Sex differences in spatial cognition have been reported for many species ranging from voles to humans. The range size hypothesis predicts that sex differences in spatial ability will only occur in species in which the mating system selects for differential range size. Consistent with this prediction, we observed sex differences in spatial ability in giant pandas, a promiscuous species in which males inhabit larger ranges than females, but did not observe sex differences in Asian small-clawed otters, a related monogamous species in which males and females share home ranges. Furthermore, the sex difference in giant pandas was observed during the period of male range expansion and outside female estrus, thus the potentially confounding influence of decreased female ability was avoided. Finally, all subjects in this study were raised in captivity and never actually inhabited different range sizes. Therefore these findings emphasize the importance of biological rather than experiential factors underlying sex differences in spatial cognition. These results are the first evidence of sex differences in spatial ability in the order Carnivora, and provide support for the range size hypothesis.