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Understanding why male mammals choose to mate with just 1 female

A male dik-dik antelope closely follows the female as she searches for food in her territory, guarding her against other males.
[Image courtesy of Peter Brotherton]

A new study in Science reports that scientists are getting closer to understanding a question they've long debated: why some male mammals – who could mate with several females – stick to just one. The new study, with authors from the University of Cambridge, is one of the field's most extensive to date.

While female mammals are pregnant, which can last from weeks to years in some species, males could mate with other females. This would allow them to produce more offspring, thus spreading their genetic influence more widely.

But a percentage of male mammals are socially monogamous—only mating with one female per breeding season. And this has long puzzled scientists, who have tried to identify the advantages social monogamy offers for decades. There are several competing ideas.

More recently, social monogamy in mammals has been explained by two hypotheses, one of which focuses on paternal care, suggesting that natural selection favored the pair-forming seen in social monogamy because care (including feeding and protection) from two parents makes it more likely the offspring will survive.

The second hypothesis suggests that males form pairs to guard their mates—something they have to do when females spread out.

Now, to better understand which hypothesis is more accurate, D. Lukas and T. H. Clutton-Brock have used data on more than 2,500 mammalian species to test the two, offering strong support for the second.

Their study benefited from a relatively newly available method, gene-based phylogenies. These are the tree-like genetic maps that researchers use to figure out how related two different species are, by comparing similarities in their genomes over time.

The researchers started their study by compiling life-history data on current mammalian species, classifying each species into one of three breeding strategies: solitary (where females of the species live alone in home ranges and mate with any male who shows up); socially monogamous (where a single breeding female and a single breeding male share a common range and mate with each other for more than one breeding season); or group-living (where females don't live in separate home ranges, but instead live together in a group).

Referring to their phylogenetic trees and using statistics, they worked to understand how females that used to be group-living or solitary might have come to be socially monogamous in the patterns we see today (which are different than patterns from long ago).

Statistics revealed that it was much more likely for a female to be socially monogamous today if her ancestors were solitary, living in large, distinct home ranges, than if her ancestors had lived in a group with others.

This finding sheds light on the setting in which social monogamy evolved—one in which females occupied separate home ranges and males, unable to roam far and wide to defend access to more than one, formed pairs with one female.

The results support the second hypothesis of social monogamy -- as a mate guarding strategy. The evolution of paternal care, meanwhile, though it characterizes most socially monogamous species today, was a secondary adaption, the authors suggest.