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The females of most species of mammals have sharply circumscribed periods of sexual activity.  The exact pattern depends on the species; some, such as rats, have an intense period of ‘heat’ (estrus) every four or five days. Others, such as female cats, have more prolonged periods lasting days or even weeks, but usually limited to certain times of the year (spring).  The same applies to female dogs; anyone owning an intact bitch will know the effect this has on the surrounding population of male dogs.  Sexual behavior is thus a cyclic process, though the form of the cycle varies in different species. 

A common factor in these various patterns is that it depends on hormones secreted from the females’ ovaries.  It is these that are wholly responsible for the periodic activation of sexual activity in the females of most species of mammal. This is easily shown: Every dog or cat owner knows that ‘neutering’ (that is, removing the ovaries) of their pet will permanently and completely prevent any sexual behavior. It is less commonly known that sexual activity can be reinstated quickly by giving ovarian hormones to such females. 

The ovaries produce two major hormones: estrogen and progesterone.  Female rats, cats and dogs all need estrogen, but some species may also need a small amount of progesterone. The common factor is that it’s the female’s ovaries producing one or both of these hormones that is essential for sexual behavior. Give them the right dose and combination for their species, and within 24 hours (or less) sexual interest and activity returns. The ovaries secrete estrogen and progesterone when there are ripe eggs (ova) in the ovaries ready for release (ovulation), ensuring that the female is sexually active at the same time as she is fertile.

Is this also true for women? The sexuality of women can vary throughout the menstrual cycle, though this is not a constant feature of all women, and the pattern of variation is also individually variable. Some women experience a mid-cycle peak, which synchronizes with ovulation, but others feel sexier just before menstruation. Others don’t really have much variation at all. 

Removal of the ovaries is sometimes necessary for medical reasons; this doesn’t have the dramatic effect on sexuality that it would have in a cat or dog. At menopause, a woman’s ovaries stop secreting estrogen and progesterone; again, there is not the universal or sharp decline in sexuality that would occur if menopause happened in other species. Some women do report less sexual activity after removal of the ovaries or the menopause, but this may be one result of dryness of the vagina, one consequence of the lack of estrogen. Local treatment puts it right in most cases.

So has the evolution of the human brain emancipated the sexuality of women from the control of hormones? There was a time when this was even thought to be true; some regarded this as a source of pride: Humans were too superior to be in the thrall of hormones, which were for lesser species. We all know that sexuality in humans is a complicated and variable part of human life.

Then some contrary facts began to appear. Occasionally it is necessary to remove both the adrenal glands and the ovaries. It was noted that these women reported a profound loss of sexual interest. It was not restored by giving them either estrogen or progesterone. Then it was realized that women’s adrenals produced testosterone, a hormone more usually associated with male sexuality. Giving these women testosterone, however, had a markedly beneficial effect on their sex lives. The ovary also secretes testosterone; the amount varies, but is around half that from the adrenals. Interestingly, postmenopausal ovaries still produce testosterone.

Since then, numerous studies have shown that testosterone, in much smaller doses than are given to males, can stimulate women’s sexual interest. Levels of testosterone in a woman's blood do not show the cyclic pattern characteristic of estrogen and progesterone, which might account for the lack of rhythmic alterations in sexual interest in most women in contrast to rats, cats and other non-primate species -  though some women do have a small mid-cycle peak; this might explain some cases of a mid-cycle peak in sexuality. 

Testosterone treatment can be helpful in some cases of low libido, and there are instances when a woman’s testosterone may fall below optimal levels; for example, levels do tend to decrease with age. There are studies that suggest that about half of those women complaining of a loss of sexual interest are helped by testosterone.

Drug companies are eager to identify a condition of 'female sexual dysfunction' in women, not because they can provide testosterone (which is not patentable) but to promote other compounds, many of extremely doubtful value, but which might be profitable. But it is important to recognize that impaired sexual function can have many causes, and thus many potential remedies. The curious fact is that there is currently no approved testosterone compound for women either in the UK or USA, so women have to use one designed for men, though in reduced amounts (it is usually given as a skin cream). Testosterone treatment has been shown to improve not only sexuality but also well-being, energy and, in some studies, cognitive function. It may even heighten sexual interest in women who do not complain of low sexual drive. 

Testosterone plays a pervasive and crucial role in the lives and careers of human males (see footnote). It's an ancient hormone; fish, birds and reptiles all use it. It has shaped human history, both personal and political. But it has a role for women as well, though one that we might not have suspected from studies on other species. 

In contrast to the voluminous literature on testosterone in men, that on women is still very sparse. The part played testosterone in female sexuality applies to other primates, such as some species of monkey. There seems to have been a significant change in the control of sexuality in primates—not a loss of hormonal control, but a change in the way it is carried out. One consequence has been to separate fertility from sexuality. Another implication is that testosterone in humans is not a ‘male’ hormone (as it it usually considered) but is important for both sexes.

There's a fascinating difference between the roles of testosterone in adult and early human life.  In the womb, testosterone has a major influence on the development of 'masculinity'. In the adult, it regulates sexuality in both sexes. A much fuller account of this, why it has occurred, and the story of the huge role that testosterone has played in human history - one that continues in our current society - is given in my recent book [1]

[1] Testosterone: sex, power and the will to win. Oxford University Press 2015

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