Patterns of maternal expenditure in Plains bison (Bison bison bison) are equivocal, with different studies yielding conflicting results. Therefore, our objective was to investigate the unsettled question of differential maternal expenditure in Plains bison. By observing the nursing behavior of known age calves, we measured post birth maternal expenditure on sons and daughters between birth and 3 mo of age. Calf weights at 6 mo of age also were measured. Because male calves grow faster and are larger than females as adults, we predicted sons would attempt to acquire more energy via differential nursing behavior and would be heavier at 6 mo of age. The results of our t-tests revealed no significant differences between sons and daughters in any nursing behavior. Male calves, however, performed all behaviors approximately 12% (range 8–20%) more frequently than female calves. For example, despite the nursing attempts of sons being rejected nearly 13% more often than that of daughters, sons nursed about 10% more frequently and 9% longer than daughters did. This occurred despite mothers terminating nursing bouts equally for both sexes. Therefore sons appear to demand (and obtain) more energy from mothers than daughters.