Sexual dimorphism
Sexual dimorphism is the difference in body size between male and female members of a species as a result of intrasexual selection, which is sexual selection that acts within a sex. High sexual dimorphism and larger body size in males is a result of male-male competition for females. Primate species in which groups are formed of many females and one male have higher sexual dimorphism than species that have both multiple females and males, or one female and one male. Polygynous primates have the highest sexual dimorphism, and polygamous and monogamous primates have less. Humans have the lowest levels of sexual dimorphism of any primate species, indicating that we have evolved decreasing levels of polygyny. Decreased polygyny is associated with increased paternal investment.