Offspring Human Fertility Behavior in Biodemographic Perspective (2003) / Chapter Skim
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9. Pubertal Maturation, Andrenarche, and the Onset of Reproduction in Human Males
9. Pubertal Maturation, Andrenarche, and the Onset of Reproduction in Human Males
Pages 260-288
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From page 260... ...
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.
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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. ~ 1 1 1 ITS ·1 , me, Here I argue that this process can be considered to have two major dimensions: 1)
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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~.
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. 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)
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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.
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. 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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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.
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From page 267... ...
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.
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From page 268... ...
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.
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From page 269... ...
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)
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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.
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So far, I have argued, on largely theoretical grounds, that the existence of adrenarche in humans gives us special reason to expand our understanding of the processes involved in human reproductive maturation beyond simply the development of sexual behavior. The next section follows through on this argument by considering empirical evidence for gonadal and adrenal hormones in the sexual behavior of adolescent boys.
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From page 272... ...
In addition, cortisol reactivity was also related to substance use, suggesting that individual differences in the HPA axis may underlie the relationship between various aspects of risk taking in adolescent boys. BIOLOGICAL MODEL: INTEGRATING PUBERTAL MATURATION AND RISK TAKING Given the evidence outlined above, pubertal maturation and risk taking can be conceived of as two separate dimensions of adolescent sexual behavior, associated with gonadal and adrenal steroids, respectively.
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From page 273... ...
Increasing levels of testosterone are known to promote sexual behavior in adolescent males through two separate pathways: by increasing secondary sexual characteristics, which are a sign of social maturity, and through libido, which plays a role in sexual initiation. On the other hand, individual variation in cortisol has been related to problem behavior that is known to be related to adolescent sexual behavior.
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From page 274... ...
Rodgers et al.'s findings fail to account for the effects of pubertal timing, which presumably has genetic elements in boys, as it does in girls (Campbell and U3ry, 1995~. On the other hand, Miller et al.'s findings that variation in genes for dopamine receptor type helps account for variation in age at first sexual intercourse in a sample of non-Hispanic European American men and women, even controlling for a variety of psychosocial factors, is compatible with individual variation in risk taking, given evidence for dopamine in impuisivity among adolescents with attention-deficit disorder with hyperactivity (Rosa Neto et al., 2002~.
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From page 275... ...
levels change little with age, suggesting a relatively fixed effect of individual variation. However, sexual behavior in adults depends much more heavily on the development of long-term relationships, in which the role of libido and risk taking in sexual behavior may be very different and may lead to changes in the observable effects of testosterone and cortisol.
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(1991) offered one such mechanism in the case of girls; internalizing behavior associated with early stress would lead to increased eating, hasten age at reproductive maturation, and lead to a greater number of sexual partners.
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This extended definition of reproductive maturation in human males highlights the role of both the testicular and adrenal axis in male reproductive behavior. Sufficient evidence exists to argue that pubertal maturation in males is part of individual variation in the reproductive axis that persists into adulthood and underlies individual continuity in sexual behavior.
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From page 278... ...
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.
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From page 279... ...
, 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.
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Bhasin 1993 Sexual function does not change when serum testosterone levels are pharmacologically varied within the normal male range. Fertility and Sterility 59:11181123.
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1987 Dissociation of adrenarche and gonadarche in precocious puberty and in isolated hypogonadotropic hypogonadism. Journal of Clinical Endocrinology and Metabolism 64(6)
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Thompson, J.R. Udry 2002 Associations between stress reactivity and sexual and non-sexual risk taking in young adult human males.
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1998 Evolution of human growth spurts. American Journal of Pl7ysical AntI7ropology 101(1)
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Sapolsky 2002 Male orangutan subadulthood: A new twist on the relationship between chronic stress and developmental arrest. American Journal of Physical Anthropology 118(1)
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Phillips 2001 Altered control of cortisol secretion in adult men with low birth weight and cardiovascular risk factors. Journal of Clinical Endocrinology and Metabolism 86( 1 )
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Azpiroz 2000 Relating testosterone levels and free play social behavior in male-female preschool children. PsycI7oneuroendocrinology 25(8)
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Fernandez-Ballart 2001 Age of pubertal onset affects the intensity and duration of pubertal growth peak but not final height. American Journal of Human Biology 13(3)
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American Journal of Physical Anthropology 71(4)
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