the capacity of breast tissue to respond to later estrogen. Underarm hair and pubic hair are usually considered secondary sex characteristics, but they may also be considered non-secondary sex characteristics because they are features of both sexes following puberty. Females In females, breasts are a 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 menstruation. Female secondary sex characteristics include: Enlargement of breasts and erection of nipples. Growth of body hair, most prominently underarm and pubic hair. Widening of hips; lower waist to hip ratio than adult males. Elbows that hyperextend 5–8° more than male adults. Upper arms approximately 2 cm longer, on average, for a given height. Labia minora, the inner lips of the vulva, may grow more prominent and undergo changes in color with the increased stimulation related to higher levels of estrogen. Males The increased secretion of testosterone from the testes during puberty causes the male secondary sexual characteristics to be manifested. Testosterone directly increases size and mass of muscles, vocal cords, and bones, 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 but may slow and eventually stop the growth of head hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion. Overall, male secondary sex characteristics include: Growth of body hair, including underarm, abdominal, chest hair and pubic hair. Growth of facial hair. Enlargement of larynx (Adam's apple) and deepening of voice. Increased stature; adult males are taller than adult females, on average. Heavier skull and bone structure. Increased muscle mass and strength. Broadening of shoulders and chest; shoulders wider than hips. Increased secretions of oil and sweat glands. References Sexual selection