|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).
Table 1: Mean age of onset
of pubescent characteristics.
Budding of breasts
|Mean age ± SD
11.2 ± 1.1
11.7 ± 1.2
12.1 ± 1.0
12.9 ± 1.0
|Mean age ± SD
11.6 ± 1.1
12.8 ± 1.0
13.4 ± 2.2
14.1 ± 0.9
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.
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
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).
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
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
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
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.
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.