to develop into an organism that survives better they will be retained in future generations, but if they make things worse, then the changes in the genes will soon be lost.
Basic differences between males and females are thus due to genetic differences that change how the embryo develops and they can affect any structure, including the brain. Such differences will almost always be related to improved reproduction, and so any biological differences between men and women will be understood in terms that explain their reproductive advantage. Of course, social factors can make men and women behave differently and can even alter their biology, and the distinction between biological and social mechanisms is the fundamental and difficult problem that we meet all too often.
One cannot explain the genetic differences between men and women without understanding the role of evolution. Darwin’s theory on sexual selection suggests that competition for mates and the careful selection of those who might be suitable as mates have played a role in shaping the evolution of sex differences. It is sex itself that was unquestionably the major factor in determining differences in female and male psychology. Men and women had to feel a need to engage in sexual behaviour, as evolution cares only about successful reproduction. It was during the evolutionary process that their physical and biological characteristics were genetically determined and sex itself evolved. Yet some of the books and articles on this topic do not even mention the word gene, and so totally avoid understanding how genes can determine behaviour. If women reproduced without men it would make life simpler, and in my view even very boring, without two sexes and the sex act itself. There are indeed animals that do just that, as we shall see. But we, like many animals, are committed to having males co-operate with females in our reproduction.
Females have had the more complex evolution, as they have to provide the egg and nurture the child. Males have only to provide a sperm. It was thus that males evolved as modified females and their evolution was mainly related to protecting and feeding females and offspring. Looking after children and being sensitive to their emotional states and needs could have led to the evolution of empathy, particularly in women; this development would have benefited the children. There is also a view that female behaviour has evolved to help facilitate harmony within the family. Men, meanwhile, needed to carry out tasks which need strength and speed, and the most successful were those who were good hunters and competitive for females. This required aggression, which became genetically determined.
But why this complication? What is the evolutionary advantage of having two sexes? This is a complex issue but it is generally accepted that there are major advantages. Most animals reproduce by having two sexes but there are exceptions, such as the single-sex invertebrates like rotifers. These are very small but their body can be divided into three main sections: a head, a trunk and a foot. For most rotifer species, males have never been discovered; females produce eggs that then develop into females. Virgin birth, in which females produce eggs that develop into young without having been fertilised, also occurs in some insects, fishes, and a few species of lizard and frog.
The evolutionary superiority of two sexes must involve the production of offspring who enjoy some advant
age over those created by asexual reproduction. Evolutionary theorists admit that identifying the specific features of sexual reproduction that confer an advantage is a difficult problem in biology. The most favoured explanation is that sexual reproduction introduces greater genetic variation and so enables some offspring to survive better in changing or novel environments. Another theory is that sexual reproduction provides the best defence against rapidly reproducing infectious species like parasites. Yet another is that a major function of sex is to repair damaged genes in the egg and sperm.
Our preoccupation with sex makes sense, since that is why we are here. Should we be sorry that there is not a more attractive explanation for our sexual lives? Evolution has programmed us to be preoccupied with sex to ensure reproduction. Sexual activity goes back a very long way. The first animals known to have copulated were a species of fish, now extinct, some 375 million years ago. They were live bearers, that is, the mother gave birth to live offspring. Before this time the females of species, for example frogs, put their eggs into the water where they were fertilised by sperm deposited by the males. For copulation, where the males insert sperm into the female, modifications in the form of extensions, like the penis from the pelvic region, had to be evolved. With reproduction being linked to copulation, animals had to evolve attitudes to the opposite sex in order to make it work. They had to co-operate with their partners but compete with others of the same sex.
Fundamental to the concept of evolution by natural selection is the idea that if we look back far enough in the history of any two species, we will find they are descended from a common ancestor. This common line of descent means that all living organisms are related to each other. It may thus be helpful to look at sexual behaviour in our closest relatives, chimpanzees, and other primates. As will be seen, they have quite complex behaviour in relation to sex and this feature is biologically determined. It is thus plausible that humans also have specific biologically determined behaviours and skills analogous to those of our closest primate relatives.
Why Can't a Man Be More Like a Woman? Page 3