Humans, less promiscuous than meerkats, but more than beavers

The study, published in Proceedings of the Royal Society: Biological Sciences, is based on the idea that if a species is monogamous, most of a pair’s offspring will be full siblings. On the contrary, in species more prone to promiscuity, it is likely that siblings are more frequently half-siblings.

The results place humans at an overall sibling ratio of 66%, giving them seventh place among the eleven species considered socially monogamous, exactly above meerkats (60%) and below beavers (73%). At the top of the ranking is the California deer mouse, which maintains its partner for life once mated, with a score of 100%. In last place is the Soay sheep from Scotland, with 0.6% siblings, since each ewe mates with several rams.

The white-handed gibbon is the most similar to the humans studied, with a monogamy rate of 63.5%. It is the only other species that typically has offspring through pregnancy, unlike litters of other monogamous mammals. The only other non-human primate in the top division is the mustachioed tamarin: a small Amazonian monkey that usually produces twins or triplets and has a full sibling rate of nearly 78%.

All other primates in the study have polygamous or polygynander mating systems (in which males and females have multiple partners) and rank very low on the monogamy chart. Mountain gorillas reach a full reproduction rate of 6%, while chimpanzees reach just 4%, the same level as dolphins. Several species of macaques, from Japanese (2.3%) to Rhesus (1%), are toward the bottom of the table.

“Based on the mating patterns of our closest relatives, such as chimpanzees and gorillas, human monogamy likely evolved from non-monogamous group living, a highly unusual transition among mammals,” says Dyble.

Strong social groups

Among the few animals showing a similar evolutionary change are the wolf and fox species, which exhibit some degree of social monogamy and cooperative care, while the ancestral canid probably lived in groups and was polygamous. The gray wolf and red fox sneak into the top league with full sibling rates of almost half (46% and 45% respectively), while African species have much higher rates: the Ethiopian wolf reaches 76.5%, and the African wild dog comes in second for monogamy with a rate of 85%.

“Almost all other monogamous mammals live in compact families, consisting only of a breeding pair and their young, or in groups where only one female breeds,” says Dyble, “while humans live in strong social groups where several females have children.”

The only other mammal thought to live in a stable, mixed, multi-adult group with multiple exclusive pair bonds is a large rabbit-like rodent called the Patagonian mara, which inhabits burrows containing multiple long-term pairs.

Cooperative societies

The question of human monogamy has been debated for centuries. It has long been considered a cornerstone of social cooperation that has allowed humans to dominate the planet. However, anthropologists find a wide range of mating norms among humans. For example, previous research shows that 85% of pre-industrial societies allowed polygamous marriage, meaning a man marrying multiple women at the same time.

To calculate rates of human monogamy, Dyble used genetic data from archaeological sites, including Bronze Age cemeteries in Europe and Neolithic sites in Anatolia, and ethnographic data from 94 human societies around the world: from the Hadza hunter-gatherers of Tanzania to the Toraja rice farmers of Indonesia.

“There is enormous cross-cultural diversity in human mating and marriage practices, but even the extremes of the spectrum remain above what we observe in most non-monogamous species,” explains the anthropologist.

For Dyble, monogamy may have served humans well as societies adopted more cooperative systems. By having a greater number of full siblings, the chances of altruistic cooperation increase, as individuals can expect a close genetic relationship with their siblings. This type of cooperation, based on kinship ties, could have been fundamental for the development of shared parental care behaviors and for the creation of complex social networks, key aspects for the development of our large brain and our cognitive abilities. In fact, it has been proposed that monogamy and paternal care are coevolutionary elements essential to our survival and success as a species.