The evolution of sex differences

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There would be no basis for writing this book if there were not an evolutionary segregation history of the sexes.   Indeed,  male and females of same species co-evolve differentially.   Sex differences can be absolutely spectacular or banale,  depending on the ecological niche of each species.  For example,  whereas human males and females are moderately different in form,  horses are very similar and praying mantises are very different.  So, on the biological scale of sex differences,  humans are somewhere near the middle.   Since the main theme of this book concerns differences between the sexes,  it will be instructive to see to which extent evolution has sometimes created extreme differences between the two sexes of a given species.  

We are accustomed to thinking that evolution proceeds by parentally transmissible mutations.   Any mutation  which confers an advantage to the progeny and to the progeny of the progeny will become more frequent over successive generations,  and deleterious mutations will simply snuff themselves out.   That's the ABC of evolutionary theory.   But how are we to explain the evolutionary mechanism by which a male animal could resemble a member of another species more than the female of its own species ?   It’s as if each sex were a species onto itself.   The key to understanding how sexual dimorphism (differences in form between the two sexes) comes about is in the next section:  it is primarily a matter of sex-chromosomes.  

But before getting into that specific issue,  I want to drive home the point that sexual dimorphism is a rich domain full of surprises throughout the animal kingdom,  and that the study of biological sex differences is  very complex,  not to say treacherous.   This will help the reader be more receptive to the theme of biological sex differences in humans which this book seeks to explore,  and hopefully,  also very critically minded.   First,  we usually think that it is the male which is supposed to be larger than the female.  This, however, is not always the case.  There are species of animals in which the male is much smaller.   For example, in a certain species of deep sea fish, the female is several thousand times larger than the male.  The male actually assumes the ecological niche of a parasite,  hooking itself onto the female, near her brow, for a free ride and a free lunch (leftovers).  It produces the required sperm at the appropriate moment  -thus rendering the type of service that a well adapted parasite should.  A roughly analogous situation is observed in the human organism:  the female germ (ovum) and the male germ (spermatozoid) are like fish to the extent that they inhabit a liquid environment which they swim around in (the female reproductive tract).  The ovum is a giant cell compared to the sperm which is a tiny cell 90,000 times smaller.  Sexual dimorphism is also observed in the insect world.   Praying mantis females are much larger than the males,   and everybody knows their nuptial tale:  the female starts eating the male's head while he is copulating with her,   and she reaches the sexually critical parts just after he has managed to impregnate her  -which he does without his head.  Actually,  this story is now believed by some to be a myth:  the female mantis’s eyesight is so poor that she mistakes the male for a lunch,  but does not usually eat him if he approaches her from behind.  Certain birds present a similar body dimorphism,  jacanases having a larger female than the male.  

We are also accustomed to thinking that it is the male which assumes the role of the hunter or defender of the territory,  at least in mammals.  But this rule also suffers exceptions.   Female lions hunt more than do the males.   Female hyenas defend the territory more than do the males.   It is also thought that males are more colored than females -especially in bird species.  This is not true of humans,  and the contrary is observed, though rarely, in certain species of birds.  Two-toned (calico) cats are more frequently female and single toned cats more frequently male. All of these sexual dimorphisms (well maybe not the cats) speak to mutual adaptations of the two sexes of a given species  -which have the net result of increasing viability of progeny.  That is the fundamental reason why men and women are biologically different, even though some of the biological differences could be fortuitous, as is probably the case of cat-coat color. In fact,  calico cats are female because the fur pigment is determined by a dominant gene located on the X chromosome.   I explain this mechanism in an upcoming section.

One of the most adventurous and also the best treatises I have read on the evolution of sex differences in humans is in the latest and recent revision of a wonderful book by Rhawn Joseph entitled Neuropsychiatry,  Neuropsychology and Clinical Neuroscience.   In this book, Joseph speculates about how recent evolution (the last two million years or so) has led to the basic behavioral sex differences known to exist in modern day humans.  Evolutionary  changes can be understood (or at least speculated about) in terms of adaptations to ecological niches (specific living environments and conditions).   To summarize Joseph’s point of view,  he states that  the big ecological change for hominids (primitive human species),  with respect to emergence of sex differences,  was big game hunting.  This put additional pressure on natural selection toward rapid encephalization for both sexes (successive selection of mutations yielding larger and more efficient brains) and neoteny  (increasingly severe and prolonged helplessness of the human newborn due to the mechanics of encephalization as well as to the newfound ability of big brained mothers to provide intensive and complex caring to the progeny).    Big game hunting led to increased segregation of men’s and women’s activities (childrearing, food gathering and tool making in small quiet female collectives,  and running, hunting and route finding for men).   Men’s bodies and brains became more specialized for visuospatial processing,  physical exertion,  and aggression.   Women’s bodies and brains became more specialized for childbearing,  particularly of newborns with larger heads (thus the wider hips,  and generally less efficient musculature for major physical exertion, other than childbearing of course).  Women’s brains,  claims Joseph, also became more specialized for intimate contact with children and other women (a “necessity” resulting mainly from neoteny).  This would explain, he states, why women,  like most female higher mammals (rats, dogs, monkeys, etc.),  have a more emotionally diverse prosody (voice tone)  and generally vocalize more often,  though less loudly, than men (whose loudness he also relates to imperatives of hunting big game),  and why women willingly and efficiently spend more time with their infants and children.   Another segregated role of primitive women,  Joseph thinks,  was food gathering and tool making in small intimate relaxed groups of females with and without children (thus the emergence of the female disposition to promote,  more than men,  attachment in general).  I add that other commentators consider that these latter activities contributed to better fine coordination in women,  an eventuality which seems to have eluded Joseph.  Joseph even goes as far as to state that the generally acknowledged female superiority in certain verbal domains draws its neurobiological origin from these roles,  involving a lot of “chatter”  - associated with select brain circuits belonging only to women.   Joseph, a man, goes further than any woman scientist I know in eulogizing women’s “social-emotional... superiority”.   In my own review of the relevant literature (see chapter 3),  I come to a more reserved conclusion,  just slightly less gynophyliac (pro-women).

Biological interpretations of sex differences in human behavior,  as is pointed out by Joseph,  may have tremendous implications for understanding sex-segregation of social roles.  For example,   why is it that women professionals overwhelmingly opt more than men for the nurturing professions (primary school teaching, daycare work, nursing,  and now... medicine) ?   Even more interestingly,   why should they not ?   Why should social reformers want to change this situation by injecting public money into programs designed to attract more women to the natural sciences, as is currently being done in several industrialized countries ?     Is not the better solution to wage struggle for equal pay for work of equal value ?   I say «Why should a child care worker make half the salary of an electronics technician,  as is the case today,  both bearing an equally demanding college degree ?».    One of the reasons for wage differences is that union militancy is stronger in male dominated occupations,   making it very very difficult for liberal governments to impose egalitarian pay scales by way of legislation.     But with enough consciousness raising,  women could become more militant,  and men more sharing,  I hope.    You cant have your cake and eat it too.  Complaining to governement is not militant enough, although it certainly does help.

In this book,  we will explore sex differences not only in behavior  but also in basic body function.  This expanded focus is necessary to fully understand the complexity of the biological underpinnings of gender specificity in humans.    Male animals are more often colorful or ostentatious, they are larger, they are more aggressive,  and they are sexually less selective (they are more promiscuous or less choosy),  and they have shorter life expectancy, than females.    What could possibly be the link between all of these sex differences ?   Females are believed to be more choosy because they are the ones who endure the more reproductively critical consequences of bad choice of mate. They invest more vital energy in reproduction (gestation, lactation,  etc).  One of their criteria for choice of a male mate is large body size and social dominance,  which suggests good health and good ability to defend the territory.  Given as much,  males must attract the females with ostentatious plumage, fur, horns, etc.,  since the latter are the ones who do the choosing,  and not the former.    It is not advantageous for females to have large body formats because in times of food penury,  they would then have trouble surviving.   It is even in the female advantage to have a reduced metabolic rate,  and special mechanisms of resistence to food shortages,  so that she can carry through the full reproductive cycle.    There is a limiting factor in evolutionary increases of body size,  and that is reduced life expectancy.   The male is more expendable.   He may reproduce efficiently and quickly,  and then it may be in the interest of the promulgation of his own genes to clear the way for availability of food for his progeny.  He does this by konking out.  Larger bodies result, among other things, from faster cell division,  and all else being equal, entail greater expenditure of energy. Consequently,  the ability of the body to replace lost cellular components reaches its limit earlier.  If this doesn’t suffice,  the male may even be more likely to get himself killed by behavioral means.  However, the male must not die before having transmitted his genes,  nor can he be entirely dispensed with when his niche calls upon him to defend the females and the young against predators.  So his shorter life expectancy has to be constrained at some point.  There are a minority of animal species which dont fit into this scheme,  because they have found special ways to adapt to particular ecological niches.   However,  even these exceptions confirm the general idea:   species in which males and females are not dimorphic present the following characteristics:  male and female life expectancy tends to be the same,  the males tend to care for the young, to have no special role in defending the territory, and to be less aggressive.    Mating patterns are more monogamous.  

Within primate species,  baboons represent an extreme example of adaptation to the sexually dimorphic niche, the female’s body weight reaching only 50% of the male’s.  They are very polygamous.  Gibbons represent the other extreme with female weight reaching 99% of the male’s. Like a 1950’s -style nuclear family, gibbons were thought to live in stable groups of five of six in which a mom and pop mate for life and raise their offspring.  Family comes first and the only excitement comes when the group spars with the neighbors. The impression was that they were monogamous and not very social with other groups,  therefore, that they were fairly boring, says Thad Bartlett, an anthropologist and Dickinson College in Carlisle, Pennsylvania.  But in a recent report at the annual meeting of the American Association of Physical Anthropologists, Bartlett showed that gibbons are anything but boring.  He and others have found that although many gibbon pairs mate for years on end, like human families of the 90s they have plenty of drama, infidelity, divorce, abandonment, and step-children from other unions, as well as much socializing and kinship among members of different groups.  The findings show how important it is to explore what "monogamy" means for primates, and underscore the social complexity of these intelligent animals.  Gibbons really have been the prototype for monogamous primates, says Phyllis Dolhinow, a biological anthropologist at the University of California, Berkeley.  It turns out things just aren’t as tightly structured as had been assumed.    Humans are situated between baboons and gibbons with regard to sexual dimorphism in body size,  the female weighing in at 85% of the male.   The general scheme I have just outlined does not apply only to primates.  It also applies to insects, fish, reptiles, birds, and lower mammals.    This scheme has been nicely elaborated by the Hoyenga couple in their wonderful book entitled  The question of sex differences:  Psychological, cultural and biological issues,  published in 1979.

To conclude this section on evolutionary underpinnings of sex differences,  let me bring up the theme of overdetermination.     Overdetermination is a concept which preoccupied the French marxist philosopher,  Louis Althusser a few years ago.   It applies to psychology,  philosophy,  biology and dynamic systems in general.   It is the idea according to which in very complex dynamic systems,  the bifurcations in the deep long term trajectories result from a convergence in space and time of a multitude of efficient causes,  which may in fact be more or less dependent on each other, or even entirely fortuitous.   This is the case for important evolutionary mutational sequences leading to intricate  complex adaptations  -including sex differences.    Let’s take just one example.    In this book,  I will give a lot of details about the different life expectancies of men and women.   In fact,  this sex difference was surely highly overdetermined in evolution:  1) because we are a sexually dimorphic species,  there is a sexual segregation in parental nurturant behavior,  which presses for shorter male longevity,  so as to optimize available resources for the group;  2) because we are sexually dimorphic,  with the male being specialized for defending the territory and hunting,  he has a higher metabolism and a specialized vascular system to drive his greater muscle mass,  all of which shorten his life expectancy;  3)  the human male’s prenatal testosterone and Y chromosome both contribute to putting him at risk for more life threatening diseases and conditions;  4) the human female’s stronger immune system is adapted to her childbearing role and protects her from fatal diseases;  5) the human male’s adaptation to territorialism and hunting puts him at risk for far more life threatening behaviors,  for himself and others.