9
Pubertal Maturation, Adrenarche, and the Onset of Reproduction in Human Males

Benjamin Campbell

Despite their essential participation in the reproductive process, the role of males in fertility variation has tended to be downplayed by demographers, who are primarily interested in fertility differentials between groups, though there is increasing interest in the role of men (see, for instance, Bledsoe et al., 2000). A focus on females makes sense for all of the standard reasons in terms of reproductive accounting because of maternal certainty and the fact that females have more constrained reproductive spans and fertility. However, the benefits of this accounting approach begin to decline as we move further afield to consider factors less directly related to pregnancy, such as behavior. Here there is more room for other perspectives with which to orient our thinking.

From an evolutionary point of view, male humans, as the sex with greater variation in reproductive success, can be expected to be subject to more intense selection (Bateman, 1948). Recently, genetic evidence supports the idea that male reproductive genes undergo more rapid evolution than females (Wyckoff et al., 2000), though it is unclear if this represents greater mutation rates in the male germ line because of more replication events (Vogel and Ratherberg, 1975) or the impact of other processes such as methylation of genes (Huttley et al., 2000) or incorporation of transposable elements (Erlandsson et al., 2000).

The impact of selection on male reproductive biology is evident in morphological differences in male genitalia across species (see Eberhard, 1985, for graphic examples), as well as functional differences in sperm production. Across primate species the ratio of testes size to body size (Harcourt et al., 1981) and even the morphology of sperm themselves



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Offspring: Human Fertility Behavior in Biodemographic Perspective 9 Pubertal Maturation, Adrenarche, and the Onset of Reproduction in Human Males Benjamin Campbell Despite their essential participation in the reproductive process, the role of males in fertility variation has tended to be downplayed by demographers, who are primarily interested in fertility differentials between groups, though there is increasing interest in the role of men (see, for instance, Bledsoe et al., 2000). A focus on females makes sense for all of the standard reasons in terms of reproductive accounting because of maternal certainty and the fact that females have more constrained reproductive spans and fertility. However, the benefits of this accounting approach begin to decline as we move further afield to consider factors less directly related to pregnancy, such as behavior. Here there is more room for other perspectives with which to orient our thinking. From an evolutionary point of view, male humans, as the sex with greater variation in reproductive success, can be expected to be subject to more intense selection (Bateman, 1948). Recently, genetic evidence supports the idea that male reproductive genes undergo more rapid evolution than females (Wyckoff et al., 2000), though it is unclear if this represents greater mutation rates in the male germ line because of more replication events (Vogel and Ratherberg, 1975) or the impact of other processes such as methylation of genes (Huttley et al., 2000) or incorporation of transposable elements (Erlandsson et al., 2000). The impact of selection on male reproductive biology is evident in morphological differences in male genitalia across species (see Eberhard, 1985, for graphic examples), as well as functional differences in sperm production. Across primate species the ratio of testes size to body size (Harcourt et al., 1981) and even the morphology of sperm themselves

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Offspring: Human Fertility Behavior in Biodemographic Perspective (Anderson and Dixson, 2002) vary between monogamous and polygynous mating systems as a result of selection for sperm competition. For instance, seasonally breeding rhesus macaques have 60 percent larger testes than do the closely related and similarly sized nonseasonally breeding pigtailed macaques, a result of the greater mating competition during seasonal breeding. In addition, rhesus males have 60 percent larger abdominal fatfolds than pigtail males (Muhlenbein et al., 2002), suggesting that substantial energy reserves are needed to offset the energetic costs of male-male competition, among male rhesus. Such findings are powerful evidence for the impact of selection on sperm production and delivery across species. However, the relatively small testicular to body size ratio exhibited by humans suggests that sperm competition in human males has played only a small role in human evolution (Dixson, 1998; Smith, 1984). Furthermore, the low percentage of normal sperm in human ejaculates is comparable to that of gorillas and very different from that of chimpanzees (Small, 1988), suggesting that the human male reproductive strategy has more in common with preinsemination mate guarding among gorillas than the postinsemination sperm competition of the chimpanzee. Thus, the fact that the size of male ejaculates has been shown to vary with time since exposure to their partners is less evidence for sperm competition in humans (Baker and Bellis, 1989) per se than it is for the importance of mate guarding. However, in the case of humans, the role of paternal investment in offspring means that selection on males includes not only behavior leading to sex, and aspects of the male reproductive tract associated with insemination, but also behaviors related to pair bonding and the provisioning of offspring. Anecdotal evidence that testosterone increases in males exposed to mates after a prolonged absence (Bribiescas, 2001) suggests a role for testosterone in sexual behavior and pair bonding (see Young, this volume). Recent reports of lower testosterone levels in fathers of young children than in nonfathers (Berg and Wynne-Edwards, 2001; Storey et al., 2000) suggest that changes in testosterone may play a role in shifting behavior from mating to parenting. With that brief orientation toward the critical elements of male reproductive strategies among humans, I turn to the central focus of this paper, the role of reproductive maturation in the development of male sexual behavior, pair bonding, and parenting. Here I argue that this process can be considered to have two major dimensions: 1) sexual behavior that is related to reproductive maturation and somatic growth and that is organized by testosterone as part of the reproductive axis, and 2) social behavior that is related to early childhood experience and to the adrenal axis, which produces cortisol and dehydroepiandrosterone and its sulfate DHEA/S. Briefly put, maturation of the

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Offspring: Human Fertility Behavior in Biodemographic Perspective reproductive axis is crucial to sexual behavior, whereas development of the adrenal axis is crucial to social behavior and attachment and hence the expression of sexual behavior, as fertility and family formation. It is only by understanding the separate roles of these two important domains of human behavior and their interaction during maturation that we can get a differentiated account of the development of male reproductive behavior. Of course, any such model represents a vast simplification of all that goes into male reproductive maturation. At the same time, it is intended to be a human model in the essential sense of the word, a model that integrates factors we encounter as part of our everyday existence, without necessarily experiencing them fully or in isolation. As such it should be applicable to variation across societies, including variation in social structures, and biological differences in the timing of adolescent maturation, as well as cultural practices that may have independent effects on behavior. Finally, much of what will be discussed here is necessarily speculative. Thus, I conclude with recommendations for research that might elaborate and test how these hormones mediate the interaction of biological, psychological, and social levels of fertility behavior in human males. THE SCOPE OF MALE REPRODUCTIVE MATURATION Most biological analyses of male reproductive maturation focus exclusively on adolescence because of the obvious connection between pubertal maturation and the onset of sexual behavior. However, the onset of reproductive capacity and sexual behavior may be considered a necessary, but not sufficient, condition for reproduction among human males for whom pair bonding and paternal investment play an additional role (Lancaster and Lancaster, 1987). Thus, changes in familial relationships (Bee, 1997), cognition (Nelson, 1996), and the subjective sense of self (Herdt and McClintock, 2000) that begin prepubertally may be considered as part of the reproductive transition. In addition, reproduction among males is generally associated with young adulthood and family formation, which do not take place until the early 20s. For instance, in the United States only 3 percent of births are fathered by men under the age of 20, while traditional Kikuyu men marry and become fathers around the age of 25 (Bogin, 2001). Thus, the establishment of productive skills and the development of social position can also be considered part of the process of male reproductive maturation (see Kaplan and Lancaster, this volume). Furthermore, juvenile, adolescent, and young adult stages appear to be linked as a coherent biological process by increases in the production of DHEA/S by the adrenal gland (because of the close relationship of the two hormones, I will refer to them here collectively as DHEA/S unless discussing

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Offspring: Human Fertility Behavior in Biodemographic Perspective FIGURE 9-1 Temporal relationship of adrenarche and puberty in human males. NOTE: This graph indicates the relative role of changes in DHEA/S and testosterone at different stages of the development of males from age 8 to 25. Both the young adult and juvenile stages are marked by changes in DHEA/S without changes in testosterone, whereas during adolescent both hormones are increasing. results that are specifically based on one of the two hormones). Based on Western populations, increases in the production of DHEA/S by the adrenal gland, a process referred to as adrenarche, begin around age 8 and continue until the mid-20s (Worthman, 1999), thus bracketing the progression of pubertal development from roughly ages 10 to 18 (see Figure 9-1). Furthermore, the increase of DHEA/S in males appears to extend some 5 years beyond that for females (Worthman, 1999) suggesting important sex differences in the onset of reproduction. DHEA/S is the most common hormone in the human body. Yet despite its well-known importance as a precursor for estrogen production during fetal development (Parker 1999) and its rise during maturation (Orentreich et al., 1992), a functional understanding of DHEA/S’s role in postnatal development remains hazy. DHEA/S is a neurosteroid, meaning that it is produced in the brain as well as the adrenal gland (Baulieu, 1998), though a specific DHEA/S receptor is lacking (Robel and Baulieu et al., 1995). However, DHEA/S is known to occupy receptors for the neurotransmitter gamma aminobutyric acid type (GABA-A) acting as an antagonist (Majewska, 1995) and presumably influencing neural mechanisms in this way. In addition, DHEA/S is converted to estrogen and testosterone in pe-

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Offspring: Human Fertility Behavior in Biodemographic Perspective ripheral tissues (Labrie et al., 1995), meaning that it may also have effects through testosterone and estrogen receptors, not only within the brain but also on immune function and bone and muscle growth. For instance, males with 21-hydroxylase deficiency, a condition that leads to the overproduction of DHEA/S, have reduced fertility, as a result of increased estrogen concentrations suppressing gonadotropin stimulation of the testes (Cabrera et al., 2001; Stikkelbroeck et al., 2001). However, such individuals are also likely to have testicular tumors, clouding the implication of these findings for normal maturation. DHEA/S is known to have a variety of physiological effects during maturation, including promoting muscle and bone growth (Argquitt et al., 1991; Zemel and Katz, 1986) and disease resistance (Kurtis et al., 2001), as well as playing a role in mood (Goodyer et al., 1996). In addition premature adrenarche has been shown to have effects on both cognition and psychosocial development (Dorn et al. 1999; Nass et al. 1990). Though it is unclear how far findings on cognition can be generalized, Goodyer et al. found that DHEA/S hyposecretion was related to major depression in a sample of 8- to 16-year olds, suggesting that DHEA/S does have important effects on mood during development in normal children. BIOLOGY OF REPRODUCTIVE MATURATION IN HUMAN MALES To understand the role of reproductive maturation in the development of reproductive behavior in human males, its important to separate various strands, including reproductive maturation, pubertal development, and somatic growth. Reproductive maturation involves maturation of the hypothalamic-pituitary testicular axis. This starts with gonadarche or the beginning of testicular growth and the onset of spermatogenesis and testosterone production by the testes, which plays some role in libido. Pubertal maturation, or the development of secondary sexual characteristics as a consequence of increasing testosterone levels, represents the outward appearance of reproductive maturation and plays a role in changing social status. Both reproductive maturation and pubertal maturation are necessary conditions for adult maturation common to mammalian species. Somatic growth, on the other hand, is not essential to the beginning of reproduction; in fact, in many species reproductive maturation begins when somatic growth has reached its nadir. In contrast, for humans somatic growth during adolescence includes a reacceleration of growth in both weight and height that has been slowing down ever since infancy, suggesting that the pubertal growth spurt may be a specifically human adaptive feature (Bogin, 1999). However, more recent work suggests that other primate species exhibit a similar growth spurt (Leigh, 1998) in weight, especially among sexually dimorphic species. Chimp males, for instance, show important gains in

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Offspring: Human Fertility Behavior in Biodemographic Perspective weight during reproductive maturation (Leigh and Shea, 1996). Thus, the distinctive element of the adolescent growth spurt in humans is not increased growth per se but, more specifically, the dramatic increases in stature. This is easily attributable to our bipedality, which makes height a more salient dimension of size. Unfortunately, definitive fossil evidence for the origins of the adolescent growth spurt in height during human evolution is not available. Current speculations focus on Homo ergaster when increases in size and loss of sexual dimorphism suggest important changes in the timing of maturation (Hawkes, 2002). However, interpretation of the almost complete skeleton of the Nariokotome boy at 1.8MYA does not provide conclusive evidence for the existence of a growth spurt during this time period. Smith (1993) argues that the relationship between dental and postcranial developmental timing demonstrated by the specimen is at variance with that seen in modern humans, while Clegg and Aiello (1999) argue that it is, in fact, within the range of variation found in modern humans. Thus, the temporal origin of the human growth spurt remains an open issue. In addition to the pubertal growth spurt, the existence of adrenarche as part of human maturation deserves attention as a distinctive clue to the human life course. Again, adrenarche is not uniquely a human attribute but rather one we share with other great apes (Collins et al., 1981; Smail et al., 1982) that may take on a distinctively human pattern. Importantly, adrenarche follows the completion of brain growth around the age of 7 and precedes the onset of puberty (Bogin, 1999), suggesting that it plays some role between the energetic demands of brain growth and those of puberty. While adrenarche was once thought to be directly related to the onset of puberty, there is no evidence for an obligate role of adrenarche in the onset of pubertal maturation, and adrenarche and gonadarche are considered functionally separate processes. Instead it appears that adrenarche may be more directly related to processes of brain maturation. DHEA/S has an impact on Gamma-aminobutyric acid (GABA(a)) neurons (Majewska, 1995) that act to inhibit dopaminergic neurons in the prefrontal cortex that regulate judgment and the control of behavioral impulses (Niehoff, 1999). In addition, the impact of DHEA/S on GABA(a) neurons in the hypothalamus may act to inhibit GnRH production and retard the onset of puberty (Genazzani et al., 2000). Thus, while adrenarche and gonadarche are fundamentally separate processes, they are not unrelated, a point that comes home when we consider the role of energetics in reproductive maturation. ENERGETICS AND MALE REPRODUCTIVE MATURATION For males, increases in stature and muscle mass come well after the onset of testicular growth and the onset of spermatogenesis (Tanner, 1978).

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Offspring: Human Fertility Behavior in Biodemographic Perspective 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

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Offspring: Human Fertility Behavior in Biodemographic Perspective FIGURE 9-2 Causal relationship of adrenarche and pubertal maturation. NOTE: The old model suggests that adrenal androgens produced by adrenarche are necessary for desensitizing the hypothalamus to the feedback effects of gonadal androgens thus allowing testicular production of testosterone to begin to rise. The new model incorporates both genetic and environmental factors in the onset of adrenarche and gonadarche. In addition to variation in the timing of hypothalamic and pituitary maturation, the onset of adrenal production of DHEA/S and gonadal production of testosterone are also related to energy availability, most likely in the form of fat storage. Adrenal production of DHEA/S may also alter hypothalamic-pituitary control of gonadotropins. The pubertal growth spurt is a product of both the onset of testicular production of testosterone and energy available for bone and muscle growth. DHEA/S contribute to the development of somatic growth as well, though their role appears to be negligible compared to that of testosterone. However, in populations where energetic constraints lead to lowered testosterone levels, the contribution of DHEA/S to the progression of pubertal maturation may increase.

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Offspring: Human Fertility Behavior in Biodemographic Perspective EVOLUTIONARY EXPLANATIONS OF HUMAN REPRODUCTIVE MATURATION The model of energetics and pubertal timing outlined above suggests that, in the case of humans, energetic constraints on the simultaneous development of the brain and reproductive maturation acted as the primary selective force in the development of the maturation process beginning with adrenarche and marked by continuing increases in DHEA/S. It is difficult to be sure when changes in human life history evolved. In the scenario suggested here, changes in life history patterns associated increased size, slowed childhood growth, and changes in the relative metabolic costs of the gut versus the brain would have started with the origins of Homo ergaster (Aiello and Wells, 2002). Initially, the increased metabolic demands of the brain may have been met by an increase in caloric intake, involving dietary shift, changes in relative size of the gut, and slower childhood growth. With further increases in brain size subsequent to Homo ergaster full development of a larger brain would have required additional energy and time. In order to allow for continued brain development without engendering the energetic costs of reproductive maturation, increases in DHEA/S production would have promoted development of the prefrontal cortex, while suppressing GnRH production and the onset of pubertal maturation. At younger ages, reproductive suppression may have been complete, creating a juvenile phase in which the brain could develop without the competing demands of reproductive maturation. However, continued increases in DHEA/S would allow for further brain maturation even as reproductive maturation and sexual behavior commenced, thus allowing the integration of sexual impulses and their behavioral expression. BEHAVIORAL IMPLICATIONS Bogin (1994) has argued that the adolescent growth spurt in human males reflects the results of male-male competition. He suggests that, once males exhibit the somatic cues of maturity, they enter into a competitive world of adult males that inevitably carries costs, including the risk of injury. Thus it would be beneficial to delay development of full secondary sexual characteristics, including growth, until one is as competitive as possible so that the benefits of engaging in male-male competition are more likely to outweigh the costs. Bercovitch and Ziegler (2002) make a similar argument for primates. Among orangutans the presence of adult males does directly suppress the development of secondary sexual characteristics in developing males without suppressing reproductive function (Maggioncalda et al., 2002), suggesting that the presence of older males is a critical social signal of

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Offspring: Human Fertility Behavior in Biodemographic Perspective reproductive opportunities. There is no evidence for such reproductive suppression in humans and the role of male-male competition during adolescence seems secondary to the role of energetic factors. However, the fact that the presence/absence of the father during early childhood has been linked to hormonal profiles (Flinn et al., 1996) suggests that male parental care may play a role as a social cue about later reproductive opportunities (see section on Cultural Variation). Nonetheless, male-male competition would make adolescence a stressful time. In addition to a potential role in suppressing the onset of reproductive maturation, increased DHEA/S production during reproductive maturation may have been selected for because of its protective role in the stress response. Under conditions of stress such as those associated with major life transitions (Dorn and Chrousos, 1997), elevated cortisol is associated with anxiety (Frankenhaeuser, 1980), which may in turn interfere with social interaction and learning. As Fredrickson (2000) has put it, activation of the hypothalamic-pituitary-adrenol (HPA) axis may be associated with execution of narrow behavioral programs mediated by the limbic system. In contrast, DHEA/S has been associated with improved mood among HIV-positive men (Cruess et al., 1999) and improved mood and cognition in aging men (van Niekerk et al., 2001). Furthermore Boudarene and Legos (2002) report that low anxiety levels are associated with increased levels of DHEA/S during cognitive stress tests. Importantly, DHEA/S is a cortisol antagonist, meaning that it acts to block the effects of cortisol on neurons in the brain (Kimonides et al., 1999; Yoo et al., 1996). Given these actions of cortisol and DHEA on the hippocampus for learning and memory (McEwen, 1999), increased levels of DHEA would alleviate the effects of cortisol in narrowing concentration (Stansbury and Gunnar, 1994) and lead to increased inputs to the hippocampus from the cortex allowing for greater possibility of flexible behavior and increased learning. In the case of adolescent males for whom increased testosterone levels are associated with increases in sexual behavior (Halpern et al., 1998), learning may be very much focused on interactions with the opposite sex as well as competitive interactions with other males (Weisfield, 1999). Increasing levels of testosterone can be thought of as making such interactions increasingly salient and promoting impulses toward sex and aggression. Cortisol and DHEA/S may be thought of as having contrasting effects on the execution of these impulses. Increased levels of DHEA/S would promote their expression and relate them to a wider arrange of social inputs through its effects on the prefrontal cortex. Cortisol, on the other hand, would interfere with the broader associations of such behavior by blocking the formation of new memories by the hippocampus (see Figure 9-3 for a diagrammatic representation).

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Offspring: Human Fertility Behavior in Biodemographic Perspective FIGURE 9-3 Schematic representation of the impact of gonadal and adrenal hormones on reproductive strategies. NOTE: This diagram, loosely based on Ledoux (1998) and Zuckerman (1995) suggests that testosterone, DHEA/S, and cortisol all play different roles in brain mechanisms that underlie the expression of sexual behavior. Testosterone is related to the perception of sexual stimuli and associated sexual impulses. DHEA/S, on the other hand, is related to disinhibition of behavior in the precortex, whereas cortisol acts on the formulation of memories in the amygdala and hippocampus and the development of learned behavioral responses to sexual interaction. In more human terms, Weisfield (1999) has argued that adolescence represents a time in which social emotions become associated with sexual behavior and the social relationships that go with them. In particular he argues that pride and shame have important roles as males start to judge achievement in terms of their ability to attract mates. If, as Damassio

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Offspring: Human Fertility Behavior in Biodemographic Perspective precortical control of behavior. In addition, DHEA/S’s antiglucocorticoid effects suggest a potentially important role for DHEA/S in memory development and the ability to produce novel behavior patterns in response to familiar stimuli, under conditions of arousal. The very fact that DHEA/S levels rise during maturation in humans represents an important evolutionary feature of the human life cycle that may shed light on the role of brain development and attachment in humans as well as fertility behavior more generally. FUTURE RESEARCH DIRECTIONS To more fully understand the implications of adrenarche for the development of boys, I suggest three important lines of investigation. The first of these is a better understanding of the interaction of DHEA/S and cortisol with testosterone in the disruption of normal emotional development in boys. The second is the study of cross-population differences in age-related patterns in adrenal and gonadal steroids and the insight they may provide into maturational timing. The third is a more detailed investigation of the role of adrenal and gonadal steroids on behavioral development in chimpanzees and gorillas as a background for understanding the truly human aspects of male reproductive maturation. There is already sufficient evidence for the effects of cortisol and DHEA on major depressive disorder in adolescence to warrant further investigation (Goodyer et al., 2001). What the perspective offered here adds is a functional understanding that for adolescent boys mood dysregulation reflects not only the shifting patterns of psychosocial adjustment during this period but also the role of testosterone in producing the underlying motivations for attachment. DHEA/S, cortisol, and testosterone are only some of the many factors involved in this process, but they can help provide insight into the specific aspects of brain functioning that fail to develop normally in these conditions. Researchers will also want to investigate the impact of variations in maturational timing across populations on the development of emotions and behavior. One very important issue in understanding the interaction of gonadal and adrenal steroids on adolescent maturation among boys is the role of DHEA/S in mediating the effects of testosterone on sexual behavior. If because of its relationship to brain growth, the timing of adrenarche is less variable than that of gonadarche (Worthman, 1993), individuals in early-maturing populations such as our own may be exposed to the maturational effects of DHEA/S for a much shorter time before being exposed to the sexually motivating effects of testosterone. Faster rates of maturation among adolescent boys may simply provide less time for the effects of

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Offspring: Human Fertility Behavior in Biodemographic Perspective DHEA/S on the brain and hence less time to integrate attachment and emotional development with sexual behavior. The final line of investigation involves comparison with our most closely related species, great apes, in order to more fully understand the relationship of adrenarche and gonadarche in reproductive maturation specifically in humans, given their large brains. While adrenarche has been demonstrated in both chimpanzees and gorillas (Smail et al., 1982; Collins et al., 1981), and DHEA/S has not demonstrated a relationship to reproductive behavior (Nadler et al., 1987), we have very little information on the timing and tempo of increases in DHEA/S and its relationship to the development of reproductive behavior. It is only by comparing such patterns from great apes with those from humans that we will be able to understand the contribution of adrenal steroids to a uniquely human reproductive maturation. REFERENCES Aiello, L.C., and J.C.K. Wells 2002 Energetics and the evolution of the genus Homo. Annual Review of Anthropology 31:323-338. Alexander, G.M., and B.B. Sherwin 1991 The association between testosterone, sexual arousal and selective attention for erotic stimuli in men. Hormones and Behavior 25(3):367-381. Anderson, M.J., and A.F. Dixon 2002 Sperm competition: Motility and the midpiece in primates. Nature 416(6880):496. Anderson, R.A., J. Bancroft, and F.C. Wu 1992 The effects of exogenous testosterone on sexuality and mood of normal men. Journal of Clinical Endocrinology and Metabolism 75(6):1503-1507. Argquitt, A.B., B.J. Stoecker, J.S. Hermann, and E.A. Winterfeldt 1991 Dehydroepiandrosterone sulfate, cholesterol, hemoglobin, and anthropometric measures related to growth in male adolescents. Journal of the American Dietetic Association 91(5):575-579. Baker, R.R., and M.A. Bellis 1989 Number of sperm in human ejaculates varies in accordance with sperm competition theory. Animal Behavior 37:867-869. Bateman, A.J. 1948 Intrasexual selection in Drosophila. Heredity 2:349-368. Baulieu, E.E. 1998 Neurosteroids: A novel function of the brain. Psychoneuroendocrinology 23(8):963-987. Bee, H. 1997 The Developing Child, 8th ed. New York: Addison-Wesley. Belsky, J., L. Steinberg, and P. Draper 1991 Childhood experience, interpersonal development and reproductive strategy: An evolutionary theory of socialization. Child Development 62(4):647-670. Bercovitch, F.B., and T.E. Ziegler 2002 Current topics in primate socioendocrinology. Annual Review of Anthropology 31:1-23.

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