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Secondary sex characteristics are traits that distinguish the two sexes of a species, but that are not directly part of the reproductive system. Some have argued that in general they evolved to give an individual an advantage over its rivals in courtship. They are opposed to the primary sexual characteristics: the sex organs.
Sexual differentiation begins during gestation, when the gonads form. General habitus and shape of body and face, as well as sex hormone levels, are similar in prepubertal boys and girls. As puberty progresses and sex hormone levels rise, obvious differences appear.
Male levels of testosterone directly induce growth of the penis, and indirectly (via Dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause breasts to develop in females. However, fetal or neonatal androgens may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen.
In males, testosterone directly increases size and mass of muscles, vocal cords, and bones, enhancing strength, deepening the voice, and changing the shape of the face and skeleton. Converted into DHT in the skin, it accelerates growth of androgen-responsive facial and body hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion.
In females, breasts are the most obvious manifestation of higher levels of estrogen; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, and menses.
In humans, secondary sex characteristics include:
- abdominal and chest hair
- more hair on other parts of body
- more facial hair
- on average, larger hands and feet than women
- broader shoulders and chest
- heavier skull and bone structure
- greater muscle mass and physical strength
- a prominent Adam's apple and deep voice
- fat deposits mainly around the abdomen and waist ("apple shape")
- coarser skin texture
- Ring finger is normally longer than Index finger
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