Secondary Sexual Characteristics
Secondary sexual characteristics are the set of anatomical structures and features unique to males and females that are not directly related (and therefore secondary) to the production of sex cells (gametes).
Although in humans, the differences in male and female secondary sexual characteristics are more easily defined and determined, in many animals the differences are subtle. Regardless, the role of secondary sexual characteristics, especially with regard to the evolutionarily essential processes of mate identification and selection, can be extremely important.
Secondary sexual characteristics are generally distinguishing of gender in the human male and female. Accordingly, characteristics such as male facial hair or the enlarged breasts of a female may also carry importance in a particular social or religious culture.
The development of easily identifiable secondary sexual characteristics begins with the onset of puberty and continues throughout the teenage years into young adulthood.
In females, secondary sexual characteristics include the development of breast and mammary tissue needed to suckle infants. During puberty, the female hips take on a more broad and rounded appearance as a result of a general widening in pelvic structure that will ultimately allow for easier passage of the fetus through the birth canal. There is also an increased amount of fatty tissue deposited throughout the body that results in a normally higher percentage of body fat for females.
Maturation of the uterus, ovaries, and the endocrine system results in the onset of menstruation, a normally 28-day (with variation) cyclic growth and shedding of the uterine mucosal lining. Changes in the endocrine system also result in the more outwardly visible secondary sexual characteristics associated with increased hair growth in the pubic region and under the arms.
In males, the outward secondary sexual characteristics include the growth of the larynx and the appearance of the "Adam's apple." Changes in the larynx and elongation of the vocal cords result in a deepening voice associated with males. The release of androgens also promote growth and the generalized development of body mass and muscle. As a result, males are, on average, heavier, taller, and stronger than females.
In males, hormonal changes, especially those associated with the release of androgens, associated with puberty also stimulate the growth of course facial hair. Across the body, hair begins to thicken and grow, especially in the axillary (underarm), chest, back, and pubic regions of the body.
Odor can also be considered a secondary sexual characteristic. The prevalence of the use of perfume in many cultures provides evidence of the strong link between olfactory sensations and sexual selection. The changes in the endocrine system during puberty greatly increase the output of apocrine glands that are located in the axillary, anal, peritoneal, genital, and breast regions. The enhanced apocrine gland production of sebum leads to sometimes distinguishing smells for males and females. Especially in males, increased sebaceous production of oils can produce acne.
At puberty, there is a general enlargement of the penis and testicles in males and an enlargement of the vulva, labia, and clitoris in the female.
The development of secondary sexual characteristics in females is generally complete by the age of 16-18. In males, most secondary sexual characteristics are well established by age 20.
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