Infanticide by male raiders is common in primate species that form one-male multi-female harems. Operational sex ratio is strongly female biased in the harems and this bias causes infanticide by sexual selection among males. However, infanticide is observed in animal species, e.g., Lethocerus deyrollei, where operational sex ratio is strongly biased without harem structure. Thus, biased operational sex ratio rather than harem structure can make infanticide evolve. Then why is harem structure so common in infanticidal primates?

We hypothesize that harem formation is not the cause of but a counter strategy against infanticide. When infanticide by males is common, females prefer to form a harem with an alpha male that defend infants of the females. Using an individual-based model with a haploid genetic system, we examined the evolution of male infanticide and defending/deserting harem strategy for varying harem size. Our model demonstrated that male infanticide always evolved to its maximum degree but the defending strategy only evolved when each harem had multiple females. Our theory integrates the evolution of infanticide in animal species with and without harem structure.