Evolution of Monogamy

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The evolution of monogamy refers to the natural history of mating systems in which species reproduce by forming pairs to raise offspring.

Contents

[edit] Animals

The evolution of mating systems in animals has received an enormous amount of attention from biologists. This section briefly reviews three main findings about the evolution of monogamy in animals.

This list is not complete. Other factors may also contribute to the evolution of social monogamy. Moreover, different sets of factors may explain the evolution of social monogamy in different species. There is no one-size-fits-all explanation of why different species evolved monogamous mating systems.


[edit] Sexual Dimorphism

Sexual dimorphism refers to differences in body characteristics between males and females. A frequently studied type of sexual dimorphism is body size. Males typically have larger bodies than females. In some species, however, females have larger bodies than males. Sexual dimorphism in body size has been linked to mating behavior. [1] [2] [3] [4] In polygamous species, males compete for control over sexual access to females. Large males have an advantage in the competition for access to females, and they consequently pass their genes along to a greater number of offspring. This eventually leads to large differences in body size between males and females. Polygamous males are often 1.5 to 2.0 times larger in size than females. In monogamous species, on the other hand, males and females have more equal access to mates, so there is little or no sexual dimorphism in body size.

Some researchers have attempted to infer the evolution of human mating systems from the evolution of sexual dimorphism. Several studies have reported a large amount of sexual dimorphism in Australopithecus, an evolutionary ancestor of human beings that lived between 2 and 4 million years ago. [2] [3] [5] [6] These studies raise the possibility that Australopithecus had a polygamous mating system. Sexual dimorphism then began to decrease. Studies suggest sexual dimorphism reached modern human levels around the time of Homo Erectus 0.5 to 2 million years ago. [2] [3] [5] [7] This line of reasoning suggests human ancestors started out polygamous and began the transition to monogamy somewhere between 0.5 million and 2 million years ago.

Attempts to infer the evolution of monogamy based on sexual dimorphism remain controversial for three reasons:

  • The skeletal remains of Australopithecus are quite fragmentary. This makes it difficult to identify the sex of the fossils. Researchers sometimes identify the sex of the fossils by their size, which, of course, can exaggerate findings of sexual dimorphism.
  • Recent studies using new methods of measurement suggest Australopithecus had the same amount of sexual dimorphism as modern humans. [8][9] This raises questions about the amount of sexual dimorphism in Australopithecus.
  • Even if future studies clearly establish sexual dimorphism in Australopithecus, other studies have shown the relationship between sexual dimorphism and mating system is unreliable. [1] [2] Some polygamous species show little or no sexual dimorphism. Some monogamous species show a large amount of sexual dimorphism.

Studies of sexual dimorphism raise the possibility that early human ancestors were polygamous rather than monogamous. But this line of research remains highly controversial. It may be that early human ancestors showed little sexual dimorphism, and it may be that sexual dimorphism in early human ancestors had no relationship to their mating systems.

[edit] Testis Size

The relative sizes of male testes often reflect mating systems. [10] [11] [12] [13] In species with promiscuous mating systems, where many males mate with many females, the testes tend to be relatively large. This appears to be the result of sperm competition. Males with large testes produce more sperm and thereby gain an advantage impregnating females. In polygynous species, where one male controls sexual access to females, the testes tend to be small. One male defends exclusive sexual access to a group of females and thereby eliminates sperm competition.

Studies of primates, including humans, support the relationship between testis size and mating system. [12] [13] [14] Chimpanzees, which have a promiscuous mating system, have large testes compared to other primates. Gorillas, which have a polygynous mating system, have smaller testes than other primates. Humans, which have a socially monogamous mating system, accompanied by moderate amounts of sexual non-monogamy, have moderately sized testes. The moderate amounts of sexual non-monogamy in humans may result in a low to moderate amount of sperm competition.

Although testis size in humans is consistent with the modern pattern of social monogamy accompanied by moderate sexual non-monogamy, this fact reveals little about when the modern pattern evolved. Did Homo Erectus have testes similar in size to modern humans? What about Australopithecus? It is not possible to measure the size of testes in the fossil remains of human ancestors. This limits the usefulness of testis size in understanding the evolution of monogamy in humans.

[edit] Monogamy as a Best Response

In species where the young are particularly vulnerable and may benefit from protection by both parents, monogamy may be an optimal strategy for males [15].

  • Spending more time with a single female maximizes the chance that he fathers her children. If he supervises those children, there is a high probability he is protecting his own offspring and not someone else's.
  • If he only has one partner all his young will be in the same geographical area and therefore easier to supervise.

Neither of these arguments can readily explain monogamous behavior by females.

[edit] Cultural Evolution

Culture has clearly increased the incidence of social monogamy. Many modern cultures have passed laws making social monogamy the only legal form of marriage. The passage of such laws in many cases reflects the spread of Christianity. However, in recent years, international organizations such as the United Nations and the African Union have started to promote social monogamy as a way to give women and men equal rights in marriage.

The United Nations started to promote social monogamy as the preferred form of marriage in 1979 when the General Assembly adopted the Convention on the Elimination of All Forms of Discrimination Against Women, an international bill of rights for women that over 180 nations have agreed to implement. Article 16 of the Convention requires nations to give women and men equal rights in marriage. Polygamy is interpreted as inconsistent with Article 16 because it extends the right of multiple spouses to men but not to women. The United Nations has established the Committee on the Elimination of Discrimination against Women, or CEDAW, to monitor the progress of nations implementing the Convention. The United Nations is thus working through the Convention and CEDAW to promote women's equality by making monogamy the only legal form of marriage worldwide.

The African Union has recently adopted the Protocol on the Rights of Women in Africa. While the protocol does not suggest making polygamous marriage illegal, article 6 of the protocol states that "monogamy is encouraged as the preferred form of marriage and that the rights of women in marriage and family, including in polygamous marital relationships are promoted and protected." [16][17] The protocol entered into force November 25, 2005.

Readers interested in the cultural evolution of monogamy may wish to visit the Wikipedia article on the history of human sexuality.

[edit] References

  1. ^ a b Owens, I.P.F. & Hartley, I.R. (1998). Sexual dimorphism in birds: why are there so many different forms of dimorphism? Proceedings of the Royal Society of London B, 265, 397-407.
  2. ^ a b c d Frayer, D.W. & Wolpoff, M.H. (1985). Sexual dimorphism. Annual Review of Anthropology, 14, 429-473.
  3. ^ a b c Geary, D.C., & Flinn, M.V. (2001). Evolution of human parental behavior and the human family. Parenting: Science and Practice, 1, 5-61.
  4. ^ Dunn, P.O., Whittingham, L.A., & Pitcher, T.E. (2001). Mating systems, sperm competition, and the evolution of sexual dimorphism in birds. Evolution, 55, 161–175.
  5. ^ a b Flinn, M.V. & Ward, C.V. (2004). Ontogeny and Evolution of the Social Child. In: Origins of the social mind: Evolutionary psychology and child development, B. Ellis & D. Bjorklund (Eds.), chapter 2, pp. 19-44. London: Guilford Press.
  6. ^ Lockwood, C.A., Richmond, B.G., Jungers, W.L., & Kimbel, W.H. (1996). Randomization procedures and sexual dimorphism in Australopithecus afarensis. Journal of Human Evolution, 31, 537-548.
  7. ^ Arsuaga, J.L., Carretero, J.M., Lorenzo, C., Gracia, A., Martínez, I., Bermúdez de Castro, J.M., & Carbonell, E. (1997). Size variation in Middle Pleistocene humans. Science, 277, 1086-1088.
  8. ^ Reno, P.L., Meindl, R.S., McCollum, M.A., & Lovejoy, C.O. (2003). Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans. Proceedings of the National Academy of Sciences, 100, 9404-9409.
  9. ^ Larsen, C.S. (2003). Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior. Proceedings of the National Academy of Sciences, 100, 9103-9104.
  10. ^ Pitcher, T.E., Dunn, P.O., & Whittingham, L.A. (2005). Sperm competition and the evolution of testes size in birds. Journal of Evolutionary Biology, 18, 557–567.
  11. ^ Simmons, L.W., Firman, R.E.C., Rhodes, G., & Peters, M. (2004). Human sperm competition: testis size, sperm production and rates of extrapair copulations. Animal Behaviour, 68, 297-302.
  12. ^ a b Dixson, A., & Anderson, M. (2001). Sexual selection and the comparative anatomy of reproduction in monkeys, apes, and human beings. Annual Review of Sex Research, 12, 121-144.
  13. ^ a b Harcourt, A.H., Harvey, P.H., Larson, S.G., & Short, R.V. (1981). Testis weight, body weight and breeding system in primates. Nature, 293, 55-57.
  14. ^ T. R. Birkhead (2000), Promiscuity: an evolutionary history of sperm competition. Harvard University Press, Cambridge, Mass.
  15. ^ http://www.philosophytalk.org/pastShows/MarriageandMonogamy.html
  16. ^ Amnesty International, 2006. The Protocol on the Rights of Women in Africa: Strengthening the promotion and protection of women’s human rights in Africa. Retrieved May 29, 2006 from http://web.amnesty.org/library/Index/ENGIOR630052004 .
  17. ^ University of Minnesota Human Rights Library, 2006. Protocol to the African Charter on Human and Peoples' Rights on the Rights of Women in Africa. Retrieved May 29, 2006 from http://www1.umn.edu/humanrts/africa/protocol- women2003.html .

[edit] Bibliography

  • Korotayev, Andrey (2004). World Religions and Social Evolution of the Old World Oikumene Civilizations: A Cross-cultural Perspective, First Edition, Lewiston, New York: Edwin Mellen Press. ISBN 0-7734-6310-0. 

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