Sexual selection theory states that males compete for mate monopolization and larger males can sire more offspring than smaller males, thereby resulting in the evolution of sexual size dimorphism. Female grouping facilitates mate monopolization and increases intensity of sexual selection. Sexual selection should be particularly intense in mammals because females have the most parental investment due to gestation and lactation, making them a limiting resource for which males compete. Nevertheless, I found evidence suggesting a minor role of sexual selection in mammals. I found low values of the standardised variances in male reproductive success, Im, and Nonacs’s B indices. Phylogenetic confirmatory path analyses indicated that sexual dimorphism evolved after the evolution of large body size, regardless of the evolution of polygyny and breeding groups. Results are explained by a ‘gender neutral’ model, in which all individuals in a population are initially subjected to the same pressures of natural selection.