Social monogamy in mammals evolved in solitary ancestors to allow males to monopolise widely-spaced females, says new research from the Department of Zoology.
Dr Dieter Lukas led the comparative study alongside Professor Tim Clutton-Brock, a renowned expert on mammalian social systems and principal investigator of the Kalahari Meerkat Project.
Their research, published in Science, shows that monogamy, a system in which a single male associates permanently with a single female, evolved as a mating strategy where males were unable to monopolise and defend multiple females over large territories. It did not evolve, as has been thought, due to a need for extra offspring care from the father, which most likely arose as a consequence of monogamy itself.
Lukas classified the social system of all mammalian species for which information exists, a sample of two and a half thousand species. He found that nine percent were socially monogamous, including large carnivores like jackals and wolves, and a number of primate species. This is much lower than estimates of the extent of social monogamy in birds, where it occurs in about ninety percent of species.
“Previously there have been different ideas about the evolution of social monogamy in mammals,” says Lukas. “With this study we were able to test all these different hypotheses at once.” Their first finding was that monogamy arose in species where individuals were solitary and rarely interacted except to breed. They were also able to dispel previous hypotheses regarding the evolution of monogamy, an advance that was possible, says Lukas, due to the volume of information they collected.
Monogamy was thought to be possibly linked to a need for the male’s support in raising offspring, for example, if the female alone could not provide enough food or adequately defend the young. This study shows that paternal care evolved after monogamy was already present. “With the amount of information that we had, we were able to really pinpoint when these transitions happened, and which happened first. Paternal care indeed evolves after monogamy is present, and seems to be a consequence. It seems to occur in about half of all socially monogamous species, and once it does evolve it provides clear benefit to the female.”
They found instead convincing support for the hypothesis that monogamy arose as a mating strategy where males could not defend access to more than one female. This is linked to population density, and indirectly, to diet. In species where social monogamy evolved, females are dispersed widely. Lukas and Clutton-Brock found that while monogamous and non-monogamous species do not differ much in female home range size, monogamous females defend their territories much more vigorously from other females.
This most likely comes down to the density of food, which determines the level of competition, and the benefits of territoriality. The study found a link between monogamy and species that rely on patchily distributed but high quality food sources, such as meat and fruit. Many carnivores, consequently, are monogamous – herbivores, which rely on low quality but widely distributed food sources such as grass, tend not to be.
Where females are spaced far apart, males cannot guard more than one. “If females are dispersed really widely,” says Lukas, “then the best strategy is to stick with her, defend her, and at least make sure you sire all her offspring. In short, the best strategy is to be monogamous.”
The analysis did not include humans, and Lukas believes that the complications of human culture may provide further reasons for monogamy in Homo sapiens. “There is a huge debate as to what the social system in humans actually is – monogamy seems to be pretty widespread these days, but we don’t, for instance, know how widespread it is in indigenous populations. Another thing that’s peculiar about humans is that we have pairs that live within larger groups, something we don’t observe in any other mammal. This makes it really difficult to understand whether what we observe in humans is parallel to what we observe in other mammals, which is unlikely to be the case.”
[I wrote the University of Cambridge press release – this is the pre-edited draft version]