In freedom, the social structure of monkeys is basically harem, although this is not the rule for all species. It is just very widespread. It exists in baboons, in all macaques, in patas monkeys, in guenons, and so on. This would explain why polygyny is more widespread in some species (a male has relationships with a different number of females) than monogamy (a male has a relationship with only one female).
Contrary to what one might think the female monkey has more decision-making power in the partnership. In essence, she chooses the partner or, at least, tries to do it, but she generally succeeds. The choice is always prudent, almost systematic and in the society in which monkeys live, it becomes fundamental. Of course, there are exceptions, but the exception does not confirm the rule, at least in this case. It is extraordinary, but female monkeys are able, without obviously having any scientific evidence, to distinguish one male from another according to the reproductive quality of their seminal fluid1. They have tools to assess the quality of their semen and, if you like, this also applies to humans. A frail-looking, submissive and insecure looking man is probably less prolific than a strong, sturdy, confident and even dominant male. Therefore, a female needs few aesthetic and behavioral elements to entrust and ensure the fertilization of her egg. She almost never does this at random. Monkeys are exceptionally good at this, but so are women, only to these elements, although not secondary, do they add the social position of the potential partner, his reputation and his economic willingness2.
Obviously, monkeys unlike us, do not use money nor do they evaluate the economic standing of the males with whom they mate, but they have other means to do so, for example, by ensuring that the male duly protects their offspring, including the one which is yet to be born and that they are willing to share the food with them. Other elements that are not secondary are the social position that the males have in the group, the number of females they have, especially in the harem social structures, their position in the hierarchical scale and the power they have to drive their rivals away.
In monkeys, the rhesus ones (rhesus macaque) in particular, females tend, in any case, to make the male genetic patrimony as variable as possible, in the sense that when they have the opportunity they betray even with submissive individuals with the precise aim of not hanging the egg. They do this with a certain dexterity, always trying to confuse the paternity of their offspring. Of course, this is very risky for them, but they know that it is always better to take a risk even if they might get pregnant. On the other hand, monkeys do not go in oestrus all year round and ovations, which give more guarantees of fertility, are a couple a year, not twelve. Then sperm is not always available and always fertile. It is therefore necessary to take advantage of every circumstance. Semen, it is true, is a very precious commodity, even if it is less than the egg, for the simple fact that the first one is produced one per month, while the other is made up of billions of spermatozoa. Then access to males within the group is not always easy as all females tend to have sexual relations with the dominant male who is not always available to all of them. In sexual relations, there is another important fact for females, which is that it is always better to have relations with the dominant male even without any guarantee of being fertilized because it is useful, in any case, to strengthen an understanding, an alliance, to protect each other and to protect the offspring. In short, in monkeys, the parental investment is measured by the energy that must be spent to deal with it, but also by cunning.
However, even males in the reproductive process are not just watching and doing as little as possible. They also think about success in order not to disperse their sperm. This is also why, in order to have a greater guarantee, they always tend to have relationships with as many females as possible. In fact, polygyny in these animals is widespread mainly for these reasons: maximum profit with minimal effort and the harem structure offers these guarantees. Among other things, it is much better for a male to have a number of females at his disposal than to have just one, because it could also be unfruitful and therefore frustrate his reproductive efforts.
Males are absolutely obsessed with fatherhood, but they must offer females every guarantee that their children are adequately protected and that they can grow up well-fed and healthy.
In the male and female reproductive strategies of monkeys an important role is played by the territory in which they are found, taking into account the presence of predatory animals which in some places are very dangerous and even numerous. In these cases, the maternal protection of the young and also that of the leader are very important. Furthermore, the male must be assiduous in guaranteeing the safety of the whole group, not just only an individual who is close to him, whether it be a female or an offspring. An environment with few dangers favors the peace of mind of the whole community and the guarantee that when searching for food it is not essential to be very attentive to the dangers. In this context, dominant males would not waste too much energy to protect the community and would have more time for reproductive activities.
Between humans and apes in general, there are considerable differences that make sexual intercourse very different between us and them: cyclical on the one hand and throughout the year on the other. Furthermore, ovulation is clear and manifest in apes. All the extraneous signs are evident: redness and swelling of the genital areas, while in women these signs are not detectable by males, on the contrary, they do not appear at all aesthetically. This has led to a significant difference between us and them over the course of the evolution of sexual relationships. The concealment of ovulation in our species has made a considerable contribution to empowering women with being able to choose a male partner. Women can therefore choose more than, for example, apes, and they prefer strong, handsome and tall men with a good degree of self-esteem, even without the possibility of spreading their genetic heritage. These physical and psychological characteristics indicate the presence in these males of good genes, in essence, they become important fitness indicators that should not be neglected by women. Therefore, what is important for a woman is a continuous availability on the part of a male who has these characteristics. How is it possible to have a desirable male always available? It is the monogamous relationship and maybe not is why monogamy is more widespread in humans than polygyny. A woman would gain nothing from a polygamous union. She would always be at the mercy of a dominant male who, among other things, disperses his genetic heritage with many females.
The conclusion we can draw that each species uses its own means for reproductive success. Therefore, all animals including apes, in a certain sense also humans, have the main purpose of spreading their genetic heritage. If to do so there is a social structure that offers more guarantees, so much the better, as in the case of living in a harem structure, without forgetting that it takes a great deal of energy to keep it under control. The reproductive strategy of an ape is not so different from that of many other animals, for example, birds, which have flashy and extravagant ornaments to attract the female, but men do have their own.
1 Small, M.F. 1993. Female choices: Sexual behavior of female primates. Ithaca
(N.Y.), Cornell University Press.
1 Buss, D.M. 2005. The handbook of evolutionary psychology. Boston, Pearson. (Psicologia evoluzionistica. Edizione italiana a cura di A. Tartabini, Milano, Pearson, 2012).