Polygynous mammals often have male-biased sexual size dimorphism due to male–male competition for mates. However, fecundity selection may enlarge female body size. The Mongolian gerbil Meriones unguiculatus (Milne-Edwards, 1867) is a social rodent of promiscuous mating. Male gerbils exhibit social dominance hierarchy, which may differentiate reproductive success among male gerbils with larger males having more reproductive opportunities. This study investigated the hypothesis that social dominance hierarchy would result in male-biased sexual size dimorphism in Mongolian gerbils. We also tested the prediction of fecundity selection, that litter size would be positively related to female body mass of Mongolian gerbils. Body mass, carcass weight and body length of male gerbils were greater than those of females. Therefore, male-biased sexual size dimorphism was supported in Mongolian gerbils. Although litter size of female gerbils increased with increasing carcass weight, ecological constraints (e.g. burrow living and increased energy expenditure during pregnancy) probably kept female body size smaller than male body size of the gerbil. Social interactions may mediate sexual and fecundity selections on the body size of social rodents.