Evolution of monogamy

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

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.

exual 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.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.] Frayer, D.W. & Wolpoff, M.H. (1985). "Sexual dimorphism". "Annual Review of Anthropology", 14, 429-473.] Geary, D.C., & Flinn, M.V. (2001). "Evolution of human parental behavior and the human family". "Parenting: Science and Practice", 1, 5-61.] 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.] In polygynous 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. Polygynous 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. From a new biological point of view, monogamy could result from mate guarding and is engaged as a result of sexual conflict [T Lodé “la guerre des sexes chez les animaux” Eds O Jacob, Paris, 2006, ISBN 2-7381-1901-8] following the hypothesis of mutual destruction. Very few animals are proved to be monogamous and are constrained to be monogamous by habitat pressures (see penguins).

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.Frayer, D.W. & Wolpoff, M.H. (1985). "Sexual dimorphism". "Annual Review of Anthropology", 14, 429-473.] Geary, D.C., & Flinn, M.V. (2001). "Evolution of human parental behavior and the human family". "Parenting: Science and Practice", 1, 5-61.] 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.] 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.] 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.Frayer, D.W. & Wolpoff, M.H. (1985). "Sexual dimorphism". "Annual Review of Anthropology", 14, 429-473.] Geary, D.C., & Flinn, M.V. (2001). "Evolution of human parental behavior and the human family". "Parenting: Science and Practice", 1, 5-61.] 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.] 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.] 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. 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.] 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.] 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. 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", London, B265, 397–407.] Frayer, D.W. & Wolpoff, M.H. (1985). "Sexual dimorphism". "Annual Review of Anthropology", 14, 429-473.] 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.

Testis size

The relative sizes of male testes often reflect mating systems. 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.] 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.] 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.] 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.] 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. 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.] 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.] T. R. Birkhead (2000), "Promiscuity: an evolutionary history of sperm competition". Harvard University Press, Cambridge, Mass.] 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" (see incidence of 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. Also, notably, in the case of an avowedly sexually monogamous society, the occurrence of sexual nonmonogamy is typically culturally stigmatized, and therefore detecting its prevalence is inherently difficult, if indeed it is at all possible. At best, such statistics can be viewed as general approximations with a wide margin of error.

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.

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.Monogamy could be social but rarely genetic. Thierry Lodé [Thierry Lodé "La Guerre des sexes chez les animaux" Eds O Jacob, Paris, 2006] argued that monogamy should result from conflict of interest between the sexes called sexual conflict. Organized from territory defense and mate guarding, monogamy appears as a response of male for the control of female sexuality, but exclusive monogamy would be rare and the biological evolution would privilege the diversity of sexual behaviors.

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. Christianity itself picked its value from Greco-Roman cultureFact|date=May 2008, though it's not clear why they had it .

ee also

Monogamy topics

* Monogamy
* Varieties of monogamy
* Incidence of monogamy
* Value of monogamy
* Psychology of monogamy

Evolution topics

* Evolution
* Evolution of sex
* Animal sexuality
* Human evolution
* History of human sexuality
* sexual conflict

References

Bibliography

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