Sex differences having to do with sexual behavior

One trait which distinguishes the male from the female mammal,  in most species,  is the greater libido  (sex drive) of males.    Some people would, I think,  disagree with that statement.  Some of the reasons why I think men have a greater libido,  at least as far as its biological dimension is concerned, are the following:  1) large scale inventories have shown that desire for more sex on the part of the husband from the wife is higher, on the average,  than on the part of the wife from the husband (by about 15%), 2) the same inventories have revealed that boys and men masturbate more than girls and women do, 3) boys (and late treated androgenital girls, i.e., prenatally hormonally masculinized girls,  see the next sections of this chapter for more details) have erotic dreams in adolescence prior to any concrete sexual experience,  whereas this happens to women only in their twenties, and 4) temporal lobe epileptics who report epileptogenic erotic or orgastic «hallucinations» are far more frequently, and disproportionately, of the male sex.   One interesting aspect of the libido of men and women is that its life-course trajectory differs slightly.  Of course,  the sex drive declines in both sexes throughout adulthood,  a phenomenon apparently related (among other things) to the general decline in steroid hormone concentrations in both sexes.   However,  it seems that the decline is a bit steeper for men from around age 30 to around age 45.  Then menopause inflects the female trajectory toward lower libido.  Even the erotic content of dreams in women could be modulated by hormones:   women have been found to have more erotic dreams during pregnancy,  a time when their estrogen level is climbing up.   Of course,  the duration of a sexual relationship ends up being a sexual inhibitor, in most polygamous species,  for both sexes.

By the way,  it is an undeniable fact that the human species, like most primates,  is polygamous -with a strong tendency for pair bonding  -and I simply state this as a matter of fact,  not as a matter of morals.  Most of humanity’s cultures, I think, have been formally polygamous (for men).  Otherwise,  the occurrence of multiple partners in contemporary western society, as estimated from survey research, is very high. The divorce and re-marriage rate is also very high,  and this particular statistic only scratches the surface of de facto  polygamy in western culture.   I suspect that a higher proportion of this polygamous behavior is driven by male drives and behavior than by female drives and behavior.    Men fantasize more about new sexual partners than women do (by about 18%, according to one large scale survey). 

Another basically biological difference in male and female mammals is the approach to sex.  Of course, those equipped with a penis are more likely to be thrusting during copulation,  especially in species which copulate "doggy style".    To me,  it just seems technically easier for most mammals,  though not at all necessary in humans of course.  And humans like to have a more imaginative and varied sex life than animals do.  Anyway,  all mammals (except humans some of the time) behave in a sex-dimorphic manner.   Females are more passive during copulation. For example, female rats adopt a species-specific position when they are ready for copulation:  they rest chest to the ground, raise their rump, and lift their tail to the side.  This is called lordosis.  So it seems to me that there is something biological about this trait,  and I do assume that this sex difference does indeed loosely characterize most human copulation.  I vaguely recall that the Hite report corroborates this in the sense that women are slightly more passive during copulation  -not in the sense that they adopt lordosis!  Indeed, humans seem to prefer the so-called “missionary” or ventro-ventral position.   It has been argued that the missionary position is sought for some degree of eye contact,  a trait we humans are said to share with dwarf chimpanzees (bonobos) who also copulate in the missionary position (more so though when they are young,  the more experienced ones preferring the “doggy” style or ventro-dorsal position). 

Finally,  men and women have a different approach to the preliminaries of sex.   Men are excited more easily by visual erotica and women seem to prefer petting for a longer period of time prior to copulation.   Is the determination of these two sex differences biological ?   Though there must surely be some cultural factors involved,  I suspect that a biological sex difference must be at play here as well.   Some ethologists (specialists of animal behavior in natural settings) have proposed that the ecology of the sex drive is sex-specific:  it is in the biological interest of female mammals to be more selective of mates  because they get limited opportunities to reproduce (gestation takes time).   However,  it is in the biological interest of male mammals to inseminate as many females as possible because the male genes thus have more chances of passing on to posterity.  Of course,  this general account does not apply to all animal species because ecological pressures are multidetermined and heterogeneous:  some species, even among primates (e.g., gorillas), form a strong pair bond for life.

Men and women have sexual relations mostly with the opposite sex.  So you would think that nature could have fine-tuned their sexualities for maximum compatibility.  But things are not so simple.  Men and women are different sexual animals.  Women have a lower sex drive,  a more passive approach to copulation, a more stable sexual orientation and sexual identity,  many fewer paraphylias (sexual perversions) and they manifest subtle cycling of sexual desire as a function of the menstrual cycle (apparently mostly linked primarily to their testosterone level),  and a not so subtle drop in sexual desire at menopause (see chapter 10).  All of these sex differences could be construed (only by a warped mind, I think) to be exclusively culturally determined.   However, here is an example of a gender difference in sexual behavior that cannot remotely be construed as cultural:    in 1994 a team of American researchers sprayed exaltolide (an industrial version of androstenol, a human male steroid hormone)  on a chair in a waiting room.   The women nearly all chose to sit on that chair and the men nearly all avoided sitting on that chair !  None were aware of the odor.   The physiology of this mechanism,  and its sexual potency, are well known in the hamster and they are just starting to be investigated in the human.   I suspect that we will be hearing a lot more about this phenomenon as the molecules in question approach commercialization.  Let me dwell a little on the subject of one particular male-prevalent sexual deviation,  namely rape.    Recidivist rapists are sometimes offered a hormonal “solution” to their “problem”,  namely antiandrogen treatment or castration.  One study reviewed and followed up a large set of such cases. Castration causes androgen deficiency and a decline in sexual behavior. Results showed greatly reduced recidivism rates for sex offenders who have been castrated. Antiandrogen and hormonal treatments are pharmacological methods of reducing the sex drive in sexual offenders. Cyproterone acetate (CPA) is a commercially available antiandrogen that has been used in the treatment of paraphilias. Many patients have reported a feeling of calm as a result of CPA treatment as well as reduced sex drive and reduced deviant fantasies and behaviors.   Human males castrated before puberty fail to develop sexual desire entirely.   Women do not lose their libido much after ovariectomy, they lose it more after adrenalectomy.  This is because the more critical androgens for female libido are secreted by her adrenal gland.  It is still not clear whether estrogen contributes anything at all to female libido.  

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The most spectacular effects of hormones on sexual behavior have frequently been observed in rats.   Prenatal or perinatal or even postnatal injections of sex hormones can produce gender reversal in sexual behavior of rats.     This is not observed nearly as much in primates such as monkeys and humans.    Rats seem to have a special brain circuitry for sex.   Male  and female rats naturally manifest both the male-specific (mounting) and female-specific (lordosis) sexual behaviors,  in different proportions.   It is as if they have more androgynous sexual brains.   These circuits can be easily masculinized or feminized with sex hormone injections,  and other manipulations such as castration,  pharmacological neurotransmitter manipulation,  etc.    I wish to caution the reader in overextending inferences about the biology of sexual behavior from rat research.   Monkey research can generally be considered more trustworthy for extrapolation (which must nevertheless remain very reserved) to humans.   That is why it is important to note that prenatal androgenization of female macaques does indeed result in more mounting and leg clasping behavior in young adulthood,  but it is only the proportion of male-typical and female-typical behaviors which changes.