Study finds humans less promiscuous than most animals

By Stephen Beech

Humans are far less likely to sleep around than our primate cousins, reveals new research.

We sit comfortably in the "Premier League of monogamy" - between meerkats and beavers - while the vast majority of other mammals take a far more "promiscuous" approach to mating, say scientists.

A new University of Cambridge study includes a league table of monogamy rates in various species of mammal.

It shows humans are far closer to meerkats and beavers for levels of exclusive mating than we are to most primates.

The most promiscuous species is Scotland’s Soay sheep, with a monogamy rating of just 0.6%, as each ewe mates with several rams, according to the study.

Top of the table is the California deermouse that stays paired for life once mated, with a 100% rating, according to the findings published in the journal Proceedings of the Royal Society B Biological Sciences.

Previous evolutionary research used fossil records and anthropological fieldwork to infer human sexual selection while, in other species, scientists have conducted long-term observations of animal societies and used paternity tests to study mating systems.

Now, Cambridge Dr. Mark Dyble has employed a new approach that analysed the proportions of full compared to half-siblings in a host of species, as well as several human populations throughout history, as a measure for monogamy.

He said species and societies with higher levels of monogamy are likely to produce more siblings that share both parents, 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, Dr. Dyble argues it 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.

Evolutionary anthropologist Dr. Dyble said: “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.

“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.”

It has long been suggested that monogamy is a "cornerstone" of the social cooperation that allowed humans to dominate the planet.

But anthropologists have found 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, Dr. Dyble used genetic data from archaeological sites, including Bronze Age burial grounds in Europe and Neolithic sites in Anatolia.

He also used ethnographic data from 94 human societies around the world: from Tanzanian hunter-gatherers the Hadza, to the rice-farming Toraja of Indonesia.

Dr. Dyble said: “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."

The study has humans at an overall 66% rate for full siblings, placing us seventh of 11 species 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, Dr. Dyble says that suggests a "significant" trend towards monogamy for the 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 usually 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 with the Soay sheep at the bottom.

Dr. Dyble said: “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."

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 (46%) and Red Fox (45%) sneak into the upper league with full sibling rates of almost half, while African species have much higher rates.

Dr. Dyble said: “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.

“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 long-term couples.

Dr. 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.”