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Secondary sex characteristic

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A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system.[1] In humans, these characteristics typically start to appear during puberty—and include enlarged breasts and widened hips of females, facial hair and Adam's apples on males, and pubic hair on both.[1][2] In non-human animals, they can start to appear at sexual maturity[3]—and include, for example, the manes of male lions,[4] the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes.

A peacock displays his long, colored tail, an example of secondary sex characteristics.
An adult human's Adam's apple, a visible secondary sex characteristic common in males.

Secondary sex characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species.[5] In evolution, secondary sex characteristics are the product of sexual selection for traits that show fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions.[6]

Many characteristics are believed to have been established 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.

Secondary sex characteristics vs. primary sex characteristics

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The reproductive organs in male or female mammals that are usually identifiable at birth are described as the primary sex characteristics or sex organs. In the male, these are the penis, testes, scrotum. In the female, these are the ovaries, fallopian tubes, uterus, cervix, vagina and vulva. The primary sex organs are different from the secondary sex organs because at maturity they produce gametes, which are haploid male or female germ cells which can 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 as the subject becomes sexually mature. In mammals, these characteristics include breasts in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding.[7]

Evolutionary roots

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See also: Mate choice
 
A red deer stag's antlers are secondary sexual characteristics.

In The Descent of Man and Selection in Relation to Sex, Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species.[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.[9] 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]

In non-human animals

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Examples of secondary sex characteristics in non-human animals include manes of male lions[4] 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, horns in many goats and antelopes,[10] and the swollen upper lip and elongated premaxillary and maxillary teeth of male spikethumb frogs.[11] Male fish develop "nuptial tubercles",[12] mainly on their snouts, in the breeding season. These are an honest signal of health, and may assist females in sexual selection for species that use lek mating, such as the roach Rutilus rutilus.[13]

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.[14]

Male jumping spiders have visual patches of UV reflectance, which are ornamentations used to attract females.[15]

In humans

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Anatomical characteristics of the human female and male
Main article: Sexual selection in human evolution

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.[16][17][18]

Underarm hair and pubic hair are usually considered secondary sex characteristics,[2] but they may also be considered non-secondary sex characteristics because they are features of both sexes following puberty.[19]

Females

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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.[1][2] Estrogen also induces growth of the uterus, proliferation of the endometrium, and menstruation.[1] Female secondary sex characteristics include:

Males

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The increased secretion of testosterone from the testes during puberty causes the male secondary sexual characteristics to be manifested.[23] Testosterone directly increases size and mass of muscles, vocal cords, and bones, deepening of the voice, and changing the shape of the face and skeleton.[1] 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] Overall, male secondary sex characteristics include:

References

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  1. ^ a b c d e f g h i j k l m n Bjorklund DF, Blasi CH (2011). Child and Adolescent Development: An Integrated Approach. Cengage Learning. pp. 152–153. ISBN 978-1133168379.
  2. ^ a b c d e f Rizzo DC (2015). Fundamentals of Anatomy and Physiology. Cengage Learning. pp. 483–484. ISBN 978-1305445420. Archived from the original on April 9, 2023. Retrieved October 14, 2019.
  3. ^ Melmed S, Polonsky KS, Larsen PR, Kronenberg HM (2011). Williams Textbook of Endocrinology E-Book. Elsevier Health Sciences. p. 1054. ISBN 978-1437736007.
  4. ^ a b c d Pack PE (2016). CliffsNotes AP Biology (5th ed.). Houghton Mifflin Harcourt. p. 219. ISBN 978-0544784178.
  5. ^ Norris DO, Lopez KH (2010). Hormones and Reproduction of Vertebrates, Volume 1. Academic Press. p. 87. ISBN 978-0080958095. Archived from the original on April 9, 2023. Retrieved October 14, 2019.
  6. ^ Campbell B (2017). Human Evolution: An Introduction to Man's Adaptations. Routledge. pp. 392–393. ISBN 978-1351514415. Archived from the original on April 9, 2023. Retrieved October 14, 2019.
  7. ^ Keathley, Christina (July 27, 2021). "Types of Sex Characteristics". study.com. Archived from the original on October 26, 2022. Retrieved October 26, 2022.
  8. ^ "Sexual selection" Archived September 1, 2022, at the Wayback Machine. Darwin Correspondence Project. University of Cambridge. Retrieved 1 September 2022.
  9. ^ Weatherhead PJ, Robertson RJ (February 1979). "Offspring quality and the polygyny threshold: "The sexy son hypothesis"". American Naturalist. 113 (2): 201–208. doi:10.1086/283379. S2CID 85283084.
  10. ^ "Primary and Secondary Sex Characteristics". Archived from the original on August 3, 2020. Retrieved August 14, 2020.
  11. ^ Duellman, W.E.; Campbell, J.A. (1992). "Hylid frogs of the genus Plectrohyla: systematics and phylogenetic relationships" (PDF). Museum of Zoology, University of Michigan (181).
  12. ^ Froese, Rainer; Pauly, Daniel (eds.). "Glossary Search for nuptial tubercle". FishBase glossary.
  13. ^ Raine Kortet; Jouni Taskinen; Anssi Vainikka & Hannu Ylönen (August 2004). "Breeding Tubercles, Papillomatosis and Dominance Behaviour of Male Roach (Rutilus rutilus) During the Spawning Period". Ethology. 110 (8): 591–601. Bibcode:2004Ethol.110..591K. doi:10.1111/j.1439-0310.2004.01002.x.
  14. ^ ^ Ronald Fisher in a letter to Charles Galton Darwin, 22 November 1932, cited in Fisher, R. A., Bennett, J. H. 1999. The genetical theory of natural selection: A complete variorum edition, Oxford University Press, Oxford, p. 308
  15. ^ Lim, Matthew L. M., and Daiqin Li. "Courtship and Male-Male Agonistic Behaviour of Comsophasis Umbratica Simon, an Ornate Jumping Spider (Araneae: Salticidae)." The Raffles Bulletin of Zoology (2004): 52(2): 435-448. National University of Singapore. 20 September 2015
  16. ^ G. Raspé (October 22, 2013). Hormones and Embryonic Development: Advances in the Biosciences. Elsevier Science. pp. 32–. ISBN 978-1-4831-5171-7. Archived from the original on April 9, 2023. Retrieved March 18, 2018. The first genetic male child with a defect in 3β-hydroxy-Δ5-steroid oxidoreductase to have reached puberty has been reported to have a high level of 3β-hydroxy-A5 steroid excretion, hypospadias at birth, salt-wasting, and a history of two siblings with congenital adrenal hyperplasia and ambiguous genitalia [33]. Although at puberty he has signs of virilization, he has developed pronounced gynecomastia. Thus, this boy demonstrates that breast development may occur in postpubertal males if the programing of the pubertal sex differentiation of the mammary gland anlagen is disturbed by an enzyme defect which causes a failure of fetal testicular testosterone production. This observation is completely consistent with the findings in the experimental models [11, 32].
  17. ^ Neumann F, Elger W (March 1967). "Steroidal stimulation of mammary glands in prenatally feminized male rats". Eur. J. Pharmacol. 1 (2): 120–3. doi:10.1016/0014-2999(67)90048-9. PMID 6070056. When administered to gravid rats during pregnancy an anti-androgenic steroid [cyproterone acetate] induced development of nipples in male fetuses. These nipples and associated glandular tissues develop after birth as in normal female animals. Progestin-estrogen treatment of adult, castrated feminized males produced stimulation of the glandular tissue similar to that seen after treatment of castrated female animals. In castrated male rats this treatment produces little glandular proliferation. It is concluded that androgens normally prevent the development of nipples and extensive formation of mammary tissue in male fetuses.
  18. ^ Rajendran KG, Shah PN, Dubey AK, Bagli NP (1977). "Mammary gland differentiation in adult male rat--effect of prenatal exposure to cyproterone acetate". Endocr Res Commun. 4 (5): 267–74. doi:10.3109/07435807709052946. PMID 608453. Breast tissue of adult male Holtzman rats exposed to cyproterone acetate during embryonic differentiation showed presence of specific estradiol receptor proteins and C-19 steroid aromatase. We reported similar findings in gynecomastia in man. It is therefore proposed that gynecomastia probably results from failure of adequate testosterone action on the breast primordia during embryonic differentiation.
  19. ^ Sherwood L (2011). Fundamentals of Human Physiology. Cengage Learning. p. 578. ISBN 978-0840062253. Archived from the original on April 9, 2023. Retrieved October 14, 2019.
  20. ^ Buss, David (March 15, 2019). Evolutionary Psychology: The New Science of the Mind (Sixth ed.). Routledge. p. 290. ISBN 9780429590061. Archived from the original on December 2, 2021. Retrieved December 2, 2021.
  21. ^ Manwatching: A Field Guide to Human Behaviour, 1977, Desmond Morris
  22. ^ Lloyd, Jillian (May 2005). "Female genital appearance: 'normality' unfolds". British Journal of Obstetrics and Gynaecology. 12 (5): 643–646. CiteSeerX 10.1.1.585.1427. doi:10.1111/j.1471-0528.2004.00517.x. PMID 15842291. S2CID 17818072.
  23. ^ Van de Graaff, Kent M.; Fox, Stuart Ira (1989). Concepts of Human Anatomy and Physiology. Dubuque, Iowa: William C. Brown Publishers. pp. 933–4. ISBN 0697056759.
  24. ^ a b Sexual reproduction Archived 2009-02-08 at the Wayback Machine
  25. ^ "Secondary Characteristics". hu-berlin.de. Archived from the original on September 27, 2011.


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