Sexual Reproduction and Male Parental Investment in Offspring

Hillard S. Kaplan, University of New Mexico

Sexual reproduction evolved as a means of increasing variability among offspring through the sharing of genetic material. A secondary effect of sexual reproduction is the divergent evolution of the two sexes. Sex is defined by gamete size; females produce larger gametes, reflecting their greater initial energetic investment in offspring. With increased investment beyond the energy supplied in gametes, the divergence between the two sexes becomes exaggerated. Females provide all investment to offspring in 97% of mammalian species. Human paternal investment is highly variable, but is often high. This paper presents an evolutionary model of differential investment by sex. It proposes that complementarity in the benefits of each sex's investments and mating market characteristics determine the relative investments of the two sexes. The model is applied to variation across and within human societies in paternal investment, marital stability, and the incidence of single parenting and sexual transmitted diseases.