Human beings are 66% monogamous, study finds; see classification with other species

Monogamous relationships are a topic that always comes up for debate – whether out of curiosity, relationship conflicts, or old questions about “what’s natural.” Now, a new study led by an evolutionary anthropologist Marc Dyblefrom the University of Cambridge (United Kingdom), adds an interesting piece to this puzzle by comparing our reproductive modes to those of 34 other mammal species.

Published last Wednesday (10) in the magazine Proceedings of the Royal Society BResearch suggests that from a reproductive perspective, humans are among the species with the highest rates of monogamy – but with important nuances. The survey places humanity in seventh place among 11 species classified as socially monogamous. Thus, it is estimated that there are 66% full siblings (i.e. brothers and sisters who share a father and a mother).

How the study ‘measures’ monogamy in the real world

Instead of asking about customs or using only behavioral descriptions, the researchers took a more direct route. Observe, in practice, how families form over time. The logic is simple. When there is more reproductive exclusivity, it is more likely to find siblings with the same two parents. In systems with more partners, half-siblings increase.

“Species with higher levels of monogamy tend to produce more siblings who share both parents”explains Dyble, in a press release. “Meanwhile, those with more polygamous or promiscuous systems tend to generate greater numbers of half-siblings.”

To make this comparison, the author created a computer model crossing recent genetic data with known reproductive strategies. In the human case, he combined information from archaeological sites with ethnographic data from current societies. In total, 103 populations from different periods were analyzed, spanning approximately 7,000 years.

What the numbers say about us

The overall result attracted attention because it placed humans well above most non-monogamous mammals. “There is a monogamous elite that humans fit comfortably into. The vast majority of other mammals take a much more promiscuous approach to mating,” Dyble said.

And here’s an important subtlety: The study acknowledges that the rules of marriage and relationships vary widely across cultures — and anthropology has already described that a large portion of pre-industrial societies accepted polygamy. Yet when we consider the “bottom line” in terms of parentage (full siblings versus half siblings), reproductive monogamy seems predominant.

“There is enormous cross-cultural diversity in mating and marriage practices among humans, but even the extremes of this spectrum are still above what we observe in most non-monogamous species,” underlines the researcher.

Humans, beavers and meerkats: similar, but not so much

In the “table” of the study, humans (66%) are close to species such as: the meerkat (60%), the white-handed gibbon (63.5%) and the beaver (73%). This proximity does not, however, mean that we live like these animals. Dyble himself points out that our social systems are very different: “Almost all other monogamous mammals live in cohesive family units, consisting only of a breeding pair and their young, or in groups where only one female reproduces.”

In humans, it is common for there to be larger, more stable social groups, with multiple monogamous adults and multiple females having children within the same network. According to the study, one of the few mammals with a relatively similar organization would be the Patagonian mara, a rodent that lives in burrows with several long-term pairs.

What about other primates? The contrast is strong

If the comparison is made with our closest evolutionary relatives, the difference is great. In many primates, polygynous (one male with several females) or polygynandre (multiple partners for both) systems predominate. The figures cited in the text are very low for chimpanzees (4%), mountain gorillas (6%), as well as several species of monkeys with even lower rates.

This supports the idea that monogamous humans may have emerged as a rare transition in the mammalian world. “Based on the mating patterns of our closest living relatives, such as chimpanzees and gorillas, human monogamy likely evolved from non-monogamous group living. This appears to be a fairly unusual transition among mammals.”underlines the Cambridge professor.

Why would this be important for human evolution?

One hypothesis discussed in the study (and in analyzes on the subject) is that more stable ties may have favored cooperative care. This is when not only the father and mother invest in their offspring, but also other members of the group. This type of support tends to be more common in monogamous species and may have been crucial for larger family networks, large-scale cooperation, and more complex societies.

Reproductive monogamy is not the same as sexual monogamy

Finally, Dyble himself draws attention to a key point: the study measures reproductive monogamy, not sexual behavior. In humans, culture, contraception and social organization can separate sex from reproduction as is the case in some mammals.

In the end, the conclusion becomes less moral and more interesting: compared to the rest of mammals, we are, of course, relatively monogamous – but in our own way, within large social networks, cultural rules and diverse emotional strategies that do not fit into a single definition.