Humans are far closer to meerkats and beavers for levels of exclusive mating than we are to most of our primate cousins, according to a new University of Cambridge study that includes a table ranking monogamy rates in various species of mammal.
Previous evolutionary research has used fossil records and anthropological fieldwork to infer human sexual selection. While in other species, researchers have conducted long-term observations of animal societies and used paternity tests to study mating systems.
Now, a new approach by Dr Mark Dyble from Cambridge's Department of Archaeology analyses the proportions of full versus half-siblings in a host of species, as well as several human populations throughout history, as a measure for monogamy.
Species and societies with higher levels of monogamy are likely to produce more siblings that share both parents, says Dyble, while those with more polygamous or promiscuous mating patterns are likely to see more half-siblings.
He devised a computational model that maps sibling data collected from recent genetic studies onto known reproductive strategies to calculate an estimated monogamy rating.
While still a rough guide, Dyble argues this is a more direct and concrete way to gauge patterns of monogamy than many previous methods when looking at a spectrum of species, and human societies over thousands of years.
"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."
The question of human monogamy has been debated for centuries. It has long been hypothesised that monogamy is a cornerstone of the social cooperation that 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 permitted polygynous marriage – where a man is married to several women at the same time.
To calculate human monogamy rates, Dyble used genetic data from archaeological sites, including Bronze Age burial grounds in Europe and Neolithic sites in Anatolia, and ethnographic data from 94 human societies around the world: from Tanzanian hunter-gatherers the Hadza, to the rice-farming Toraja 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 study, published in Proceedings of the Royal Society: Biological Sciences, has humans at an overall 66% rate for full siblings, placing us seventh of eleven species in the study considered socially monogamous and preferring long-term pair bonds.
Meerkats come in at a 60% full sibling rate while beavers just beat humans for monogamy with a 73% rate. As with humans, this suggests a significant trend towards monogamy for these species, but with a solid amount of flexibility.
The white-handed gibbon comes closest to humans in the study, with a monogamy rate of 63.5%. It's the only other top-ranked "monotocous" species, meaning it usually has one offspring per pregnancy, unlike the litters had by other monogamous mammals.
The only other non-human primate in the top division is the moustached tamarin: a small Amazonian monkey that typically produces twins or triplets, and has a full sibling rate of almost 78%.
All other primates in the study are known to have either polygynous or polygynandrous (where both males and females have multiple partners) mating systems, and rank way down the monogamy table.
Mountain gorillas manage a 6% full sibling rate, while chimpanzees come in at just 4% – on a par with dolphins. Various macaque species, from Japanese (2.3%) to Rhesus (1%), sit almost at the bottom of the table.
"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.
Among the few with a similar evolutionary shift are species of wolf and fox, which have a degree of social monogamy and cooperative care, whereas the ancestral canid was likely to have been group-living and polygynous.
The Grey Wolf and Red Fox sneak into the upper league with full sibling rates of almost half (46% and 45% respectively), while African species have much higher rates: the Ethiopian wolf comes in at 76.5%, and the African Wild dog is ranked second for monogamy with a rating of 85%.
Top of the table is the California deermouse that stays paired for life once mated, with a 100% rating. Ranked bottom is Scotland's Soay sheep, with 0.6% full siblings, as each ewe mates with several rams.
"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 believed to live in a stable, mixed-sex, multi-adult group with several exclusive pair bonds is a large rabbit-like rodent called the Patagonian mara, which inhabits warrens containing a number of long-term couples.
Dyble added: "This study measures reproductive monogamy rather than sexual behaviour. 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."
