The modern human is part of a small minority of mammals that live in monogamous pairings—and when it comes to partner fidelity, we sit somewhere in the middle between the Eurasian beaver and the white-handed gibbon. This is what evolutionary anthropologist Mark Dyble of Cambridge University concludes after comparing Homo sapiens with 34 other mammal species.
In the study, the researcher examined the offspring of these species and looked at the ratio of full siblings to half-siblings — that is, siblings who share both parents versus those who share only one parent. Humans, on average, have about 66 percent full siblings.
For comparison: the leader is the California mouse (Peromyscus californicus), with a 100 percent full-sibling rate. The African wild dog (Lycaon pictus) sits at 85 percent, and the Damara mole-rat (Fukomys damarensis) and the Ethiopian wolf (Canis simensis) each hover around 80 percent.
When placed on the list, humans sit between the Eurasian beaver (Castor fiber, 73 percent) and the white-handed gibbon (Hylobates lar; just under 64 percent), according to Dyble in the journal Proceedings of the Royal Society B: Biological Sciences. In contrast, many non-monogamous mammals include the black rhinoceros (Diceros bicornis) and the European badger (Meles meles), where the share of full siblings is roughly 20 percent.
In total, only nine percent of mammals are monogamous
Toward the bottom of the list are creatures such as the bottlenose dolphin (Tursiops truncatus) with about 4 percent, the orca (Orcinus orca) at around 3 percent, and various macaque species at roughly 1 to 2 percent. All the way at the end of the monogamy spectrum sits the Soay sheep. This wild Scottish island sheep shows a full-sibling share of just 0.6 percent.
“Monogamy is the dominant mating pattern in our species,” Dyble states in a university press release. “The vast majority of other mammals adopt a far more promiscuous approach to mating.” He notes that only about nine percent of mammal species are monogamous.
To gauge the level of monogamy in a species, Dyble used genetic data on siblings across different mammals. While the data can be imperfect due to gaps, he argues that this is the most direct and concrete method available for such a comparison.
Monogamy tops among primates: the moustached tamarin
Dyble also says that monogamy is observed in human societies even when men have multiple wives or women have multiple husbands. “There is a huge cultural diversity in human mating and marriage practices,” he explains. “But even the extremes within this spectrum sit above what we see in most non-human species.”
In stark contrast to humans, our closest relatives—the great apes—are markedly promiscuous. For example, the eastern gorilla (Gorilla beringei) shows full-sibling shares of only about 6 percent, and the common chimpanzee (Pan troglodytes) around 4 percent—an order of magnitude higher promiscuity than many monogamous mammals.
A leading example of monogamy among nonhuman primates is the moustached tamarin (Saguinus mystax) from the Amazon. This small primate, part of the Callitrichidae family, has full-siblings accounting for roughly 78 percent of its offspring.
Dyble suggests that human monogamy may have helped foster the growth of expansive kin networks, the first step toward forming large societies and networks of cultural exchange that have been crucial to human success.
