We retrospectively tested whether differences in activity patterns and foraging efficiency by males and females were responsible for sexual segregation in mule deer (Odocoileus hemionus). These deer spatially segregated by sex during the dry season (May–October), which encompassed parturition, and aggregated during the wet season (November–April), which included rut. Activity did not differ among types of social groups of deer (mixed-sex, adult male, or adult female), but was greater during the dry season than in the wet season. Foraging efficiency (percentage of active deer feeding) was greater in mixed-sex groups than in groups of either adult males or adult females during both wet and dry seasons. We rejected the hypothesis that differences in activity or foraging efficiency lead to spatial segregation of the sexes in mule deer; no aspect of that hypothesis will explain why sexes use space or other resources differentially. We propose a new approach that incorporates niche theory and reconciles past difficulties in how best to interpret sexual segregation in ungulates.