Sexual selection for male body mass and the evolution of litter size in mammals

Interspecific variation in litter size is one of the most puzzling features of mammalian life-history diversity. In polygynous mammals, sexual selection by male-male competition often favors increased body size. Because adult body size, and hence reproductive success, are commonly related to maternal expenditure, I hypothesize that sexual selection for male body size can favor female reproductive strategies of single-birth parental allocation. To test this hypothesis, I made comparisons of sexual dimorphism in body size and litter size variations, after controlling for the effect of body mass, for 106 species of mammals. The results show that increases in sexual dimorphism are accompanied by decreases in the number of offspring per litter. Body mass is related to reductions in litter size only for larger mammals, whereas sexual dimorphism is negatively correlated with litter size throughout the whole range of species of all included taxa. Thus, the results support the hypothesis that sexual selection for male body mass reduces the size of mammalian litters, establishing a new consequence of sexual selection.