Monogamy through our evolution

The origin of sex

Most living things include some kind of sexual behaviour. From bacteria to primates to all types of life forms. A question arises, perhaps an obvious one: Why?

Sexual reproduction allows a recombination of maternal and paternal genes to occur in each generation. In each reproductive cycle, the dice are “rolled again”, within an order, and the generation of genetic variability is facilitated. In this way, the repertoire of individuals on which natural selection can act is increased.

What would happen if more than two individuals were involved in reproduction? Indeed, in each generation, a greater genetic diversity would be obtained. But it could be excessive. In other words, if in each generation there was an extensive redistribution of genes, the variants that are formed and are advantageous, would have little chance of being transmitted intact to the next generation.

There is a certain conflict between facilitating the emergence of individuals with new genetic variants and avoiding such a high degree of turnover that successful gene combinations are destroyed in each generation. It seems that the strategy of involving two individuals in breeding allows a good balance to be struck between these two options. (Laurence, 1996).

Chimpanzees and bonobos. Sex beyond reproduction

Sex, therefore, in its origin, is a vital function comparable to nutrition, respiration, temperature control… Its function is to maintain in populations an adequate representation of gene combinations (each individual would be associated with a “gene combination”). However, in the course of evolution, sex has acquired new functions. These new functions are most evident in humans and their closest “relatives”, the chimpanzees and bonobos.

We share approximately 99% of our DNA with chimpanzees and bonobos. It is therefore not surprising that they are the animals that most resemble us. These similarities are not only obvious in our external appearance, but also include emotional, cognitive and behavioural aspects.

There are two species of chimpanzee, the common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus). Both species are found exclusively in Africa. They are very similar in appearance, although the chimpanzee is more robust and the bonobo more graceful.

Chimpanzees form flexible groups of 20-40 individuals. They are characterised by a strict social hierarchy, with a dominant male at the top. Confrontations and aggressive behaviour related to the place of group members in this hierarchy are frequent.

They may show different patterns of sexual behaviour. A promiscuous relationship pattern is common, in which a female in oestrus mates with all the males in the group. Although the highest-ranking males in the hierarchy have priority. Relationships with a tendency towards exclusivity may also be observed. In these cases, the couple may stay in the group or move away from it, living a kind of “honeymoon”. In either case, the relationship ends a few days after the end of the female’s oestrus period (Goodall, 1999 and Tutin, 1979).

Bonobos also live in groups, but their behaviour is quite different from that of chimpanzees. In this case, females occupy the dominant positions in the social hierarchy. On the other hand, conflicts between group members are less frequent and are usually resolved friendly.

In this species, sex plays a very important role in their social relationships. In fact, sexual touching and rubbing is used almost as a form of greeting. Thus, bonobos are very promiscuous, with frequent sexual encounters among all members of the group, including individuals of different ages and the same gender. They mate, even when the female is out of oestrus period, and their sexual behaviours are very varied, including oral sex and face-to-face mating. It is precisely this almost permanent sexual activity that is responsible for lowering the degree of aggression in the group. (Furuichi, 2019).

In both chimpanzees and bonobos, sexual behaviour is complex and goes beyond its reproductive function. In fact, it plays an important role in the relationships among individuals and conditions the group dynamics.

However, these two species show clear differences in their sexual behaviour. While chimpanzees reflect their rivalries through hierarchical conflicts over access to females in heat, bonobos use sex as a mechanism to relax tensions in their social relationships.

Our ancestors and effectiveness of monogamy

Sexual behaviour is associated with the partner pattern, which can be monogamous or polygamous (including both polygyny and polyandry).

In general, polygamy allows a greater number of parings and it is associated with a higher probability of leaving offspring. In mammals, polygamy has different consequences for males and females. The female is “programmed” to take on pregnancy, lactation and breeding, thus she would benefit from a monogamous model that ensures the male’s “collaboration” in breeding. On the other hand, a polygamous “no-strings-attached” model might be more convenient for the male, leaving him free to mate with as many females as possible and to have as many offspring as possible.

However, this reasoning may be too simplistic when applied to some species, such as ours, with long and demanding breeding. In these cases, a single female has difficulty in performing this task adequately. If, for this reason, the survival of the offspring is endangered, the model is inappropriate for both the male and the female. It is of little use to leave many offspring if they have little chance of reaching adulthood.

Note that the monogamous couple structure refers to a “social” monogamy, regardless of whether it is accompanied by a “sexual” monogamy. The male needs to “believe” that he is the father of the offspring. Even if he is not. In fact, when genetic analysis is done on offspring of monogamous couples, there is always a percentage of offspring whose DNA gives away that it is the result of a mating outside the supposedly monogamous relationship. Logically, this situation has no consequence for the evolutionary success of the species (Estupinyà, 2013).

Chimpanzees and bonobos can help us to imagine what our common ancestor behaviour was like about 7 million years ago. Probably, he displayed a complex sexual behaviour that was not restricted to reproduction and partnership models that facilitated the formation of groups. It seems logical that this should be the case, but it is an approximation that we cannot verified.

Chimpanzees and bonobos have found a balance in this dilemma between monogamy and polygamy through a strategy that we can consider intermediate. In principle they are polygamous, with more or less restrictions, everyone mates with everyone. But the males do not disengage from the females after mating. All members of the chimpanzee or bonobo group form a kind of team that ensures the group’s well-being. The females take on most of the task of breeding, but the males take on other functions such as protecting the group, both from predators and from other groups of the same species, and hunting small mammals including other primate species.

On the other hand, chimpanzees and bonobos follow a patrilineal pattern. The females leave the group when they reach the sexual maturity. As consequence, the males who belong to the group use to be related. Finally, the fatherhood may be considered secondary.

We can continue to trace the evolution that led to the emergence of our species. Probably, Australopithecus and earlier hominids, suffered a major drought that transformed their environment. The rainforest turned into savanna where the food was scarce and the predator pressure was high. In this difficult scenario, it was especially important the close collaboration between male and female. They must form a team to share tasks and ensure the survival of offspring. This circumstance may have favoured the emergence of a high degree of monogamy in our species and we would have abandoned the model attributed to the common ancestor of chimpanzees (Edgar, 2016). Over time this model proved to be very successful.

In turn, this family pattern would allow these hominid species to form groups that could resemble current hunter gatherer populations. In these groups we can observe a tendency towards monogamy but they maintain certain aspects of polygamy. In fact, among different hunter-gatherer groups, there is a wide variation of relationship patterns that rarely involve strict monogamy.

Thus, the Yanomami of the Amazon have different family patterns which, in addition to the monogamous relationship, they include the polygynous and polyandrous relationship (Chagnon, 2014). Also, the pygmy tribes of Central Africa show a high flexibility degree in their couple relationships. For example, promiscuity is allowed among young people as well as and the possibility of separation in monogamous adult couples (Turnbull, 1962).

Sexuality in the service of monogamy

The strengthening of monogamy was brought about by the progressive disconnection between reproduction and sex. This process allowed sexuality to take on an important role as a facilitator of emotional bonding in the couple. In this sense, our species shows very particular aspects related to sex, such as the presence of orgasm and the absence of an oestrus period that regulates sexual relations. (Morris, 1994).

If a female rat has her clitoris stroked with a small brush when placed in a particular spot in the cage, the rat tends to spend more time in that spot than in the rest of the cage. The same is true for males who are allowed to mate in restricted areas of the cage. Such experiments seem to show that, in all mammals, sexual intercourse could be associated with pleasure (Cibrian-Landeral, 2010). However, male and female orgasms far exceed in intensity and complexity the degree of pleasure observed in members of other species.

On the other hand, in most mammals, females have an oestrus period in which they are receptive to mating. These periods are often seasonal. Nature attempts to regulate the birth of young at a time of year, usually spring, when food is plentiful and the temperature is warm.

During oestrus, females show that she is in a fertile period. For example, female dogs produce substances, pheromones, with a very strong odour in the perianal area. This signals to males that she is sexually receptive. Thus, sexual behaviour is limited to the fertile periods of the female dog, which coincide with ovulation. Outside these periods, interest in sex disappears.

However, in our species and some other rare exceptions, such as bonobos, the fertile period is hidden, there is no oestrus and the sexual receptivity of females is maintained independently of ovulation.

The association of sexual intercourse with a pleasure that is provided reciprocally and can be accessed independently of fertile periods allows for the emergence of a very intense and special bond in the human couple. It is an emotional connection associated with the feeling of love. In this way, the capacity for commitment between human partners is very high and can shift the balance from polygamy to a greater degree of monogamy.

On the other hand, the human capacity for high emotional commitment in relationships is not restricted to the realm of the couple. It is, of course, evident in the mother-child relationship, but it also affects all interpersonal relationships including other family relationships and friends.

Monogamy and the construction of societies

the sexual permissiveness observed in the hunter gatherer groups tended to disappear with the arrival of Neolithic societies around 14,000 years ago. What was the reason? Possibly the emergence of private property and specific social roles that did not exist in these groups.

In agricultural and farming societies, such as medieval European societies, social roles are inherited. The son of a farmer will be a farmer and the son of a blacksmith will be a blacksmith. Of course, the king must be a descendant of kings. Land and herds of domestic animals are also inherited. These societies could not survive if roles and property were redistributed in each generation.

Therefore, there is a need to regulate inheritance. It is necessary to know who is the father’s direct descendant. The mother is obviously always known. The question is to identify the father in order to maintain the social order through the generations. In this sense, a family model based on the monogamous couple relationship can be very useful.

From his point of view, a polygynous relationship (one male partner mates with several females) would also be acceptable. However, a widespread pattern of polygyny without polyandry would mean that many men would be left without partners.

In a sense, the evolutionary heritage provides a solid foundation for building societies based on monogamy. This meeting point is entrenched through high social pressure, often endorsed by religion, which will condemn and punish sexual relations outside the monogamous couple. Thus, these relationships will tend to be infrequent and clandestine.

As an exception, men with greater political power and economic capacity could afford the luxury of polygyny. Sometimes with social complacency and, on other occasions, through veiled permissiveness. These exceptions would have little impact on the social system.

Over time, agricultural and livestock-raising societies have driven hunter-gatherer groups to near extinction. This is due to their greater efficiency in producing food, which in turn implies a larger population size (Cavalli-Sforza & Cavalli-Sforza, 2015). As a result, monogamy is imposed as a model of couple relationship and family structure in human populations.

The capacity for emotional commitment is not restricted to the partners. It also affects family relationships (parental and sibling) and between non-related members. The strength of these relationships is a key contributor to the possibility of forming societies and to their success throughout most of our recent history.

The industrial revolution, approximately 300 years ago, brings many changes. Social classes appear and power is associated with money. Therefore, the inheritance of roles and property becomes less important in these societies. Wealth is inherited and it is not easy to change social class. However, the rigidity of the medieval system decreases and the possibilities for individuals to change their role and economic power in relation to their parents’ generation increase.

These changes allow the necessity of maintaining the monogamous structure to be questioned again. The “recovery” of more flexible models of pairing is evident in these societies. The legalisation of divorce, the recognition that people have the right to have their sexual orientation respected, the use of contraceptives, fertility and adoption programmes, among others are moving sexuality even further away from the reproductive function. On the other hand, the differences between classical gender roles become fewer. Moreover, the couple shares the caregiving work with other institutions related to education.

As is common in group and social dynamics, it is a circular feedback process where it is difficult to distinguish causes from consequences.

The technological revolution accentuates the changes that took place in the industrial revolution. We keep our old inherited trend towards monogamy but it is not so necessary as it was in the past. To some extent, the monogamy is desire based rather than needed based. This situation offers a great opportunity to have a deeper and more satisfactory couple relationship.

However, it’s not all advantages! the relations based on desire involve a high effort. The external pressures (financial, familiar, social) are less intense and the stability of the couple increasingly depends on its bond.

This situation creates a new dilemma. We can enter into a mature relationship based on desire, but this type of relationship requires a great deal of emotional effort. At the same time, the majority of the population in the technological society puts a lot of effort into work, which makes it difficult to engage in personal relationships, including relationships with a partner. It is a complicated situation that is likely to generate a lot of conflict in today’s couples.

The “Be Right Back” episode of the Netflix Black Mirror series (2013) proposes a kind of solution to allow us to have a desire-based relationship without paying its emotional cost. You can get an android as partner!

At first glance, it may seem absurd. However, without knowing it, we can have bots as friends on our social networking websites. On the other hand, some commercial calls and our mobile assistant service (for example “Siri”) are handled by a robot too. It doesn’t seem that we are far away from creating robots to help us with household chores. What could be the next step?

If we were to create the right android physically, emotionally and intellectually we would solve the dilemma. We would have a high-quality relationship for little effort. Of course, it would have a high fantasy component, but so do normal relationships. Science fiction? The future? Who knows!

General overview

The human being presents a complex couple relationship with a high prominence of monogamy. Genetics and culture have contributed to the development of a dynamic model of partnership that has varied over time.

The foundations of this relationship were built millions of years ago. At an early stage, sex acquired the function of generating emotional bonds in the couple, as we can see in chimpanzees and bonobos. This tendency became progressively more pronounced and it facilitated the emergence of a couple model with a high monogamy component. This model allowed the formation of groups whose organisation could be similar to that of today’s hunter-gatherer.

In turn, this group model allowed the emergence of societies that implies a more complex level in the hierarchy of systems. The agricultural society based on the inheritance of property and social roles “demanded” a relationship model with a high component of monogamy.

In industrial and technological societies, social organisation ceased to be based on inheritance and the strict monogamous couple relationship model was relaxed. Among many other factors, the advent of contraceptives and greater equality in the roles of men and women contributed to this change.

Nevertheless, maintaining a partnership model based on mutual desire rather than need requires a great deal of effort. Given that in these societies there is often little time devoted to relationships, a new problem arises that is probably not yet solved. On the other hand, new technologies applied to robotics are invading all areas including personal relationships. However, it may be hard to imagine how it will affect us in the future.

Bibliography

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Luis Palacios
lpalacios@rivendelsl.com