Monogamy (from ancient Greek μόνος mónos "alone, unique" and γάμος gamos "marriage", German "monogamy" ) describes a lifelong exclusive reproductive community between two individuals of a species . In humans, mating behavior is not necessarily associated with reproduction. In contrast to monogamy are patterns of polygamy (in humans and polygamy ) such. B. polygamy , polyamory or polyamory . In polyandry or polygyny , only one of the partners can have other sexual partners while the other partner behaves monogamous. With humans, since the increasing social acceptance of same-sex partnerships, one can speak of monogamy here too, provided that these represent a relationship of loyalty .
Biological considerations on monogamy
In biology, a distinction is made between social and sexual monogamy. In social monogamy, the individuals raise the young as a couple, but can have additional sexual contacts. Only a few mammalian species are socially monogamous, at least during the rearing phases , but more than 90 percent of all birds. 14% of birds are genetically monogamous. In some species such as B. the Grevy's zebra , after the birth of one or more young animals , females change from polyandric to monogamous behavior for a limited time to a male conspecific who has a territory . Genetic studies suggest that the coelacanth has a monogamous mating behavior.
Sexual conflicts can lead to monogamous behavior if one of the sexes succeeds in restricting the sexual behavior of the opposite sex and in forcing monoandric or monogynous behavior.
The size of the testicles and the amount of sperm produced is often associated with mating systems. For example, in bird species that breed in colonies or in which the male does not take part in rearing young, the testes are larger than in solitary breeding species. Investigations on z. B. Onthophagus taurus and Drosophila melanogaster show that monogamy reduces testes and spermatogenesis in the generation sequence. Even primate species, which are subject to sperm competition due to polyandric behavior , produce larger amounts of sperm and have larger testes relative to body mass than polygynous species.
According to one hypothesis, the female reproductive tract in mammals and birds prevents fertilization by males who are not fit enough, among other things through the vaginal pH value.
Red-billed gull females studied in New Zealand behave monogamous if they have been well fed with food by the male during advertising and then mate with the same male again in the following year. Badly cared for females leave the male or accept or offer themselves for extra pair copulation (EPC). In marmots , the male is monogamous and invests in the offspring when the territory is small and isolated. Otherwise, males invest more in EPCs and partner guarding. Among the birds of paradise , the Manucodia keraudrenii is monogamous because of its specialization in low-nutritional fruits and both parents take part in raising young. The reproductive success of lapwing examined in Northern England correlates for the males with the territory. Polygynous males produce 58% to 100% more young on average than monogamous males, while the reproductive success between polygynous and monogamous females does not differ significantly.
Raising youngsters together
Behavioral biologists postulate a correlation between the amount of parental care required for the offspring and the intensity of partner ties. Thereafter, the probability of socially monogamous behavior increases with the necessary parental effort to raise the young. Under precocial birds therefore monogamy should be less common than among nest stools . Genetic studies of socially monogamous birds show that a significant proportion of the young were not conceived by males, who seem to look after “their” offspring in an apparently tight couple. Raising boys together does not therefore mean that a couple only has sexual contact with one another. A connection between monogamy and rearing the offspring together has not been empirically proven.
According to some evolutionary biologists , the choice of partner is determined by the total investment of the parents in conception, birth and care of the offspring. According to this, the sex with the larger investment behaves more choosy when choosing a partner, but is also more limited in this choice. According to the “differential allocation” hypothesis, the investment of the parents in the offspring should depend on the perceived attractiveness of the partner. Another hypothesis is that attractive males with many "extra pair" populations (EPC) should invest less in rearing boys. According to this, females with unattractive males would be susceptible to EPCs, which is empirically supported in birds . At z. For example, in the case of the blue tit , the paternal investment in rearing young is dependent on the female's body decoration, and young females in particular invest more in rearing if the young are descended from an attractive male. With blue tits and zebra finches , unattractive males invest more in rearing their young than attractive males. Overall, attractive males were found to play a less important role in rearing young birds when EPCs are common. On the other hand, attractive males perform in z. B. Kestrels as better hunters make a greater direct contribution to the rearing of young than unattractive males.
The males of the dunnock participate in the rearing of the young of a female depending on their possible paternity.
"Mate Guarding" hypothesis
According to the “mate guarding” hypothesis, risks and the expense of guarding a partner can result in a gain in reproduction. Different strategies of partner guarding result from different goals of the sexes during reproduction.
According to studies, the conflict of interests of the sexes in mating leads to different strategies of the females to maintain the mating status.
The intraspecific aggressiveness of females during the breeding season can have various functions, such as: B. Monopoly of existing resources, defense against intraspecific brood parasitism or defense of the mating status. Research shows that monogamy in some bird species is also caused by the aggressiveness of the females towards other females. The females of the European star , the house sparrow and the flycatcher aggressively drive away other females who have not yet mated. The females of the European star are more aggressive during the advertising period, but also when the male has access to another nest with which it can attract a second female to mate. With this behavior, the females seem to secure the undivided contribution of the male to rearing young. In other types such as B. the red grouse or grouse , in which the males make no contribution to the rearing of young, the females seem to secure limited food resources for their offspring.
Also with the common spotted iguana , the aggressiveness of the females in their territory prevents one male from monopolizing more than one female. In mammals z. For example, baboon females are aggressive towards females who show interest in their male or in which the male shows interest. In other socially organized mammals such as the African wild dogs , wolves and jackals , female aggressiveness prevents the pregnancy of lower-ranking females. The alpha male of these animals is therefore socially polygynous, but monogamous in terms of reproduction.
The females of the gravedigger Nicrophorus defodiens , a beetle, behave aggressively towards their male and attack it e.g. B. with bites as soon as it attracts other females to copulate with pheromones , whose eggs would then be placed in the same corpse.
According to the “mate guarding” hypothesis, females should offer sex services to control the mating behavior of the male if there is a risk that he might want to mate with another female. However, this form of guarding is rarely found in socially monogamous species.
In the case of the European star, some females offer themselves to their male repeatedly every hour for copulation, e.g. B. when the male tries to attract another female with his song. This behavior is practiced by females for up to four days after the young have hatched, which can be interpreted as female control over the male's mating behavior. In the blue tit, females guard a male depending on his attractiveness. Also in some mammals, such as B. the Dikdiks , monogamy seems to have developed from the partner guarding.
Excessively frequent copulations e.g. B. in lions , kestrels or monogamous porcupines are initiated by the female. The classic explanation for this behavior is a strengthening of the couple bond. In a more recent view, females seem to judge male vitality and health through sexual performance .
The males z. B. the mountain hut singer behave aggressively against rivals and then turn against the female, sometimes until it leaves the nest. In mallards may be forced pairing come by one or more males. In this case, the partner of the mallard reacts aggressively against the “rapist” and immediately tries to copulate with his partner.
According to the "Mate-Guarding Hypothesis", males should guard their females especially during the fertility period in order to increase the probability of their fatherhood . Successful partner guarding reduces the costs for the production of ejaculate , which is limited and, in contrast to the earlier view, not free.
The males of z. B. rodents , mongooses , ungulates and primates guard the female after copulation and prevent other males from copulating with the female by displaying possession or by force. In some species, the male mates repeatedly with the female, increasing the likelihood of his fatherhood. With the barn swallows , a male guards the female especially during her fertility period. If a female barn swallow leaves the nest alone while laying eggs, the male will often give a warning call , causing nearby swallows to flee and often preventing an "extra pair" population. The female of the spider Linyphia litigiosa attracts males with pheromones exuded from their web. After mating, the male destroys the net, reducing the likelihood of rivals finding their way to the female.
In some species, such as B. Boobies , the males throw the first egg out of the nest when paternity is in doubt.
With a "mating plug" (also "copulation plug", "sperm plug", "vaginal plug" or sphragis ) the males close z. B. some insects , lizards , spiders and mammals after copulation the female cloaca or vagina to prevent further copulation of the female. For example, in the case of the garter snakes , scorpions and dried fruit moths, the male closes the female cloaca with a gelatinous plug after copulation. In bumblebees , the plug also contains linoleic acid , which tends to prevent the females from further copulations.
Genetic foundations of monogamous behavior
A group of US neurobiologists investigated the genetic basis of monogamous behavior and published the results in the journal Nature in 2004 . The researchers examined two closely related species of the vole , the meadow vole ( Microtus pennsylvanicus ) and the prairie vole ( Microtus ochrogaster ). The male of the meadow vole live solitary and polygamous, while the males of the prairie vole live in a lifelong brood care community in nature , which, however, is not always sexually exclusive. The researchers found a neurophysiological difference in the males of both species. The monogamous prairie voles had significantly more receptors for vasopressin than the meadow voles. The researchers isolated the gene that is responsible for the production of the vasopressin receptor and inserted this gene into the forebrains of polygamous males. The authors described the result as “we substantially increase partner preference formation” (“we substantially increase the formation of pairing behavior”). A single gene could affect so complex social behavior and therefore be an explanation for that, the social behavior in the course of evolutionary history sometimes seems to change relatively quickly. Larry Young, one of the authors of the study, pointed out that the results cannot be applied to human behavior because the arrangement of the vasopressin receptors in the human brain is not comparable to that in voles.
The population geneticist Dr. Gerald Heckel from the Zoological Institute at the University of Bern demonstrated in July 2006 that the gene identified by the US researchers alone cannot be responsible for the monogamous behavior of the mice. The Bern-based researcher analyzed the DNA of 25 mouse species and discovered the monogamous variant of the so-called “loyalty gene” in all examined animal species, except in the polygamous mouse species mentioned and in another, likewise polygamous, mouse species. Nevertheless, almost all of the species examined live polygamously despite the presence of the “loyalty gene”. Accordingly, there can be no general connection between genetic and social behavior patterns and the absence or presence of certain, natural variants of the “loyalty gene”. In a press release from the University of Bern, Dr. Heckel said that the results of his study showed “that monogamy developed in mammals regardless of the slight change in only this single gene: the simple genetic programming of such a complex and important behavior as mating behavior is very unlikely.” A genetically rigidly programmed mating behavior would also be "Certainly a disadvantage for organisms such as rodents , which have strongly varying population densities and have to constantly adapt their reproductive strategy to changing conditions." In 2019, 24 new candidate genes were described which are divided into four monogamous classes of vertebrates (mammals, birds, amphibians, Fish) are upregulated.
The sex hormone oxytocin , which has important functions at birth in humans and, according to some studies, causes a short-term increase in confidence after administration into the nose and appears to influence a man's social distance from strangers, has an effect on couple bonding .
Monogamy is not very common among mammalian species at around 3% to 5%. Among others are among the monogamous mammals some bat - species , a few mouse - and rat species, several rodents in South America (eg. Agoutis , pacas , acouchi and Maras ), some seal species, the giant otter , the Canadian beaver , some African antelope species (e.g. duiker , dikdiks or klipspringer ) as well as some primates , including gibbons and some New World monkeys (e.g. marmosets and tamarins ).
According to the evolutionary biologists and ethologists van Schaik and Dunbar, monogamy, such as that found in gibbons, marmosets and tamarins, has emerged as protection against infanticide , the killing of the young of a female by their new sexual partner. Also, the man could have been protected by the establishment of monogamous lifestyles before infanticide.
Monogamy has been attributed to around 14 of the approximately 200 or 10% to 15% of the primate species . According to recent studies by Fuentes, there is purely monogamous behavior in seven species of primate and is therefore just as little common at 3% as it is in other mammalian species. In the meantime, infidelities in males and females have also been observed in gibbons , for example in the white-handed gibbon . However, since there is primarily a couple relationship and the male cannot know whether a child is from another, this is still protected from infanticide. A distinction is made between two types of monogamous behavior; in which the male participates directly in looking after the young or indirectly by defending the territory . However, primates of the first group are occasionally polyandric and primates of the second group are occasionally polygynous. In 2014 it was reported that the southern red throated owl monkey ( Aotus azarae ) "actually seems to be one hundred percent faithful": "Biologists were able to show that all 35 young animals examined actually descended from the parents who raised them."
Monogamy in humans
Since there is no data available about the social life of the direct ancestors of man in terms of phylogenetic history , one examines "originally" living human societies instead. In 1949, anthropologist George P. Murdock published studies of the social structure of 238 different human communities around the world. The system of monogamous marriage existed in 43 communities. It was concluded that before contact with the western world, 80% of human communities were polygynous. Since a harem was reserved for men with power and social status, most men lived with a woman.
Other estimates by anthropologists about the frequency of monogamous human societies range between around 20 and 50 percent, but have the deficiency that they only reflect the official conditions and not the actual practice. Nevertheless, these estimates can be interpreted in such a way that strictly adhered to monogamy is a rather rare behavior in human societies. Recent anthropological studies, such as B. by Helen Fisher , show behavioral patterns such as affair and the change of partners as in all epochs up to early history recurring characteristics of the human mating behavior.
In humans, female and male mating behavior is very variable and has developed depending on environmental and learned factors. Through genetic studies on the X chromosome and autosomes arguments (eg. As in the Aka in for polygyny Central African Republic , Mandinka in Senegal , San in Namibia , Basques , Han and Melanesians in Papua New Guinea ) for long periods of human evolution found , whereby the results differ geographically . For the time of human expansion from Africa 80,000 to 100,000 years ago, studies of mitochondrial DNA show an increase in the female population , but not a substantial increase in the male population of early humans . The descent of today's people from a few men is interpreted , among other things, as a result of sexual selection . It was not until 18,000 years ago that the male population of humans increased, which coincides with the advent of agriculture . From the results z. B. concluded that with the advent of agriculture there was a change towards monogamy.
The Roman marriage law with the ideal of monogamy shaped European cultural history for centuries. Irrespective of state regulations, there were wedding rites, such as the Roman wedding . A particularly strict legal regulation was the Lex Iulia et Papia from the time of Augustus, which was valid for over 500 years.
The marriage law of the Catholic Church and also the Civil Code (BGB), which came into force on January 1, 1900, prescribe monogamy. There are laws similar to BGB in Germany in many countries around the world.
In Family, Private Property and the State, Friedrich Engels took the view that omnigame early humans lived without exclusive social or sexual ties and that children could therefore not be assigned to a father. With the advent of private property , according to Engels, men established monogamy to control female sexuality and thus her fatherhood, in order to be able to pass on their wealth to their children.
Monogamy and marriage
The term monogamy is often used for "monogamy" in which the legal institution of marriage is only open for a couple's relationship and bigamy , polygamy or harem education are excluded. Some legal systems sanction both plural marriage and adultery . For example, until 2011 there was no possibility of divorce in Malta . In some cultures, the expectation of marital fidelity extends beyond the death of a spouse.
The combination of the terms monogamy and marriage is rather unusual in the western world today; Instead, the term monogamy is often used for the type of cohabitation in a civil partnership - regardless of the legal form.
Today's sexual practice and freedom of partner choice in modern western culture, as well as the simplicity of divorce and remarriage within the same national legal system, have led to an increase in so-called “serial monogamy”. In serial monogamy, people have several consecutive monogamous relationships (in the case of marriage, time marriages ) that they end after a certain period of time. This behavior has a smooth transition to promiscuity due to the associated change of sexual partner .
Due to the increasing emancipation of women, there are also more and more women who are breaking out of life relationships that are unsatisfactory for them and looking for a new partner.
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