Secondary sexual characteristic

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File:Peacock.displaying.better.800pix.jpg
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 distinguished from 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.

In humans

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, 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. 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.

File:Woman labeled.jpg
female sex characteristics
File:Man labeled.jpg
male sex characteristics

In humans, secondary sex characteristics include:

See also

Notes

References

"Sexual Maturity." Technical Issues in Reproductive Health. Columbia University. May 2, 2008. <http://www.columbia.edu/itc/hs/pubhealth/modules/reproductiveHealth/anatomy.html>.

Steinhardt, A. "Sexual Reproduction." Hartnell College. May 2, 2008. <http://www.hartnell.edu/faculty/asteinhardt/>.

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