Abstract
In theory, biased adult sex ratios combined with high population densities in physically coercive mating systems could trigger an extinction vortex, regardless of external factors such as predation or habitat loss. However, this has never been documented in the wild. In an exceptionally dense island population of Hermann's tortoises in Lake Prespa in North Macedonia, sexually coercive males dramatically overnumber females, inflict severe copulatory injuries and put them at risk of fatal falls from the island plateau's sheer rock faces. Harassed females are emaciated, reproduce less frequently, produce smaller clutches and have lower annual survival rates compared to females from a neighbouring mainland population. Sixteen years of capture‐recapture data reveal an ongoing extinction event and predict that the last island female will die in 2083. Paradoxically, while high population density might suggest prosperity, it can trigger population collapse under highly skewed adult sex ratio in a coercive mating system.