Tom Minor
Kathy Rastle What is known of the biological mechanisms that underlie sexual motivation in humans and other mammals?
This essay aims to detail some of the findings of studies investigating the biological mechanisms which underpin human and mammalian sexual motivation and also to explain what they mean in terms of how biological substances and internal neural systems translate into this innate preoccupation with sex, that we call a ‘sex drive’. Kaplan (1989) defined sexual desire as a drive or appetite, produced by the activation of a specific neural mechanisms in the brain, and he states that the excitement and orgasm phases involve genital organs. This definition focuses on some prominent topics that will appear in this essay: the role of the brain and the role of gonadal hormones in producing a sex drive. Other factors not directly referred to by Kaplan will also be dealt with but in the main this essay will focus on hormonal and neural mechanisms of sexual motivation. Finally, this essay will consider humans and mammals quite separately because although there are similarities, the mechanisms that underlie sexual motivation are fundamentally different. We shall begin with humans at adolescence, where sexual interest really begins. It is no coincidence that during puberty, the onset of sexual maturity, sexual interest heightens. However, it is something that is put on hold until the body has ample energy supplies to cope with the extra demands of being sexually active and potentially reproducing offspring. The biological instrument responsible for this is the hypothalamus, a small region in the centre of the brain that organises the production and circulation of hormones. The hypothalamus begins releasing gonadotropin releasing factors between the 9th and 14th year, generally earlier for
females. These stimulate the secretion of gonadotropins from the pituitary gland. There are two divisions of gonadotropins. The first is follicle-stimulating hormone (FSH). In females this induces the growth of follicles in the ovaries, which are responsible for supporting the egg and secreting oestrogen. In males FSH encourages the production of spermatozoa in the testes. The second division is called Luteinizing hormone (LH) in females and Interstitial-cell stimulating hormone (ICSH) in males. For females LH instigates ovulation and through a feedback system causes the follicle to secrete progesterone once the egg has been released. In males, ICSH promotes the production of the male androgen, testosterone. These hormones are responsible for all the pubescent changes that occur to males and females in terms of body shape, facial and bodily hair, muscular development and genital growth as well as sexual arousal levels. When investigating biological mechanisms that underlie sexual motivation in adults it is interesting to examine the role of the adrenal glands. Work done by Money, Wiedeking, Walker and Gein, (1976) and Walker, (1978) found that in cases where men had undergone chemical castration due to cancer, which involves administering synthetic hormones to suppress the use of androgens, some men lost interest in sex whilst other did not. The adrenal glands seem to play a crucial role in the endocrine system and produce androgens in males and females. Waxemberg et al., (1959) discovered that whilst terminally ill breast cancer patients who had ovarectomies suffered minimally in terms of frequency of sexual intercourse, patients who had adrenalectomies as well suffered almost complete cessation of any sexual activity. These studies strongly support the theory that it is indeed androgens that are responsible for a sex drive. I would suggest that the reason why some of the men in the Money et al. study didn’t suffer a complete loss of sexual interest was because
they still had functioning adrenal glands, a secondary source of androgens. It is apparent that hormones aren’t the sole contributors to a sex drive as some of the men in Money et al’s study did still have an interest in sex even though their hormone levels were considerately lower, but clearly they play an important part. Neural mechanisms seem to contribute toward the control of sexual motivation. Obviously ethics restrict the extent of an investigation but nonetheless cases exist that can help us view the role of certain brain regions. The temporal lobes have been studied extensively with respect to epilepsy and in the case of sex offenders. Lesions or damage to this area are strongly associated with a reduction in sex drive and the loss of perversions and fetishes. The amgydala is situated very near to the temporal cortex and could also play a part in sexual motivation and orientation in humans because it certainly is the case with other mammals. (Smith 2002) The hormonal situation appears to be the same for lower primates, with the exception of prosimians such as lemurs, galagos and lorises where there is a vaginal membrane present on the female that disappears once she becomes fertile, (Doyle, 1974). Wallen (2001) has found that elimination of testosterone through castration or other chemical means did not eliminate male rhesus monkeys’ sexual behaviours. Gorski, Gordon, Shryne and Southan, (1978) offer that the sexually dimorphic nucleus in the medial preoptic hypothalamus is linked to sexual behaviour; it is larger in males, which could account for the notion of a constant sex drive. They also found that part of the female hypothalamus generates a cyclic pattern of hormone release linked to the menstrual cycle and quite possibly sex drive. The hypothalamus seems to be the main centre for sexual desire but it is likely that other areas of the brain are involved as well.
Lower mammals have a much more predictable set of behaviours when it comes to sexual motivation. The male has a constant sex drive influenced by steroid hormones, principally testosterone. The female only becomes receptive and interested when she is at the point in her oestrus cycle that is optimal for fertilisation. Young (1961) named estradiol and progesterone as responsible for modulating females’ sexual desire. Hormones not only influence motivation to copulate in lower mammals but the ability to do so as well. Young, (1937) notes that female guinea pigs have imperforate vaginas except when they are fertile and hormone levels are high, limiting copulation to a small portion of the ovarian cycle. Another example is that many mammals require gonadal steroids in order to produce an erection rigid enough for sexual intercourse to occur, (Beach & Levinson 1950; Phoenix, Copenhaver & Brenner 1976). Just as with humans, mammals suffer a dramatic decrease in sexual interest if the primary source of androgen production is removed through castration and again the existence of adrenal glands explain why there isn’t always a complete extinction of sexual motivation and activity in these cases. Artificial injection of testosterone and its two major metabolites, estradiol and dihydrotestosterone, seem to restore a sex drive with these animals. (M.J. Brown & Vreeburg, 1973) Neural control of sexual motivation in lower mammals has been linked to the hypothalamus, in particular the anterior hypothalamus, because lesions in this area result in sex drives disappearing. Artificially eliciting sexual desire can be achieved by electrically stimulating this area and in one case a male rat was kindled in this way for seven and a half-hours, provided with a liberal supply of receptive females and mated 155 times! The amygdala appears to have an opposing, inhibitory function in lower mammals as removal results in hyper-sexuality and electrical stimulation results
in any sexual activity stopping, (Smith 2002). There are many empirically founded arguments that sexual motivation is also influenced by environmental factors. Ward, (1972) demonstrates how the immediate chemical environment during pre-natal development can be included within these explanations. He exposed female pregnant rats to short, regular and severe bursts of stress by squashing them into a tube. The male offspring of these rats developed normally until they reached sexual maturity when they became only interested in mating the other male rats in the environment. In this example, hormone levels, triggered by a stress response changed the sexual orientation of the mammals. Sexual orientation has a strong connection to sexual motivation and research into the biological mechanisms that underlie it in all species is rather limited. Richard Pillard and James Weinrich have been pioneers in this area since the early 1980s and in particular have generated a lot of interest into the genetics of homosexuality. They looked at a sample of homosexual and heterosexual men’s brothers and found that the brothers of heterosexual men were 4% likely to be homosexual, which is unsurprisingly similar to the average for the population, whereas the brothers of homosexual men were 22% likely to be homosexual themselves. This forms the basis for a strong case that genetics does play a part in sexual orientation as a function of motivation. The exact mode of inheritance may help explain why the case is stronger for males than females but this issue was not introduced for an in depth analysis: it was purely to illustrate the other factors involved in sexual motivation. Wallen (1996) studied male rhesus monkeys’ mounting behaviour and found that they were quite indiscriminate as juveniles until they experienced ejaculation with females when they then transitioned to exclusively female mounting. I mention this to
differentiate the behaviour of lower primates as hyper-sexuality rather than homosexuality. Many rat studies follow this assumption and as a rationale I would comment that human sexual orientation is a result of a more complex and flexible network of control structures subsequent to evolution. In conclusion, this essay has discussed, in brief, a variety of different biological explanations for sexual motivation in humans and other mammals. There are of course social incentives for engaging in sexual activity: Wallen (2001) found that the social structure even in primates affects sexual motivation, but these issues are for another critique. To close: perhaps evolution can explain sexual motivation. Gregory, (1998) implies this evolutionary basis with reference to procreation and the continuation of a race. Evolution as a mechanism of biology may provide us with some answers as to the differences between species in relation to their drive to be sexual. Much more research into this subject would be required for these answers to be found.
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