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

Secondary sex characteristics are features 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.[citation needed] They are distinguished from the primary sex 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. Other dramatic examples include the tusks of male narwhals, enlarged proboscises in male elephant seals and proboscis monkeys, the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. Male birds and fish of many species have brighter coloration or other external ornaments. Differences in size between sexes are also considered secondary sexual characteristics.

In humans, visible secondary sex characteristics include enlarged breasts of females and facial hair on males.

Contents

Evolutionary roots

Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right.

Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species.[1] Biologists today distinguish between "male to male combat" and "mate choice", usually female choice of male mates. Sexual characteristics due to combat are such things as antlers, horns and greater size. Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration and other features that have no immediate purpose for survival or combat.

Ornamentation might arise because of some arbitrary female preference that is initially amplified by random genetic drift, eventually being reinforced by active selection for males with the appropriate ornament. This is known as the sexy son hypothesis.[2] An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism. This idea is known as the good genes hypothesis.

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, menses, and makes the skin clearer and gives it a pink to red hue (increases pheomelanin, reduces eumelanin).

Androgenic hair growth, pubic hair, in males and females.

In humans, secondary sex characteristics include:

  • Male
    • Growth of body hair, including underarm, abdominal, chest, and pubic hair. Loss of scalp hair androgenic alopecia can also occur
    • 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[3]
    • 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[4]
    • Increased secretions of oil and sweat glands, often causing acne and body odor [3]
    • Coarsening or rigidity of skin texture, due to less subcutaneous fat
    • A prominent Adam's apple
    • Fat deposits mainly around the abdomen and waist[citation needed]
    • 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[citation needed]
    • Lower digit ratio, on average[citation needed]

See also

Notes

  1. ^ Darwin, C. (1871) The Descent of Man and Selection in Relation to Sex John Murray, London
  2. ^ Weatherhead PJ, Robertson RJ (Feb 1979). "Offspring quality and the polygyny threshold: 'The sexy son hypothesis'". Am Nat. 113 (2): 201–8. doi:10.1086/283379. 
  3. ^ a b Sexual reproduction
  4. ^ a b The Secondary Sexual Characteristics, Magnus Hirschfeld Archive of Sexology
  5. ^ Cite error: Invalid <ref> tag; no text was provided for refs named reproductive_anatomy; see Help:Cite error.
  6. ^ Manwatching a Field Guide to Human Behaviour, 1977, Desmond Morris

References

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

Judson, Olivia (2003) Dr.Tatiana's Sex Advice to All Creation: Definitive Guide to the Evolutionary Biology of Sex. ISBN: 978-0099283751 Steinhardt, A. "Sexual Reproduction." Hartnell College. May 2, 2008.








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