Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals.[1][2] These characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species,[3] but unlike the sex organs (primary sex characteristics), are not directly part of the reproductive system.[4] Secondary sex characteristics are believed to be the product of sexual selection for traits which display fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions.[5]
Secondary sex characteristics include, for example, the manes of male lions,[2] the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. These characteristics are believed to be produced by a positive feedback loop known as the Fisherian runaway produced by the secondary characteristic in one sex and the desire for that characteristic in the other sex. 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.
female secondary sex characteristics
The reproductive organs in male or female organisms are usually identifiable at birth and are ascribed as the Primary Somatic Sex Characteristics. In the male, this would be the penis, scrotum, and the ability to produce sperm that will help form a zygote. In the female, this would be the uterus, vagina, fallopian tubes, clitoris, cervix, and the ability to have offspring. The primary sex organs are different from the secondary sex organs because they produce gametes, which is a mature haploid germ cell male or female which will unite with another of the opposite sex during sexual reproduction to form a zygote. The secondary sex characteristics differ in that they will not be identifiable at birth, they will develop over time as the subject matures and becomes sexually active. Those characteristics are breast in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding.[7]In the animal kingdom, an extraordinary diversity of structures exists that cannot be explained by natural selection (Darwin 1871).[8]
Ronald Fisher, the English biologist, developed a number of ideas concerning secondary characteristics in his 1930 book The Genetical Theory of Natural Selection, including the concept of Fisherian runaway which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified. The 1975 handicap principle extends this idea, stating that a peacock's tail, for instance, displays fitness by being a useless impediment that is very hard to fake. Another of Fisher's ideas is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexually attractive in order to maximize the number of grandchildren they produce.[10] 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.[citation needed]
Secondary sex characteristics in non-human animals include manes of male lions[2] and long feathers of male peafowl, 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. [11]
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.[12]
Sexual differentiation begins during gestation, when the gonads are formed. The general structure and shape of the body and face, as well as sex hormone levels, are similar in preadolescent boys and girls. As puberty begins and sex hormone levels rise, differences appear, though some changes are similar in males and females. Male levels of testosterone directly induce the growth of the genitals, 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.[14][15][16]
Underarm hair and pubic hair are usually considered secondary sex characteristics,[6] but they may also be considered non-secondary sex characteristics because they are features of both sexes following puberty.[17]
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.[4][6] Estrogen also induces growth of the uterus, proliferation of the endometrium, and menstruation.[4] Female secondary sex characteristics include:
The increased secretion of testosterone from the testes during puberty causes the male secondary sexual characteristics to be manifested.[22] 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.[4] 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.[citation needed] Taller stature is largely a result of later puberty and slower epiphyseal fusion.[citation needed] Male secondary sex characteristics include:
The resultant increase in sex steroid production will ensure the appearance and maintenance of sexual characteristics and the capacity for reproduction. The entire endocrine system is altered during adolescence. However, it is essentially the activation of the hypothalamic-pituitary-gonadal axis that induces and enhances the progressive ovarian and testicular sex hormone secretion that are responsible for the profound biological, morphological, and psychological changes to which the adolescent is subjected.
Puberty proceeds through five stages from childhood to full maturity (P1 to P5) as described by Marshall and Tanner. In both sexes, these stages reflect the progressive modifications of the external genitalia and of sexual hair. Secondary sex characteristics appear at a mean age of 10.5 years in girls and 11.5 to 12 years in boys.
First ovulatory cycles usually occur at a median age of 9 to 10 months after menarche. However, the time sequence in the appearance of sex characteristics may vary. If breast development, pubic and/or axillary hair, and menses occur earlier than normal variations from the mean, the terms premature thelarche, pubarche and/or adrenarche, and menarche are used. Puberty is completed usually within 3 to 4 years of its onset, and the final height resulting from complete fusion of the epiphyses occurs within approximately 2 years after menarche.
The rising levels of plasma gonadotropins stimulate the ovary to produce increasing amounts of estradiol. Estradiol is responsible for the development of secondary sexual characteristics, that is, growth and development of the breasts and reproductive organs, fat redistribution (hips, breasts), and bone maturation. The maturation of the ovary at adolescence correlates well with estradiol secretion and the stages of puberty.
During puberty, plasma estradiol levels fluctuate widely, probably reflecting successive waves of follicular development that fail to reach the ovulatory stage. The uterine endometrium is affected by these changes and undergoes cycles of proliferation and regression, until a point is reached when substantial growth occurs so that withdrawal of estrogen results in the first menstruation (menarche). Plasma testosterone levels also increase at puberty although not as markedly as in males. Plasma progesterone remains at low levels even if secondary sexual characteristics have appeared. A rise in progesterone after menarche is, in general, indicative that ovulation has occured. The first ovulation does not take place until 6-9 months after menarche because the positive feedback mechanism of estrogen is not developed.
17 alpha(α)-hydroxylase/17,20-lyase deficiency is a condition that affects the function of certain hormone-producing glands called the gonads (ovaries in females and testes in males) and the adrenal glands. The gonads direct sexual development before birth and during puberty and are important for reproduction. The adrenal glands, which are located on top of the kidneys, regulate the production of certain hormones, including those that control salt levels in the body. People with 17α-hydroxylase/17,20-lyase deficiency have an imbalance of many of the hormones that are made in these glands. 17α-hydroxylase/17,20-lyase deficiency is one of a group of disorders, known as congenital adrenal hyperplasias, that impair hormone production and disrupt sexual development and maturation.
Males and females are affected by disruptions to sexual development differently. Females (who have two X chromosomes) with 17α-hydroxylase/17,20-lyase deficiency are born with normal external female genitalia; however, the internal reproductive organs, including the uterus and ovaries, may be underdeveloped. Women with complete 17α-hydroxylase/17,20-lyase deficiency do not develop secondary sex characteristics, such as breasts and pubic hair, and do not menstruate (amenorrhea). Women with partial 17α-hydroxylase/17,20-lyase deficiency may develop some secondary sex characteristics; menstruation is typically irregular or absent. Either form of the disorder results in an inability to conceive a baby (infertility). 2ff7e9595c
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