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The pros and cons of monogamy and polygamy


By Dr Penny Morgan


Penny is a retired zoologist who specialised in bird behaviour. Post retirement, she took a degree in law and now she is writing thrillers with an animal welfare theme. In this article, Penny compares the reproductive strategies of animals and humans.



Many varieties of pair bonds, or reproductive strategies, exist between sexually reproducing individuals, and some fleeting relationships that can scarcely be called ‘bonds’. But is there some factor determining which type of bond a species like ours adopts? And how does this affect our social structures?


Polygamy

Polygamy is defined as being married to two or more persons at the same time. There are several types:

  • Polygyny is the marriage of a man to several women at the same time

  • Polyandry is the marriage of one woman to several husbands, who may sometimes be brothers (fraternal polyandry). Polyandry is rare and it limits population growth. It may be found in poorer agricultural areas such as the Himalayas where it makes more sense to have several males to tend the poor land.

  • Polygynandry where both males and females have multiple partners

Polygamy seems to have been more prevalent in the past. King Solomon, for example, is rumoured to have had 700 wives. Louis XIV of France, the Sun King, had a great number of mistresses, both official and unofficial. His chief mistress at any one time carried the title of maîtresse-en-titre, and the most celebrated one, the Marquise de Montespan, bore him seven children.


Polygamy is fairly common in sub-Saharan countries such as Mali, Burkina Faso and Niger, and it frequently involves a 'bride price'. This refers to the payment or exchange of goods, money, or other valuable assets given by the groom or his family to the family of the bride as a condition for marriage. It is a common practice in some cultures where polygamy is accepted or prevalent. The bride price serves as a form of compensation or recognition for the bride's family, and it is often seen as a way to strengthen the social and economic ties between the two families involved.


Five principal reasons for men to maintain more than one wife have been suggested. Having more than one wife allows the husband to:

  1. have as many children as he desires

  2. heighten his prestige and boost his ego among his peers

  3. signal his high social status within the community

  4. ensure a sufficient availability of labour to perform the necessary farm work

  5. satisfy his sexual urges

There are, unsurprisingly, fewer reasons for women to favour polygamy. Women in such an arrangement may enjoy each other’s company and they can share the domestic duties!


In agricultural societies men perform physical labour such as felling trees, hunting, fishing, etc., while women do the harvesting, processing and preparation of food, in addition to other domestic duties. Older males benefit from having several young wives to share duties and provide sons as labourers. But it seems pretty clear that polygyny sanctions and perpetuates gender inequality, with co-wives being subordinated to their husband. Second and subsequent wives are often treated as dogsbodies. Women in polygynous unions tend to marry at a younger age into an arrangement that, by its very nature, is likely to foster jealousy, competition, and conflict. Although the husband ought, in principle, to treat his co-wives equally, in practice he will almost inevitably favour one over the others – most likely the youngest and most recent one. There is also the risk of disease spreading from one partner to another and inheritance disputes.


Two swans mated for life?

Monogamy

About 5 per cent of mammalian species are monogamous. Birds, however, are widely considered as model species for studies of monogamy, since it is the most common avian mating strategy (around 90 per cent of species). Both partners benefit from social monogamy through biparental provision and protection, the parental time investment needed to raise chicks being so great. It also provides paternity assurance for males. By the time the chicks leave the nest it is almost breeding season again, leaving little time to find a new mate and form a bond. Many species live in unpredictable environments where advantageous conditions for breeding cannot be predicted. In such cases, a pre-existing pair bond makes sense.


Social monogamy as opposed to sexual monogamy

Many species assumed to be monogamous are not quite as faithful as we (or their mates) may believe. This is quaintly termed 'extra-pair copulation' (i.e. cheating!). DNA tests of baby birds have shown that in over 75 per cent of these allegedly monogamous species, birds have mated with one or more individuals other than their ‘social mate’. The humble Dunnock is a prime example. As both males and females want as many healthy chicks as possible, the result is a series of fleeting trysts by both sexes. Whilst remaining socially monogamous, they are not sexually monogamous and the hazard of extra-pair copulation is that it may lead to sexual conflict and questionable paternity.


Today, most human societies are monogamous. Genomic and ethnographic studies suggest that there was a shift, around 5,000-10,000 years ago, from polygyny to monogamy and this is related to the shift from mobile to sedentary communities.


Costs and benefits

Are countries that practice polygamy less economically stable than those which practice monogamy? It is telling, perhaps, that polygamy is practiced in almost all of the twenty most unstable countries on the Fragile States Index, perhaps because richer men can afford more wives while poorer men cannot. In cultures that permit men to take multiple wives, intra-sexual competition that occurs causes greater levels of crime, violence, poverty and gender inequality than in societies that institutionalise and practise monogamous marriage.


Females may derive benefits from engaging in extra-pair copulation if they mate with specific males who possess good genes for their offspring, while trusting that their primary partners will continue to invest in the young, provided that they are unaware of the extra-pair copulation. Females could also avoid 'brood failure' due to an infertile mate by engaging in extra-pair copulation although this is relatively rare in humans when compared to socially monogamous birds. Ecological determinants of monogamy and extra-pair copulation show a certain flexibility, responsive to the social and physical environment.


So polygamy may benefit males more than females, and monogamy may benefit females more than males. According to Matt Ridley (see further reading) there is a 'monogamy threshold' which is reached when there are too many females mating with just one male and lots of males are left without mates. For these males, and for the less-favoured females, it then becomes beneficial to switch to monogamy. By mating with just one female, and helping her care for her young, these males are guaranteed to pass their genes on to at least one descendant.


Biological basis for monogamy

Neurobiological studies have suggested that the reward system in the brain, which utilises dopamine, reinforces monogamous behaviour. Two other hormones, vasopressin and oxytocin, are also implicated. Blocking oxytocin (the ‘love hormone’) in monogamous prairie voles changes their behaviour and they rush around after any furball!


'Humans originally evolved in multimale polygynous bands, like gorillas.'

Conclusion

According to scientific studies, the human mating system was considered to be moderately polygynous, based both on surveys of world populations, and on characteristics of human reproductive physiology. But there is genomic evidence of a relatively recent shift from polygyny to monogamy in humans. In the ethnological literature, there is general agreement that

humans originally evolved in multimale polygynous bands, like gorillas. But the development of extensive farming resulted in a decrease in the levels of polygyny and the change towards monogamy. Biparental care is essential in many species with high maintenance (like ours), increasing the likelihood of offspring survival. This favours monogamy. The most commonly-observed pair bond in humans is monogamous marriage, characterised by low levels of extra-pair paternity and high levels of paternal care.


Further reading

The Red Queen: Sex and the Evolution of Human Nature (1994) by Matt Ridley






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dowdle.vm
2023年8月07日

A recent TV programme involving Humanists UK Vice President Professor Alice Roberts featured the role of grandparents and prolonged breast feeding as a way of regulating the numbers of children being born to couples in Africa, while also ensuring greater survivability for children being born. So, it may be that a monogamous relationship brings with it greater commitment if there is only two sets of grandparents, who may feel a greater sense of responsibility towards their children and their children's children.

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