Another major change that occurs during sexual arousal is a dramatic shift in the distribution of blood, from the deeper regions to the surface areas of the body. This overall forcing of additional blood into the skin leads to a number of striking results. It produces not only a body that feels generally hotter to the touch—a sexual glow, or fire—but also certain specific changes in a number of specialised areas. At high intensities of arousal a characteristic sexual flush appears. It is most commonly seen in the female, where it usually begins in the region of skin over the stomach and upper abdomen, then spreads to the upper part of the breasts, then the upper chest, then the sides and middle region of the breasts and finally the undersides of the breasts. The face and neck may also be involved. In very intensely responding females it may also spread over the lower abdomen, the shoulders, the elbows, and, with orgasm, to the, thighs, buttocks and back. In certain cases it may cover almost the whole body surface. It has been described as a measles-like rash and appears to be a visual sexual signal. It also occurs, but in fewer cases, in the male where, again, it starts in the region of the upper abdomen, spreads over the chest and then the neck and face. It occasionally also covers the shoulders, forearms and thighs. Once orgasm has been reached, the sex flush rapidly disappears, vanishing in reverse order to its sequence of appearance.
In addition to the sex flush and general vaso-dilation, there is also marked vasocongestion of various distensible organs. This blood congestion is caused by the arteries pumping blood into these organs faster than the veins can carry it away. The condition can be maintained for considerable periods of time because the engorgement of the blood vessels in the organs itself helps to dose off the veins that are attempting to carry the blood away. This occurs in the lips, nose, ear-lobes, nipples and genitals of both sexes and also in the breasts of the female. The lips become swollen, redder and more protuberant than at any other time. The soft parts of the noses become swollen and the nostrils expanded. The ear-lobes also become thickened and swollen. The nipples become enlarged and erect in both sexes, but more so in the female. This is not due to vaso-congestion alone, but also to nipple muscle contraction.) Female nipple length increases by as much as one centimetre, and nipple diameter as much as a half a centimetre. The areola region of pigmented skin around the nipples also becomes tumescent and deeper in colour in the female, but not in the male. The female breast also shows a significant increase in size. By the time orgasm has been reached the breast of the average female will have increased by anything up to 25 per cent of its normal dimensions. It becomes firmer, more rounded and more protuberant.
The genitals of both sexes undergo considerable changes as arousal proceeds. The vaginal walls of the female experience massive vaso-congestion leading to lubrication of the vaginal tube. In some cases this may occur within seconds of the beginning of the pre-copulatory activity. There is also a lengthening and distension of the inner two thirds of the vaginal tube, the overall length of the vagina increasing up to ten centimetres at the phase of high sexual excitement. As orgasm approaches, there is a swelling of the outer one third of the vaginal tube, and during orgasm itself there is a two to four second muscle spasm contraction of this region, followed by rhythmic contractions at intervals of 0.8 of a second. There are from three to fifteen of these rhythmic contractions in each orgasmic experience.
During arousal the external female genitals become considerably swollen. The outer labia open and swell, and may show size increases of up to two or three times the normal proportions. The inner labia also become distended to two or three times their normal diameter and they protrude through the protective curtain of the outer labia, adding as they do so an extra centimetre to the overall vaginal length. As arousal progresses there is a second striking change in the inner labia. Having already become vaso-congested and protuberant, they now change colour, turning bright red.
The clitoris (the female counterpart of the male penis) also becomes enlarged and more protuberant as sexual arousal begins, but as higher levels of excitement are reached, the labial swelling tends to mask this change and the clitoris is retracted under the labial hood. It cannot at this later stage be stimulated directly by the male’s penis, but in its swollen and sensitive condition can still be affected indirectly by the rhythmic pressures applied to that region by the thrusting movements of the male.
The penis of the male undergoes a dramatic modification with sexual arousal. From a limp, flaccid condition it expands, stiffens and erects by means of intensive vasocongestion. Its normal, average length of nine and a half centimetres is increased by seven to eight centimetres. The diameter is also considerably increased, giving the species the largest erect penis of any living primate.
At the moment of male sexual climax there are several powerful muscle contractions of the penis that expel the seminal fluid into the vaginal tube. The first of these contractions are the strongest ones and occur at intervals of 0.8 of a second—the same rate as the orgasmic vaginal contractions of the female.
During arousal the scrotal skin of the male becomes constricted and the mobility of the testes is reduced. They are elevated by a shortening of the spermatic cords (as, indeed, they are in states of cold, fear and anger) and are held tighter against the body. Vaso-congestion of the region results in a testicular increase of up to fifty or even a hundred per cent.
These, then, are the principal ways in which the male and female bodies become modified by sexual activity. Once the climax has been reached, all the changes noted are rapidly reversed and the resting, post-sexual individual quickly returns to the normal quiescent physiological state. There is one final, post-orgasmic response that is worth mentioning. There may be a copious sweating by both male and female immediately following sexual climax and this may occur regardless of how much or how little physical effort has been put into the preceding sexual activities. However, although it is not related to total physical expenditure, it does bear a relationship to the intensity of the orgasm itself. The film of sweat develops on the back, the thighs and the upper chest. Sweat may run from the armpits. In intense cases, the whole of the trunk, from shoulders to thighs may be involved. The palms of the hands and soles of the feet also perspire and, where the face has become mottled with the sexual flush, there may be sweating on the forehead and upper lip.
This brief summary of the sexual stimuli of our species and the responses given to them can now serve as a basis for discussing the significance of our sexual behaviour in relation to our ancestry and our general way of life, but first it is worth pointing out that the various stimuli and responses mentioned do not all occur with equal frequency. Some occur inevitably whenever a male and female come together for sexual activity, but others appear only in a proportion of the cases. Even so, they still occur with a sufficiently high frequency to be counted as ‘species characteristics’. As regards the body responses, the sex flush is seen in 75 per cent of females and about 25 per cent of males. Nipple erection is universal for females, but only occurs in 6o per cent of males. Copious sweating after orgasm is a feature of 33 per cent of both males and females. Apart from these specific cases, most of the other body.responses mentioned apply in all cases, although, of course, their actual intensity and duration will vary according to the circumstances.
Another point that requires clarification is the way in which these sexual activities are distributed throughout the individual’s lifetime. During the first decade of life no true sexual activity can occur in either sex. A great deal of so-called ‘sex-play’ can be observed in young children, but until the female has begun to ovulate and the male to ejaculate, functional sexual patterns obviously cannot occur. Menstruation begins for some females at the age of ten and by the age of fourteen. 80 per cent of young females are actively menstruating. All are doing so by the age of nineteen. The development of pubic hair, the broadening of the hips, and the swelling of the breasts accompanies this change and, in fact, slightly precedes it. General body growth proceeds at a slower rate and is not completed until the twenty-second year.
The first ejaculation in boys does not usually occur until they have reached eleven years, so that they are sexually slower starters than the girls. (The earliest recorded successful ejaculation is for a boy of eight, but this is most unusual.) By the age of twelve, 25 per cent of boys have experienced their first ejaculation and by fourteen 8o per cent have done so. (At this point, therefore, they have caught up with the girls.) The mean age for the first ejaculation is thirteen years and ten months. As with the girls, there are characteristic accompanying changes. Body hair begins to grow, especially in the pubic region and on the face. The typical sequence of appearance of this hairiness is: pubic, armpit, upper lip, cheeks, chin, and then, much more gradually, the chest and other parts of the body. Instead of a broading of the hips, there is a widening of the shoulders. The voice becomes deeper. This last change also takes place in the girls but to a much smaller extent. In both sexes there is also an acceleration of the growth of the genital organs themselves.
It is interesting that, if one measures sexual responsiveness in term of frequency of orgasm, the male is much quicker to reach his peak of performance than the female. Although males begin their sexual maturation process a year or so behind the girls, they nevertheless attain their orgasmic peak while they are still in their teens, whereas the girls do not reach theirs until their mid-twenties or even thirties. In fact, the female of our species has to reach the age of twenty nine before she can match the orgasm rate of the fifteen-year-old male. Only 23 per cent of fifteen-year-old females will have experienced orgasm at all, and this figure has only risen to 53 per cent by the age of twenty. By thirty-five it is go per cent.
The adult male achieves an average of about three orgasms a week, and over seven per cent experience daily or more than daily ejaculation. The frequency of orgasm for the average male is highest between the ages of fifteen and thirty, and then drops steadily from thirty to old age. The ability to achieve multiple ejaculation fades, and the angle at which the erect pen is is carried also drops. Erection can be maintained for an average of nearly an hour in the late teens, but it has fallen to only seven minutes at the age of seventy. Nevertheless, 70 per cent of all males are still sexually active at the age of seventy.
A similar picture of waning sexuality with increasing age is found in the female. The more or less abrupt cessation of ovulation at around the age of fifty does not markedly reduce the degree of sexual responsiveness, when the population is taken as a whole. There are, however, great individual variations in its influence on sexual behaviour. The vast majority of all the copulatory activity we have been discussing occurs when the partners are in a pair-bonded state. This may take the form of an officially recognised marriage, or an informal liaison of some sort. The high frequency of nonmarital copulation that is known to take place should not be taken to imply a random promiscuity. In most cases it involves typical courtship and pair-formation behaviour, even if the resulting pair-bond is not particularly long lasting. Approximately go per cent of the population becomes formally paired, but 5o per cent of females and 84 per cent of males will have experienced copulation before marriage. By the age of forty, 26 per cent of married females and 50 per cent of married males will have experienced extra-marital copulation. Official pair-bonds also break down completely in a number of cases and are abandoned. The pairbonding mechanism in our species, although very powerful, is far from perfect.
Now that we have all these facts before us we can start to ask questions. How does the way we behave sexually help us to survive? Why do we behave in the way we do, rather than in some other way? We may be helped in these questions if we ask another one: How does our sexual behaviour compare with that of other living primates?
Straight away we can see that there is much more intense sexual activity in our own species than in any other primates, including our closest relations. For them, the lengthy courtship phase is missing. Hardly any of the monkeys and apes develop a prolonged pair-bond relationship. The pre-copulatory patterns are brief and usually consist of no more than a few facial expressions and simple vocalisations. Copulation itself is also very brief. (In baboons, for instance, the time taken from mounting to ejaculation is no more than seven to eight seconds, with a total of no more than fifteen pelvic thrusts, often fewer). The female does not appear to experience any kind of climax. If there is anything that could be called an orgasm it is a trivial response when compared with that of the female of our own species.
The period of sexual receptivity of the female monkey or ape is more restricted. It usually only lasts for about a week, or a little more, of their monthly cycle. Even this is an advance on the lower mammals, where it is limited more severely to the actual time of ovulation, but in our own species the primate trend towards longer receptivity has been pushed to the very limit, so that the female is receptive at virtually all times. Once a female monkey or ape becomes pregnant, or is nursing a baby, she ceases to be sexually active. Again, our species has spread its sexual activities into these periods, so that there is only a brief time just before and just after parturition when mating is seriously limited.
Clearly, the naked ape is the sexiest primate alive. To find the reason for this we have to look back again at his origins. What happened? First, he had to hunt if he was to survive. Second, he had to have a better brain to make up for his poor hunting body. Third, he had to have a longer childhood to grow the bigger brain and to educate it. Fourth, the females had to stay put and mind the babies while the males went for hunting. Fifth, the males had to co-operate with one another on the hunt. Sixth, they had to stand straight and use weapons for the hunt to succeed. am not implying that these changes happened in that order; on the contrary they undoubtedly all developed gradually at the same time, each modification helping the others along. I am simply enumerating the six basic, major changes that took place as the hunting ape evolved. Inherent in these changes there are, I believe, all the ingredients necessary to make up our present sexual complexity.