Secondary sex characteristic
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. 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 male 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.
Evolutionary roots 
Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species. 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. 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.
- 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 (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
- Larger hands and feet than women, prepubescent boys, and girls
- Larger bodies
- Square face
- Small waist, but wider than females
- Broadening of shoulders and chest; shoulders wider than hips
- Increased secretions of oil and sweat glands, often causing acne and body odor
- Coarsening or rigidity of skin texture due to less subcutaneous fat
- Higher waist-to-hip ratio than prepubescent or adult females or prepubescent males, on average
- Lower bodyfat percentage than prepubescent or adult females or prepubescent males, on average
- Enlargement (growth) of the penis
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.
- Enlargement of breasts and erection of nipples.
- Growth of body hair, most prominently underarm and pubic hair
- Greater development of thigh muscles behind the femur, rather than in front of it
- Widening of hips; lower waist to hip ratio than adult males
- Smaller hands and feet than men
- Elbows that extend further than 180 degrees
- Rounder face
- Smaller waist than men
- Upper arms approximately 2 cm longer, on average, for a given height
- Changed distribution in weight and fat; more subcutaneous fat and fat deposits, mainly around the buttocks, thighs, and hips
See also 
- Body shape
- Estrogenic fat
- Female body shape
- Menstrual cycle
- Sex differences in humans
- Sexual dimorphism
- Darwin, C. (1871) The Descent of Man and Selection in Relation to Sex John Murray, London
- 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.
- Sexual reproduction
- The Secondary Sexual Characteristics, Magnus Hirschfeld Archive of Sexology
- Reproductive Anatomy and Physiology, Technical Issues In Reproductive Health, Columbia Mailman School of Public Health
- Manwatching: A Field Guide to Human Behaviour, 1977, Desmond Morris
- "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. London: Vintage. ISBN 978-0-09-928375-1.