Thirdly, it implies that in pair-bonding primates the couple are held together by frequent passionate sexual activity. This is the reverse of the truth. The only monogamous apes are the gibbons. The male gibbon has a hundred per cent certainty that any offspring born to his mate will carry his own genes, and consequently does not need to over-exert himself. Sexual activity is at a lower level in gibbons than in other ape species.
Fourthly, it implies that ‘permanent receptivity’ would automatically promote pair-bonding – in other words, that living in a society where all the females were on heat all of the time would ensure that each male remained faithful to one of them for life. It sounds highly improbable.
Finally, it makes the large assumption that the hominids were pair-bonded. This claim is frequently made. It is a central plank in some of the conventional scenarios of foraging males and stay-at-home females. But it remains unproven.
It is quite true, as implied by Tim White’s terse comment at the head of this chapter, that fossilised bones do not tell us very much about the sexual behaviour of our prehistoric ancestors. On the other hand, it is often possible for an anatomist, by studying the physiology of a species, to learn a great deal about its social organisation even before its behaviour has been studied by ethologists in the wild.
For example, one common kind of social grouping is described as ‘harem-type’. It consists of one dominant male accompanied by a number of females. Because the male is monopolising more than his share of the females, his position is frequently challenged by rival males and he must be equipped to defend it. The typical hallmarks of harem-type societies are that the male is much larger than the female and is equipped with natural weapons such as tusks, horns or antlers.
The gorilla conforms to this pattern. The male is twice as heavy as the female and has long, strong canine teeth, which are not found in the females. Man is obviously not cast in this mould. The size difference between men and women is very much less and the natural weapons are absent. A man’s canine teeth are relatively no larger than a woman’s.
Chimpanzees live in larger and more anarchic groups. Occasionally a male will establish a short-term monopoly of one particular female, but most of the sexual activity in a troop of chimps is opportunistic – and opportunities are frequent. Not only do females tend to outnumber males in a chimp community, but each female is in oestrus for ten days of every cycle as compared with two days in the gorilla, and during that period she may be mated by most or all of the males.
The most effective male strategy in such a promiscuous society is frequent hasty copulation, which demands an abundance of sperm. So the hallmarks of that kind of organisation are, firstly, the retention of natural weapons – because the system gives rise to a good deal of male rivalry and aggression – and secondly, the possession of very large testes. Relative to his body size, a chimpanzee’s testes are four times the size of a gorilla’s. They are also four times the size of a man’s.
Gibbons are monogamous, and they display the hallmarks of monogamous species in that there is virtually no difference between the sexes either in body size or in the length of the canine teeth.
In physical terms man conforms to none of these patterns. The size difference between the sexes (20 per cent on average, as compared with 100 per cent in the gorilla) is not great enough to indicate a harem-type society, but is too marked to suggest a monogamous one. Nor does the size of the human testes indicate monogamy. While they cannot compare with the chimpanzee’s, they are appreciably larger than the gorilla’s, and a man can attain an ejaculation frequency of 3.5 times a week before his sperm count begins to drop. This suggests a society which allows for a fair amount of opportunistic sexual activity. The large comparative size of the penis in adult male humans (man 13 cm; chimpanzee 8cm; gorilla 3cm) is not related to frequency of deployment. It is a necessary corollary of the retraction and relative inaccessibility of the vagina.
An aquatic environment seems to have had a broadly similar effect on some other species – that is, relative retraction of the female sex organ leading to a corresponding extension of that of the males. For example, most birds and reptiles do not possess a penis; the pressing together of the cloacal apertures seems to suffice for the transference of the sperm. But many species of aquatic reptiles (crocodiles and turtles) and aquatic birds (swans, ducks, geese) have found it necessary to evolve a penis as part of their adaptation to a watery habitat.
Overall, the anatomical evidence in respect of Homo suggests a species in a state of transition. There is nothing very surprising in this conclusion, for a similar transition must have taken place also among the great apes. The chimp/gorilla split occurred almost as recently as the chimp/human split, yet one or both of the African apes must have changed its sexual strategy quite radically since then, or they would not behave so differently today.
The transition, in the case of the hominids, appears to have been in the direction of a more monogamous society. Humans are more prone to form ongoing one-to-one sexual relationships than chimpanzees are, and their consortships, once formed, tend to last longer. A trend towards pair-bonding would be an entirely natural development in a species where the business of rearing the young became increasingly onerous as the period of infantile dependency grew longer.
It is true that monogamy does not come to us quite as naturally as Western cultures have often implied, and that of societies untouched by Western teaching only 26 per cent are monogamous. The existence of adultery and the high divorce rates are evidence that our instinct for pair-bonding is by no means absolute. On the other hand, the majority of divorcees re-marry; serial monogamy is still monogamy. And even among pair-bonded species such as gibbons, each of the pair is occasionally tempted to stray unless the would-be interloping males and females are chased away by the aggrieved partner. It would seem that the price of pair-bonding in gibbon society, like the price of freedom, is constant vigilance.
It is tempting to blame all sexual tensions and difficulties on ‘society’ – on more and more taboos – on inculcated feelings of shame and guilt, economic pressures, religious dogmas and artificial gender stereotypes. All these undoubtedly add to the difficulties. But it would be a mistake to imagine that there was a time, prior to civilisation, when Nature had arrived at a perfect solution.
All the orthodox theories on oestrus suffer from the same weakness as the other savannah scenarios, namely, that numerous other mammal species have faced precisely the same problems but none has resorted to the same solution. She-wolves, for example, depend on male co-operation in raising their young. The strategy adopted by wolves exactly mirrors the popular foraging-male scenarios proposed for the hominids. There is a female staying at home in a lair with the cubs; there is a male hunting far afield and bringing back food to his family. There is lifelong pair-bonding. Yet the female retains oestrus, because in a pair-bonded species its value is, if anything, enhanced. It continues to promote harmony between the sexes, without any danger of causing friction between the males, since each of them is interested only in its own mate.
When we are confronted with a unique characteristic like the suppression of oestrus, it is surely more probable that it was a response to a unique predicament, rather than an anomalous reaction to a very common one.
Th predicament in this case could have been an aquatic habitat. In mammals, oestrous status is communicated by scent signalling – a pheromonal message emitted by the female. Being airborne, it may be carried quite a long way – as evidenced by the distance a dog will travel to locate a bitch on heat. But in a wading or swimming ape the pheromones would be washed away almost as soon as they were secreted.
Some anthropologists are inclined to dispute this proposition by querying the premise: that oestrus is always primarily a scent signal. This is because in some anthropoids there are also visual signs of oestrus. In chimpanzees, for example, the skin around the vagina periodically swells up and becomes pink and conspicuous, reaching a peak of tumesce
nce during oestrus. A similar phenomenon occurs in baboons and macaques. It has been interpreted as a visual signal directed at the male.
It is unlikely that it originally evolved for that purpose. Sexual swelling is also found in the gorilla, but since it is less extensive and the skin is covered with hair, its value as a visual signal is nil. The degree of sexual swelling, as between species, does not correlate with anything in the sex life of the animals concerned. It correlates with the habitat. With the exception of the talapoin monkey and forest-dwelling chimpanzees, the species concerned are all ground dwellers, and the degree of sexual swelling appears to relate to the extent to which the monkeys or apes spend their time squatting on hard ground. It is most extensive in species living on the open savannah or in rocky habitats (for example, baboons and geladas) and least extensive in species dwelling in the forests (such as gorillas and mandrills) where the terrain is softer and leafier. It could have evolved originally as a protection for the female, to minimise direct contact between the ground and the vaginal opening during oestrus, when the female’s own secretions plus traces of semen could cause dust and grit to adhere and be driven in during intercourse.
Ethologists, being human, have a very limited sense of smell. To them the visual signal conveyed by the pink backside is the most conspicuous sign of oestrus in the chimpanzee; it has been an invaluable aid to them in relating the animals’ behaviour to the stages of their sexual cycle. It must be tempting for them to assume that it is also the most obvious and important signal to a male chimpanzee. But experiments have shown that the male is responding primarily to a pheromonal signal. A male chimp’s response to an oestrous female is unaffected when the visual signal is concealed or disguised, whereas he rapidly loses interest if the olfactory signal is counteracted.
On balance there seems no good reason to doubt that in oestrous mammals the signal perceived by the male is primarily olfactory. But in humans the ability to receive and interpret scent signals is very low. The olfactory lobe in our brains is proportionately smaller than in the brains of apes. (This is a common feature in aquatic mammals. In whales and seals the olfactory lobe has diminished almost to vanishing point.) So one reason for the ending of oestrus could be that it ceased to work properly. As a result of the pheromonal secretions being washed away, plus diminished scent perception, the signal was simply not getting across.
Another possibility is that it ceased to matter whether this signal got across or not, because male behaviour was no longer being regulated by the rhythms of the female cycle. To understand how this can happen, it is necessary to consider a major landmark in human sexual evolution – the change to face-to-face (ventro-ventral) copulation, the commonest mode in humans. This was a direct result of the stretched posture, with spine and hind limbs in a straight line, which results from either swimming or bipedalism.
In most quadrupedal mammals the vagina is accessibly located at the rear, with the opening more or less flush with the surface of the body and covered only by the tail (if there is one – apes have no tails.) In Homo sapiens, as a result of the physical modification for upright walking, the vagina is directed ventrally – towards the front. It is also retracted inside the body wall and covered by thick folds of skin (the labia majora) which are not found in the apes.
Bipedalism alone does not account for all the changes. It is rare in land animals, but quite common among aquatic ones, for the vagina to be retracted and covered with a flap of skin. The same is true of another human feature – the hymen. The hymen is commonly found in small primitive animals, and its original function was probably to ensure that sexual activity was limited to the period when conception was most likely. In the guinea pig, for example, the membrane reseals the vagina after each reproductive period. The hymen is still found in the lemurs of Madagascar and other prosimians, but not in monkeys.
It seems unlikely that its presence in Homo can be accounted for by thinking of it as a primitive hangover. An aquatic connection seems the likeliest explanation. K. E. Fichtelius has pointed out that a well developed hymen is also a feature of various aquatic mammals quite unrelated to one another, such as the toothed whales, and the seals, and the dugongs.
Ventro-ventral copulation, very rare in land mammals, is the commonest mode in aquatic mammals except for those which go ashore to breed. Whales and dolphins, dugongs and manatees, beavers, and sea otters are among the numerous aquatic species which mate face to face. Swimming promotes this method of copulation in the same way that bipedalism does, because in both cases the spine and the hind limbs are realigned, forming a continuous straight line instead of the 90-degree angle found in most quadrupeds.
In the early ’70s the suggestion was made that for a land mammal, changing to the ventro-ventral position would initially have proved traumatic for the females. In all quadrupeds the supine position is a particularly vulnerable one. Among primates there are fairly strict rules governing the infringement of personal space. Close encounters among adults for benign purposes – copulation or grooming – are normally conducted by an approach from the rear. A purposive approach by an adult male from the front usually signifies aggressive intentions. And primates – unlike many birds – have no repertoire of courtship ritual or foreplay which might give assurance to the female.
This suggestion was discounted as pure speculation. It could only be substantiated by finding another anthropoid species in which ventro-ventral copulation was the commonest mode, and observing how it affected the relationship between the sexes. No such species was known to exist. Since then studies have been made of the sexual behaviour of orang-utans and have resulted in some remarkable findings.
In a study by R. D. Nadler in 1977, four pairs of orangutans were kept under observation. The male of each pair was allowed daily access to the females. Three of the four pairs mated every day throughout the female’s sexual cycle, sometimes more than once. (The female orang-utan is therefore nowadays technically classified, like the female human, as ‘permanently receptive’.) Copulation was ventro-ventral.
Essentially all copulations, moreover, were initiated by the male in a manner resembling rape … At the beginning of each test, the male pursued the female, who fled and frequently sought to escape by climbing to the top of the cage. The male quickly caught the female, wrestling her, struggling and screaming, to the ground and forcibly initiated copulation. Once the male achieved intromission and initiated thrusting, the female became passive.
However, primate behaviour in zoo and laboratory conditions is often atypical, so it was important to confirm these findings by studies of behaviour in the wild. B.M.F. Galdikas observed wild orang-utans in Borneo over a period of four years from 1971–1975. Copulation was less frequent than under laboratory conditions but, in the wild as in captivity, it was ‘generally ventro-ventral’ and frequently carried out by force. The females’ struggles ‘… ranged in intensity and duration all the way from brief tussles with squealing and some pushing and slapping at the male’s hands to protracted violent fights in which the female struggled throughout the length of the copulation, emitted loud rape grunts, and bit the male whenever she could.’
The frontal approach in orang-utans is not attributable either to water or to bipedalism, but to a combination of size and habitat. The Asian apes – the gibbon and the orang-utan – are far more arboreal than the African ones. The gibbon is the smallest and most agile of all the apes and swings through the branches as lithely as any monkey, but the orang-utan is much larger and slower. It cannot afford to launch itself into space because of the risk of landing on a branch that would not bear its weight. The gorilla, when it grew large enough to be confronted by the same problem, came down to the ground where it now spends virtually all its waking hours. That option was not open to the orang-utan because it lives on fruit rather than greenery. It stayed aloft to be near the food supply and it lives and moves ponderously, suspended from the branches by its hands and often by one or both of its prehensile feet as well
.
For such a large animal, sex in the tree tops presents problems. The male lives alone but his territory overlaps with that of around four females who constitute his ‘dispersed harem’. Mating in the normal quadrupedal position is impossible because a female could never keep her balance on top of a branch while sustaining the weight of a male twice her size. Copulation from the rear with both animals in suspension would also be difficult: the male would have at best one hand free to grasp the female and intromission would be hard to effect with her body swinging free in the air like a pendulum. In practice, he corners her in a suitable place where she can hold on to one branch and recline on another, and forcibly manoeuvres her into a supine position. It creates a good deal of commotion, but that is of no significance because the animals are not threatened by any predators other than man.
There is no way of establishing just how recently these animals made the transition to ventro-ventral mating as a standard procedure. It is almost certainly much more recent than in the case of our own species. The reasons for thinking this are anatomical and behavioural. The orangutan’s sexual organs are less modified than in humans to accommodate the ventro-ventral approach, and the females’ behaviour suggests either that the males’ approach is inept or even that they do not at first understand what is happening. In the wild, as in the laboratory, the fighting usually – though not always – ceases once the male achieves intromission and the female realises that he is bent on sex and not on mayhem. From that point on resistance is often succeeded by restive resignation – (‘fidgeted and looked around’) – or a detached passivity – (‘resumed eating, reaching out and plucking fruits as the male was thrusting’). On some occasions the mating is wholly co-operative with no fighting, when the female is mature, and experienced, and is in oestrus.
The Scars of Evolution Page 15