Some mammals may have turned to pair-living because of infanticide or isolated females.
Using an evolutionary tree of 230 primates as a framework, Christopher Opie of University College London and his colleagues ran simulations of evolutionary history to investigate what conditions might produce the behaviours of modern primates. They conclude that monogamy arose after males began guarding females to stop rivals from killing their offspring.
Tim Clutton-Brock and Dieter Lukas at the University of Cambridge, UK, used a similar method to study how monogamy came about in mammals generally. Using an evolutionary tree of more than 2,000 species, they found that monogamy tended to arise when females lived alone and were widely dispersed. Pair-living probably arose because males could not cover a large enough area to monopolize more than one female.
For a longer story on this research, see go.nature.com/glatpz