Alternative mating strategies occur in many animal lineages, often because males adopt tactics best suited to their own phenotypes or to spatiotemporal heterogeneity in the distribution of females. Garter snakes near a communal overwintering den in Manitoba show courtship in two contexts: competition from rival males is intense close to the den, but weak or absent when males court solitary dispersing females in the surrounding woodland. Larger size enhances male mating success near the den, but not mate location rates in the woodland. As predicted by the hypothesis that males match their tactics to their competitive abilities, our mark-recapture data show that larger, heavier individuals remained near the den, whereas smaller and more emaciated males moved to the woodland. To locate mates, woodland males relied upon substrate-deposited pheromonal trails and visual cues (rapid movement), whereas males in the crowded den environment ignored such cues and instead tongue-flicked every snake they encountered to check for sex pheromones. In arena trials, den males adjusted their courtship intensity to the presence of rival males, whereas woodland males did not (perhaps reflecting the lower probability of interruption by a rival). Thus, male garter snakes adjust the times, places, form and intensity of their reproductive behaviours (mate-searching tactics, intensity of courtship) relative to both their own competitive abilities and spatial heterogeneity in mating opportunities.