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 fea tures of mammalian life-history diversity. In polygynous mammals, sexu al selection by male-male competition often favors increased body size . Because adult body size, and hence reproductive success, are commonl y 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 comparison s of sexual dimorphism in body size and litter size variations, after controlling for the effect of body mass, for 106 species of mammals. T he 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 wh ole range of species of all included taxa. Thus, the results support t he hypothesis that sexual selection for male body mass reduces the siz e of mammalian litters, establishing a new consequence of sexual selec tion.