The majority (98%) of the 174 female elephants in South Africa's Addo Elephant National Park are tuskless. Long-term records are used to assess trends in tusklessness in this elephant population throughout the 20th century. These data are then used to test hypotheses of possible causes of this tusklessness. It is shown that attributes of the region's vegetation (structure and/or nutrient composition) and selective hunting cannot provide adequate explanations for the high frequency of tusklessness among the Addo elephants. Non-selective genetic changes resulting from the population's isolation, small size and bottlenecks (during the 1800s and 1920s) are suggested to be primarily responsible for the observed tusklessness. The rapid increase in the frequency of tusklessness during the past 70 years provides a phenotypic indicator of underlying genetic drift.