Scientists have classified monogamy between mammals and humans

For decades, scientists studying human evolution have relied on fossil evidence and anthropological fieldwork to draw conclusions about mating behavior. In nonhuman animals, researchers have instead turned to long-term observation of social groups and genetic paternity testing to understand reproductive patterns.

Measuring monogamy through siblings

New research takes a different route. Dr. Mark Dyble of Cambridge’s Department of Archeology has investigated the ratio of full-siblings to half-siblings in many species of mammals, as well as among human populations spanning thousands of years. This sibling balance serves as a proxy for how exclusive mating tends to be.

According to Dyble, species or societies with higher levels of monogamy tend to produce more children who share both parents. In contrast, populations with more polygamous or promiscuous mating systems generate a higher proportion of half-siblings.

To quantify this pattern, Dyble developed a computational model that combines sibling data from recent genetic studies with known reproductive strategies. The result is an estimated rating of monogamy that can be compared across species and cultures.

While the model isn’t intended to be perfectly accurate, Dyble says it offers a more tangible way to compare mating systems across animal and human societies over long periods of time.

“There is a first league of monogamy in which humans sit comfortably, while the vast majority of other mammals take a much more promiscuous approach to mating,” said Dyble, an evolutionary anthropologist at the University of Cambridge.

“The finding that human sibship rates overlap with the range seen in socially monogamous mammals adds further weight to the view that monogamy is the dominant mating pattern for our species.”

The long debate about human mating

Whether humans are naturally monogamous has been debated for centuries. Many scientists have suggested that stable pair bonding helped foster the cooperation that allowed humans to thrive globally.

At the same time, anthropologists have documented vast differences in human marriage systems. Earlier research shows that 85% of pre-industrial societies allowed polygynous marriage – where a man is married to several women at the same time.

Genetic data from ancient and modern societies

To estimate the level of human monogamy, Dyble analyzed genetic evidence from archaeological sites, including Bronze Age burials in Europe and Neolithic settlements in Anatolia. He combined this with ethnographic data from 94 human societies around the world, from the Hadza hunter-gatherers of Tanzania to the rice-growing Toraja people of Indonesia.

“There is a huge amount of cross-cultural diversity in human mating and marriage practices, but even the extremes of the spectrum still sit above what we see in most non-monogamous species,” Dyble said.

Findings published in Proceedings of the Royal Society: Biological Sciencesshow that humans have an overall full sibling rate of 66%. This ranks our species seventh out of the eleven studied and firmly in the group considered to be socially monogamous, with a preference for long-term pair bonds.

How humans compare to other mammals

Meerkats show a 60% full sibling rate, while beavers are slightly higher than humans at 73%. In both cases, the data show a strong tendency toward monogamy, along with some flexibility.

The most human-like species in the study is the white-handed gibbon, with a monogamy rate of 63.5%. It is the only other highly rated “monocot” species, meaning it usually produces a single offspring per pregnancy rather than litters.

Another notable record is the bearded tamarin, a small monkey from the Amazon. It is the only subhuman primate in the highest group and usually gives birth to twins or triplets, resulting in a full sibling rate of nearly 78%.

All of the remaining primates in the study exhibit either polygynous or polygynandrous (where both males and females have multiple partners) mating systems and fall much lower in the rankings.

Mountain gorillas show a full sibling count of just 6%, while chimpanzees only 4% – on par with dolphins. Macaque species score even lower, ranging from 2.3% in Japanese macaques to just 1% in Rhesus macaques.

An unusual evolutionary shift

“Based on the mating patterns of our closest living relatives, such as chimpanzees and gorillas, human monogamy probably evolved from non-monogamous group living, a transition that is very unusual in mammals,” Dyble said.

A similar shift occurs in some species of wolves and foxes, which practice forms of social monogamy and cooperative care, even though their canid ancestors were probably gregarious and polygynous.

Gray wolves and red foxes enter the upper tier with full sibling ratios of almost half (46% and 45%, respectively). African species score even higher, with Ethiopian wolves at 76.5% and African wild dogs in second place overall with a monogamy rating of 85%.

At the top of the list is the California deermouse, which pairs for life after mating and achieves a full sibship rate of 100%. At the opposite extreme is the Scottish Soay sheep with only 0.6% full siblings as each ewe is mated to several rams.

What makes people different

“Almost all other monogamous mammals either live in tight family units of only the breeding pair and their offspring, or in groups where only one female reproduces,” Dyble said. “While people live in strong social groups in which several women have children.”

The only other mammal thought to maintain stable, mixed, multi-adult groups with few exclusive pair bonds is the Patagonian mara, a large rabbit-like rodent that lives in communal corridors formed by long-term pairs.

Dyble emphasized that the study focused on reproductive outcomes rather than sexual behavior.

“This study measures reproductive monogamy rather than sexual behavior. In most mammals, mating and reproduction are closely linked. In humans, contraceptive methods and cultural practices break this link.”

“Humans have a range of partnerships that create the conditions for a mix of full and half-siblings with strong parental investment, from serial monogamy to stable polygamy.”