Lukas and Clutton-Brock view "social monogamy" somewhat differently: as a breeding male-female pair ranging together, with or without their offspring, associating with each other for at least one breeding season.
They suggest that in some mammalian species, females started finding higher-quality foods in areas that were farther apart from each other. As a result, the females would aggressively defend territories that contained this food, keeping other females out.
"As they then started to spread out further, that's when males seem to have changed their mating strategies," said Lukas, lead author of the study.
Other species that came to specialize in more abundant food, particularly in the grasslands, evolved to be polygynous, Clutton-Brock said.
The Cambridge team did a separate analysis of primates -- using more species than Opie's group did -- and did not find the same association between social monogamy and male infanticide as the other study. They haven't pinpointed the discrepancy, but the independent research groups may have classified species differently, or it may have to do with different sample sizes, researchers said.
Huck also had issues with some of the assumptions regarding various species in this study. It also appears, she said, that these results focus on mating monopolization -- breeding with only one mate -- rather than how some animals evolved to live in pairs.
"Unless we are clear about the classifications and what exactly we are talking about, we cannot be clear in what the results we get out of a complicated evolutionary model actually signify," she said.
What about humans?
Clutton-Brock cautioned against drawing any definite conclusions about humans from the study but said it is possible that the dietary and resource patterns his paper described could have something to do with the evolution of human breeding strategies, as well as a need for extended paternal care in our species.
Given that other great apes are polygynous and that human males and females differ so markedly in their average body size and longevity, Clutton-Brock says "the ancestral condition for humans is probably polygyny."
Clutton-Brock and Lukas said they were unaware of Opie's team's study until just a few days ago; otherwise they would have hooked up beforehand to compare notes.
It's not too late. The Cambridge scientists say they intend to connect with Opie's group to trace the origins of their differences, which may in turn put them closer to the roots of human togetherness.