Secondary sex characteristic explained

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 be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship, and aggressive interactions. They are distinguished from the primary sexual characteristics: the sex organs, which are directly necessary for reproduction to occur.

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

In humans

Sexual differentiation begins during gestation, when the gonads are formed. 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, differences appear, though puberty causes some similar changes in male and female bodies.

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, but may slow and eventually stop the growth of head hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion.

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 menses.

In humans, secondary sex characteristics include:

• Male
  • Growth of body hair, including underarm, abdominal, chest, and pubic hair
  • Greater mass of thigh muscles in front of the femur, rather than behind it as is typical in mature females
  • Growth of facial hair
  • Enlargement of larynx and deepening of voice^[1]
  • Increased stature; adult males 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^[2]
  • Increased secretions of oil and sweat glands, often causing acne and body odor ^[1]
  • Coarsening or rigidity of skin texture, due to less subcutaneous fat
  • A prominent Adam's apple
  • Fat deposits mainly around the abdomen and waist
  • Higher waist to hip ratio than prepubescent or adult females or prepubescent males, on average
  • On average, larger hands and feet than prepubescent or adult females or prepubescent males
  • Lower digit ratio, on average
• Female
  • Enlargement of breasts^[2]
  • Growth of body hair, most prominently underarm and pubic hair
  • Greater development of thigh muscles in back (behind the femur) than in front of it
  • Vaginal and uterine growth ^[3]
  • Decreased stature; adult females shorter stature than adult males, on average
  • Widening of hips^[3] ; lower waist to hip ratio than adult males, on average
  • Decreased secretions of oil and sweat glands, causing less acne and body odor
  • Changed distribution in weight and fat; more subcutaneous fat and fat deposits mainly around the buttocks, thighs and hips
  • Higher digit ratio, on average

See also

• Sexual selection in human evolution
• Puberty
• Virilization
• Estrogenic fat
• Sex differences in humans
• Sexual dimorphism
• Body shape
• Female body shape

References

"Sexual Maturity." Technical Issues in Reproductive Health. Columbia University. May 2, 2008. .

Steinhardt, A. "Sexual Reproduction." Hartnell College. May 2, 2008. .

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

Notes and References

1. http://www.hartnell.edu/faculty/asteinhardt/mywebs/sexual_reproduction.htm Sexual reproduction
2. http://www2.hu-berlin.de/sexology/ATLAS_EN/html/secondary_characteristics.html The Secondary Sexual Characteristics
3. http://www.columbia.edu/itc/hs/pubhealth/modules/reproductiveHealth/anatomy.html Reproductive Anatomy and Physiology