Scientists ranked monogamy across mammals and humans stand out

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

Measuring Monogamy Through Siblings

The new research takes a different path. Dr. Mark Dyble of Cambridge's Department of Archaeology examined the ratio of full siblings to half-siblings across many mammal species, 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 links sibling data from recent genetic studies to known reproductive strategies. The result is an estimated monogamy rating that can be compared across species and cultures.

While the model is not meant to be perfectly precise, Dyble says it offers a more tangible way to compare mating systems across both animals and human societies over long stretches of time.

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

"The finding that human rates of full siblings overlap with the range seen in socially monogamous mammals lends further weight to the view that monogamy is the dominant mating pattern for our species."

A Longstanding Debate About Human Pairing

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

At the same time, anthropologists have documented enormous variation 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 human monogamy levels, Dyble analyzed genetic evidence from archaeological sites, including Bronze Age burial grounds in Europe and Neolithic settlements in Anatolia. He combined this with ethnographic data from 94 human societies worldwide, ranging from the Hadza hunter-gatherers of Tanzania to the rice-farming 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," said Dyble.

The findings, published in Proceedings of the Royal Society: Biological Sciences, show that humans have an overall full sibling rate of 66%. That places our species seventh out of eleven studied and firmly within the group considered socially monogamous, with a preference for long-term pair bonds.

How Humans Compare to Other Mammals

Meerkats show a 60% rate of full siblings, while beavers rank slightly higher than humans at 73%. In both cases, the data point to a strong tendency toward monogamy alongside some flexibility.

The species most similar to humans in the study is the white-handed gibbon, with a monogamy rate of 63.5%. It is the only other highly ranked "monotocous" species, meaning it typically produces a single offspring per pregnancy rather than litters.

Another notable entry is the moustached tamarin, a small monkey from the Amazon. It is the only non-human primate in the top group and usually gives birth to twins or triplets, resulting in a full sibling rate of nearly 78%.

All remaining primates in the study display 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 rate of just 6%, while chimpanzees come in at only 4% -- on a 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 highly unusual among mammals," said Dyble.

A similar shift appears in some wolf and fox species, which practice forms of social monogamy and cooperative care, even though their ancestral canids were likely group-living and polygynous.

Grey Wolves and Red Foxes enter the upper tier with full sibling rates close to half (46% and 45% respectively). African species score even higher, with Ethiopian wolves at 76.5% and African Wild dogs ranking second overall with an 85% monogamy rating.

At the top of the list is the California deermouse, which pairs for life once mated and achieves a full sibling rate of 100%. At the opposite extreme is Scotland's Soay sheep, with just 0.6% full siblings because each ewe mates with multiple rams.

What Makes Humans Different

"Almost all other monogamous mammals either live in tight family units of just a breeding pair and their offspring, or in groups where only one female breeds," said Dyble. "Whereas humans live in strong social groups in which multiple females have children."

The only other mammal thought to maintain stable, mixed-sex, multi-adult groups with several exclusive pair bonds is the Patagonian mara, a large rabbit-like rodent that lives in communal warrens made up of long-term couples.

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

"This study measures reproductive monogamy rather than sexual behavior. In most mammals, mating and reproduction are tightly linked. In humans, birth control methods and cultural practices break that link."

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