Of course, when Hollywood stars or politicians have extramarital affairs, the whole world rumbles. But if we peek into human biology, anthropology and sociology, the monogamous human appears as a very weird notion.
We are mammals, and if we look to the mammalian world, just 3 to 5% of the about 5,000 species of mammals form lifelong, monogamous bonds - this is the case of beavers, wolves, gibbons, jackals, foxes, some bats, dwarf deer and antelopes (like dik-dik).
A strictly monogamous animal mates only inside the pair. For example, in the case of geese, albatrosses or some parrots, the death of a partner totally compromises mating for the other, for that season or for life.
But biologists say that strictly sexually monogamous species are almost non existent. Most mammals have just a social monogamy: they pair up to mate and raise offspring, but still have flings. For example, in the case of the Arctic foxes, 25 % of the litters are not fathered by the male of the pair. Having offspring from multiple fathers allows a female to increase the genetic variation in her cubs. This increase in variation improves the chances that at least one cub in a litter will have the genetically proper stuff to survive for a long term in such a harsh and changing environment.
Monogamy is a breeding behavior that is considered to give offspring a better survival chances, as in monogamous couples females receive all the support of the male in raising newborns to adulthood, from food to protection.
It's clear: a pair achieves more food and survives better than the bachelors. The "married" jackals were found to live on average 3-4 years longer than the solitary ones.
In beaver families, there is a strong need for cooperation to maintain their dams and pools, that's why beaver social units are so tight. Thus, monogamy evolved in situations where young need a better cooperation of both parents in raising them. That's why humans, with their long childhood, form monogamous pairs.
The Story of the Primates
But how do our closest relatives behave? The Old World monkeys have basically two type of mating systems: harems of females (polygamy), in which one male only mates with the females of the group (like in gelada monkeys, colobus monkeys or proboscis monkeys) or a promiscuous system, in which all the females mate with all the males (like in many macaques), as the males form coalitions (they do not compete or fight inside the group, except for the hierarchy-establishing conflicts).
A very interesting case occurs in some more primitive monkeys of the new world, like marmosets and tamarins: polyandry (which is rare in general in the animal world, more common being amongst some birds, 1% of them, like nandu, cassowaries, some shore birds (like phalaropes), lily-trotters or buttonquails), i.e. a female having several partners (2-3 in this case), which mate only with her. Polyandry could be stimulated for various reasons, one of them being that monogamy means investing your genes in just one variant, and, just like gambling, he/she may be the best, right, not right or the worst. At the same time, the female receives the support of several males in raising the offspring, as none of them can be excluded as the father. Other mammals known to be polyandrous are some Australian marsupials (Antechinus).
But in apes, we find the only case of real monogamy amongst primates: the gibbons. Even if as the swans, gibbons are symbols of faithfulness, they are now known to cheat, abandon and even "divorce" one another, exactly like the humans. After the age of 18, for the "married" gibbons the retirement period starts. They can no longer produce offspring, losing the parent quality, but they are accepted by the "family" of one of their offspring, as grandparents. This way, they benefit from the community protection and, when it's about feeding, they can get some scraps. The solitary gibbons do not go beyond the retirement age, as they are not able to defend and feed themselves.
This is not the case of our closest relatives: orangutans form "lose" harems (a male's territory overlaps with that of several females, which will mate only with him, like in the case of many carnivorous mammals), while gorillas live in strict harem societies.
What about our closest relatives, chimps and bonobos? That's total promiscuity.
The Sperm Cue
In the case of harem societies and real monogamous system, the female mates only with one male, that's why there's no sperm competition. Sperm competition appears in promiscuous or polyandrous species. What does human sperm says about this?
A 2007 research shed light on this, investigating sperm samples from humans, gorillas, chimpanzees and rhesus macaques (which practice the promiscuous system). The human sperm was found to travel at about 0.2 km/hour. The sperm from chimpanzees and macaques had a speed of 0.7 km/h. A chimp female can have multiple sex partners in one hour, thus the sperm competition is much stronger in this case. But in the case of the gorillas, the sperm speed was only of 0.1 km/h. Female gorillas have just one sex partner at a given time. The chimp and macaque sperm also appeared to be more powerful, at about 50 piconewtons, while human sperm developed just about 5 piconewtons, and gorillas some lousy 2 piconewtons.
These results point that evolutionary, humans are mildly polygynous, balancing more towards the harem system. Evolutionary psychologists suggested that men are more likely to have extramarital sex, because of the male's urge to "spread genes" by broadcasting sperm. Both males and females attempt to increase their evolutionary progress by seeking out high-quality mates.
Other studies suggest that humans are equipped for sperm competition, which is widespread amongst promiscuous species. Women have "affairs" (extra pair copulations), and this is not a surprise, as we recently evolved (4-5 million years ago) from highly promiscuous chimpanzee-like species.
But in a species like ours, where the male invests all his resources in raising children inside a monogamous couple, spending them into genetically unrelated offspring means a biological disaster. For example, when men spend more time away from their partners (when their partners could get the opportunity to mate with other males), the number of sperm cells per similar sperm volumes rises sharply.
In one research, phalluses made after molds of human penises removed a sperm-like substance from an artificial vagina, pointing that the penis developed its shape to act as an anatomical squeegee. There are also sexual behaviors pointing to sperm competition. Women report that men thrust more deeply and quickly into the vagina after allegations of infidelity, a mechanism researchers believe is directed to sperm removal.
The authors believe that not only the increase in sperm cells after period of separation is a sign of sperm competition, but also their greatly increased libido in the same situation: the male wants to copulate as soon as possible and as much as possible, as insurance against possible extra-pair fecundation. When partners are separated for periods of time, males are more likely to arouse easily, produce more sperm, and even rape their partners.
Sexual conflict between males and females triggers a coevolutionary race between the sexes, in which an advantage gained by one sex selects for counteradaptations in the other sex.
It would be interesting to see in future studies if females developed mechanisms for increasing retention of sperm, after being inseminated by males with the best genes.
The human committed partnership between a man and a woman evolved for raising the children. Monogamy is invented for order and investment, not necessarily because it's natural, warn many researchers, which point that both social and sexual monogamy in humans is not a natural state. In fact, most primitive human societies and many evolved societies have been practicing the harem system. Researchers believe that monogamy only became established as hunter-gatherer societies took up agriculture and settled in houses, allowing the social roles of men and women to become more fixed.
It is clear that in humans, there is more paternal investment than in most other primates. Still, it is clear that males have less to lose than females by having extramarital sex. Women, on the other hand, would lose resources, and female promiscuity clearly does not boost the welfare of her children.
Still, human cultures have evolved so much that, amazingly, there are societies that forced our biology even beyond monogamy into polyandry. The marriage of a woman with more than one husband is extremely rare, but it does exist. The most common type of human polyandry is the fraternal one in which two (or more) brothers marry the same wife.
Various Himalayan nations practiced it: in Tibet, Kashmir, Nepal, Bhutan, Ladakh, Arunachal Pradesh (northeastern India) and Mosuo people (in southwestern China). The extinct culture of the Marquesan Islands (Pacific) practiced polyandry, but the phenomenon was also encountered amongst Amerindians (in the Canadian Arctic), Ceylon, Mongolia, South India (by Toda people), some Sub-Saharan African tribes and Guanches, the original inhabitants of the Canary Islands.
There are tribal societies considering that a child could and should possess more than one father. In many cases (like that of Tibet), polyandry was caused by a need to retain aristocratic titles or lands within the family or due to frequent absence of the husband from the household for long periods (so that usually only one husband was present). Poor farmers, too, could not afford to divide their small agricultural lands. Some anthropologists see in human polyandry a method of birth control, as the woman will have only one pregnancy, no matter the number of partners, while in polygyny, a man impregnates several women, resulting more children.