Desmond Morris’s ‘The Naked Ape’: A Pre-Sociobiological Work of Human Ethology 

First published in 1967, ‘The Naked Ape’, a popular science classic authored by the already famous British zoologist and TV presenter Desmond Morris, belongs to the pre-sociobiological tradition of human ethology

In the most general sense, the approach adopted by the human ethologists, who included, not only Morris, but also playwright Robert Ardrey, anthropologists Lionel Tiger and Robin Fox and the brilliant Nobel-prize winning ethologist, naturalist, zoologist, pioneering evolutionary epistemologist and part-time Nazi sympathizer Konrad Lorenz, was correct. 

They sought to study the human species from the perspective of zoology. In other words, they sought to adopt the disinterested perspective, and detachment, of, as Edward O Wilson was later to put it, “zoologists from another planet” (Sociobiology: The New Synthesis: p547). 

Thus, Morris proposed cultivating: 

An attitude of humility that is becoming to proper scientific investigation… by deliberately and rather coyly approaching the human being as if he were another species, a strange form of life on the dissecting table” (p14-5).  

In short, Morris proposed to study humans just as a zoologist would any other species of non-human animal. 

Such an approach was an obvious affront to anthropocentric notions of human exceptionalism – and also a direct challenge to the rather less scientific approach of most sociologists, psychologists, social and cultural anthropologists and other such ‘professional damned fools’, who, at that time, almost all studied human behavior in isolation from, and largely ignorance of, biology, zoology, and the scientific study of the behavior of all animals other than humans. 

As a result, such books inevitably attracted controversy and criticism. Such criticism, however, invariably missed the point. 

The real problem was not that the ethologists sought to study human behavior in just the same way a zoologist would study the behavior of any nonhuman animal, but rather that the study of the behavior of nonhuman animals itself remained, at this time, very much in its infancy. 

Thus, the field of animal behavior was to be revolutionized just a decade or so after the publication of ‘The Naked Ape’ by the approach that came to be known as, first, sociobiology, now more often as behavioral ecology, or, when applied to humans, evolutionary psychology

These approaches sought to understand behavior in terms of fitness maximization – in other words, on the basis of the recognition that organisms have evolved to engage in behaviors which tended to maximize their reproductive success in ancestral environments. 

Mathematical models, often drawn from economics and game theory, were increasingly employed. In short, behavioral biology was becoming a mature science. 

In contrast, the earlier ethological tradition was, even at its best, very much a soft science. 

Indeed, much such work, for example Jane Goodall’s rightly-celebrated studies of the chimpanzees of Gombe, was almost pre-scientific in its approach, involving observation, recording and description of behaviors, but rarely the actual testing or falsification of hypotheses. 

Such research was obviously important. Indeed, Goodall’s was positively groundbreaking. 

After all, the observation of the behavior or an organism is almost a prerequisite for the framing of hypotheses about the behavior of that organism, since hypotheses are, in practice, rarely generated in an informational vacuum from pure abstract theory. 

However, such research was hardly characteristic of a mature and rigorous science. 

When hypotheses regarding the evolutionary significance of behavior patterns were formulated by early ethologists, this was done on a rather casual ad hoc basis, involving a kind of ‘armchair adaptationism’, which could perhaps legitimately be dismissed as the spinning of, in Stephen Jay Gould’s famous phrase, just so stories

Thus, a crude group selectionism went largely unchallenged. Yet, as George C Williams was to show, and Richard Dawkins later to forcefully reiterate in The Selfish Gene (reviewed here), behaviors are unlikely to evolve that benefit the group or species if they involve a cost to the inclusive fitness of the individual engaging in the behavior. 

Robert Wright picks out a good example of this crude group selectionism from ‘The Naked Ape’ itself, quoting Morris’s claim that, over the course of human evolution: 

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” (p64). 

To anyone schooled in the rudiments of Dawkinsian selfish gene theory, the fallacy should be obvious. But, just in case we didn’t spot it, Wright has picked it out for us: 

Stop right there. It was in the reproductive interests of the males for the females to develop a tendency toward fidelity? So natural selection obliged the males by making the necessary changes in the females? Morris never got around to explaining how, exactly, natural selection would perform this generous feat” (The Moral Animal: p56). 

In reality, couples have a conflict of interest here, and the onus is clearly on the male to evolve some mechanism of mate-guarding, though a female might conceivably evolve some way to advertise her fidelity if, by so doing, she secured increased male parental investment and provisioning, hence increasing her own reproductive success.[1]

In short, mating is Machiavellian. A more realistic view of human sexuality, rooted in selfish gene theory, is provided by Donald Symons in his seminal The Evolution of Human Sexuality (which I have reviewed here). 

Unsuccessful Societies? 

The problems with ‘The Naked Ape’ begin in the very first chapter, where Morris announces, rather oddly, that, in studying the human animal, he is largely uninterested in the behavior of contemporary foraging groups or other so-called ‘primitive’ peoples. Thus, he bemoans: 

The earlier anthropologists rushed off to all kinds of unlikely corners of the world… scattering to remote cultural backwaters so atypical and unsuccessful that they are nearly extinct. They then returned with startling facts about the bizarre mating customs, strange kinship systems, or weird ritual procedures of these tribes, and used this material as though it were of central importance to the behaviour of our species as a whole. The work done by these investigators… did not tell us was anything about the typical behaviour of typical naked apes. This can only be done by examining the common behaviour patterns that are shared by all the ordinary, successful members of the major cultures-the mainstream specimens who together represent the vast majority. Biologically, this is the only sound approach” (p10).[2]

Thus, today, political correctness has wholly banished the word ‘primitive’ from the anthropological lexicon. It is, modern anthropologists insist, demeaning and pejorative.  

Indeed, post-Boasian cultural anthropologists in America typically reject the very notion that some societies are more advanced than others, championing instead a radical cultural relativism and insisting we have much to learn from the lifestyle and traditions of hunter-gatherers, foragers, savage cannibals and other such ‘indigenous peoples’. 

Morris also rejects the term ‘primitive’ as a useful descriptor for hunter-gatherer and other technologically-backward peoples, but for diametrically opposite reasons. 

Thus, for Morris, to describe foraging groups as ‘primitive’ is to rather give them altogether too much credit: 

The simple tribal groups that are living today are not primitive, they are stultified. Truly primitive tribes have not existed for thousands of years. The naked ape is essentially an exploratory species and any society that has failed to advance has in some sense failed, ‘gone wrong’. Something has happened to it to hold it back, something that is working against the natural tendencies of the species to explore and investigate the world around it” (p10). 

Instead, Morris proposes to focus on contemporary western societies, declaring: 

North America… is biologically a very large and successful culture and can, without undue fear of distortion, be taken as representative of the modern naked ape” (p51) 

It is indeed true that, with the diffusion of American media and consumer goods, American culture is fast becoming ubiquitous. However, this is a very recent development in historical terms, let alone on the evolutionary timescale of most interest to biologists. 

Indeed, viewed historically and cross-culturally, it is we westerners who are the odd, aberrant ones. 

Thus, we even have been termed, in a memorable backcronym, WEIRD (Western, Educated, Industrialized, Rich and Democratic), and hence quite aberrant, not only in terms of our lifestyle and prosperity, but also in terms of our psychology and modes of thinking

Moreover, while foraging groups, and other pre-modern peoples, may now indeed now be tottering on the brink of extinction, this again is a very recent development. 

Indeed, far from being aberrant, this was the lifestyle adopted by all humans throughout most of the time we have existed as a species, including during the period when most of our unique physical and behavioural adaptations evolved

In short, although we may inhabit western cities today, this is not the environment where we evolved, nor that to which our brains and bodies are primarily adapted.[3]

Therefore, given that it represents the lifestyle of our ancestors during the period when most of our behavioral and bodily adaptations evolved, primitive peoples must necessarily have a special place in any evolutionary theory of human behaviour.[4]

Indeed, Morris himself admits as much himself just a few pages later, where he acknowledges that: 

The fundamental patterns of behavior laid down in our early days as hunting apes still shine through all our affairs, no matter how lofty they may be” (p40). 

Indeed, a major theme of ‘The Naked Ape’ is the extent to which the behaviour even of wealthy white westerners is nevertheless fundamentally shaped and dictated by the patterns of foraging set out in our ancient hunter-gatherer past. 

This, of course, anticipates the concept of the environment of evolutionary adaptedness (or EEA) in modern evolutionary psychology

Thus, Morris suggests that the pattern of men going out to work to financially provision wives and mothers who stay home with dependent offspring reflects the ancient role of men as hunters provisioning their wives and children: 

“Behind the façade of modern city life there is the same old naked ape. Only the names have been changed: for ‘hunting’ read ‘working’, for ‘hunting grounds’ read ‘place of business’, for ‘home base’ read ‘house’, for ‘pair-bond’ read ‘marriage’, for ‘mate’ read ‘wife’, and so on” (p84).[5]

In short, while we must explain the behaviors of contemporary westerners, no less than those of primitive foragers, in the light of Darwinian evolution, nevertheless all such behaviors must be explained ultimately in terms of adaptations that evolved over previous generations under very different conditions. 

Indeed, in the sequel to ‘The Naked Ape’, Morris further focuses on this very point, arguing that modern cities, in particular, are unnatural environments for humans, rejecting the then-familiar description of cities as concrete jungles on the grounds that, whereas jungles are the “natural habitat” of animals, modern cities are very much an unnatural habitat for humans. 

Instead, he argues, the better analogy for modern cities is a Human Zoo

The comparison we must make is not between the city dweller and the wild animal but between the city dweller and the captive animal. The city dweller is no longer living in conditions natural for his species. Trapped, not by a zoo collector, but by his own brainy brilliance, he has set himself up in a huge restless menagerie where he is in constant danger of cracking under the strain” (The Human Zoo: pvii). 

Nakedness 

Morris adopts what he calls a zoological approach. Thus, unlike modern evolutionary psychologists, he focuses as much on explaining our physiology as our behavior and psychology. Indeed, it is in explaining the peculiarities of human anatomy that Morris’s book is at his best.[6]

This begins, appropriately enough, with the trait that gives him his preferred name for our species, and also furnishes his book with its title – namely our apparent nakedness or hairlessness. 

Having justified calling us ‘The Naked Ape’ on zoological grounds, namely on the ground that this is the first thing the naturalist would notice upon observing our species, Morris then comes close to contradicting himself, admitting that we actually have more hairs on our bodies than do chimpanzees.[7]

However, Morris summarily dispatches this objection: 

It is like saying that because a blind man has a pair of eyes, he is not blind. Functionally, we are stark naked and our skin is fully exposed” (p42). 

Why then are we so strangely hairless? Neoteny, Morris proposes, provides part of the answer. 

This refers to the tendency of humans to retain into maturity traits that are, in other primates, restricted to juveniles, nakedness among them. 

Neoteny is a major theme in Morris’s book – and indeed in human evolution

Besides our hairlessness, other human anatomical features that have been explained either partly or wholly in terms of neoteny, whether by Morris or by other evolutionists, include our brain size, growth patterns, inventiveness, upright posture, spinal curvature, smaller jaws and teeth, forward facing vaginas, lack of a penis bone, the length of our limbs and the retention of the hymen into sexual maturity (see below). Indeed, many of these traits are explicitly discussed by Morris himself as resulting from neoteny

However, while neoteny may supply the means by which our relative hairlessness evolved, it is not a sufficient explanation for why this development occurred, because, as Morris points out: 

The process of neoteny is one of the differential retarding of developmental processes” (p43). 

In other words, humans are neotenous in respect of only some of our characters, not all of them. After all, an ape that remained infantile in all respects would never evolve, for the simple reason that it would never reach sexual maturity and hence remain unable to reproduce. 

Instead, only certain specific juvenile or infantile traits are retained into adulthood, and the question then becomes why these specific traits were the ones chosen by natural selection to be retained. 

Thus, Morris concludes: 

It is hardly likely… that an infantile trait as potentially dangerous as nakedness was going to be allowed to persist simply because other changes were slowing down unless it had some special value to the new species” (p43). 

As to what this “special value” (i.e. selective advantage) might have been, Morris considers, in turn, various candidates.  

One theory considered by Morris theory relates to our susceptibility to insect parasites.  

Because humans, unlike many other primates, return to a home base to sleep most nights, we are, Morris reports, afflicted with fleas as well as lice (p28-9). Yet fur, Morris observes, is a good breeding ground for such parasites (p38-9). 

Perhaps, then, Morris imagines, we might have evolved hairlessness in order to minimize the problems posed by such parasites. 

However, Morris rejects this as an adequate explanation, since, he observes: 

Few other den dwelling mammals… have taken this step” (p43). 

An alternative explanation implicates sexual selection in the evolution of human hairlessness.  

Substantial sex differences in hairiness, as well as the retention of pubic hairs around the genitalia, suggests that sexual selection may indeed have played a role in the evolution of our relative hairlessness as compared to other mammals. 

Morris, however, rejects this explanation on the grounds that: 

The loss of bodily insulation would be a high price to pay for a sexy appearance alone” (p46). 

But other species often often pay a high price for sexually selected bodily adornments. For example, the peacock sports a huge, brightly coloured and elaborate tail that is thought to have evolved through sexual selection or female choice, which is costly to grow and maintain, impedes his mobility and is conspicuous to predators. 

Indeed, according to Amotz Zahavi’s handicap principle, it is precisely the high cost of such sexually-selected adornments that made them reliable fitness indicators and hence attractive to potential mates, because only a highly ‘fit’ male can afford to grow such a costly, inconvenient and otherwise useless appendage. 

Morris also gives unusually respectful consideration to the highly-controversial aquatic ape theory as an explanation for human hairlessness. 

Thus, if humans did indeed pass through an aquatic, or at least amphibious, stage during our evolution, then, Morris agrees, this may indeed explain our hairlessness, since it is indeed true that other aquatic or semiaquatic mammals, such as whales, dolphins and seals, also seem to have jettisoned most of their fur over the course of their evolution. 

This is presumably because fur increases frictional drag while in the water and hence impedes swimming ability, and is among the reasons that elite swimmers also remove their body-hair before competition. 

Indeed, our loss of body hair is among the human anatomical peculiarities that are most often cited by champions of aquatic ape theory in favor of the theory that humans did indeed pass through an aquatic phase during our evolution. 

However, aquatic ape theory is highly controversial, and is rejected by almost all mainstream evolutionists and biological anthropologists.  

As I have said, Morris, for his part, gives respectful consideration to the theory, and, unlike many other anthropologists and evolutionists, does not dismiss it out of hand as entirely preposterous and unworthy even of further consideration.[8]

On the contrary, Morris credits the theory as “ingenious”, acknowledging that, if true, it might explain many otherwise odd features of human anatomy, including not just our relative hairlessness, but also the retention of hairs on our head, the direction of the hairs on our backs, our upright posture, ‘streamlined’ bodies, dexterity of our hands and the thick extra layer of sub-cutaneous fat beneath our skin that is lacking in other primates. 

However, while acknowledging that the theory explains many curious anomalies of human physiology, Morris ultimately rejects ‘aquatic ape theory’ as altogether too speculative given the complete lack of fossil evidence in support of the theory – the same reason that most other evolutionists also reject the theory. 

Thus, he concludes: 

It demands… the acceptance of a hypothetical major evolutionary phase for which there is no direct evidence” (p45-6). 

Morris also rejects the theory that was, according to Morris himself, the most widely accepted explanation for our hairlessness among other evolutionists at the time he was writing – namely the theory that our hairlessness evolved as a cooling mechanism when our ancestors left the shaded forests for the open African savannah

The problem with this theory, as Morris explains it, is that:  

Exposure of the naked skin to the air certainly increases the chances of heat loss, but it also increases heat gain at the same time and risks damage from the sun’s rays” (p47). 

Thus, it is not at all clear that moving into the open savannah would indeed select for hairlessness. Otherwise, as Morris points out, we might expect other carnivorous, predatory mammals such as lions and jackals, who also inhabit the savannah, to have similarly jettisoned most of their fur. 

Ultimately, however, Morris accepts instead a variant on this idea – namely that hairlessness evolved to prevent overheating while chasing prey when hunting. 

However, this fails to explain why it is men’s bodies that are generally much hairier than those of women, even though, cross-culturally, in most foraging societies, it is men who do most, if not all, of the hunting. 

It also raises the question as to why other mammalian carnivores, including some that also inhabit the African Savannah and other similar environments, such as lions and jackals, have not similarly shed their body hair, especially since the latter rely more on their speed to catch prey species, whereas humans, armed with arrows and javelins as well as hunting dogs, do not always have to catch a prey themselves in order to kill it. 

I would tentatively venture an alternative theory, one which evidently did not occur to Morris – namely, perhaps our hairlessness evolved in concert with our invention and use of clothing (e.g. animal hides) – i.e. a case of gene-culture coevolution

Clothing would provide an alternative means of protect from both sun and cold alike, but one that has the advantage that, unlike bodily fur, it can be discarded (and put back on) on demand. 

This explanation suggests that, paradoxically, we became naked apes at the same time, and indeed precisely because, we had also become clothed apes. 

The Sexiest Primate? 

One factor said to have contributed to the book’s commercial success was the extent to which its thesis chimed with the prevailing spirit of the age during which it was first published, namely the 1960s. 

Thus, as already alluded to, it presented, in many ways, an idealized and romantic version of human nature, with its crude group-selectionism and emphasis on cooperation within groups without a concomitant emphasis on conflict between groups, and its depiction of humans as a naturally monogamous pair-bonding species, without a concomitant emphasis on the prevalence of infidelity, desertion, polygamy, Machiavellian mating strategies and even rape.  

Another element that jibed with the zeitgeist of the sixties was Morris’s emphasis on human sexuality, with Morris famously declaring: 

The naked ape is the sexiest primate alive” (p64). 

Are humans indeed the ‘sexiest’ of primates? How can we assess this claim? It depends, of course, on precisely how we define ‘sexiness’. 

Obviously, if beauty is in the eye of the beholder, then sexiness is located in a rather different part of the male anatomy, but equally subjective in nature. 

Thus, humans like ourselves find other humans more sexy than other primates because we have evolved to do so. A male chimpanzee, however, would likely disagree and regard a female chimpanzee as sexier. 

However, Morris presumably has something else in mind when he describes humans as the “sexiest” of primates. 

What he seems to mean is that sexuality and sexual behavior permeates the life of humans to a greater degree than for other primates. Thus, for example, he cites as evidence the extended or continuous sexual receptivity of human females, writing: 

There is much more intense sexual activity in our own species than in any other primates” (p56) 

However, the claim that sexuality and sexual behavior permeates the life of humans to a greater degree than for other primates is difficult to maintain when you have studied the behavior of some of our primate cousins. Thus, for example, both chimpanzees and especially bonobos, our closest relatives among extant non-human primates, are far more promiscuous than all but the sluttiest of humans

Indeed, one might cynically suggest that what Morris had most in mind when he described humans as “the sexiest primate alive” was simply a catchy marketing soundbite that very much tapped into the zeitgeist of the era (i.e. the 1960s) and might help boost sales for his book. 

Penis Size

As further evidence for our species’ alleged “sexiness” Morris also supposedly unusually large size of the human penis, reporting: 

The [human] male has the largest penis of any primate. It is not only extremely long when fully erect, but also very thick when compared with the penises of other species” (p80). 

This claim, namely that the human male has an unusually large penis, may originate with Morris, and has certainly since enjoyed wide currency in subsequent decades. 

Thus, competing theories have been formulated to account for the (supposedly) unusual size of our penes, including the idea that penis size evolved through sexual selection (e.g. The Mating Mind: p234-6), or that our large penes are designed to remove sperm deposited by rival males in the female reproductive tract (Human Sperm Competition: p170-171; Gallup & Burch 2004; Gallup et al 2004; Goetz et al 2005; Goetz et al 2007

Yet, according to Alan F Dixson, the human penis is, in fact, not unusually long by primate standards, being roughly the same length as that of the chimpanzee (Sexual Selection and the Origins of Human Mating Systems: p64). 

Instead, Dixson reports: 

The erect human penis is comparable in length to those of other primates, in relation to body size. Only its circumference is unusual when compared to the penes of other hominids” (Sexual Selection and the Origins of Human Mating Systems: p65). 

The human penis is unusual, then, only in its width or girth. 

As to why our penes are so wide, the answer is quite straightforward, and has little to do with the alleged ‘sexiness’ of the human species, whatever that means. 

Instead, it is a simple, if indirect, reflection of our increased brain-size.

Increased brain-size first selected for changes in the size and shape of female reproductive anatomy. This, in turn, led to changes in male reporoductive anatomy. Thus, Bowman suggests: 

As the diameter of the bony pelvis increased over time to permit passage of an infant with a larger cranium, the size of the vaginal canal also became larger” (Bowman 2008). 

Similarly, Robin Baker and Mark Bellis write: 

The dimensions and elasticity of the vagina in mammals are dictated to a large extent by the dimensions of the baby at birth. The large head of the neonatal human baby (384g brain weight compared with only 227g for the gorilla…) has led to the human vagina when fully distended being large, both absolutely and relative to the female body… particularly once the vagina and vestibule have been stretched during the process of giving birth, the vagina never really returning to its nulliparous dimensions” (Human Sperm Competition: Copulation, Masturbation and Infidelity: p171). 

In turn, larger vaginas select for larger penises in order to fill this larger vagina (Bowman 2008).  

Interestingly, this theory directly contradicts the alleged claim of Philippe Rushton that there is an inverse correlation between brain-size and penis-size, which relationship supposedly explains race differences in brain and genital size. Thus, Rushton was infamously quoted as observing: 

It’s a trade off, more brains or more penis. You can’t have everything.[9]

On the contrary, this analysis suggests that, at least as between species (and presumably as between sub-species, i.e. races, as well), there is a positive correlation between brain-size and penis-size.[10]

According to Baker and Bellis, one reason male penis size tracks that of female vagina size (both being relatively large, and especially wide, in humans) is that the penis functions as, in Baker and Bellis’s words, a “suction piston”, functioning as a mechanism of sperm competition by removing any sperm previously deposited by rival males. 

Thus, they report: 

In order to distend the vagina sufficiently to act as a suction piston, the penis needs to be a suitable size [and] the relatively large size… and distendibility of the human vagina (especially after giving birth) thus imposes selection, via sperm competition, for a relatively large penis” (Human Sperm Competition: p171). 

Interestingly, this theory – namely that the human penis functions as a sperm displacement device – although seemingly fanciful, actually explains some otherwise puzzling aspect of human coitus, such as its relatively extended duration, the male refractory period and related Coolidge effect – i.e. why a male cannot immediately recommence intercourse immediately after orgasm, unless perhaps with a new female (though this exception has yet to be experimentally demonstrated in humans). 

It even has some empirical support (Gallup & Burch 2004; Goetz et al 2005; Goetz et al 2007), including some delightful experiments involving sex toys of various shapes (Gallup et al 2004). 

Morris writes: 

“[Man] is proud that he has the biggest brain of all the primates, but attempts to conceal the fact that he also has the biggest penis, preferring to accord this honor falsely to the mighty gorilla” (p9). 

Actually, the gorilla, mighty though he indeed may be, has relatively small genitalia. This is on account of his polygynous, but non-polyandrous, mating system, which involves minimal sperm competition.[11]

Moreover, the largeness of our brains, in which, according to Morris, we take such pride, may actually be the cause of the largeness of our penes, for which, according to Morris, we have such shame (here, he speaks for few men). 

Thus, large brains required larger heads which, in turn, required larger vaginas in order to successfully birth larger-headed babies. This in turn selected for larger penises to fill the larger vagina. 

In short, the large size, or rather large girth/width, of our penes has less to do with our being the “sexiest primate” and more to do with our being the brainiest

Female Breasts 

In addition to his discussion of human penis size, Morris also argues that various other features of human anatomy that not usually associated with sex nevertheless evolved, in part, due to their role in sexual signaling. These include our earlobes (p66-7), everted lips (p68-70) and, tentatively and rather bizarrely, perhaps even our large fleshy noses (p67). 

He makes the most developed and persuasive case, however, in respect of another physiological peculiarity of the human species, and of human females in particular, namely the female breasts

Thus, Morris argues: 

For our species, breast design is primarily sexual rather than maternal in function” (p106). 

The evolution of protruding breasts of a characteristic shape appears to be yet another example of sexual signalling” (p70). 

As evidence, he cites the differences in shape between women’s breasts and both the breasts of other primates and the design of baby bottles (p93). In short, the shape of human breasts do not seem ideally conducive to nursing alone. 

The notion that breasts have a secondary function as sexual advertisements is indeed compelling. In most other mammals, large breasts develop only during pregnancy, but human breasts are permanent, developing at puberty, and, except during pregnancy and lactation, composed predominantly of fat not milk (see Møller et al 1995; Manning et al 1997; Havlíček et al 2016). 

On the other hand, it is difficult to envisage how breasts ever first became co-opted as a sexually-selected ornament. 

After all, the presence of developed breasts on a female would originally, as among other primates, have indicated that the female in question was pregnant, and hence infertile. There would therefore initially have been strong selection pressure among males against ever finding breasts sexually attractive, since it would lead to their pursuing infertile women whom they could not possibly impregnate. 

How then did breasts ever make the switch to a sexually attractive, sexually-selected ornament? This is what George Francis, at his blog, ‘Anglo Reaction’, terms the breast paradox.[12]

Morris does not address this not insignificant problem. However, he does suggest that two other human traits unique among primates may have facilitated the process. 

Our so-called ‘nakedness’ (i.e. relative hairlessness), the trait that furnished Morris’s book with its title, and Morris himself with his preferred name for our species, is the first of these traits. 

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” (p70-1). 

Secondly, Morris argues that our bipedalism (i.e. the fact we walk on two legs) and resulting vertical posture, necessarily put the female reproductive organs out of sight underneath a woman when she adopts a standing position, and hence generally out of the sight of potential mates. There was therefore, Morris suggests, a need for some frontal sexual-signaling. 

This, he argues, was further necessitated by what he argues is our species’ natural preference for ventro-ventral (i.e. missionary position) intercourse. 

In particular, Morris argues that human female breasts evolved in order to mimic the appearance of the female buttocks, a form of what he terms ‘self-mimicry’. 

The protuberant, hemispherical breasts of the female must surely be copies of the fleshy buttocks” (p76). 

Everted Lips 

Interestingly, he makes a similar argument in respect of another trait of humans not shared by other extant primates – namely, our inverted lips.

The word ‘everted’ refers to the fact that our lips are turned outwards, as is easily perceived by comparing human lips with the much thinner lips of our closest non-human relatives

Again, this seems intuitively plausible, since, like female breasts, lips do indeed seem to be a much-sexualized part of the human anatomy, at least in western societies, and in at least some non-western cultures as well, if erotic art is to be taken as evidence.[13]

These everted lips, he argues, evolved to mimic the appearance of the female labia

As with Morris’s idea that female breasts evolved to mimic the appearance of female buttocks, the idea that our lips, and women’s use of lipstick, is designed to imitate the appearance of the female sexual organs has been much mocked.[14]

However, the similarity in appearance of the labia and human lips can hardly be doubted. After all, it is even attested to in the very etymology of the word

Of course, inverted lips reach their most extreme form among extant sub-species of hominid among black Africans. This Morris argues is because: 

If climatic conditions demand a darker skin, then this will work against the visual signalling 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” (p69-70).[15]

Thus, rejecting the politically-incorrect notion that black Africans are, as a race, somehow more ‘primitive than other humans, Morris instead emphasizes the fact that, in respect of this trait (i.e. everted lips), they are actually the most differentiated from non-human primates.  

Thus, all humans, compared to non-human primates, have everted lips, but black African lips are the most everted. Therefore, Morris concludes, using the word ‘primitive’ is in the special phylogenetic sense

Anatomically, these negroid characters do not appear to be primitive, but rather represent a positive advance in the specialization of the lip region” (p70).

In other words, whereas whites and Asians may be more advanced than blacks when it comes to intelligence, brain-size, science, technology and building civilizations, when it comes to everted lips, black Africans have us all beaten! 

Female Orgasm

Morris also discusses the function of the female orgasm, a topic which has subsequently been the subject of much speculation and no little controversy among evolutionists.  

Again, Morris suggests that humans’ unusual vertical posture, brought on by our bipedal means of locomotion, may have been central to the evolution of this trait. 

Thus, if a female were to walk off immediately after sexual intercourse had occurred, then: 

Under the simple influence of gravity the seminal fluid would flow back down the vaginal tract and much of it would be lost” (p79).  

This obviously makes successful impregnation less likely. As a result, Morris concludes: 

There is therefore a great advantage in any reaction that tends to keep the female horizontal when the male ejaculates and stops copulating” (p79). 

The chief adaptive function of the female orgasm therefore, according to Morris, is the tiredness, and perhaps post-coital tristesse, that immediately follows orgasm, and motivates the female experiencing these emotions to remain in a horizontal position even after intercourse has ended, and hence retain the male ejaculate within her reproductive tract. 

The violent response of female orgasm, leaving the female sexually satiated and exhausted has precisely this effect” (p79).[16]

However, the main problem with Morris’s theory is that it predicts that female orgasm should be confined to humans, since, at least among extant primates, we represent the only bipedal ape.  

Morris does indeed argue that the female organism is, like our nakedness, bipedal locomotion and large brains, an exclusively human trait, describing how, among most, if not all, non-human primates: 

At the end of a copulation, when the male ejaculates and dismounts, the female monkey shows little sign of emotional upheaval and usually wanders off as if nothing had happened” (p79). 

Unfortunately for Morris’s theory, however, evidence has subsequently accumulated that some non-human (and non-bipedal) female primates do indeed seem to sometimes experience responses seemingly akin to orgasm during copulation. 

Thus, Alan Dixson reports: 

Female orgasm is not confined to Homo sapiens. Putatively homologous responses [have] been reported in a number of non-human primates, including stump-tail and Japanese Macaques, rhesus monkeys and chimpanzees… Pre-human ancestors of Homo sapiens, such as the australopithecines, probably possessed a capacity to exhibit female orgasm, as do various extant ape and monkey species. The best documented example concerns the stump tailed macaque (Macaca arctoides), in which orgasmic uterine contractions have been recorded during female-female mounts… as well as during copulation… De Waal… estimates that female stump-tails show their distinctive ‘climax face’ (which correlates with the occurrence of uterine contractions) once in every six copulations. Vaginal spasms were noted in two female rhesus monkeys as a result of extended periods of stimulation (using an artificial penis) by an experimenter… Likewise, a female chimpanzee exhibited rhythmical vaginal contractions, clitoral erection, limb spasms, and body tension in response to manual stimulation of its genitalia… Masturbatory behaviour, accompanied by behavioural and physiological responses indicative of orgasm, has also been noted in Japanese macaques… and chimpanzees” (Sexual Selection and the Origins of Human Mating Systems: p77). 

Thus, in relation to Morris’s theory, Dixson concludes that the theory lacks “comparative depth” because: 

Monkey and apes exhibit female orgasm in association with dorso-ventral copulatory postures and an absence of post-mating rest periods” (Sexual Selection and the Origins of Human Mating Systems: p77). 

Certainly orgasm is not required for successful impregnation. 

Thus, American physician, Robert Dickson, in his book, Human Sex Anatomy (1933), reports that, in a study of a thousand women who attended his medical practice afflicted with so-called ‘frigitity’ (i.e incapable of orgasmic response during intercourse): 

The frigid were not notably infertile, having the expected quota of living children, and somewhat less than the average incidence of sterility” (Human Sex Anatomy: p92). 

Thus, as argued by Donald Symons in his groundbreaking The Evolution of Human Sexuality (which I have reviewed here), the most parsomonious theory of the evolution of female orgasm is that it represents simply a non-adaptive byproduct of male orgasm, which is, of course, itself adaptive (see Sherman 1989Case Of The Female Orgasm: Bias in the Science of Evolution).

It thus represents, if you like, the female equivalent of male nipples – only more fun.

Hymen

Interestingly, Morris also hypothesizes regarding the evolutionary function of another peculiarity of human female reproductive anatomy which, in contrast to the controversy regarding the evolutionary function, if any, of the female orgasm and clitoris (and of the female breasts), has received surprisingly scant attention from evolutionists – namely, the hymen

In most mammals, Morris reports, “it occurs as an embryonic stage in the development of the urogenital system” (p82). However, only in humans, he reports, is it, when not ruptured, retained into adulthood. 

Regarding the means by which it evolved, the trait is then, Morris concludes, like our large brains, upright posture and hairlessness, “part of the naked ape’s neoteny” (p82). 

However, as with our hairlessness, neoteny only the means by which this trait was retained into adulthood among humans, not the evolutionary reason for its retention.  

In other words, he suggests, the hymen, like other traits retained into adulthood among humans, must serve some evolutionary function. 

What is this evolutionary function? 

Morris suggests that, by making first intercourse painful for females, it deters young women from engaging in intercourse too early, and hence risking pregnancy, without first entering a relationship (‘pair-bond’) of sufficient stability to ensure that male parental investment, and provisioning, will be forthcoming (p73). 

However, pain experienced during intercourse occurs rather too late to deter first intercourse, because, by the time this pain is experienced, intercourse has already occurred. 

Of course, given our species’ unique capacity for speech and communication, the pain experienced during first intercourse could be communicated to young virginal women through conversation with other non-virginal women who had already experienced first intercourse.  

However, this would be an unreliable method of inducing fear and avoidance regarding first intercourse, especially given the sort of taboos regarding discussion of sexual activities which are common in many cultures. 

At any rate, why would natural, or sexual, selection not instead simply directly select for fear and anxiety regarding first intercourse – i.e. a psychological rather than a physiological adaptation. After all, as evolutionary psychologists and sociobiologists have convincingly demonstrated, our psychology is no less subject to natural selection than is our physiology. 

Although, as already noted, the evolutionary function, if any, of the female hymen has received surprisingly little attention from evolutionists, I can think of at least three rival hypotheses regarding the evolutionary significance of the hymen. 

First, it may have evolved among humans as a means of advertising to prospective suitors a prospective bride’s chastity, and hence reassuring the suitor of the paternity of offspring that subsequently result and encouraging paternal investment in offspring. 

This would, in turn, increase the perceived attractiveness of the female in question, and help secure her a better match with a higher-status male, and hence increase her own reproductive success

Thus, it is notable that, in many cultures, prospective brides are inspected for virginity, a so-called virginity test, sometimes by the prospective mother-in-law or another older woman, before being considered marriageable and accepted as brides. 

Alternatively, and more prosaically, the hymen may simply function to protect against infection, by preventing dirt and germs from entering a woman’s body by this route. 

This, of course, would raise the question as to why, at least according to Morris, the trait is retained into sexual maturity only among humans?  

Actually, however, as with his claim that the female orgasm is unique to humans, Morris’s claim that only humans retain the hymen into sexual maturity is disputed by other sources. Thus, for example, Catherine Blackledge reports: 

Hymens, or vaginal closure membranes or vaginal constrictions, as they are often referred to, are found in a number of mammals, including llamas, guinea-pigs, elephants, rats, toothed whales, seals, dugongs, and some primates, including some species of galagos, or bushbabys, and the ruffed lemur” (The story of V: p145). 

Finally, even more prosaically, the hymen may simply represent a nonadaptive vestige of the developmental process, or a nonadaptive by-product of our species’ neoteny

This would be consistent with the apparent variation with which the trait presents itself, suggesting that it has not been subject to strong selection pressure that has weeded out suboptimal variations. 

This then would appear to be the most parsimonious explanation. 

Zoological Nomenclature 

The works on human ethology of both Richard Ardrey and Konrad Lorenz attracted much attention and no little controversy in their day. Indeed, they perhaps attracted even more controversy than Morris’s own ‘The Naked Ape’, not least because they tended to place greater emphasis on humankind’s capacity, and alleged innate proclivity, towards violence. 

In contrast, Morris’s own work, placing less emphasis on violence, and more on sex, perhaps jibed better with the zeitgeist of the era, namely the 1960s, with its hippy exhortations to ‘make love not war’. 

Yet, although all these works were first published at around the same time, the mid- to late-sixties (though Adrey continued publishing books of this subject into the 1970s), Morris’s ‘The Naked Ape’ seems to be the only of these books that remains widely read, widely known and still in print, to this day. 

Partly, I suspect, this reflects its brilliant and provocative title, which works on several levels, scientific and literary.  

Morris, as we have seen, justifies referring to humans by this perhaps unflattering moniker on zoological grounds.  

Certainly, he acknowledges that humans possess many other exceptional traits that distinguish us from all other extant apes, and indeed all other extant mammals. 

Thus, we walk on two legs, use and make tools, have large brains and communicate via a spoken language. Thus, the zoologist could refer to us by any number of descriptors – “the vertical ape, the tool-making ape, the brainy ape” are a few of Morris’s own suggestions (p41).  

But, he continues, adopting the disinterested detachment of the proverbial alien zoologist: 

These were not the first things we noticed. Regarded simply as a zoological specimen in a museum, it is the nakedness that has the immediate impact” (p41) 

This name has, Morris observes, several advantages, including “bringing [humans] into line with other zoological studies”, emphasizing the zoological approach, and hence challenging human vanity. 

Thus, he cautions: 

The naked ape is in danger of being dazzled by [his own achievements] and forgetting that beneath the surface gloss he is still very much a primate. (‘An ape’s an ape, a varlet’s a valet, though they be clad in silk or scarlet’). Even a space ape must urinate” (p23). 

Thus, the title works also on another metaphoric level, which also contributed to the title’s power.  

The title ‘Naked Ape’ promises to reveal, if you like, the ‘naked’ truth about humanity—to strip humanity down in order to reveal the naked truth that lies beneath the façade and finery. 

Morris’s title reduces us to a zoological specimen in the laboratory, stripped naked on the laboratory table, for the purposes of zoological classification and dissection. 

Interestingly, humans have historically liked to regard ourselves as superior to other animals, in part, precisely because we are the only ones who did clothe ourselves. 

Thus, beside Adam and Eve, it was only primitive tropical savages who went around in nothing but a loincloth, and they were disparaged as uncivilized precisely on this account. 

Yet even tropical savages wore loincloths. Indeed, clothing, in some form, is sometimes claimed to be a human universal

Yet animals, on the other hand, go completely unclothed – or so we formerly believed. 

But Morris turns this reasoning on its head. In the zoological sense, it is humans who are the naked ones, being largely bereft of hairs sufficient to cover most of our bodies. 

Stripping humanity down in this way, Morris reveals the naked truth that beneath, the finery and façade of civilization, we are indeed an animal, an ape and a naked one at that. 

The power of Morris’s chosen title ensures that, even if, like all science, his book has quickly dated, his title alone has stood the test of time and will, I suspect, be remembered, and employed as a descriptor of the human species, long after Morris himself, and the books he authored, are forgotten and cease to be read. 

Endnotes

[1] In fact, as I discuss in a later section of this review, it is possible that the female hymen evolved through just such a process, namely as a means of advertising female virginity and premarital chastity (and perhaps implying post-marital fidelity), and hence as a paternity assurance mechanism, which benefited the female by helping secure male parental investment, provisioning and hypergamy.

[2] Morris is certainly right that anthropologists have overemphasized the exotic and unfamiliar (“bizarre mating customs, strange kinship systems, or weird ritual procedures”, as Morris puts it). Partly, this is simply because, when first encountering an alien culture, it is the unfamiliar differences that invariably stand out, whereas the similarities are often the very things which we tend to take for granted.
Thus, for example, on arriving in a foreign country, we are often struck by the fact that everyone speaks a foreign unintelligible language. However, we often take for granted the more remarkable fact that all cultures around the world do indeed have a spoken language, and also that all languages supposedly even share in common a universal grammar.
However, anthropologists have also emphasized the alien and bizarre for other reasons, not least to support theories of radical cultural malleability, sometimes almost to the verge of outright fabrication (e.g. Margaret Mead’s studies in Samoa).

[3] It is true that there has been some significant human evolution since the dawn of agriculture, notably the evolution of lactase persistence in populations with a history of dairy agriculture. Indeed, as Cochran and Harpending emphasize in their book The 10,000 Year Explosion, far from evolution having stopped at the dawn of agriculture or the rise of ‘civilization’, it has in fact sped up, as a natural reflection of the rapid change in environmental conditions that resulted. Thus, as Nicholas Wade concludes in A Troublesome Inheritance, much human evolution has been “recent, copious and regional”, leading to substantial differentiation between populations (i.e. race differences), including in psychological traits such as intelligence. Nevertheless, despite such tinkering, the core adaptations that identify us as a species were undoubtedly molded in ancient prehistory, and are universal across the human species.

[4] However, it is indeed important to recognize that the lifestyle of our own ancestors was not necessarily identical to that of those few extant hunter-gatherer groups that have survived into modern times, not least because the latter tend to be concentrated in marginal and arid environments (e.g. the San people of the Kalahari DesertEskimos of the Arctic region, Aboriginals of the Australian outback), with those formerly inhabiting more favorable environments having either themselves transitioned to agriculture or else been displaced or absorbed by more advanced invading agriculturalists with higher population densities and superior weapons and other technologies.

[5] This passage is, of course, sure to annoy feminists (always a good thing), and is likely to be disavowed even by many modern evolutionary psychologists since it relies on a rather crude analogy. However, Morris acknowledges that, since “’hunting’… has now been replaced by ‘working‘”: 

The males who set off on their daily working trips are liable to find themselves in heterosexual groups instead of the old all-male parties. All too often it [the pair bond] collapses under the strain” (p81). 

This factor, Morris suggests, explains the prevalence of marital infidelity. It may also explain the recent hysteria, and accompanying witch-hunts, regarding so-called ‘sexual harassment’ in the workplace.
Relatedly, and also likely to annoy feminists, Morris champions the then-popular man the hunter theory of hominid evolution, which posited that the key development in human evolution, and the development of human intelligence in particular, was the switch from a largely, if not wholly, herbivorous diet and lifestyle, to one based largely on hunting and the consumption of meat. On this view, it was the cognitive demands that hunting placed on humans that selected for increased intelligence among humans, and also the nutritional value of meat that made possible increases in  highly metabolically expensive brain tissue.
This theory has since fallen into disfavor. This seems to be primarily because it gives the starring role in human evolution to men, since men do most of the hunting, and relegates women to a mere supporting role. It hence runs counter to the prevailing feminist zietgeist.
The main substantive argument given against the ‘man the hunter theory’ is that other carnivorous mammals (e.g. lions, wolves) adapted to carnivory without any similar increase in brain-size or intelligence. Yet Morris actually has an answer to this objection.
Our ancestors, fresh from the forests, were relative latecomers to carnivory. Therefore, Morris contends, had we sought to compete with tigers and wolves by mimicking them (i.e. growing our fangs and claws instead of our brains) we would inevitably have been playing a losing game of evolutionary catch-up. 

Instead, an entirely new approach was made, using artificial weapons instead of natural ones, and it worked” (p22).

However, this theory fails to explain how female intelligence evolved. One possibility is that increases in female intelligence are an epiphenomenal byproduct of selection for male intelligence, rather like the female equivalent of male nipples.
On this view, men would be expected to have higher intelligence than women, just as male nipples are smaller than female nipples, and the male penis is bigger than the female clitoris. That adult men have greater intelligence than adult women is indeed the conclusion of a recent controversial theory, though the difference is very modest (Lynn 1999). There is also evidence this sexual division of labour between hunting and gathering led to sex dithfferences spatio-visual intelligence (Eals & Silverman 1994).

[6] Another difference from modern evolutionary psychologists derives from Morris’s ethological approach, which involves a focus on human-typical behaviour patterns. For example, he discusses the significance of body language and facial expressions, such as smiling, which is supposedly homologous with an appeasement gesture (baring clenched teeth, aka a ‘fear grin’) common to many primates, and staring, which represents a form of threat across many species.

[7] Interestingly, however, he acknowledges that this statement does not apply to all human races. Thus, he observes: 

Negroes have undergone a real as well as an apparent hair loss” (p42). 

Thus, it seems blacks, unlike Caucasians, have fewer hairs on their body than do chimpanzees. This fact is further evidence that, contrary to the politically correct orthodoxy, race differences are real and important, though this fact is, of course, played down by Morris and other popular science writers.

[8] Edward O Wilson, for example, in Sociobiology: The New Synthesis (which I have reviewed here) dismisses aquatic ape theory, as then championed by Elaine Morgan in The Descent of Woman, as feminist-inspired pop-science “contain[ing] numerous errors” and as being “far less critical in its handling of the evidence than the earlier popular books”, including that of Morris (who is mentioned by name in the same paragraph (Sociobiology: The New Synthesis: p29).

[9] Actually, I suspect this infamous quotation may be apocryphal, or at best a misconstrued joke. Certainly, while I think Rushton’s theory of race differences (which he calls ‘differential K theory’) is flawed, as I explain in my review of his work, there is nothing in it to suggest a direct trade-off between penis-size and brain-size. Indeed, one problem with Rushton’s theory, or at least his presentation of it, is that he never directly explains how traits such as penis-size actually relate to r/K selection in the first place.
The quotation is usually traced to a hit piece in Rolling Stone, a leftist hippie rag with a reputation for low editorial standards and fake news. However, Jon Entine, in his book on race differences in athletic ability, instead traces it to a supposed interview between Rushton and Geraldo Rivera broadcast on the Geraldo’ show in 1989 (Taboo: Why Black Athletes Dominate Sports: p74).
Interestingly, one study has indeed reported that there is a “demonstrated negative evolutionary relationship”, not between brain-size and penis-size, but rather between brain-size and testicle size, if only on account of the fact that each contain “metabolically expensive tissues” (Pitnick et al 2006).

[10] Interestingly, Baker and Bellis attribute race differences in penis-size, not to race differences in brain-size, but rather to race differences in birth weight. Thus, they conclude:

Racial differences in size of penis (Mongoloid < Caucasoid < Negroid…) reflects racial differences in birth weight… and hence presumably, racial differences in size of vagina” (Human Sperm Competition: p171). 

[11] In other words, a male silverback gorilla may mate with the multiple females in his harem, but each of the females in his harem likely have sex with only one male, namely that silverback. This means that sperm from rival males are rarely simultaneously present in the same female’s oviduct, resulting in minimal levels of sperm competition, which is known to select from larger testicles in particular, and also often more elaborate penes as well.

[12] Alternative theories for the evolution of permanent fatty breasts in women is that they function analogously to camel humps, i.e. as a storehouse of nutrients to guard against and provide reserves in the event of future scarcity or famine. On this view, the sexually dimorphic presentation (i.e. the fact that fatty breasts are largely restricted to women) might reflect the caloric demands of pregnancy. Indeed, this might explain why women have higher levels of fat throughout their bodies. (For a recent review of rival theories for human breast evolution see Pawłowski & Żelaźniewicz 2021.)

[13] However, to be pedantic, this phraseology is perhaps problematic, since, to say that breasts and lips are ‘sexualized’ in western, and at least some non-western, cultures implicitly presupposes that they are not already inherently sexual parts of our anatomy by virtue of biology, which is, of course, the precisely what Morris is arguing. 

[14] For example, if I recall correctly, extremely annoying, left-wing 1980s-era British comedian Ben Elton once commented in a one of his stand-up routines that the male anthropologist (i.e. Morris, actually not an anthropologist, at least not by training) who came up with this idea (namely, that lips and lipstick mimiced the appearance of the labia) had obviously never seen a vagina in his life. He also, if I recall correctly, attributed this theory to the supposed male-dominated, androcentric nature of the field of anthropology – an odd notion given that Morris is not an anthropologist by training, and cultural anthropology is, in fact, one of the most leftist-dominated, feminist-infested, politically correct fields in the whole of academia, this side of ‘gender studies’, which, in the present, politically-correct world of academia, is saying a great deal.

[15] To test this theory, we might look at other relatively dark-skinned, but non-Negroid, populations. Here, the theory receives, at best, only partial support. Thus, Australian Aboriginals, another dark-skinned but unrelated group, do indeed tend to have quite large lips. However, these lips are not especially everted. 
On the other hand, the dark-skinned Dravidian populations of Southern India are not generally especially large-lipped, but are rather quite Caucasoid in facial morphology, and indeed, like the generally lighter-complexioned, Indo-European speaking, ‘Aryan’ populations of northern India, were generally classified as ‘Caucasoid by most early-twentieth century racial anthropologists.

[16] This theory is rather simpler, and has hence always struck me as more plausible, than the more elaborate, but also more widely championed so-called ‘upsuck hypothesis’, whereby female orgasm is envisaged as somehow functioning to suck semen deeper into the cervix. This idea is largely based on a single study involving two experiments on a single subject (Fox et al 1970). However, two other studies failed to produce any empirical support for the theory (Grafenberg 1950; Masters & Johnson 1966). Baker and Bellis’s methodologically problematic work on what they call ‘flowback’ provides, at best, ambivalent evidence (Baker & Bellis 1993). For detailed critique, see Dixson’s Sexual Selection and the Origins of Human Mating Systems: p74-6.

References 

Baker & Bellis (1993) Human sperm competition: ejaculate manipulation by females and a function for the female orgasm. Animal Behaviour 46:887–909. 
Bowman EA (2008) Why the human penis is larger than in the great apes. Archives of Sexual Behavior 37(3): 361. 
Eals & Silverman (1994) The Hunter-Gatherer theory of spatial sex differences: Proximate factors mediating the female advantage in recall of object arrays. Ethology and Sociobiology 15(2): 95-105.
Fox et al 1970. Measurement of intra-vaginaland intra-uterine pressures during human coitus by radio-telemetry. Journal of Reproduction and Fertility 22:243–251. 
Gallup et al (2004). The human penis as a semen displacement device. Evolution and Human Behavior, 24, 277–289 
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Pawłowski & Żelaźniewicz (2021) The evolution of perennially enlarged breasts in women: a critical review and a novel hypothesis. Biological reviews of the Cambridge Philosophical Society 96(6): 2794-2809. 
Pitnick et al (2006) Mating system and brain size in bats. Proceedings of the Royal Society B: Biological Sciences 273(1587): 719-24.