This is in clear contrast to human females for whom menarche, the first clear sign of reproductive function, occurs after peak height velocity. At the broadest level, this difference in the relative timing of reproductive maturation and somatic growth between the sexes can be explained in terms of energetics. Reproduction is energetically costly in females and so reproductive function matures only after the energetic demands of growth are nearly finished. In contrast, the low relatively energetic cost of sperm production in males means that its early onset does not detract from further growth in stature and muscle mass (Ellison, 2001).
This energetic perspective can be applied to understanding the process of reproductive maturation in males as well. As mentioned earlier, previous attempts to link adrenarche and pubertal onset have focused on the potential role of adrenarche in the onset of pubertal maturation. Increases in adrenal androgens were thought to desensitize the hypothamalus to negative feedback from gonadal steroids thus allowing gonadal production to rise and initiating puberty (Grumbach et al., 1974; see Figure 9-2A). However, it is now clear that adrenarche is not necessary for pubertal maturation (Counts et al., 1987), and the idea of adrenarche as a specific event that causes the onset of pubertal maturation has been largely discarded (Parker, 1999).
Nonetheless, adrenarche and gonadarche may be related by a third factor on which both depend—energetic status, as suggested by the model presented in Figure 9-2B. In this model the timing of adrenarche and gonadarche is the product of two separate processes: maturation of the hypothalamus and pituitary and maturation of the adrenal gland and the testes. Hypothalamic and pituitary maturation appears to be a largely internal process reflecting more general developmental processes extending back to the womb. More importantly, maturation of the hypothalamus and pituitary appears to precede maturation of the adrenal gland and testes, suggesting that variation in the timing of adrenarche and gonadarche is largely dependent on development of adrenal gland and gonadals.
In fact, the onset of adrenal and gonadal androgen production has been tied to variations in energetic availability, specifically fat stores. Recent evidence indicates that the timing of adrenarche is associated with maximum increases in body mass index (Remer, 2000; Remer and Manz, 1999), while the timing of pubertal onset—that is, testicular growth—is directly related to fat stores (Vizmanos and Marti-Henneberg, 2000). Thus, greater food availability would lead to earlier onset of adrenarche as well as an earlier onset of puberty and a correlation of testosterone and DHEA/S levels without any direct relationship between the two processes.
Once testosterone production has begun, pubertal development and somatic growth are related to both increasing testicular production of testosterone and energetic availability through the effects of blood glucose, insulin, and activity, as shown in Figure 9-2B. Continued increases in