In both males and females, sexual behavior is influenced by biology and experience. The biological part includes the secretions of hormones by the ovaries, the testes, and certain other glands. The production of hormones in the testes is controlled by two hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—secreted by the pituitary gland. In males, the maturation of sperm depends on FSH, and the production of testosterone by the testes depends on LH. In females, the growth of the ovarian follicles, in which ova mature, depends on FSH, whereas ovulation and the production of estrogen and progesterone depend on LH. The pituitary gland continues secreting FSH and LH even after menopause, when the production of estrogen declines. The ovaries and the adrenal glands of females also produce androgens, some of which are converted to estrogen.
The pituitary gland and the testes form a closed feedback loop in which a decline in testosterone by the testes causes the pituitary to increase its secretion of LH, and an increase in testosterone level causes the pituitary to decrease its secretion of LH. The testosterone level usually declines somewhat in old age, and older men do not show the morning peak in testosterone level that is characteristic of young men. Furthermore, increased sexual activity appears to increase the testosterone level in the blood, whereas decreased sexual activity produces the opposite effect. Among older men, in particular, low testosterone levels usually accompany reduced sexual activity (Harman & Talbert, 1985). There is truth in the saying that if you don't use it, you'll lose it.
Regarding changes in the number and motility (activity level) of sperm with aging, the number of sperm is not affected as much as their motility. Unlike the continued production of sperm by males, females are born with all the ova (albeit in immature form) they will ever have. In addition, with each passing decade of later life, an increasing number of men stop producing sperm altogether. As described later in this chapter, other structural and functional changes in the sex organs also occur in older men.
Though surgery and hormonal treatments can affect sexual behavior, the effects are not always predictable. Castration of prepubertal boys inhibits the appearance of sexual behavior, but postpubertal castration does not always result in a decline in sexual activity. Following a hysterectomy or mastectomy, women, like men who have undergone prostate surgery, may lose interest in sexual intercourse. This result is, however, by no means invariable. The effects of removing the ovaries (ovariectomy) on the sex drive of women are highly variable, often increasing it. It should also be emphasized that the sexual behavior of humans depends not only on biology but also on sensory stimulation, learning, and psychological factors.
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