Evolution of Monogamy
Encyclopedia
Monogamous pairing in animals refers to the natural history of mating system
s in which species pair bond
to raise offspring. This is associated, usually implicitly, with sexual monogamy.
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.
The amount of social monogamy in animals varies across taxa, with over 90% of birds engaging in social monogamy while only 7% of mammals were known to do the same.
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.
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.
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
following the hypothesis of mutual destruction.
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 5 million years ago.
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.
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:
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.
Studies of primates, including humans, support the relationship between testis size and mating system. 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.
The selection factors in favor of different mating strategies for a species of animal, however, may potentially operate on a large number of factors throughout that animal's life cycle. For instance, with many species of bear, the female will often drive a male off soon after mating, and will later guard her cubs from him similar to that of other animals after they are born. It is thought that this might be due to fact that too many bears close to one another may deplete an area of food resources for the relatively small but growing cubs. Monogamy could be social but rarely genetic. Thierry Lodé
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.
Evolution topics:
Mating system
A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to higher animals, it specifies which males mate with which females, under which circumstances; recognised animal mating systems include monogamy,...
s in which species pair bond
Pair bond
In biology, a pair bond is the strong affinity that develops in some species between the males and females in a pair, potentially leading to breeding. Pair-bonding is a term coined in the 1940s that is frequently used in sociobiology and evolutionary psychology circles...
to raise offspring. This is associated, usually implicitly, with sexual monogamy.
Animals
The evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
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.
The amount of social monogamy in animals varies across taxa, with over 90% of birds engaging in social monogamy while only 7% of mammals were known to do the same.
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.
Sexual dimorphism
Sexual dimorphismSexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
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.
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
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
following the hypothesis of mutual destruction.
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
Australopithecus
Australopithecus is a genus of hominids that is now extinct. From the evidence gathered by palaeontologists and archaeologists, it appears that the Australopithecus genus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct...
, an evolutionary ancestor of human beings that lived between 2 and 5 million years ago.
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
Homo Erectus
Homo Erectus is a 2007 comedy film about cavemen that was written and directed by Adam Rifkin, and starring Giuseppe Andrews, Gary Busey, David Carradine, Ron Jeremy, Ali Larter, Hayes MacArthur, Adam Rifkin, and Talia Shire. It premiered at the 2007 Slamdance Film Festival in January 2007...
0.5 to 2 million years ago.
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. This raises questions about the amount of sexual dimorphism in Australopithecus.
- Humans may have been partially unique in that selection pressures for sexual dimorphism might have been related to the new niches that humans were entering at the time, and how that might have interacted with potential early cultures and tool use. If these early humans had a differentiation of gender roles, with men hunting and women gathering, selection pressures in favor of increased size may have been distributed unequally between the sexes.
- Even if future studies clearly establish sexual dimorphism in Australopithecus, other studies have shown the relationship between sexual dimorphism and mating system is unreliable. 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. 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. 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.The selection factors in favor of different mating strategies for a species of animal, however, may potentially operate on a large number of factors throughout that animal's life cycle. For instance, with many species of bear, the female will often drive a male off soon after mating, and will later guard her cubs from him similar to that of other animals after they are born. It is thought that this might be due to fact that too many bears close to one another may deplete an area of food resources for the relatively small but growing cubs. Monogamy could be social but rarely genetic. Thierry Lodé
Thierry Lodé
Thierry Lodé is a French biologist and professor who teaches evolutionary ecology in the CNRS Units ETHOS...
argued that monogamy should result from conflict of interest between the sexes called sexual conflict
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
. 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.
See also
Monogamy topics:- MonogamyMonogamyMonogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...
- Varieties of monogamy
- Incidence of monogamy
- Value of monogamyValue of MonogamyThe value of monogamy refers to people's views about the contributions monogamy makes, good or bad, to individual and social well-being.Some cultures value monogamy as an ideal form of family organization. However, many cultures prefer other forms of family organization...
- Psychology of monogamyPsychology of MonogamyThe psychology of sexual monogamy deals with the thoughts, feelings, and behaviors of human beings in sexually monogamous relationships.Psychological studies of sexual monogamy are sparse...
Evolution topics:
- EvolutionEvolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
- Evolution of sexEvolution of sexThe evolution of sexual reproduction is currently described by several competing scientific hypotheses. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi...
- Animal sexualityAnimal sexualityAnimal sexual behaviour takes many different forms, even within the same species. Among animals other than humans, researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, sex apparently via duress or coercion, copulation with dead animals,...
- Human evolutionHuman evolutionHuman evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...
- History of human sexualityHistory of human sexualityThe social construction of sexual behavior—its taboos, regulation and social and political impact—has had a profound effect on the various cultures of the world since prehistoric times.- Sources :...
- r/K selection theoryR/K selection theoryIn ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...
- Sexual conflictSexual conflictSexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...