occurs prenatally, over an extended period in midgestation (Phoenix et al., 1968). There is also recent evidence in male macaques that neonatal exposure to testosterone may play a role in determining the extent of adult sexual behavior (Mann et al., 1998).

Pubertal Activation of the Reproductive Axis

Pubertal reawakening of the reproductive axis occurs in late childhood and is marked by a cascade of hormonal, physical, psychological, and behavioral changes. One of the earliest signs of puberty is an elevation of gonadotropin and gonadal steroid hormone levels specifically at night (Boyar et al., 1974), although because detection of this rise requires collecting blood samples at night it is virtually never examined in demographic studies. Investigation into the mechanisms controlling the pubertal reawakening of the GnRH pulse generator has been an area of intense investigation for the past two decades (Ojeda and Bilger, 2000; Plant, 2001). Although the mechanisms are not fully understood, significant progress has been made in identifying central changes in the hypothalamus that appear to play a role in this process. There is strong evidence of both increases in stimulatory neural input into GnRH neurons and decreases in inhibitory input. Despite an increased understanding of the neural changes occurring at puberty, the question of what signals trigger the pubertal awakening of the reproductive axis is unanswered at this time. Availability of food and nutritional status have been shown to affect the timing of puberty; however, these signals appear to be only modulators of the pubertal process in that puberty can only be moderately advanced by increasing food availability (Frisch and MacArthur, 1974). Whether there is a genetic timing mechanism that regulates puberty or whether other signals from the body are responsible for timing the reactivation of the reproductive axis awaits further research.

Changes in body habitus are the first signs of puberty detected by most individuals, although these emanate from increased levels of gonadal steroid hormones and are thus relatively late events in the reawakening of the reproductive axis. Likewise, in girls, menarche is a very late event, heralding the point where the adult cyclic interplay between the hypothalamic-pituitary-ovarian axis is initiated. As described in the section above, the increase in testosterone at puberty in males leads to development of the secondary sexual characteristics, including increased growth of facial, axillary, and pubic hair; deepening of the voice; increase in muscle mass; enlargement of the testes and penis; increased incidence of erections and ejaculations; and attainment of fertility. Sexual behavior is also dramatically increased in males at puberty, and there is a strong correlation at this



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