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A peacock displays his long, colored feathers, an example of his secondary sexual characteristics.

Secondary sex characteristics are traits that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to have evolved to give an individual an advantage over its rivals in courtship. They are opposed to the primary sexual characteristics: the sex organs.

Well known secondary sex characteristics include facial hairs of male lions, and long feathers of peacock. In humans, the most visible are breasts of females and beard and moustache of males. Secondary sex characteristics include the tusks of sea lions, the plumage of many male birds, the chemical indicators of many insects, etc.

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 testicles and 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, 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.

female sex characteristics

male sex characteristics

In humans, secondary sex characteristics include:

• Male
  • abdominal and chest hair, also more hair on other parts of body
  • more facial hair
  • taller stature, on average
  • higher waist to hip ratio, on average
  • broader shoulders and chest, wider than hips^[1]
  • heavier skull and bone structure
  • greater muscle mass for the same degree of strength (women can be as strong or stronger than men if they engage in physical activity, but their muscles will not grow as large)
  • a prominent Adam's apple and deep voice
  • fat deposits mainly around the abdomen and waist^{[citation needed]} ("apple shape")
  • coarser skin texture
  • on average, larger hands and feet than women^{[citation needed]}
  • lower digit ratio, on average^{[citation needed]}

• Female
  • breasts^[2]
  • shorter stature, on average
  • lower waist to hip ratio, on average
  • wider in hips^[3]
  • less facial hair
  • higher voice
  • more subcutaneous fat
  • fat deposits mainly around the buttocks, thighs and hips ("pear shape")
  • smoother skin texture
  • higher digit ratio, on average^{[citation needed]}

• Puberty
• Virilization
• Sex differences in humans
• Sexual dimorphism

• http://www.columbia.edu/itc/hs/pubhealth/modules/reproductiveHealth/anatomy.html