Humans are playing in the premier league of monogamous mammals, according to a new ranking of animals by their reproductive habits, but we may need a new manager to beat the beavers.
In the study from University of Cambridge, humans ranked 7th out of 35 species on the monogamy scale, pipping white-handed gibbons and meerkats, but lagging behind moustached tamarins and Eurasian beavers.
Dr Mark Dyble, an evolutionary anthropologist at Cambridge, said humans sit comfortably in the top flight for monogamous species, but the vast majority of mammals take “a far more promiscuous approach to mating”.
Animals in the lower ranks include feral cats, bottlenose dolphins and our close genetic relatives, chimpanzees and mountain gorillas. Scotland’s Soay sheep is at the bottom due to each ewe mating with several rams.
Scientists have reported rates of monogamy for human and animal populations before, but Dyble wanted to know where humans sat relative to other mammals. To do this, he analysed genetic data from animal and human studies and calculated the proportions of full versus half-siblings for each.
Societies and animals with higher levels of monogamy tend to produce more siblings that share the same parents, while those with more promiscuous mating habits are likely to have a higher proportion of half-siblings.
Dyble found that levels of monogamy varied substantially across more than 100 human populations he assessed. The lowest rate was seen at an Early Neolithic site in the Cotswolds where 26% of siblings were full siblings. Meanwhile, in four Neolithic populations in northern France, 100% were full siblings.
He then ranked humans and 34 other mammal species by the average proportion of full siblings. The top 11, led by the California deermouse, are all considered to be monogamous, while the bottom 24 are regarded as non-monogamous species.
Humans had a 66% rate of full siblings, meaning full siblings outnumbered half-siblings two-to-one. Beavers were ahead at 72% with meerkats just behind at 60%. Mountain gorillas came in at 6%, with chimpanzees and dolphins at 4%.
“As anthropologists, we’re interested in understanding the variation across human societies, but this is taking a step back from that and saying, OK, if we were any other species of mammal, we’d be broadly content with characterising ourselves as a monogamous species,” Dyble said. Details are published in Proceedings of the Royal Society B.
Though chimps and gorillas are close genetic relatives of humans, their societies are structured in very different ways. Chimps are largely promiscuous, with many males mating with many females. Gorillas have a polygynous system where a silverback male mates with half a dozen or so females.
Based on the mating patterns of chimps and gorillas, human monogamy probably evolved in a highly unusual transition away from non-monogamous group living. Why it evolved is unclear, but monogamous mating is strongly linked to the evolution of paternal care across the animal world.
Robin Dunbar, professor of evolutionary psychology at the University of Oxford, said previous work has put humans “right on the cusp between monogamous and polygamous species”. While some animals pair up for life, humans are often kept together by religious proscriptions and other social pressures, he added.
“If these religions lose their force, serial monogamy, or polygamy by any other name, quickly emerges,” he said. “There’s a risk here of confusing desire with reality: humans desire polygamy but are constrained into a grudging form of monogamy by social or religious threat.”
Dr Kit Opie, an evolutionary anthropologist at the University of Bristol, said the interesting question was how humans became monogamous in the first place. “Our closest living relatives, chimpanzees and bonobos have an entirely different mating system. I would argue that both the promiscuity of chimpanzees and bonobos and monogamy in humans are counterstrategies to male infanticide, which is acute in large-brained primate species,” he said.
“Females either try to confuse paternity, through promiscuity, so that all males in the group might be the father of the offspring, or provide paternity certainty, more or less, so that a single male is invested in the offspring and protects them,” he added.
Top 10 (percentage of siblings that are full siblings)
California deermouse (100)
African wild dog (85)
Damaraland mole-rat (79.5)
Moustached Tamarin (77.6)
Ethiopian wolf (76.5)
Eurasian beaver (72.9)
Humans (66)
White-handed gibbon (63.5)
Meerkat (59.9)
Grey wolf (46.2)
Red fox (45.2)
