His number-crunching found that these factors alone were not enough to move a species away from promiscuity. The models did work, though, with a few adjustments.

First, he stopped assuming that all males would act the same. Instead, he tested what would happen if only the low-ranking males in the group offered food to females in return for mating opportunities. These weaker males had less to lose by switching strategies because they wouldn’t get very far through fighting anyway.

The other key change was realizing that these low-ranking males would select faithful females.

“When I factored those things in, then things start to happen with the formation of pair bonds,” Gavrilets said. Pair-bonding ultimately swept through almost the entire group.