Puberty term paper
The following paper was submitted as a paper to accompany a presentation for graduate credit in my Endocrinology lecture.



The gender-specific difference in age of onset of puberty in humans
Although it is an almost universal experience, puberty is something of a nebulous concept.  Norris (1997) defines it as “the achievement by the gonads of their full hormonal and gametogenic capacity” (p.364).  This constrains these changes to endocrine events, but excludes their consequences.  Gruber and Lucas (1975) emphasize these explaining that puberty is “that period of time during which an individual undergoes rapid sexual and somatic growth” (p.123).

Puberty is, of course, a cluster of changes which differ between the genders. Both genders undergo dramatic spurts of growth, development of axillary and pubic hair.  Females develop breasts, menarche, and a widening of the pelvis.  Males experience beard growth and changes in genital size, shape, and coloration.

Discussions of the age of onset of puberty typically focus on menarche in females, thus timing of development in males is a topic frequently ignored.  The primary reason for this is that menarche is a distinct event which can usually be remembered and, therefore, recorded with precision.  By contrast, the onset of puberty is more difficult to pinpoint in males (Hafez, 1975).


Variability of pubertal onset

Because of the highly variable time of onset of puberty even within one gender, “typical” patterns in place of average are presented in the literature wherever appropriate.  For example, averages across subjects would tend to diminish dramatic but transient changes such as the growth rate during the adolescent growth spurt (Fig 1).
Fig. 1: Typical patterns of growth.
Normal onset of puberty is regarded as + or - 2.5 standard deviations (Grumbach & Stein, 1998).  While this includes 99.4% of the general population, this is a very wide range in terms of years since the standard deviation for the appearance or development of most pubescent characteristics is a full year or more (Table 1).
Female 
     Budding of breasts
     Sparse pubic hair
     Peak vertical growth rate
     Menarche
Mean age SD
11.2 1.1
11.7 1.2
12.1 1.0
12.9 1.0
Male
     Enlargement of testes, scrotum
     Lengthening of penis
     Sparse pubic hair
     Peak vertical growth rate
Mean age SD
11.6 1.1
12.8 1.0
13.4 2.2
14.1 0.9
Table 1: Mean age of onset of pubescent characteristics.

The growth spurt

Hafez (1975) highlights four major areas of somatic changes associated with puberty:
1) Development of the reproductive system and secondary sexual characteristics.
2) The adolescent growth spurt: increase in the growth rate of the skeleton, muscles, and viscera.
3) Sexually dimorphic increases in growth rate which exceed the growth spurt.  For example, broadening of shoulders in males; hips for females.
4) An increase in muscle and decrease in fat (this is more pronounced in males).
Ironically, while reproductive maturity is the goal most frequently associated with puberty, the onset of this life stage is generally identified by the additional forms of physical growth highlighted above.  This leads to a major misconception in assessing puberty.

Contrary to popular belief, there is no sexual dimorphism between males and females in the timing of adrenarche, the peak of synthesis of adrenal androgens.  This is a misconception based on growth as marker of puberty (common to both sexes).  Generally males and females enter adrenarche and puberty at the same ages.  However for boys, visible secondary signs of puberty such as height gain (see below), beard growth, muscle building, and voice change are late in the sequence (Low, 2000).

Puberty is associated with a marked increase in gonadotropin releasing hormone (GnRH) release from the hypothalamus due to elevated gonadotropin secretion.  While gonadotropin secretion is double its daytime level at night in children, during production, this level is matched during the day as well.  These pulses also increase in amplitude as the sensitivity of pituitary gonadotropes to GnRH is enhanced (Norris, 1997).

Throughout puberty the growth spurt and appearance of secondary sexual characteristics appear concomitant with an increase of sex steroids, growth hormone (GH), and insulin-like growth factor 1 (IGF-1).  Through studies of various forms of precocious and delayed puberty it has been determined that sex steroids such as testosterone and estradiol cause an increase in serum levels of GH.  However, the mechanism(s) of this action are still unknown” (Caufriez, 1997).

For the duration of their pre-pubescent development, the growth rates of both males and females parallel one another However, girls enter a period of rapid growth before their male counterparts begin what is characteristically termed a "growth spurt."  The male equivalent lasts just as long on average, but is greater in magnitude.  Further, because of the delay in onset of this "spurt," males are granted on average an additional two years of overall growth.  Thus males catch up to and eventually surpass the members of their female cohort who outpaced them early on.  One source (Kahn & Cataio, 1984) attributes this to different rates of bone growth between the genders along with the appearance of ossification centers and cessation of growth earlier in the long bones of girls.

Several gender-specific differences result from this period of growth.  For example, males conclude puberty with longer arms and legs relative to their trunks than females (due to different times of growth for these regions), accentuated by the prolonged period of growth during puberty in males.  Another example is the changes due to selective responses of cartilage in the genders.  Specifically, in females hip width increases because the cartilage cells respond to female sex hormones. In contrast, the increase in shoulder breadth is due to the opposite influence in males.

Other sexual dimorphisms include human males are a maximum of 9% taller than females, and are 20% more muscular than women, yet retain less fat.  Both sexes increase lipid storage during puberty, but males subsequently lose it.  At end of adolescence males have about 12% body fat by weight while females have 25%.


Consequences of pubertal growth for males

Given the social complexity of the human species, it can be assumed that puberty will have broader implications than the ability to produce gametes.  The evolutionary significance of many of these physical changes relate to the ability to attain dominance in a social hierarchy and thereby acquire mates.

In males the flood of testosterone brings about changes in behavior, particularly increases in displays of aggression and confidence.  In addition to the obvious advantages, these changes also serve to counter the physical awkwardness brought about by the rapid spurt of growth.

Since physical stature is an important goal ultimately, it is reasonable that males mature later than females as this grants them two extra years of bone growth.  In fact, earlier maturing males stop growing much sooner, and, thus, are outpaced by later bloomers (Low, 2000).

However, in return for the enhanced competitiveness in getting mates through testosterone’s influence, males require higher maintenance due to more size and musculature.  They are typically 5-10% taller than females; muscles are metabolically expensive compared to fat (which females retain in a greater proportion); and testosterone depresses immune system function.


Consequences of pubertal growth for females

Following adrenarche females appear to exhibit most of the changes associated with puberty prior to males of the same age.  Just as late onset of puberty conferred an evolutionary advantage to males, early onset grants advantages to females.

Females achieve reproductive competence very late in sequence (their regular ovulation takes about 4-5 years after peak height gains, while males attain their first emission before they peak).  As a result males are much more likely to defer their growth because of sensitivity to environmental quality while the reverse is true for females (Low, 2000).  In spite of this late reproductive viability females tend to attract mates at younger ages than their male counterparts.  For example, no population has been studied where men marry before females on average.  Males usually marry 2 to 7 years later (Low, 2000).

Females will attract mates even before they are reproductively viable.  Early maturity of secondary characteristics allows earlier acquisition of a mate and, thus, for a greater proportion of years spent childbearing and rearing.  For example, breast development typically occurs between 7 and 13 years of age.  Breast development in females prior to reproduction is unique to humans.


References
Austin, C.R. & Edwards, R. G.  (1981). Mechanisms of Sex Differentiation in Animals and Man. Academic Press: New York.

Caufriez, A. (1997). The pubertal spurt: effects of sex steroids on growth hormone and insulin-like growth factor I.  European Journal of Obstetrics & Gynecology and Reproductive Biology 71, 215-217.

Gruber, J.S. & Lucas, C.P. (1975). Endocrinology of Puberty in Sexual Maturity: Physiological and Clinical Parameters.  Eds. Hafez, E.S.E. & Peluso, J.J.

Grumbach, M.M. & Stein, D.M. (1998). Puberty: Ontogeny, Neuroendocrinology, Physiology, and Disorders in Williams Textbook of Endocrinology.  Eds. Wilson, J.D.; Forster, D.W.; Kronenberg, H.M.; & Larsen, P.R.

Hafez, E.S.E (1975). Parameters of Sexual Maturity in Man in Sexual Maturity: Physiological and Clinical Parameters.  Eds. Hafez, E.S.E. & Peluso, J.J.

Kahn, A.U & Cataio, J. (1984). Men and Women in Biological Perspective: A Review of the Literature.  Praeger: New York.

Low, B.S. (2000). Why Sex Matters: A Darwinian Look at Human Behavior. Princeton University Press: Princeton, NJ.

Norris, D.O. (1997). Vertebrate Endocrinology. Academic Press: New York.





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