The classical view of mammalian mating competition focuses on combat and threat. By contrast, field studies have revealed that male mating success in some species is more strongly determined by mate location ability than by physical dominance. In thirteen–lined ground squirrels, competition in locating oestrous females is exacerbated by sperm competition that favours the first male to mate with a female. We used a female–removal experiment to identify the distinguishing characteristics of males that were the first at finding and mating with females. Each focal female was observed the day before she entered oestrus; the identities of males that made contact with her and the locations of their interactions were recorded. The following morning the females were temporarily removed, and we monitored male search behaviour in their absence. Males that arrived first were those that had spent more time with the female the previous day relative to their rivals. Time invested the day before, in turn, was highly correlated with male search persistence and familiarity with the female's likely whereabouts. These results demonstrate that differential fertilization success can arise from information asymmetries among males: the advantaged individuals are those that have greater a priori knowledge of the reproductive state and spatial locations of prospective mates than rivals.