https://www.pnas.org/doi/full/10.1073/pnas.1200717109

Discussion

The transition from promiscuity to pair-bonding in a species living in hierarchically organized groups requires a mechanism that would resolve the male’s dilemma, i.e., the conflict between investing in appropriation and production, in favor of the latter (10). Several such mechanisms have been recently advanced in the literature, including those focusing on communal care, mate guarding, and mate provisioning (4–8). Using a series of simple models that build on earlier work (10), I have shown that, under biologically realistic conditions (e.g., when the group size is not too small, competition between males is strong, and the effects of male provisioning and care are not too large), the population is not able to escape the low fitness state at which males invest exclusively into competition for mating. This conclusion is not changed qualitatively, even if one accounts for an elevated genetic relatedness between males arising from their philopatry. Note that communal care provided by females, the importance of which has been stressed in a number of recent publications (4, 6–8), is even less likely to become established because of low relatedness between females who disperse to different groups on maturity. Moreover, females may benefit from multiple matings (46–48), which implies additional selection against pair-bonding.

The power and implications of the male’s dilemma discussed above have not been generally acknowledged in the discussions of human transition to pair-bonding (10).The solution of the male’s dilemma proposed here builds on the idea of mate provisioning augmented by the explicit consideration of (i) females’ evolutionary response to provisioning and (ii) the role of males’ dominance ranks in determining their preferred actions.

Mate provisioning has double benefits, one of which (mating) is immediate and another (increased fertility and decreased between-birth interval) is delayed. These benefits are most pronounced for low-ranked males who have a low chance of winning a mate in competition with top-ranked males. One, therefore, should expect that it is low-ranked males who will attempt to buy mating by provisioning.

Note that, if there are more males at the bottom than at the top of the hierarchy, selection benefiting the “masses” may become stronger than selection benefiting the “elite”. Top-ranked males can easily beat out or chase away the low-ranked males and steal the paternity, making the investment of low-ranked males in production wasteful. However, after females start developing preferences for being provisioned, the low-ranked males’ investments start to pay off. In the model presented here, male provisioning and female faithfulness coevolve in a self-reinforcing manner. At the end, except for a very small proportion of the top-ranked individuals, males invest exclusively in provisioning females who have evolved very high fidelity to their mates.

Overall, females are not predicted to become completely faithful, but rather, the level of their faithfulness is expected to be controlled by a balance between selection for better genes (potentially supplied by top-ranked males) and better access for food and care (provided largely by low-ranked males).

Overall, my results confirm the theoretical plausibility of what has been viewed as a critical step in the evolution of our own species—the transition from promiscuity to strong pair-bonding. The model shows that such a sexual revolution could have been initiated by low-ranked males who started provisioning females to get matings; after the process got underway, it would lead to a kind of self-domestication, and the end result is a group-living species comprised of provisioning males and largely faithful females.

​