New approaches to modeling primate socioecology: Does small female group size BEGET loyal males?

Humans are unusual in having stable male-female breeding bonds within multi-level societies. Such societies are not found in other apes, but have evolved independently in other primates, including several African papionins: hamadryas and Guinea baboons and gelada monkeys. Stable breeding bonds have been proposed to evolve either (1) because males can monopolize females when food distribution forces females to forage in small groups or (2) because females exchange exclusive mating for male services, such as protection from infanticide. Comparative studies are needed to test these hypotheses. In the meantime, we used an agent-based computer model to test the plausibility of these hypotheses. We simulated primates living in social groups within a larger population using a model we call BEGET (Behavior, Ecology, Genetics, Evolution, and Tradeoffs), which employed decision vectors, experimental evolution, realistic trade-offs, and phenotypic plasticity. We employed experimental evolution to generate male genotypes that varied in their competitive ability and in their long-term mating strategy. "Rover" males searched for and mated with any sexually receptive females whereas "Loyalist" males formed stable associations with particular groups of females. Much like living primates, the virtual primates exhibited tradeoffs between contest and scramble competition for access to females: Loyalists evolved larger body size than Rovers. We tested the effect of female foraging group size and the presence of infanticide and infant protection on the relative success of these strategies. We found that Loyalists achieved greater reproductive success than Rovers only when females were in groups smaller than four. Both Rovers and Loyalists sometimes evolved infanticidal behavior, but the presence of infanticide benefited Rovers rather than Loyalists, suggesting that the evolution of stable breeding bonds depends on the spatial distribution of females, rather than the risk of infanticide.