Monogamous pairing in animals

Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.

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.

The amount of social monogamy in animals varies across taxa, with over 90% of birds engaging in social monogamy while only 3% 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 dimorphism refers to differences in body characteristics between females and males. 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 females and males. Polygynous males are often 1.5 to 2.0 times larger in size than females. In monogamous species, on the other hand, femaes and males 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.
 * 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.

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. It is thought that this may be due to fact that too many bears close to one another may deplete the food available to the relatively small but growing cubs. Monogamy may be social but rarely genetic. Thierry Lodé argued that monogamy should result from conflict of interest between the sexes called sexual conflict. Organized for territorial defense and mate guarding, monogamy appears as a male attempt to control female sexuality, but exclusive monogamy would be rare and biological evolution would privilege the diversity of sexual behaviour.