To begin with, the males had to be sure that their females were going to be faithful to them when they left them alone to go hunting. So the females had to develop a pairing tendency. Also, if the weaker males were going to be expected to co-operate on the hunt, they had to be given more sexual rights. The females would have to be more shared out, the sexual organisation more democratic, less tyrannical. Each male too, would need a strong pairing tendency. Furthermore, the males were now armed with deadly weapons and sexual rivalries would be much more dangerous: again, a good reason for each male being satisfied with one female. On top of that there were the much heavier parental demands being made by the slow-growing infants. Paternal behaviour would have to be developed and the parental duties shared between the mother and the father: another good reason for a strong pair-bond.
Given this situation as a starting point we can now see how other things grew from it. The naked ape had to develop the capacity for falling in love, for becoming sexually imprinted on a single partner, for evolving a pair-bond. Whichever way you put it, it comes to the same thing. How did he manage to do this? What were the factors that helped him in this trend? As a primate, he will already have had a tendency to form brief mateships lasting a few hours, or perhaps even a few days, but these now had to be intensified and extended. One thing that will have come to his aid is his own prolonged childhood. During the long, growing years he will have had the chance to develop a deep personal relationship with his parents, a relationship much more powerful and lasting than anything a young monkey could experience. The loss of this parental bond with maturation and independence would create a ‘relationship void’—a gap that had to be filled. He would therefore already be primed for the development of a new, equally powerful bond to replace it.
Even if this was enough to intensify his need for forming a new pair-bond, there would still have to be additional assistance to maintain it. It would have to last long enough for the lengthy process of rearing a family. Having fallen in love, he would have to stay in love. By developing a prolonged and exciting courtship phase he could ensure the former, but something more would be needed after that. The simplest and most direct method of doing this was to make the shared activities of the pair more complicated and more rewarding. In other words, to make sex sexier.
How was this done? In every possible way, seems to be the answer. If we look back now at the behaviour of the present-day naked ape we can see the pattern taking shape. The increased receptivity of the female cannot be explained only in terms of increasing the birth-rate. It is true that by being prepared to copulate while still at the maternal phase of rearing a baby, the female does increase the birth-rate. With the very long dependency period, it would be a disaster if she did not. But this cannot explain why she is ready to receive the male and become sexually aroused throughout each of her monthly cycles. She only ovulates at one point during the cycle, so that mating at all other times can have no procreative function. The vast bulk of copulation in our species is obviously concerned, not with producing offspring, but with cementing the pair-bond by providing mutual rewards for the sexual partners. The repeated attainment of sexual consummation for a mated pair is clearly, then, not some kind of sophisticated, decadent outgrowth of modern civilisation, but a deep-rooted, biologically based, and evolutionary sound tendency of our species.
Even when she has stopped going through her monthly cycles—in other words, when she is pregnant—the female remains responsive to the male. This, too, is particularly important because, with a one-male one-female system, it would be dangerous to frustrate the male for too long a period. It might endanger the pair-bond.
In addition to increasing the amount of time when sexual activities can take place, the activities themselves have been elaborated. The hunting life that gave us naked skins and more sensitive hands has given us much greater scope for sexually stimulating body-to-body contacts. During pre-copulatory behaviour these play a major role. Stroking, rubbing, pressing and caressing occur in abundance and far exceed anything found in other primate species. Also, specialised organs such as the lips, earlobes, nipples, breasts and genitals are richly endowed with nerve-endings and have become highly sensitised to erotic tactile stimulation. The ear-lobes, indeed, appear to have been exclusively evolved to this end. Anatomists have often referred to them as meaningless appendages, of ‘useless, fatty excrescences’. In general parlance they are explained away as ‘remnants’ of the time when we had big ears. But if we look at other primate species, we find that they do not possess fleshy ear-lobes. It seems that, far from being a remnant, they are something new, and when we discover that, under the influence of sexual arousal, they become engorged with blood, swollen and hypersensitive, there can be little doubt that their evolution has been exclusively concerned with the production of yet another erogenous zone. (Surprisingly, the humble ear-lobe has been rather overlooked in this context, but it is worth noting that there are cases on record of both males and females actually reaching orgasm as a result of ear-lobe stimulation.) It is interesting to note that the protuberant, fleshy nose of our species is another unique and mysterious feature that the anatomists cannot explain. One has referred to it as a ‘mere exuberant variation of no functional significance’. It is hard to believe that something so positive and distinctive in the way of appearance appendages should have evolved without a function. When one reads that the side walls of the nose contain a spongy erectile tissue that leads to nasal enlargement and nostril expansion by vaso-congestion during sexual arousal, one begins to wonder.
As well as the improved tactile repertoire, there are some rather unique visual developments. Complex facial expressions play an important part here, although heir evolution is concerned with improved communication in many other contexts as well. As a primate species we have the best developed and most complex facial musculature of the entire group. Indeed, we have the most subtle and complex facial expression system of all living animals. By making tiny movements of the flesh around the mouth, nose, eyes, eyebrows, and on the forehead, and by recombining the movements in a wide variety of ways, we can convey a whole range of complex moodchanges. During sexual encounters, especially during the early courtship phase, these expressions are of paramount importance. (Their exact form will be discussed in another chapter.) Pupil dilation also occurs during sexual arousal and, although it is a small change, we may be more responsive to it than we realise. The eye surface also glistens.
Like the ear-lobes and the protruding nose, the lips of our species are a unique feature, not found elsewhere in the primates. Of course, all primates have lips, but not turned inside-out like ours. A chimpanzee can protrude and turn back its lips in an exaggerated pout, exposing as it does so the mucous membrane that normally lies concealed inside the mouth. But the lips are only briefly held in this posture before the animal reverts to its normal ‘thin—upped’ face. We, on the other hand, have permanently everted, rolled-back lips. To a chimpanzee we must appear to be in a permanent pout. If you ever have occasion to be embraced by a friendly chimpanzee, the kiss that it may then vigorously apply to your neck will leave you in no doubt about its ability to deliver a tactile signal with its lips. For the chimpanzee this is a greeting signal rather than a sexual one, but in our species it is used in both contexts, the kissing contact becoming particularly frequent and prolonged during the pre-copulatory phase. In connection with this development it was presumably more convenient to have the sensitive mucous surfaces permanently exposed, so that special muscle contractions around the mouth did not have to be maintained throughout the prolonged kissing contacts, but this is not the whole story. The exposed, mucous lips, evolved a well-defined and characteristic shape. They did not grade off inconspicuously into the surrounding facial skin, but developed a fixed boundary line. In this way they also became important visual signalling devices. We have already seen that sexual arousal produces a swelling and reddening of the lips, and the clear demarcation of this area obviously assisted in the refinement of these signals, making subtle changes in lip condition more easily recognisable. Also, of course, even in their un-aroused condition they are redder than the rest of the face skin and, simply by their very existence, without indicating changes in physiological condition, they will act as advertising signals, drawing attention to the presence of a tactile sexual structure.
Puzzling over the significance of our unique mucous lips, anatomists have stated that their evolution ‘is not as yet clearly understood’ and have suggested that perhaps it has something to do with the increased amount of sucking that is required of the infant at the breast. But the young chimpanzee also does a great deal of very efficient sucking and its more muscular and prehensile lips would seem, if anything, to be better equipped for the job. Also, this cannot explain the evolution of a sharp margin between the lips and the surrounding face. Nor can it explain the striking differences in the lips of light and dark-skinned populations. If, on the other hand, the lips are regarded as visual signalling devices, these differences are easier to understand. If climatic conditions demand a darker skin, then this will work against the visual signaling capacity of the lips by reducing their colour contrast. If they really are important as visual signals, then some kind of compensating development might be expected, and this is precisely what seems to have occurred, the negroid lips maintaining their conspicuousness by becoming larger and more protuberant. What they have lost in colour contrast, they have made up for in size and shape. Also, the margins of the negroid lips are more strikingly delineated. The ‘lip-seams’ of the paler races become exaggerated into more prominent rides that are lighter in colour than the rest of the skin. Anatomically, these negroid characters do not appear to be primitive, but rather represent a positive advance in the specialisation of the lip region.
There are a number of other obvious visual sexual signals. At puberty, as I have already mentioned, the arrival of a fully operative breeding condition is signalled by the development of conspicuous tufts of hair, especially in the region of the genitals and armpits and, in the male, on the face. In the female there is rapid growth of the breasts. The body shape also changes, becoming broader at the shoulders in the male and at the pelvis in the female. These changes not only differentiate the sexually mature individual from the immature, but most of them also distinguish the mature male from the mature female. They not only act as signals revealing that the sexual system is now functional, but also indicate in each case whether it is masculine or feminine.
The enlarged female breasts are usually thought of primarily as maternal rather than sexual developments, but there seems to be little evidence for this. Other species of primates provide an abundant milk supply for their offspring and yet they fail to develop clearly defined hemispherical breast swellings. The female of our species is unique amongst primates in this respect. The evolution of protruding breasts of a characteristic shape appears to be yet another example of sexual signalling. This would be made possible and encouraged by the evolution of the naked skin. Swollen breast-patches in a shaggy-coated female would be far less conspicuous as signalling devices, but once the hair has vanished they would stand out clearly. In addition to their own conspicuous shape, they also serve to concentrate visual attention on to the nipples and to make the nipple erection that accompanies sexual arousal more conspicuous. The pigmented area of skin around the nipple, that deepens in colour during sexual arousal, also helps in the same way.
The nakedness of the skin also makes possible certain colour-change signals. These occur in limited areas in other animals where there are small naked patches, but have become more extensive in our own species. Blushing occurs with particularly high frequency during the earlier courtship stages of sexual behaviour and, at the later phases of more intense arousal, there is the characteristic mottling of the sex flush. (Again, this is a form of signalling that the darker-skinned races have had to sacrifice to climatic demands. We know that they still undergo these changes, however, because, although they are invisible as colour transformations, close examination reveals significant changes in skin texture.)
Before leaving this array of visual sexual signals we must consider a rather unusual aspect of they evolution. To do so, we must take a sidelong glance at some rather strange things that have been happening to the bodies of a number of our lowlier primate cousins, the monkeys. Recent German research has revealed that certain species have started to mimic themselves. The most dramatic examples of this are the mandrill and the gelada baboon. The male mandrill has a bright red penis with blue scrotal patches on either side of it. This colour arrangement is repeated on its face, its nose being bright red and its swollen, naked cheeks an intense blue. It is as if the animal’s face is mimicking its genital region by giving the same set of visual signals. When the male mandrill approaches another animal, its genital display tends to be concealed by its body posture, but it can still apparently transmit the vital messages by using its phallic face. The female gelada indulges in a similar self copying device. Around her genitals there is a bright red skin patch, bordered with white papillae. The lips of the vulva in the centre of this area are a deeper, richer red. This visual pattern is repeated on her chest region, where again there is a patch of naked red skin surrounded by the same kind of white papillae. In the centre of this chest patch the deep red nipples have come to lie so close together that they are strongly reminiscent of the lips of the vulva. (They are indeed so close to one another that the infant sucks from both teats at the same time.) Like the true genital patch, the chest patch varies in intensity of colour during the different stages of the monthly sexual cycle. The inescapable conclusion is that the mandrill and the gelada have brought their genital signals forward to a frontal position for some reason. We know too little about the life of mandrills in the wild to be able to speculate as to the reasons for this strange occurrence in this particular species, but we do know that wild geladas spend a great deal more of their time in an upright sitting posture than most other similar monkey species. If this is a more typical posture for them, then it follows that by having signals on their chests they can more readily transmit these signals to other members of the group than if the markings only existed on their rear ends. Many species of primates have brightly coloured genitals, but these frontal mimics are rare.